Book summary of Brain and Cognition Interim 2 - UvA

What is hemispheric specialization? - Chapter 4

How did the investigation into hemispheric specialization start?

For centuries, the effects of unilateral brain damage have revealed major functional differences between the two hemispheres. The most dramatic and most studied has been the effect of the left-hemisphere damage on language functions.

The major lobes (frontal, parietal, occipital and temporal) appear, at last superficially, to be symmetrical, and each half of the cerebral cortex of the human brain has approximately the same size and surface area. The two hemispheres are offset, however. The right protrudes in front, and the left protrudes in back. Anatomists of the 19th century found that the Sylvian fissure - the large sulcus that defines the superior border of the temporal lobe - has a more prominent upward curl in the right hemisphere than it does in the left hemisphere, where it is flat. Geschwind measured the temporal lobe and came to the conclusion that the planum temporale, around Wernicke's area, was larger in the left hemisphere.

The asymmetry of the planum temporale is one of the few examples in which an anatomical index is correlated with a well-defined functional asymmetry. By studying the cellular basis of hemispheric specialization, we seek to understand whether differences in neural circuits between the hemispheres might underlie functional asymmetries in tasks such as language. A promising approach is to look for specializations in cortical circuitry within homotopic areas - such as differences in the cortical microcircuitry between the two hemispheres both anterior and posterior.

Additional structural differences have been documented in both anterior and posterior language cortex. These asymmetries include cell sizes differences between the hemispheres. Cortical areas have a basic underlying organization, documenting cortical locations involved in certain functions should be distinguished in terms of form and variety, between the neural structures common to all regions and the structures critical for a region to carry out particular cognitive functions.

What is the anatomy of communication?

The corpus callosum and the commissures

The left and right cerebral hemispheres are connected by the largest white matter structure in the brain (the corpus callosum) and the two much smaller fiber tracts (the anterior and posterior commissures). The corpus callosum is divided on macroscopic level into the genu, the body and the splenium. The splenium is the most posterior portion of the corpus callosum. When the posterior half of the callosum is sectioned in humans, the transfer of visual, tactile and auditory sensory information is severely disrupted. By using the diffusion tensor imaging (DTI) technique researchers have traced the white fiber tracts from one hemisphere across the corpus callosum to the other hemisphere. The corpus callosum can be partitioned into vertical segments carrying homotopic connections - those that go into the corresponding region in the other hemisphere - and heterotopic connections - those that travel to a different region in the other hemisphere.

The callosal fibers also connect heterotopic areas. These are regions with different locations in the two hemispheres. These projections generally mirror the ones found within a hemisphere. The anterior commissure is a much smaller band of fibers connecting the two hemispheres, including the two amygdalae. The posterior commisssure is even smaller and also carries some interhemispheric fibers.

What is the function of the corpus callosum?

The corpus callosum is the primary highway between the two cerebral hemispheres, and people want to know how and what exactly is being transported between the two hemispheres. In young developing humans and animals callosal projections are diffuse and more evenly distributed across the cortical surface. Cats and monkeys, for instance, lose about 70% of their callosal axons during development. But the reason axon loss doesn't lead to cell loss in both hemispheres is that a signal cell body can send out more than one axon terminal.

The difference in some corpus callosum sizes may also be attributed to differences in brain size. For instance, the size of the corpus callosum is bigger in men than women.

Can you split the brain? Cortical disconnection

Myers and Sperry did some experiments with animals to assess whether the corpus callosum was crucial for unified cortical function. They first trained the cats to choose a 'plus' stimulus versus a 'circle' stimulus. Then they made the discovery that when the corpus callosum and anterior commissure were sectioned, such visual discriminations learned by one hemisphere did not transfer to the other hemisphere.

Corpus callosotomy, or split-brain surgery, is used to treat intractable epilepsy when other forms of treatment, such as medication, fail to function. The first time this was executed, it was done by a professor in NY, named Van Wagenen. To everyone's relief, the surgery was a great success. The patient appeared and felt completely normal. The main method of testing the perceptual and cognitive functions of each hemisphere has changed little over the past 50 years. The ability to communicate solely to one hemisphere is based on the anatomy of the optic nerve. When you look at an object in front of you, information from the right side of you visual field hits the left side of the retina (both eyes), and information from the left side of your visual field hits the right side of the retina.

There are a number of methodological issues that arise in evaluations of the performance of split-brain patients:

1. Bear in mind that these patients were not neurologically normal before their callosotomy. Therefore it is unreasonable to ask whether they provide an appropriate barometer of noral hemispheric function after the operation.

2. It is also important to consider whether the transcortical connections were completely sectioned, or whether some fibers remained intact.

3. Experiments must be meticulously designed to eliminate the possibility of cross-cuing, which occurs when one hemisphere initiates a behavior that the other hemisphere detects externally, giving it a cue about the answer to a test.

When the corpus callosum is fully sectioned, little or no perceptual or cognitive interaction can occur between the two hemispheres. Surgeons therefor sometimes perform the split-brain procedure in stages, first the anterior or posterior part of the corpus callosum. The remaining fibers are sectioned in a second operation only if the seizures continue to persist.

When the posterior half of the callosum is sectioned, transfer of visual, tactile, and auditory sensory information is severely disrupted, but the remaining intact anterior region of the callosum is still able to transfer higher-order information.

What is the evidence of lateralized brain functions from split-brain patients?

When you want to understand the neural bases of language, it is useful to distinguish between grammatical and lexical functions. Grammar is the rule-based system that humans have for ordering words to facilitate communication. The lexicon is the dictionary of the mind, where words are associated with specific meanings. The grammar-lexicon distinction is more apparent when you are learning a new language. You often learn stock phrases that you speak as a unit rather than struggling with the grammar.

Language and speech are rarely present in both hemispheres; they are either in one or the other. The left hemisphere normally comprehends all aspect of language, the right hemisphere does have linguistic capabilities, although they are uncommon. Both hemispheres also show a word superiority effect. This means that people are better able to identify letters in the context of a real word, than in the context of a pseudoword. In sum, there appear to be two lexicons, one in each hemisphere.

What is visuospatial processing?

Early testing made it clear that the two hemispheres have different visuo-spatial capabilities. The right hemisphere is specialized for efficiently detecting upright faces and discriminating among similar faces. The left hemisphere is not good at distinguishing among similar faces, but is able to distinguish among dissimilar ones when it can tag the feature differences with words. Both hemispheres can generate spontaneous facial expressions, but you need your left hemisphere to produce voluntary facial expressions. When a split-brain patient gives it left hemisphere the command to smile, the lower-right side of the face responds first, while the left side responds about 180 ms later. Why does it respond at all? Most likely, the signal is rerouted through secondary ipsilateral pathways that connect to both facial nuclei, which then eventually send the signal over to the left-side facial muscles. So, the left hemisphere can trigger voluntary facial expressions, but both hemispheres can trigger involuntary expressions.

What is the interaction of attention and perception?

After cortical disconnection, perceptual information is not shared between the two cerebral hemispheres. We noted earlier that split-brain patients cannot integrate visual information between the two visual fields. The same is true for certain types of somatosensory information presented to each hand. Thus, when holding an object in the left hand, a split-brain patient is unable to find an identical object with the right hand. Experiments showed that spatial attention can be directed with ease to either visual fields, and this raised the question of whether each separate cognitive system in the split-brain patient, if instructed to do so, could independently and simultaneously direct attention to a part of its own visual field. Some forms of attention are integrated at the subcortical level, and other forms act independently in the separated hemispheres. Split-brain patients can use either hemisphere to direct attention to positions in either the left or right visual field.

The interpreter

A hallmark of human intelligence is that it is our ability to make causal interpretations about the world around us. For instance, when you walk outside and see a gray sky and a wet ground you probably automatically assume that it has rained. Even though you did not witness the rain and also nobody told you it had rained. A large part of the right hemisphere's impoverishment can be attributed to the finding that causal inferences and interpretations appear to be a specialized ability of the left hemisphere. The left hemisphere appears to have a specialized ability to make causal inferences and form hypotheses. This unique specialization of the left hemisphere is also called interpreter.

A typical observation occurs when the speaking left hemisphere offers some kind of rationalization to explain actions that were initiated by the right hemisphere but were spurred on by a motivation unknown to the left hemisphere. For example, when a split-brain patient was getting a command to stand up, only available to the right hemisphere, the patient stood up. When asked the patient why he was getting up the left hemisphere immediately came up with a plausible explanation: ''I felt like getting a coke''. If the corpus callosum would be intact, the patient would have responded that he stood up because that was the instruction he had received. When predicting which of the two events will occur, the left hemisphere uses a frequency-matching strategy, where-as the right hemisphere uses a maximizing strategy. The left hemisphere is also better at making causal inferences, but the right one is better at judgments of causal perception.

What is the evidence of lateral brain functions, comparing the normal and malfunctioning brain?

Researchers have also designed experiments to test the differential processing of the two hemispheres in people with intact brains. Studies of auditory perception similarly attempt to isolate the input to one hemisphere. As in vision work - the stimuli can be presented monaurally - that is, restricted to one ear. An alternative methodology for isolating the input is the dichotic listening task. In this task two competing messages are presented simultaneously, one to each ear, and the participant tries to report both messages. But there are some limitations to this kind of studies:

• The effects are small and inconsistent, perhaps because healthy people have two functioning hemispheres connected by an intact corpus callosum that transfers information quite rapidly.

• There is an bias in the scientific review process towards publishing papers that find significant differences over papers that report no differences. It is much more exciting to report asymmetries in the way we remember lateralized pictures of faces than to report that effects are similar.

• Interpretation is problematic. What can be inferred from an observed asymmetry in performance with lateralized stimuli?

How do you map functional and anatomical connectivity?

Researchers can also use fMRI techniques to explore hemispheric differences in healthy individuals. On measuring the functional connectivity of brain regions within the same hemisphere and between the two hemispheres, they found that the left and right hemispheres had different patterns of functional connectivity. Neurologically healthy participants exhibit a right-ear advantage when performing the dichotic listening task. When listening to songs, however, while there is a right-ear advantage for the song's words, there is a left-ear advantage for the melodies of the songs.

What is the evolutionary basis of hemispheric specialization?

In this chapter we have reviewed general principles of hemispheric specializations in humans. Because of the central role of language in hemispheric specialization, laterality research has focused primarily on humans. But the evolutionary pressures that underlie hemispheric specialization would also be potentially advantageous to other species. Humans show handedness, favoring either the left or right hand, dogs and cats show pawedness. But males and females show opposite preferences. Males favor their left paws and females favor their right paws.

What is the function of attention and how does it work? - Chapter 7

What is selective attention, and what holds the anatomy of attention?

William James made an astute observation in the late 19th century. He insightfully captured key characteristics of attentional phenomena that are under investigation today. 'It is the taking possession by the mind' that we can choose the focus of our attention, that attention can be voluntary. Since James, knowledge about attention has blossomed, and researchers have identified multiple types and levels of attentive behavior.

Arousal refers to the global physiological and psychological state of the organism, and it is best thought of on a continuum ranging from deep sleep to hyperalertness. In contrast, selective attention is not a global brain state. Instead, at any level of arousal, it is the allocation of attention among relevant inputs, thoughts, and actions while simultaneously ignoring irrelevant or distracting ones. Selective attention is the ability to prioritize and attend to some things and not to others. This is goal-driven control, steered by an individual's current behavior goals and shaped by learned priorities based on personal experience and evolutionary adaptions.

Your reaction is stimulus-driven and therefor also stimulus-driven control, which is much less dependent on current behavior goals. The mechanisms that determine where and on what our attention is focused are referred to as attentional control mechanisms. Several cortical areas are important for attention: portions of the posterior superior temporal cortex, as well as more medial brain structures, including the anterior cingulate cortex.

The superior colliculus in the midbrain and the pulvinar nucleus of the thalamus, located between the midbrain and the cortex, are involved in the control of attention. Damage to these structures can lead to deficits in the ability to orient both overt and covert attention. Overt attention is for instance eye gaze direction, the covert attention holds the attention directed without changing the eyes, head, or body orientation. Also, attention acts on sensory systems, and therefore much work on attention investigates the effect of attention on sensory signal processing.

What is the neuropsychology of attention?

Much of what neuroscientists know about brain attention systems has been gathered from examinations of patients who have brain damage that influences attentional behavior. Though the best-known disorder of attention, attention deficit hyperactivity disorder (ADHD), has heterogeneous genetic and environmental risk factors, it is characterized by disturbances in neural processing that may result from anatomical variations of white matter throughout the attention network.

What is neglect?

A patient with neglect may notice you more easily when you are on her right side, etc. And she may deny having any problems. Unilateral spatial neglect, or neglect, is quite common. It results when the brain's attention network is damaged in just one hemisphere, typically as the result of a stroke. More severe and persistent effects occur when the right hemisphere is damaged. The right-hemisphere lesion biases attention toward the right, resulting i n a neglect of what is going on the left visual field. The patient behaves as though the left regions of space and the left parts of objects simply do not exist, and has limited or no awareness of her lesion and deficit.

Neuropsychological tests are used to diagnose neglect.

• In the line cancellation test, patients are given a sheet of paper containing may horizontal lines and are asked to bisect them in the middle. Patients with left-sided neglect tend to bisect the lines to the right of the middle.

• There is also an related test that asks patients to copy objects or scenes. When you ask a patient to copy a clock with a right-hemispheric neglect, the patient shows an inability to draw the entire clock and tends to neglect the left side of the clock.

Visual field testing shows that the patients are not 'blind' in their left visual field. They are able to detect stimuli normally when those stimuli are salient and presented in isolation. When simple flashes of light or the wiggling fingers of a neurologist are shown at different angles within the visual field of the patient, the patient can see each stimulus. But when you present simultaneously two stimuli, one in each hemifield, the patient fails to perceive or act on the contralesional stimulus. This is known as extinction, because the presence of the competing stimulus in the ipsilateral hemifield prevents the patient from detecting the contralesional stimulus.

What is the difference between neglect and Bálint's Syndrome?

In contrast to the patient with neglect, a Bálint's syndrome patient demonstrates three main deficits that are characteristic of the disorder:

• Simultanagnosia is a difficulty in perceiving the visual field as a whole scene

• Ocular apraxia is a deficit in making eye movements to scan the visual field, resulting in the inability to guide eye movements voluntarily

• Optic apraxia is a problem in making visually guided hand movements

The patterns of perceptual deficits in neglect and Bálint´s syndrome are quite different, however, because different brain areas are damaged in each disorder. Neglect is the result of unilateral lesions of the parietal posterior temporal, and frontal cortex. It can also be due to damage in subcortical areas including the basal ganglia, thalamus and midbrain. Bálint´s syndrome patients suffer from bilateral occipitoparietal lesions, neglect shows us that disruption of a network of cortical and subcortical areas, especially in the right hemisphere results in disturbances of spatial attention

What are the models of attention?

Attention can be divided into two main forms: voluntary attention, also known as endogenous attention, is our ability to intentionally attend to something, such as a book. It is a top-down, goal-driven process, meaning that our goals, expectations and rewards guide what we attend. Reflexive attention, or exogenous attention, is a bottom-up, stimulus-driven process in which a sensory event - a loud bang, sting of a mosquito - captures our attention.

It is useful to think that these two attention systems as being in perfect balance, so that we are neither so focused on something like a beautiful flower that we miss the tiger sneaking up behind us.

What is the cocktail party effect?

Imagine yourself at a Super Bowl party having a conversation with a friend. How can you focus on this single conversation while the TV is blaring out and boisterous conversations around you are present? This is called the cocktail party effect. Selective auditory attention enables you to participate in a conversation at a busy restaurant or a party while ignoring the rest of the sounds around you. By selective attending, you can perceive the signal of interest amid the other noises.

Bottlenecks in information processing - stages through which only a limited amount of information can pass - seem to occur at stages of perceptual analysis that have a limited capacity. There are many stages of processing between the time information enters your eardrum and the time you become aware of what was said. At which stages are there bottlenecks that make attention necessary to favor one signal over another? This question has led to one of the most debated issues in psychology over the past six decades: Are the effects of selective attention evident early in sensory processing or only later, after sensory and perceptual processing are complete? Does the brain faithfully process all incoming sensory inputs to create a representation of the external world biased by the current goals and stored knowledge of your internal worlds.

Broadbent elaborated on the idea that the information-processing system has processing bottlenecks. The sensory inputs that can enter higher levels of the brain for processing are screened early in the information-processing stream by a gating mechanism so that only the 'most important' or attended, events pass through. Early selection is the idea that a stimulus can be selected for further processing or be tossed out as irrelevant before perceptual analysis of the stimulus is complete.

Models of late selection hypothesize that the perceptual system first processes all inputs equally, and then selection takes place at higher states of information processing that determine whether the stimuli gain access to awareness, are encoded in memory, or initiate a response. One way to measure the effect of attention on information processing is to examine how participants respond to target stimuli under differing conditions of attention. One popular method is to provide cues that direct the participant´s attention to a particular location or target feature before presenting the task-relevant target stimulus. Endogenous cueing is when the orienting of attention to the cue is voluntary and driven by the participant's goal. When a cue correctly predicts the location of the subsequent target, it is a valid trial. Sometimes, though, because the target may be presented at a location not indicated by the cue, the participant is misled in a invalid trial. Also, the researcher may include some cues that give no information about the most likely location of the impending target - this is the neutral cue.

According to most theories, a highly predictive cue induces participants to direct their covert attention internally, shining a sort of mental 'spotlight' of attention onto the cued visual field location.

What are the neural mechanisms of attention and perceptual selection?

Although most of the experiments in this chapter focus on visual attention, this should not be taken to suggest that attention is only a visual phenomenon. Selective attention operates in all sensory modalities.

What is voluntary visuospatial attention?

Visuospatial attention involves selecting a stimulus on the basis of its spatial location. It can be voluntary, such as when you attend to this page, or it can be reflexive, such as when motion at the door of the classroom attracts your attention and you look up. Spatial attention influences the processing of visual inputs: attended stimuli produce greater neural responses than do ignored stimuli, and this process takes place in multiple visual cortical areas.

Many stages of neural processing take place within the visual area. Different neurons display characteristics receptive-field proper ties: some are called simple cells other are called complex cells. The simple cells exhibit orientation tuning and respond to contrast borders. Researchers found that spatial attention enhanced the responses of the simple cells, but did not affect the spatial or temporal organization of their receptive fields, which remained unchanged over the trials.

We now understand that visuospatial attention can influence stimulus processing at many stages of cortical visual processing. Are the effects of attention the same at these different stages of processing, or does attention act at different stages of the visual hierarchy to accomplish different processing goals? One prominent model is known as the biased competition model for selective attention. This model may help answer two questions: (1) why are the effects of attention larger when multiple competing stimuli fall within a neuron's receptive field, (2) how does attention operate at different levels of the visual hierarchy as neuronal receptive fields change their properties? In this model the idea is that when different stimuli in a visual scene fall within the receptive field of a visual neuron, the bottom-up signals from the two stimuli compete like two snarling dogs to control these neuron's firing. Attention can help resolve this competition by favoring one stimulus.

Could attentional filtering or selection occur even earlier along the visual processing pathways - in the thalamus or in the retina? Unlike the cochlea, the human retina contains no descending neuronal projections that could be used to modulate retinal activity by attention. But there are massive neuronal projections that extend from the visual cortex back to the thalamus. These projections synapse on neurons in a portion of the thalamic reticular nucleus (TRN) that surrounds the lateral geniculate nucleus. Research shows that highly focused visuospatial attention can modulate activity in the thalamus.

What is reflective visuospatial attention?

Sometimes things in the environment attract out attention without our cooperation. This is called reflective attention, and it is activated by stimuli that are salient in some way. The more salient the stimulus, the more easily our attention is captured. So, the question that comes from this is, are reflexive and voluntary attention processes in the same way? To tackle this question, attention researchers have used a variant of the cuing method. These studies examine how a task-irrelevant event somewhere in the visual field, like a flash of light, affects the speed of responses to subsequent task-relevant target stimuli that might appear at the same or some other location. This method is referred to as reflexive cuing or exogenous cuing. Interestingly, when more than about 300 ms pass between the task-irrelevant light flash and the target, the pattern of effects on reaction time is reversed. Participants respond more slowly to stimuli that appear in the vicinity of where the flash has been. This phenomenon is called the inhibition of return (IOR) - that is, inhibition of the return of attention to that location. The recently reflexively attended location becomes inhibited over time such that responses to stimuli occurring there are slowed.

Our automatic orienting systems has built-in mechanisms to prevent reflexively directed attention from becoming stuck at a location for more than a couple hundred milliseconds. Responses to endogenous and exogenous cues result in attention shifts that enhances the processing of attended sensory stimuli and decrease the processing of unattended stimuli.

Does spatial attention automatically move freely from item to item until the target is located, or does visual information in the array help guide the movements of spatial attention among the array items? Researchers compared spatial attention and feature attention in a voluntary cuing paradigm. The researchers found that prior knowledge from the cue produced the typical voluntary cuing effect for spatial attention: participants were more accurate at detecting the presence of the target at the cued location compared to when the cue did not signal one location over another.

When attention is focused on a stimulus, neurons in the visual system that code that stimulus increase their postsynaptic responses and firing rates. How does this happen in a selective fashion so that attended information is routed appropriately to influence subsequent stages of processing? One model suggests that at different stages of visual analysis, neurons that code the receptive-field location of an attended stimulus show increased synchrony in their activity.

What are the attentional control networks?

As we know now, attention can be either goal directed (top-down) or stimulus-driven (bottom-up). Top-down neuronal projections from attentional control systems contact neurons in sensory-specific cortical areas to alter their excitability. As a result, the response in the sensory areas to a stimulus may be enhanced if the stimulus is given high priority, or attenuated if it is irrelevant to the current goal. Current models of attentional control suggest that two separate cortical systems are at play in supporting different attentional operations during selective attention: a dorsal attention network - concerned primarily with voluntary attention based on spatial location, features, and object properties, and a ventral attention network - concerned with stimulus novelty and salience. The two control systems interact and cooperate to produce normal behavior, and these interactions are disrupted in patients with neglect.

The dorsal frontoparietal attention network is bilateral and includes the superior frontal cortex, inferior parietal cortex, superior temporal cortex, and portions of the posterior cingulate cortex and insula. The ventral network is strongly lateralized to the right hemisphere and includes the posterior parietal cortex of the temporoparietal junction (TPJ) and the ventral frontal cortex (VFC), made up of the inferior and middle frontal gyri.

What is the importance of action and the motor system? - Chapter 8

What holds the anatomy and control of motor structures?

To understand motor control, we have to consider the organization and function of much of the dorsal territory of the cerebral cortex, as well as much of the subcortex. The lowest level of the hierarchy centers on the spinal cord. Axons from the spinal cord provide the point of contact between the nervous system and muscles, with incoming sensory signals from the body going to ascending neurons in the spinal cord, and outgoing motor signals to the muscles coming from descending spinal motor neurons. At the top of the hierarchy are cortical regions that help translate abstract intentions and goals into movement patterns. Between the association areas and the spinal cord sit the primary motor cortex and brain structures, which with the basal ganglia and cerebellum, convert these patterns into commands to the muscles.

What are the roles of muscles, motor neurons and the spinal cord?

Action, or motor movement, is generated by stimulating skeletal muscle fiber of an effector. An effector is a part of the body that can move. All forms of movement result from changes in the state of muscles that control an effector or group of effectors. Muscles consist of elastic fibers, tissue that can change in length and tension and are activated by motor neurons, which are the final neural elements of the motor system. Alpha motor neurons innervate muscle fibers and produce contractions of the fibers. Input to the alpha motor neurons comes from a variety of sources. Alpha motor neurons receive peripheral input from muscle spindles, sensory receptors embedded in the muscles that provide information about how much the muscle is stretched.

The axons of the spindles form an afferent nerve that enters the dorsal root of the spinal cord and synapses on spinal interneurons that project to alpha motor neurons. Reflexes allow postural stability to be maintained without any help from the cortex.

What are the subcortical motor structures?

There are 12 cranial nerves, essential for critical reflexes associated with breathing, eating, eye movements, and facial expressions that originate in the brainstem. The substantia nigra sends out direct projections down the spinal cord. This motor pathway is referred to as the extrapyramidal tract, meaning that this is not a part of the pyramidal part, the axons that travel directly from the cortex to the spinal segments. The cerebellum is a massive structure containing over 75% of all the neurons in the brain. Damage to the cerebellum from stroke, tumor or degenerative processes results in a syndrome called ataxia. Patients with ataxia have a lot of difficulty maintaining balance and producing well-coordinated movements.

The basal ganglia is the other major subcortical motor structure next to the cerebellum and substantia nigra. It consists out of a collection of five nuclei: caudata nucleus, putamen, globus pallidus, subthalamic nucleus, and the substantia nigra. I will use the term motor areas to refer to cortical regions involved in voluntary motor functions, including planning, control and execution of movement.

Which cortical regions are involved in motor control?

The motor cortex regulates the activity of spinal neurons in direct and indirect ways. The corticospinal tract (CST) consists of axons that exit the cortex and project directly to the spinal cord. The CST is also referred to as the pyramidal tract, because the mass of axons resembles a pyramid as it passes through the medulla oblongata.

The primary motor cortex (M1) is located in the most posterior portion of the frontal lobe, spanning the anterior wall of the central sulcus and extending onto the precentral gyrus. M1 receives input from almost all cortical areas implicated in motor control. M1 includes two anatomical subdivisions: an evolutionairy older rostral region and a more recently evolved caudal region. Corticospinal neurons that originate in the caudal region may terminate on interneurons or directly stimulate alpha motor neurons. The latter known as corticomotorneurons or CM neurons, include prominent projections to muscles of the upper limb, and they support the dexterous control of our fingers and hands.

The preeminent status of the primary motor cortex for movement control is underscored by the fact that lesions to this area, or to the corticospinal tract, produce a devastating loss of motor control. Lesions of the primary motor cortex usually result in hemiplegia, the loss of voluntary movements on the contralateral side of the body. Reflexes are absent immediately after a stroke that produces hemiplegia. But within a couple of weeks the reflexes return and are frequently hyperactive or even spastic.

The lateral and medial aspects of Brodmann area 6 are referred to as premotor cortex and supplementary motor area (SMA). The secondary motor areas are involved in planning and control of movements, but they do not accomplish this feat alone. The premotor cortex has a strong reciprocal connection with the parietal lobe.

The dorso-dorsal stream passes through the superior parietal lobe and projects to the dorsal premotor cortex. This pathway plays a dominant role in one of the most important motor activities: reaching. Patients with lesions in the dorso-dorsal stream have optic ataxia: they are unable to reach accurately for objects, especially those in their peripheral vision.

The ventro-dorsal stream passes through the inferior parietal lobe and projects to the ventral premotor cotex. This pathway is associated with producing both transitive gestures and intransitive gestures. Lesions along this processing stream can result in apraxia - a condition that affects motor planning, as well as the knowledge of which actions are possible with a given object.

What are computational issues in motor control?

The spinal cord is capable of producing orderly movement, and the stretch reflex provides an elegant mechanism to maintain postural stability even in the absence of higher-level processing. Sherrington observed by looking at a cat whose spinal cord was disconnected from the spinal apparatus, the cat was still able, without the appropriate stimulus, to make movements like he/she was walking. Thus, neurons in the spinal cord could produce an entire sequence of actions without any descending commands or external feedback signals. These neurons have come to be called central pattern generators. They most likely evolved to trigger actions essential for survival, such as locomotion.

But if the cortical neurons are not coding specific patterns of motor commands, what are they doing? Researchers did an experiment with monkeys that had been deprived of all somatosensory, or afferent, signals from the limbs. The monkeys were trained to simple point at a light. If, the animal generated a motor command specifying the desired position, it should have achieved this goal once the opposing force was removed. The results show that when the torque motor was on, the limb stayed at the starting location. As soon as it was turned off, the limb rapidly moved to the correct location. This experiment provides evidence that the location isn't the only thing being coded by humans. Although endpoint control reveals a fundamental capability of the motor control system, distance and trajectory planning demonstrates additional flexibility in the control processes.

What are the hierarchical representations of action sequences?

Hierarchical representational structures organize movement elements into integrated chunks. Researchers originally developed the idea of 'chunking' when studying memory capacity, but it has also proved relevant to the representation of action. So, the motor system is also hierarchically organized. Subcortical and cortical areas represent movement goals at various levels of abstraction.

What is the physiological analysis of motor pathways?

In this chapter we have stressed two critical points of movement: (1) motor control depends on several distributed anatomical structures, (2) these distributed structures operate in hierarchical fashion.

Neuropsychologists have long puzzled over how best to describe cellular activity in the motor structures of the CNS. Stimulation of the primary motor cortex, either during neurosurgery or via TMS, can produce discrete movements about single joints, providing a picture of the somatotopic organization of the motor cortex. Research also shows that activity of the cells in the primary motor cortex correlates much better with movement direction than with target location. Many cells in the motor cortex show directional tuning, or exhibit what is referred to as a preferred direction. Directional tuning is not just observed in the primary motor cortex; similar tuning properties are found in cells in premotor and parietal cortical areas, as well as in the cerebellum and basal ganglia. We can assume that activity is distributed across many cells, each with its unique preferred direction. Researchers introduced the concept of population vector: each neuron can be considered to be contributing a 'vote' to the overall activity level. The strength of the vote will correspond to how closely the movement matches the cell's preferred direction: if the match is close, the cell will fire strongly. Thus, the activity of each neuron can be described as a vector, oriented to the cell's preferred direction with a strength equal to its firing rate.

What are alternative perspectives on the neural representation of movement?

The population vector is dynamic and can be calculated continuously over time. After defining the preferred direction of a set of neurons, we can calculate the population vector from the activation of that set of neurons even before the animal starts to move. This shows that the population vector shifts in the direction of the upcoming movement well before the movement is produced, suggesting that at least some of the cells are involved in planning the movement and not simply recruited once execution of the movement has begun. Even though directional tuning and population vector have become cornerstone concepts in motor neurophysiology, it is also important tot consider that many cells do not show strong directional tuning. Even more puzzling, the tuning may be inconsistent: the tuning exhibited by a cell before movement begins may shift during the actual movement. Researchers are saying that we should take up on a radically different perspective on motor neurophysiology. Rather than viewing neurons as static representational devices, we should focus on the dynamic properties of neurons, recognizing that movement arises as the neurons move from one state to another.

Although scientist refer to one part of the brain as the motor cortex and one part of the brain as the sensory cortex, we know that these areas are closely intertwined. People produce movements in anticipation of their sensory consequences: we increase the force used to grip and lift in anticipation of the weight we expect to experience.

How does goal selection and action planning work?

Motor representation are hierarchically and need to encompass the goals of an action in addition to the activation patterns required to produce the movement necessary to achieve those goals. Including sensory information and feedback enables the motor cortex to have more than one option for achieving those goals.

One hypothesis about how we set our goals and plan action is from Cisek. He says that in incorporates many of the ideas and findings that we are going to look at, providing a general framework for action selection. His affordance competition hypothesis is deeply rooted in an evolutionary perspective. Our ancestors evolved in a world where they engaged in interactions with a changing, and sometimes hostile environment that held a variety of opportunities and demands for action. To survive and reproduce, early humans had to be ever ready, anticipating the next predator etc. Many interaction don't allow time for carefully evaluating plans and goals, considering options: this is called serial processing.

A better idea is to develop multiple plans in parallel. The affordance competition hypothesis proposes that the processes of action selection and specification occur simultaneously within an interactive neural network, and they evolve continuously. Even when performing one action, we are already preparing the next one. Then there is the competition part. At some point, one option wins out over the other competitors. An action is selected an executed. This selection process involves many parts of the motor pathway, where interactions within frontoparietal circuits have a prominent role.

There are cells in the premotor cortex that have been shown to represent action goals more abstractly. Some cells are preferentially activated when the animal reaches for an object, other cells become active when the animal makes a gesture to hold an object.

What are the representational variations across motor areas of the cortex?

The lateral premotor cortex is more heavily connected with the parietal cortex - a finding consistent with the hypothesis that this region plays a role in sensory-guided action. The SMA has strong connections with the medial frontal cortex, and is likely to bias or influence action selection and planning that are based on internal goals and personal experiences. The SMA has also been hypothesized to play an important role in more complex actions, such as those involving sequential movements or those requiring coordinated movements of the two limbs. Damage in the SMA can lead to impaired performance on tasks that require integrated use of the two hands, even though the individual gestures performed by either hand alone are unaffected. Lesions in SMA can also result in alien hand syndrome, a condition in which one limb produces a seemingly meaningful action but the person denies responsibility for the action.

The lateral premotor cortex is part of a network for stimulus-guided movement, whereas the more medial supplementary motor area is important for movements based on internal goals and personal experience, including skilled movements that require coordination between the two hands. Parietal motor areas also show topography; different regions of the intraparietal cortex are associated with hand, arm, and eye movements. Therefor the parietal motor representations are more goal oriented, whereas premotor-motor representations are more closely linked to the movement itself. The conscious awareness of the movement appears to be related to the neural processing of action intention rather than the movement itself.

What links are there between action and perception?

The most common known link between perception and action is neurons that are called mirror neurons. These are neurons that are active during action but also active during action perception. You might suppose that the activity in MN's reflects the similar visual properties of the action and perception conditions. Additional experiments ruled out this hypothesis: (1) the same MN that is activated when the monkey cracks a peanut itself is activated when the monkey merely hears a peanut being cracked, (2) MNs are also active when a monkey watches someone reach behind a screen for a peanut but cannot see grasping of the peanut. In fact, there doesn't even need to be a peanut behind the screen, as long as the monkey thinks there's a hidden peanut. The work on a mirror neuron network has revealed the intimate links between perception and action, suggesting that our ability to understand these actions of others depends on the neural structures that would be engaged if we were to produce the action ourselves.

Coaches and sport psychologists recognize the intimate relationship between action observation an action production. The skier can mentally visualize his movements of the slope. This process is thought to strengthen perception-action links.

How do you recoupe motor loss?

Lesions to the primary motor cortex or the spinal motor neurons can result in hemiplegia, or in hemiparesis, a unilateral weakness. Such lesions severely impact the patient's ability to use the affected limbs on the contralesional side: unfortunately, these patients rarely regain significant control over the limbs.

How do you regain movement after loss of motor cortex?

Two critical questions for the clinician and patient are, what is the patient's potential for recovery? what is the best strategy for rehabilitation? A panel looked at biomarkers for predicting motor recovery from stroke. Some biomarkers look promising when the assessment is performed after the patient has recovered from the critical initial post-stroke period, outperforming the location of the lesion and even the behavioral asymmetry between the affected and unaffected limbs.

The main treatment, traditionally, to regain motor function is physical therapy, a behavioral intervention that seeks to retrain the affected limbs. But physical therapy only produces modest recovery. Also the idea that more therapy is better, has been contradicted with research. A different behavioral method is based on the idea that the brain may favor short-term solutions over long-term gains. Scientists are currently seeking novel interventions that more directly target specific neural mechanisms.

What is the brain-machine interface?

Can neural signals be used to control a movement directly with the brain, bypassing the intermediate stage of muscles? Could you plan an action in your motor cortex, somehow connect those motor cortex neurons to a computer, and send the planned action to a robot, which would fold the laundry? These systems are called brain-machine interfaces using decoding algorithms to control prosthetic devices with neural signals.

Brain-machine interfaces are also called BMI's. BMI's offer a promising avenue for rehabilitation of people with mostly severe movement disorders, such as those resulting from, for instance, a spinal cord injury. In the early BMI systems the decoders were built from recordings of neural activity made while the animal produces movements. The output of these decoders was then used to drive the prosthetic device. More recent work has revealed that the brain's plasticity enables it to spontaneously learn how to adapt neural activity to control an arbitrary decoder, eliminating the need for a training phase to build the decoder. This insight is essential if BMI systems will be useful for individuals who have lost the ability to move their limbs by themselves.

How does movement initiation of basal ganglia work?

When multiple plans are present in the cortex, how do we decide which plan to execute? The basal ganglia plays a critical role in movement initiation. The afferent fibers to the basal ganglia terminate in the striatum, composed in primates of two nuclei: the caudate and the putamen. Processing within the basal ganglia takes place along two pathways that originate with GABAergic projection neurons from the striatum. The direct pathway involves fast, direct, inhibitory connections from the striatum to the basal ganglia. The indirect pathway takes on a slower, roundabout route to the basal ganglia. Stiatal axons inhibit the external segment of the globus pallidus, which in turn inhibits the subthalamic nucleus and GP. The final internal pathway of note is the projection from the pars compacta of the substantia nigra to the striatum, known as the dopamine pathway. The substantia nigra excites the direct pathway by acting on one type of dopamine receptor (D1), and inhibits the indirect pathway by acting on a different type of dopamine receptor (D2).

When the direct pathway is activated, it sends inhibitory signals to the target neurons in the output nuclei of the basal ganglia, which results in the inhibition of inhibiting signals to the thalamus. This sum effect is disinhibition of the thalamus, resulting in increased excitation of the cortex. So, activation of the direct pathway will promote movement if the disinhibition is along a circuit that terminates in the primary motor cortex.

Activation along the indirect pathway will result in increased inhibition from the basal ganglia and as such, reduces excitation of the cortex. This puzzling arrangement seems to be an important mechanism for helping the motor system both to maintain stability and to rapidly change when the situation changes. So, the basal ganglia can be seen to play a critical role in the initiation of actions.

What are disorders of the basal ganglia?

Huntington's disease is a hereditary neurodegenerative disorder. Patients with this gene develop symptoms in the fourth or fifth decade of life, experiencing rapid progression and die within 12 years of onset. Within a year from onset movement abnormalities are noticed: clumsiness, balance problems, and a general restlessness. The excessive movements, or hyperkinesia, can be understand by considering how the pathology affects information flow through the basal ganglia. The striatal changes occur primarily in inhibitory neurons forming the indirect pathway.

Parkinson's disease is the most common and well-known disorder affecting the basal ganglia, results from a loss of dopamine-producing neurons in the substantia nigra (SN). As with most brain tissue, these neurons atrophy with age. Motor symptoms of Parkinson are related to locomotion called hypokinesia and bradykinesia. Hypokinesia is a reduced ability to initiate voluntary movements, bradykinesia refers to a slowing in the rate of movement. At the extreme end of these symptoms lies akinesia, the total absence of voluntary movement. One of the biggest breakthroughs in neurology occurred with the development of L-dopa, a synthetic precursor of dopamine. But, over time, the dopamine-producing cells continue to die off and striatal neurons become sensitized to L-dopa, so the amount of required medication tends to increase. More recently, the success of invasive techniques such as pallidotomy and deep brain stimulation, where an electrode is placed in the skull to initiate the signal for movement by an Parkinson's disease patient, has inspired neurosurgeons to consider similar interventions for other disorders.

How do you learn and perform a new skill?

People frequently attribute motor learning to the lower levels of the hierarchical representation of action sequences. We speak of 'muscle memory' as if our muscles have learned how to respond in a way that seems automatic. The fact that we have great difficulty verbalizing how we perform these skills reinforces the notion that learning is noncognitives.

When people are acquiring new action, the first effects of learning likely will be at a more abstract level. Learning a skill takes practice, and becoming very skilled at anything requires a lot of practice. Our motor system has some basic movement patterns that can be controlled by subcortical circuits. Learning a new skill can involve building on these basic patterns, linking together a series of gestures in a novel way.

When you come off a boat, you feel that you first few steps are wobbly, it takes a moment or two to become acclimated to the stability of the dock, and to abandon you rolling gait. You sea legs are a form of sensorimotor adaptation. We cannot simply switch back to the normal state, but rather must relearn how to control our limbs in the absence od a visual or force distortion. Sensimotor learning is improvement, through practice, in the performance of motor behavior. The acquisition of a motor skill involves the formation of new movement patterns that can result in changes in both structure and connectivity.

How does memory work? - Chapter 9

What is the associated anatomy between learning and memory?

Despite the vast stores of information contained in our brains, we continuously acquire new information. Learning is the process of acquiring new information, and the outcome of learning is memory. Memory is created when something is learned, and this learning may occur either by a single exposure or by the repetition of information, experiences, or actions. Researchers believe that humans and animals have several types of memory mediated by different systems: sensory memory, short-term memory (STM) or working memory and long-term memory (LTM). Researchers also make distinctions among the types of information stored. LTM is commonly divided into declarative memory, which consists of our conscious memory for both facts we have learned (semantic) and events we have experienced (episodic); and nondeclarative memory, which is nonconscious memory that cannot be verbally reported, often expressed through the performing of procedures (procedural memory).

Researchers divide learning and memory into three major processing stages:

1. Encoding: is the processing of incoming information and experiences, which creates memory traces, traditionally thought to be alterations in the synaptic strength and number of neuronal connections. Encoding has two separate steps, (1) acquisition; sensory systems are constantly being bombarded by stimuli and most responses fade quickly and don't come near the STM. But all the stimuli are still available for processing, this is known as the sensory buffer. Not all memory traces appear to get past the second step, (2) consolidation; in which changes in the brain stabilize a memory over time, resulting in LTM.

2. Storage: is the retention of memory traces. It is a result of acquisition and consolidation, and it represents the permanent record of the information.

3. Retrieval: involves accessing stored memory traces, which may aid in decision making and change behavior. We have conscious access to some but not all the information stored in memory.

The brain has the ability to learn, which means that at neuronal level changes occur in the synaptic connections between neurons. Learning can be accomplished in different kind of ways, an it appears that different parts of the brain are specialized for different types of learning. The hippocampus is the memory component in the brain and is a portion of the medial temporal lobe that is shaped like a seahorse.

What is amnesia?

Memory deficit and loss, known collectively as amnesia, can result from brain damage caused by surgery, disease, and physical or psychological trauma. Typically, people with amnesia display deficits in specific types of memory or in aspects of memory processing. The loss of memory for events that occur after a lesion or other physiological trauma is called anterograde amnesia. It results from the inability to learn new things. A loss of memory for events and knowledge that occurred before the lesion or other physiological trauma is called retrograde amnesia. Retrograde amnesia can sometimes be temporally limited, extending back a few minutes or hours.

A lot of information about the organization of human memory was first derived from patients left accidentally amnesic after surgical treatments. The most interesting and famous of these patients was patient HM. His case holds a prominent position in the history of memory research for several reasons, one of them being that he had a memory deficit, but no other cognitive deficits. HM knew some of the autobiographical details of his life, and he retained all the other knowledge about his life and the world that he had learned up to two years immediately before his surgery. HM also changed scientist's understanding of the brain's memory processes. It had previously been thought that memory could not be separated from perceptual and intellectual functions, but these functions were completely intact by HM.

What is dementia?

Memory loss can also be caused by diseases that result in dementia. Dementia is an umbrella term for the loss of cognitive functions in different domains beyond what is expected to be normal aging. The most common types of dementia are irreversible and are the result of neurodegenarative disease, vascular disease, or a combination of the two. The most common of these protein-associated neurodegenerative diseases is Alzheimer's disease, which contributes up to 60-70% of the dementia cases. AD is characterized by the extracellular deposition of aggregated beta-amyloid proteins, negatively affecting synapse formation and neuroplasticity, and also by intercellular accumulation of neurofibrillary tangles, which are aggregations of microtubules associated with hyper-phosphorylated tau protein. The medial temporal lobe are the first to be affected by AD, later it extends to lateral, temporal, parietal, and frontal neocortices.

Vascular dementia is the second most common type of dementia, making up for 15% of the dementia cases. It is caused by decreased oxygenation of neural tissue and cell death, resulting from ischemic or hemorrhagic infarcts, rupture of small arterial vessels in the brain associated with diabetes, and rupture of cerebral arteries caused by the accumulation of beta-amyloid plaques in the walls of the vessels, which damages and weakens them. VD can have an impact on multiple brain areas, resulting in diverse symptoms, and can co-occur with AD.

Less common are the frontotemporal lobar dementias, a heterogenous group of neurodegenerative diseases characterized by accumularions of different proteins in the frontal and temporal lobes but not the parietal and occipital lobes, resulting in language and behavioral changes that may overlap with AD.

What are the mechanisms of memory?

What are the short-term forms of memory?

Short-term memories persist for milliseconds, seconds, or minutes. They include transient retention of sensory information in sensory structures, short-term stores for information about yourself and the world, and memory used in the service of other cognitive functions.

• Sensory memory: When your mother suddenly walks into the room, beginning an argument and you are watching an important part of the football game, the auditory verbal information she just presented to you seems to persist as a sort of echo in your head, even when you are not really paying attention to it. We refer to this type of memory as sensory memory.

• Short-term memory: Has a longer time course - seconds to minutes - and a more limited capacity. The Modal Model proposes that information is first stored in sensory memory. From there, items selected by attentional processes can move into ST storage. Once in the STM, if the item is rehearsed, it can be moved into LTM. The modal model suggests that, at each stage, information can be lost due to decay (information degrades and is lost over time), and interference (new information displaces old information), or because of a combination of the two.

Studies of patients with brain damage enable us to test the hierarchically structured model model of memory. A typical test to evaluate STM is the digit span test, which involves reading and remembering a list of digits and, after a delay for a few seconds, repeating the numbers. Remarkably, however, in a LTM test of associative learning, in which words are paired, a patient KF retained the ability to form certain types of new LTM that could last much longer than a few seconds. This displayed an interesting dissociation between the STM (which had been damaged) and the LTM. If this interpretation of the finding is true, than STM might not be required in order to form LTM.

• Working memory: The concept of working memory was developed to extend the concept of STM and to elaborate the kinds of mental processes that are involved when information is retained over a period of seconds to minutes. Working memory represents a limited-capacity store for retaining information over the short term and for performing mental operations on the contents of this store.

Psychologists Baddeley and Hitch argued that the idea of a unitary short-term memory was insufficient to explain the maintenance and processing of information over short periods. They proposed a three-part working memory systems consisting of a central executive mechanism that presides over and coordinates the interactions between two subordinate STM stores and LTM stores. The phonological loop is a hypothesized mechanism for acoustically coding information in working memory. The visuospatial sketch pad is a short-term memory store that parallels the phonological loop and permits information storage in either purely visual or visuospatial codes.

Deficits in STM abilities, such as remembering items on a digit span test, can be correlated with damage to subcomponents of the working memory system.

What are the long-term forms of memory?

Information retained for a significant time is referred to as long-term memory. Theorists have tended to split the LTM into two major divisions, taking into account the observable fact that people with amnesia may retain one type of LTM and not another.

• Declarative memory: is defined as memory for events and for facts, both personal and general, to which we have conscious access and which we can verbally report. This form of memory is sometimes referred to as explicit memory. Episodic memory comprised memories of events that the person has experienced that include what happened, where it happened, when, and with whom. Episodic memory differs from personal knowledge. You have personal knowledge about the day you were born, but you do not remember the experience. Semantic memory is objective knowledge that is factual in nature but does not include the context in which it was learned. Semantic memory reflects knowing facts and concepts such as how to tell time.

• Nondeclarative memory: is so named because it is not expressed verbally. It is also known as implicit memory because it is knowledge that we are not conscious of. Several types of memory fall under this category. Nondeclarative memory is revealed when previous experiences facilitate performance on a task that does not require intentional recollection of experiences.

  • Procedural memory: is one form of nondeclarative memory, which is required for tasks that include learning motor skills - such as riding a bike or swimming - and cognitive skills, such as reading. One test of procedural memory is the serial reaction-time task. The idea is that healthy participants respond faster to the complex repeating sequence than they do to a totally random sequence.

  • Priming: is another form of nondeclarative memory. Priming refers to a change in response to a stimulus, or in the ability to identify a stimulus, following prior exposure to that stimulus.

What holds the medial temporal lobe memory system?

The formation of new declarative memories depends on the medial temporal lobe. This region includes the; amygdala, the hippocampus, and the surrounding parahippocampal, entorhinal, and peririhinal cortical areas. They are all involved in the long-term memory.

The case of HM shows that the anterior portions of the hippocampus, the perirhinal and entorhinal cortices, were completely removed. Another case of RB shows the story of a patient who lost his memory due to a ischemic episode during heart bypass surgery. He could no longer form long-term memories. He also had a mild temporal retrograde amnesia that went back to about one-two years before surgery. The findings of his specific hippocampal damage in patient RB supports the idea that the hippocampus is crucial for the formation of long-term memories.

Further evidence that the hippocampus is involved in the long-term memory acquisition comes from patients with transient global amnesia (TGA). This syndrome has a number of causes, but it is triggered most commonly by physical exertion in men over 50 and by emotional stress in women over 50. The vertebrobasilar artery system, which supplies blood to the medial temporal lobe and the diencephalon, has been implicated as a critical site. High-resolution imaging data now suggest that the lesions caused by an ischemic episode are located in the CA1 subfield of the hippocampus and that these neurons are selectively vulnerable to metabolic stress.

Patients with TGA have similar symptoms as those of people with permanent damage to the medial temporal lobe, such as HM. But we do not know whether TGA patients have normal implicit learning of memory, in part because their impairment does not last long enough for researchers to adequately index things like procedural learning. But, the answer to this question would improve our understanding of human memory and of a form of amnesia that any of us could experience later in life.

Further evidence of hippocampal involvement in long-term memory formation comes form patients with Alzheimer's disease (AD), in whom the hippocampus deteriorates more rapidly than in people undergoing the normal aging process. But, some patients with anterior temporal lobe damage and the consequent dense retrograde amnesia, however, can still form new long-term episodic memories. This condition is known as isolated retrograde amnesia.

Is there evidence from animals with medial temporal lobe lesions?

To test whether the amygdala plays an essential part in memory formation, surgical lesions were created in the medial temporal lobe and amygdala of monkeys. The brain-lesioned monkeys were tested with a population behavioral task, known as the delayed non-match-to-sample task: a monkey is placed in a box with a rectractable door in the front. While the door is closed so that the monkey cannot see out, a food reward opened, and the monkey is allowed to pick up the object again, and the same object plus a new object are put in position. The new object now covers the food reward, and after a delay that can be varied, the door is reopened and the monkey must pick up the new object to get the food reward. With training, the monkey can pick new, or nonmatching objects. It was found that the monkey's memory was impaired only if the hippocampus ánd amygdala were lesioned. This finding led to the (incorrect) idea that the amygdala is a key structure in memory.

Researchers indicated that lesions of the hippocampus and amygdala produced even more severe memory deficits, but only when the cortex surrounding these regions was also lesioned. When lesions of the hippocampus and amygdala were made, but the surrounding cortex was spared, the presence or absence of the amygdala lesion did not affect the monkey's memory.

Another key question that animal researchers have addressed involves the kind of memory and learning that is impaired with lesions to the hippocampus. When electrodes were implanted in the rat hippocampus, certain cells, place cells, fired only when the rat was situated in a particular location and facing a particular direction. They provide evidence that the hippocampus has cells that encode contextual information.

Damage to the temporal lobe outside of the hippocampus can produce the loss of semantic memory, even while the ability to acquire new episodic memories remains intact.

Can you distinguish human memory systems with imaging?

Aggleton and Brown proposed the idea that encoding processes that merely identify an item as being familiar (recognition) and encoding processes that correctly identify an item as having been seen before (recollection) depend on different regions of the medial temporal lobe. A study revealed that the hippocampus is activated when information is correctly recollected. The findings of studies strongly suggest that the hippocampus is involved in both encoding and retrieval of episodic memories, but not of memories based on familiarity. Such data raised the question of which brain regions are involved in episodic versus nonepisodic memory encoding and retrieval. Results of studies demonstrate a double association in the medial temporal lobe for encoding different forms of memory: one medial temporal lobe mechanism involving the perirhinal cortex that supports familiarity-based recognition memory, and a second system involving the hippocampus and posterior parahippocampal cortex that supports recognition based on the recollection of sources (episodic) information.

When you think back on the first concert you even saw, you probably recall where an when you saw it. An early theory proposed that the fundamental role of the hippocampus is to build and maintain spatial maps. The main support of this theory was the discovery of the place cells identified in the hippocampus. How the brain solves the problem of bundling all this information - question known as the binding problem - is central to understanding episodic memory. The binding-of-items-and-contexts model proposes that the perirhinal cortex represents information about specific items, the parahippocampal cortex represents information about the context in which these items were encountered, and the processing in the hippocampus binds the representation of items with their context. As a result, the hippocampus is able to relate the various types of information about something that the individual encounters. This form of memory is referred to as relational memory.

In sum, the evidence from a number of studies indicates that the medial temporal lobe supports different forms of memory and that these different forms of memory are supported by different subdivisions of this brain region. Relational memory is memory for relations among the constituent elements of an experience - time, place, person etc. The relationsal memory theory proposes that the hippocampus supports memory for all manner of relations.

When our memory fails, we usually forget events that happened in the past. Sometimes, however, something more surprising occurs, we remember events that have never happened to us before. You can investigate falls memories using a technique. In this technique, participants are presented with a list of words that are all highly associated with a word that is not presented. When participants are asked subsequently to recall or recognize the words in the list, they show a strong tendency to falsely remember the associated word that was not presented. This memory illusion is so powerful that participants often report having a vivid memory of seeing the nonpresented critical item in the study list. The vividness of such memories make it difficult to separate the cognitive and neural basis of true and false memories.

True memories are associated with a greater activity in the medial temporal lobe and sensory areas, which are activated when a true item is first presented. False memories do not activate sensory areas; instead, regions associated with top-down cognitive control are more active for false memories.

What is memory consolidation?

Consolidation is the process that stabilizes a memory over time after it is first acquired. Consolidation processes occur at the cellular level, as well as at the system level.

The medial temporal lobes are essential for the early consolidation and initial storage of information for episodic and semantic memories. The mechanisms of the slower consolidation process, however, remain more controversial. There are two main theories:

• Standard consolidation theory: it considers the neocortex to be crucial for the storage of fully consolidated long-term memories, whereas the hippocampus plays only a temporary role. The representations of an events that are distributed throughout the cortex come together in the medial temporal lobe, where the hippocampus binds them. Consolidation occurs after repeated reactivation of the memory creates direct connection within the cortex itself between the various representations so that it no longer requires the hippocampus as the middle man to bind them.

• Multiple trace theory: it suggests that the long-term stores for semantic information rely solely on the neocortex, while episodic memory, consolidated or not, continues to rely on the hippocampus for retrieval. A new memory trace is set down in the hippocampus every time an episodic memory is retrieved: the more times a memory is retrieved, the more traces are set down. This theory suggests that episodic memories degrade over time and are slowly converted into semantic memory.

Evidence also shows that sleep plays an important role in memory consolidation after learning. The idea is that hippocampal neurons replay patterns of firing that were experienced during learning. Research also shows that stress can have a great impact on episodic memory consolidation when high levels of cortisol influence the hippocampal function.

What is the cellular basis of learning and memory?

Researchers have long believed that the synapse, with its dynamic connections, was a structure involved in the mechanisms of memory. Most models of the cellular bases of memory hold that memory is the result of changes in the strength of synaptic interactions among neurons in neural networks. Hebb proposed that synaptic connections between coactivated cells change in a manner dependent on their activity. This theory, Hebb's law, is commonly summarized as 'Cells that fire together, wire together'. Hebb proposed that the strengthening of synaptic connections results when a weak input and a strong input act on a cell at the same time. This learning theory is called Hebbian learning.

There are three major excitatory neural pathways of the hippocampus that extend from the CA1 cells:

1. The perforant pathway is the way between the entorhinal cortex and subiculum.

2. The granule cells have distinctive-looking unmyelinated axons, known as the mossy fibers, which connect the dentate gyrus to the dendritic spines of the hippocampal CA3 pyramidal cells.

3. The CA3 cells are connected to the CA1 by axon collaterals, known as the Schaffer collaterals.

Stimulation leads to greater synaptic strength in the perforant pathway so that, when the axons were stimulated again later, larger postsynaptic responses resulted in the granule cells of the dentate gyrus. This phenomenon is called the long-term potentiation (LTP) and its discovery confirmed the Hebb's law. The NDMA receptors are seen to be key in forming LTP, but they are not in maintaining it.

How does emotion work? - Chapter 10

People have been struggling to define emotion for several thousand years. As a psychological state, emotion has some unique qualities that have to be taken into account. Emotions are embodied, you feel them. They are uniquely recognizable; they are associated with characteristic facial expressions and behavioral patterns of comportment and arousal. Emotions are neurological processes that have evolved to guide behavior in such a manner as to increase survival and reproduction. They improve our ability to learn from the environment and the past. Many researchers claim that a feeling is the subjective experience of the emotion, but not the emotion itself. The most current models posit that emotions are valenced responses (positive/negative) to external stimuli and/or internal mental representations that

• involve changes across multiple response systems

• are distinct from moods, in that they often have identifiable objects or triggers

• can be either unlearned responses to stimuli with intrinsic affective properties, or learned responses to stimuli with acquired emotional value

• can involve multiple types of appraisal processes that assess the significance of stimuli to current goals

• depend of different neural systems

Others agree that emotions involve highly coordinated behavior, body and brain effects, they disagree that these various effects are part of the emotion state. They view them as a result or consequence of the emotion state

• an 'emotion' constitutes an internal, central state

• this state is triggered by specific stimuli

• this state is encoded by the activity of particular neural circuits

• activation of these specific circuits gives rise, in a causal sense, to externally observable behaviors, and to separately associated cognitive, somatic and physiological responses

Emotions fall under the umbrella term of affect, which includes not only discrete emotions that have a relatively short duration, but also more diffuse, longer-lasting states such as chronic stress and mood. Encountering a stimulus or event that threatens us in some way triggers stress, a fixed pattern of physiological and neurohormonal changes. These changes disrupt homeostasis, leading to immediate activation of the sympathetic nervous system's fight-or-flight responses, but also to activation of the hypothalamic-pituitary-adrenal axis and release of stress hormones, such as cortisol.

A mood is a long-lasting diffuse affective state that is characterized by primarily a predominance of enduring subjective feelings without an identifiable object or trigger. Moods do not have a well-defined neurohormonal or physiological substrate, and the neural correlates of different moods are still poorly understood.

What are the neural systems that are involved in emotion processing?

The identifying of the neural systems involved in emotion processing is difficult: technical issues with the various methods used to study emotion, controversies over whether emotional feeling, which are subjective, can be studied in humans and animals, and the various interpretations of research findings.

When emotions are triggered by an external event or stimulus, our sensory system plays a role. The autonome nervous system, or ANS ,is made up of the parasympathetic and sympathetic nervous systems. The two systems work in combination to achieve homeostasis. The ANS is regulated by the hypothalamus, which also control the release of multiple hormones through the HPA axis, made up of the paraventricular nucleus (PVN) of the hypothalamus, the anterior lobe of the pituitary gland, and the cortex of the adrenal glands, which sit above the kidney. Arousal is a critical part of many theories of emotion. The arousal system is regulated by the reticular activating system, which is composed of sets of neurons running from the brainstem to the cortex.

Papez came up with a circuit that has to do with emotion. The Papez circuit describes the brain areas that James Papez believed were involved in emotion. They include the hypothalamus, anterior thalamus, cingulate gyrus, and hippocampus. The limbic system includes these structures and the amygdala, orbitofrontal cortex, and the portions of the basal ganglia. MacLean proposed that the human brain had three regions that had developed gradually and sequentially over the course of evolution. He used the term limbic system to describe the complex neural circuits involved with the processing of emotion.

Over the last decades, scientific investigation of emotion has been focused more on human emotion and has also became more detailed and complex. Investigators no longer think there is only one neural circuit of emotion. Rather, depending on the emotional task or situation, we can expect different neural systems to be involved. The neuroimaging approaches based on machine learning revealed that specific emotional states activate several brain networks.

How do you categorize emotions?

In this section we discuss the basic versus dimensional categorization of emotions. Fearful, sad, anxious, elated, etc. are some of the terms we use to describe our emotional lives. Researchers made up three primary categories:

1. Basic emotions: a closed set of emotions, each with unique characteristics, carved by evolution and reflected through facial expressions

2. Complex emotions: combincations of basic emotions, some of which may be socially or culturally learned, that can be identified as evolved, long lasting feelings

3. Dimensional theories of emotion describe emotions that are fundamentally the same but that differ along one or more dimensions, such as valence and arousal, in reaction to events or stimuli

What are the basic emotions?

There are seven primary-process emotional systems, or core emotional systems, produced by ancient subcortical neural circuits common to all higher animals, which generate both emotional actions and specific autonomic changes that support those actions: SEEKING/desire, RAGE/anger, FEAR/anxiety, LUST/sex, CARE/maternal, GRIEF/seperation distress and PLAY/physiological social engagement.

For the past 150 years, many investigators of human emotions have considered facial expressions to be one of those predictable changes sparked by an emotional stimulus. Duchenne was the first in doing experiments with facial expressions in a man with facial anesthesia. He electrically stimulated the man's facial muscles and methodically triggered muscle contractions. Duchenne believed that facial expression revealed the underlying emotions. Ekman took up this work and the study of facial expression and came up with the six basic human facial expressions: anger, fear, sadness, happiness, disgust and surprise. Some of the basic emotions have been confirmed in nonhuman mammals, which show dedicated subcortical circuitry for such emotions.

What are the complex emotions?

Even if we accept that the basic emotions exist, we are still faced with identifying which emotions are basic and which are complex. Jealousy is one of the most interesting of the complex emotions. Also, romantic love is far more complicated than researchers initially thought. These emotions are associated with higher-order cortical areas and are involved with social cognition, theory of mind, and interpretation of actions performed by others.

Most researchers agree that emotional reactions to stimuli and events can be characterized by two factors: valence (positive and negative) and arousal (high or low). Although, sometimes it is the case that a person feels positive and negative at the same time. By using the dimensional approach - tracking valence and arousal - researchers can be more concretely in assessing emotional reactions elicited by stimuli. So, the dimensional approach, instead of describing discrete states of emotion, describes emotions as reactions that vary along a continuum.

What are the theories of emotion generation?

Most emotion researchers agree that the response to emotional stimuli is adaptive and that it can be separated into three components. Every theory of emotion generation is an attempt to explain:

• The physiological reaction (racing heart)

• The behavioral reaction (fight-or-flight response)

• The subjective experiential feeling ('I'm scared!')

What the theories don't agree on are the underlying mechanisms, and what causes what. The crux of the disagreement involves the timing of these three components and whether cognition is required to generate an emotional response and subjective feeling or, alternatively, whether an emotional stimulus leads directly to quick automatic processing that results in a characteristic response and feeling.

James-Lange theory of emotion:

James proposed that emotions were the perceptual results of somatovisceral feedback from bodily responses to an emotion-provoking stimulus. A sense organ relayed information about that stimulus to the cortex, which then sent this information to the muscles and viscera. The muscles and viscere than react by sending information back to the cortex, and the stimulus that had simply been apprehended has now emotionally felt.

Thus, when you run away because you see a bear and you're scared of bears, in the James's view, you don't run because you are afraid, you run first and you are afraid later, because you become aware of your body's physiological changes when you run, and then you cognitively interpret you physical reactions and conclude that what you're feeling is fright. Your emotional reaction depends on how you interpret those physical reactions. Lange went further to test this idea, due the name James-Lange theory.

Cannon-Bard theory of emotion:

They believed that physiological responses were not distinct enough to distinguish among fear, anger, and sexual attraction, for example. They proposed that we simultaneously experience emotions and physiological reactions: the thalamus processes the emotional stimuli and sends this information simultaneously to the neocortex and to the hypothalamus, which produces peripheral response. The Cannon-Bard theory remains important because it showed that reactions to emotional stimuli could occur without the cortex, at least in nonhuman animals.

Appraisal theory of emotion:

The Appraisal theory is a group of theories that say that emotion processing depends on an interaction between the stimulus properties and their interpretation. The theories differ about what is appraised and the criteria used for this appraisal. Lazarus proposed a version of appraisal theory in which emotions are a response to the reckoning of the ratio of harm versus benefit in a person's encounter with something. Thus, the cause of emotion is both the stimulus and its significance. Cognitive appraisal comes before emotional response or feeling, this appraisal step may be automatic and unconscious.

Singer-Schachter theory: cognitive interpretation and arousal

Singer and Schachter agreed with James and Lange that the perception of the body's reaction was the emotion, but they also agreed with Cannon and Bard that there were too many emotions for there to be a specific and unique autonomic pattern for each. The theory proposes that emotional arousal and then reasoning are required to appraise a stimulus before the emotion can be identified.

LeDoux's fast and slow roads to emotion:

LeDoux has proposed that humans have two emotion systems operating in parallel. One is a neural system for our emotional responses that bypasses the cortex and was hardwired by evolution to produce fast responses that increase our chances of survival and reproduction. The other system, which includes cognition, is slower and more accurate, this system generates the conscious feeling of emotion. Brain circuits that detect and respond to threats should be referred to as defensive circuits, and behaviors that occur in response to threats should be referred to as defensive behaviors.

Evolutionary psychology approach to emotion:

They suggest that emotions can be an overarching program that directs the cognitive subprograms and their interactions. An emotion is not reducible to its effects on physiology, behavioral inclinations, cognitive appraisal, or feeling states, because it involves coordinated, evolved instructions for all of these aspects together.

Panksepp's hierarchical-processing theory of emotion:

He hypothesized that emotions are subject to a control system with hierarchical processing. Emotion is processed in one of three ways; the most basic are the basic emotions and these arise straight from the neural networks in the subcortex. Cognition plays no further role when it comes to feeling these emotions. The core emotions arise from conditioning, and the tertiary-process emotions are elaborated by cognition.

What is the amygdala?

The amygdala is a small, almond-shaped structure in the medial temporal lobe adjacent to the anterior portion of the hippocampus. It is a collection of 13 nuclei that can be grouped into three main amygdaloid complexes: basolateral nuclear complex, centromedial complex and the cortical nucleus. The amygdala is known for that it is the most connected structure in the forebrain in humans. It has receptors for many different neurotransmitters and for various hormones that are present in the brain.

What is the influence of emotion on learning?

Claparède was the first to provide evidence that two types of learning, implicit and explicit, are apparently associated with two different pathways. He was a doctor who had a client with Korsakoff's syndrome. Every morning he had a ritual of shaking the hands of the clients he met. On a day he put a pin needle in his hand when he shaked hers. The next morning she, of course, didn't remember who the doctor was but as soon as he extended his hand to greet her, she hesitated for the first time. Implicit learning is a type of Pavlovian learning in which a neutral stimulus acquires aversive properties when paired with an aversive event. This is a classic example of fear conditioning. There is also a phenomenon called extinction; it represents new learning about the stimulus that inhibits expression of the original memory.

The amygdala is necessary for implicit learning, but it is not necessary for explicit or emotional learning. The conscious knowledge of an upcoming shock (in for instance an experiment of conditioning) cannot generate physiological changes normally associated with fear if there is no link present to link it to the amygdala and its midbrain connections. Information can come to the amygdala via two separate pathways; the 'low' road - goes directly from the thalamus to the amygdala, and the 'high' road - goes from the cortex to the amygdala.

Next to the amygdala is the hippocampus (in the state of learning) only necessary for the acquisition of a memory, but if arousal accompanies memory acquisition the strength and duration of that memory is modulated by amygdala activity.

What are the interactions between emotion and other cognitive processes?

Much research has been focused on the effects of emotion on learning and memory, its effects on other cognitive processes are also being unraveled.

We have a increased awareness for and pay attention to emotionally salient stimuli. Researchers often use the paradigm of the attentional blink to test this, in which stimuli are presented so quickly in succession that an individual stimulus is difficult to identify. But if participants are told to disregard all the other words, they are able to identify the targets. The proposed mechanism for this attentional change is that early in the perceptual processing of the stimulus, the amygdala receives input about its emotional significance, and, through projections to sensory cortical regions, modulates the attentional and perceptual processes. Fearful stimuli are not the only stimuli processed by the amygdala, especially fearful and disgusting ones have the priority.

The amygdala is critical in bringing an unattended but emotional stimulus into the realm of conscious awareness by providing some feedback to the primary sensory cortices, thus affecting perceptual processing.

A popular idea is that emotion leads people to make suboptimal and sometimes irrational decisions. The hypothesis that emotion and reason are separable in the brain and compete for control of behavior is often called 'dual-systems theory' and it has dominated Western thought since Plato. But the dualism theory has not been substantiated. Our current understanding suggests that there are two ways by which emotion influences decision making:

1. Incidental affect: current emotional state, unrelated to the decision at hand, incidentally influences the decision

2. Integral emotion: emotions elicited by the choice options are incorporated into the decision. This process may include emotions that you anticipate feeling after you have made the decision, which humans are notoriously bad at predicting.

What is the link between emotion and social stimuli?

Studies have shown that there is an dissociation between identifying an individual's face and recognizing the emotional expression on that face. Neuroimaging in normal patients and patients with anxiety disorders have reported that increased amygdala activation in response to brief representations of faces with fearful expressions are higher compared to faces with neutral expressions. The amygdala response/activation is significantly greater when in response to fear. One interesting part is that the participant does not need to be aware of the fearful face for the amygdala to respond. People who have extensive damage to the amygdala are not able to recognize fearful or untrustworthy facial expression.

What are other important areas or emotions?

The insula is tucked between the frontal and temporal lobes in the Sylvian fissure. It has reciprocal connections with areas associated with emotion, such as the amygdala, medial prefrontal cortex, and anterior cingulate gyrus. There is a significant correlation between the insular activity and the perception of internal bodily states, known as interoception. The connections and activation profile of the insula suggest that is integrates visceral and somatic input and forms a representation of the state of the body.

Depending on how the information is analyzed, different neuroimaging studies have been interpreted to both support and refute the theory that there are different brain areas or circuits associated with the processing of different emotions.

How does the cognitive control of emotion work?

Emotion regulation refers to the processes that influence the types of emotions we have, when we have them, and how we express and experience them. Emotion regulation processes can intervene at multiple points during the generation of emotion, some early on and some after the fact. Researchers investigate how we regulate emotion proceed by changing the input or the output. Change to the input can consist of avoiding the stimulus altogether, or altering the emotional impact of the stimulus by reappraisal. There is also a phenomenon, suppression, where the response to a emotional stimulus is altered a certain way. It reinterprets an emotion-laden stimulus in non-emotional terms. Research shows that reappraisal can lead to reduced emotional experience, suppression on the other hand caused participants to be more aroused.

What is language? - Chapter 11

What is the anatomy of language and language deficits?

Of all the higher functions that human possess, language is perhaps the most specialized and refined, and it may well be what most clearly distinguishes us from other species. Language input can be auditory or visual, so both of the sensory and perceptual systems are involved with language comprehension. Split-brain patients, as well as patient with lateralized, focal brain lesions have taught us that a great deal of language processing is lateralized to the left-hemisphere regions surrounding the Sylvian fissure. The language areas of the left hemisphere include Wernicke's area and Broca's area. These brain areas and their interconnections via white matter tracts form the left perisylvian language network.

Before neuroimaging, most of what was discerned about the neural bases of language processing came from studying patients who had brain lesions that resulted in various types of aphasia. Aphasia is a broad term referring to the collective deficits in language comprehension and production that accompany neurological damage. Aphasia may also be accompanied by speech problems caused by the loss of control over articulatory muscles, known as dysarthia, and deficits in the motor planning of articulations, apraxia. There is also a form of aphasia were the patient is unable to name objects, this is called anomia.

Broca's aphasia

Broca's aphasia is the oldest and perhaps the most-studied form of aphasia. Broca observed by patient Leborgne that he had a brain lesion in the posterior portion of the left inferior frontal gyrus, now referred to as Broca's area. In the most severe form of Broca's aphasia, singleutterance patterns of speech are often observed. The speech of patients with Broca's aphasia is often telegraphic (containing only content words and leaving out the function words that have only grammatical significance, such as prepositions and articles). Broca's aphasia patients are often aware of their errors and have a low tolerance for frustration. Broca's aphasia patients also have a comprehension deficit related to the syntax, the rules governing how words have to be put together in a sentence. Often only the most basic and overlearned grammatical forms are produced and comprehended - this is known as agrammatic aphasia.

Wernicke's aphasia

Wernicke's aphasia is a disorder primarily of language comprehension: patients with this syndrome have difficulty understanding spoken or written language and can sometimes not understand language at all. Their speech is fluently with normal prosody and grammar, but what they say is often nonsensical. Wernicke performed autopsies with his patients and came to the core of the language problem, the posterior regions of the superior temporal gyrus, now known as Wernicke's area.

Wernicke proposed a model for how the known language areas of the brain were connected. He and others found that large neural fiber tracts, arcuate fasciculus, connected Broca's and Wernicke's area. Wernicke predicted that damage to this fiber tract would disconnect the two areas in a fashion that would result in another aphasia, known as conduction aphasia. Patients understand words that they hear or see, and they are able to hear their own speech errors but cannot repair them. They also have a problem with spontaneous speech, as well as repeating speech, and sometimes they use words incorrectly. Lichtheim also proposed that this hypothetical brain region stored conceptual information about words. Once a word was retrieved from word storage, it was sent to the concept area, which supplied all information that was associated with the word. These ideas led to the Wernicke- Lichtheim model. This model proposes that language processing, from sound to motor outputs, involved interconnections of different key brain regions. And damage to different segments of this network would result in the various observed and proposed forms of aphasia.

What are the fundamentals of language in the human brain?

The human language is called natural language because it arises from the abilities of the brain. It can be spoken, gestured and written. So, how does the brain cope with spoken, gestured and written input to derive meaning? And how does the brain produce spoken, gestured and written output to communicate meaning to others? The brain must store representations of words and their associated concepts. A word in a spoken language has two properties: a meaning and a phonological form. A word written also has a orthographic form. One of the central ideas in word representation is the mental lexicon - a mental storage of information about words that includes semantic information (meaning), syntactic and the details of word forms (how the words combine to form sentences), and the details of word forms (spelling and sound patterns).

There are three general functions involving the mental lexicon:

1. Lexical access: the stage of processing in which the output of perceptual analysis activates word form representations in the mental lexicon.

2. Lexical selection: the stage in which the representation that best matches the input is identified

3. Lexical integration: the final stage, in which words are integrated into the full sentence, discourse, or larger context to facilitate understanding of the whole message.

A normal adult speaker has passive knowledge of about 50,000 words, yet can easily recognize and produce about three words per second. The mental lexicon is proposed to have other features, linguistic evidence supports the following four organizing principles:

1. The smallest meaningful representational unit in a language is called a morpheme

2. Most frequently used words are accessed more quickly than less frequently used words

3. A phoneme is the smallest unit of sound that makes a difference to the meaning of a word

4. Representations in the mental lexicon are organized according to semantic relationships between words

When you look at the patterns of deficits in patients with language disabilities, we can infer a number of things about the functional organization of the mental lexicon. Patients of Wernicke's aphaisa make errors in speech production that are known as semantic paraphasias. They might use the word 'horse' when they intend to use the word 'cow'. The categories of semantic information of words are represented in the left temporal lobe, with a progression from posterior to anterior for general to more specific information, respectively.

How does language comprehension work (the early steps)?

The brain uses some of the same processes to understand both spoken and written language, but there are also some striking differences in how spoken and written inputs are analyzed. When you are listening to spoken language, the listener has to decode the acoustic input, this input is then translated into a phonological loop. The representations in the mental lexicon that match the auditory input are then accessed and selected. The word's meaning results in activation of the conceptual information.

Infants have the perceptual ability to distinguish all possible phonemes during their first year of life, but during the first year of life, the perceptual sensitivities became tuned to the phonemes of language they experienced on a daily basis. They, therefor, loose the ability to distinguish phonemes that are not part of the English language.

In humans, the superior temporal cortex is important for sound perception. People with damage to this area may develop pure word deafness. When the speech signal hits the ear, it is first processed by pathways in the brain that are not specialized for speech but are used for hearing them in general. The Heschl's gyri of both hemispheres are activated by speech and nonspeech sounds alike, but the activation in the superior temporal sulcus, STS, of each hemisphere is modulated by whether the incoming auditory signal is a speech sound or not. Further in the brain, the brain becomes less sensitive to changes in nonspeech sounds but more sensitive to speech sounds.

Reading is the perception and comprehension of written language. Our brain is very good at pattern recognition, but reading is a quite recent invention. Learning to read requires linking arbitrary visual symbols into meaningful words. The identification of orhtographic units may take place in occipitotemporal regions of the left hemisphere, and it has been known for over a hundred years that lesions in this area can give rise to pure alexia, a condition in which patients cannot read words, even though other aspects of language are normal. In humans, written information from the left visual field arrives first via visual inputs to the contralateral right occipitical cortex and is sent to the left-hemisphere visual word form area via the corpus callosum. The visual word area is heavily interconnected with regions of the left perisylvian language system, including the frontal, temporal and inferior parietal cortical regions.

What are the later steps of language comprehension?

Once a phonological or visual representation is identified as a word, then for it to gain any meaning, semantic and syntactic information must be retrieved. Words are often not processed in isolation, but in the context of other words. To understand words in their context, we have to integrate syntactic and semantic properties of the recognized words into a representation of the whole utterance.

Is it possible to retrieve word meanings before words are heard or seen when the word meanings are highly predictable in the context? When you hear the sentence 'the tall man planted a tree on the bank'. Here 'bank' has multiple meanings. But the context of the sentence enables us to interpret bank as the 'side of the river' and not 'the financial institution'. There are lower-level representations, those constructed from the sensory input, and higher-level representations, those constructed from the context preceding the word to be processed.

There are three classes of models attempt to explain word comprehension:

1. Modular models: claim that normal language comprehension is executed within seperate and independent modules. Higher-level representations cannot influence lower-level ones, and therefore the flow is strictly data driven, bottom-up.

2. Interactive models: maintain that all types of information can participate in word recognition. Context can have its influence even before the sensory information is available, by changing the activational status of the word-form representations

3. Hybrid models: which fall between the modular and interactive extremes, are based on the notion that lexical access is autonomous and not influenced by higher-level information.

How do we process the structure of sentences? When we hear or read sentences, we activate word forms that activate the grammatical and semantic information in the mental lexion. But representations of whole sentences are not stored in the brain. Instead, the brain has to assign a syntactic structure to words in sentences, in a process called syntactic parsing. Lexical access and selection involve a network that includes the medial temporal gyrus (MTG), superior temporal gyrus (STG), and ventral inferior and bilateral dorsal inferior frontal gyri (IFG) of the left hemisphere. When you are talking about the ERP method, the N400 method is a negative-polarity brain wave related to semantic processes in language. The P600/SPS is a large positive component elicited after a syntactic and some semantic violations.

What are the neural models of language comprehension?

One neural model of language that combines work in brain and language analysis has been proposed by Hagoort. His model divides language processing into three functional components:

• Memory: refers to the linguistic knowledge that is encoded and consolidated in neocortical memory structures.

• Unification: refers to the integration of lexically retrieved phonological, semantic, and syntactic information into an overall representation of the whole utterance.

• Control: relates language to social interactions and joint action

How are these brain regions in the left hemisphere organized to create a language network in the brain? White matter tracts in the left hemisphere connect inferior frontal cortex, inferior parietal cortex, and temporal cortex to create specific circuits for linguistic operations.

What are the neural models for speech production?

Motor control involves creating internal forward models, which enable the motor circuit to make predictions about the position and trajectory of a movement and its sensory consequences, and sensory feedback which measures the actual sensory consequences of an action. Feedback control has been documented in the production of speech. Researchers have altered sensory feedback and found that people adjust their speech to correct for sensory feedback 'errors'.

Levelt came up with a influential cognitive model for language production. The first step in speech production is to prepare the message. There are two crucial aspects to message preparation: macroplanning, in which the speaker determines what she wants to express, and microplanning, in which she plans how to express it. The speech production also depends on the use of orofacial muscles that are controlled by processes using internal forward models and sensory feedback. Hickok's model of speech production involves the parallel processing and two levels of hierarchical control.

The models of language production must account for the processes of selecting the information to be contained n the message; retrieving words from the lexicon, planning sentences and encoding grammar using semantic and syntactic properties of the word, using morphological and phonoloical properties for syllabification and prodosy; thus preparing articulartory gestures for each syllable.