Cognitive Neuroscience, the biology of the mind, by M. Gazzaniga (fourth edition) – Summary chapter 5

Each sense collects, filers and amplifies information. Each system has specialized receptor cells that transduce the environmental stimulus into neural signals. The sensory nerves of the body travel up the spinal cord and enter the brain through the medulla and then go to the thalamus. After going through the thalamus, they travel to the primary sensory cortex and the secondary sensory cortex.

Receptor cells share a few general properties:

1. Range
   Each sensory modality responds to a limited range of stimuli.
2. Adaptation
   This refers to how sensory systems stay fine-tuned. It’s the adjusting of the sensitivity of the sensory system to the current environment.
3. Acuity
   This is the ability to distinguish between different stimuli.

Each level of processing (e.g: attention, memory, emotion) contributes to the end product of sensory stimulus. This leads to a subjective experience of sensory information. All senses pass through the thalamus before going to the appropriate sensory cortex. The nuclei in the thalamus are interconnected, which provides an opportunity for multisensory integration.

Sound waves arriving at the ear enter the auditory canal. Here, the signals are amplified. It makes the eardrum vibrate and these vibrations travel through air-filled middle ear and rattle tiny bones which causes another membrane (oval window) to vibrate. The oval window is the door to the fluid-filled cochlea. There are tiny hair cells in the cochlea and these are the sensory receptors of the auditory system. The location of a hair cell determines its frequency tuning.

The hair cells act as mechanoreceptors when deflected by the membrane, mechanically gated ion channels open in the cells allowing positively charged ions to flow into the cell. This will lead to depolarization and a release of a transmitter into a synapse between the hair cell and the nerve fibre.

Axons from the cochlear and the olivary nuclei projects to the inferior colliculus where it can access motor structures. Neurons throughout the auditory pathway continue to have frequency tuning and maintain their tonotopic arrangement as they travel up to the cortex. A neuron does not only respond to a specific frequency, but a range of frequency, but is fine-tuned to one specific frequency. The range differs across species.

The interaural time is the difference in the time it takes for sound to reach the two ears. Sound localization makes use of interaural time and the difference in sound intensity between the two ears.

There are different somatosensory receptors for touch, including corpuscles. Pain is signalled by nociceptors. There are three types of nociceptors: thermal, mechanical and polymodal (wide range of stimuli). Myelinated nociceptors are responsible for immediate pain and unmyelinated nociceptors are responsible for longer-lasting pain. Proprioception refers to information about the body’s position.

The initial receiving area is called the primary somatosensory cortex. It contains a somatotopic representation of the body. This is called sensory homunculus. The relative amount of cortical representation corresponds to the relative importance of somatosensory information of that part of the body (e.g: hands have a bigger cortical area than the knee). The secondary sensory cortex builds more complex representations (e.g: texture).

Functional reorganization (cortical plasticity) might occur in the brain through the remodelling of neuronal connections. The phantom limb sensation might occur because a body part is touched that has appropriated the missing limb’s old area in the cortex.

Long term plasticity may result from the growth of new synapses and/or axons. Immediate effects are likely to be due to a sudden reduction in inhibition that normally suppresses inputs from neighbouring regions.

Exteroceptive perception is perceiving information at a distance. The image we perceive is inverted and focused to project on the retina. It is composed of photoreceptors, which includes rods and cones. Rods and cones do not fire action potentials but trigger action potentials in downstream neurons. Rods are useful for little light and cones are useful with a lot of light. There are three types of cones, defined by their sensitivity to different regions of the visible spectrum. A cone that responds to short-wavelength (1) (blue part of the vision), cones that respond to medium wavelength (2) (green part of the spectrum) and cones that respond to long-wavelength (3) (red part of the spectrum). The fovea is the centre of the retina. Cones are densely packed here and the rods are more at the outer layer of the retina.

The optic nerve is a bundle of axons of ganglion cells that transmits information to the central nervous system. Rods transmit information to ganglion cells. Multiple rods transmit information to a single ganglion cell. By summing the output, rods can activate a ganglion cell even in low light situations. Each ganglion cell is activated by only a few cones. The crossover place of the optic nerve is called the optic chiasm.

There are different cortical visual areas, areas responsible for a distinct region of visual processing. One hypothesis for the multiple brain areas dedicated to vision is that it is hierarchical. Another hypothesis is that visual perception is an analytical process. This means that each area provides information about different attributes. Processing is distributed and specialized.

Conscious perception is linked to higher-area activity. Our primary sensory regions provide a representation that is closely linked to the physical stimulus and our perceptual experience is more dependent on activity in secondary and association sensory regions. Anatomical differences among people in the size of V1 affect the extent of visual illusion.

Dichromats are people who can only see two photopigments (two colour spectrums). Anomalous trichromats have all three photopigments but one has abnormal sensitivity. Achromatopsia are disorders of colour perception that arise from disturbances of the central nervous system. People with achromatopsia see colours as grey or black.

Akinetopsia is a selective loss of motion perception. Severe forms of akinetopsia result from bilateral lesions. With unilateral lesions, motion perception deficits are more subtle. Perception moves from primary to secondary areas, but also the other way around.

Hemianopia refers to incomplete blindness because a lesion is restricted to half of the visual field. If that happens, then also one half of the visual field is lost. Scotomas are small discrete regions of blindness produced by lesions. It is possible that there are two systems: one dedicated to spatial orientation and one dedicated to object identification. The superior colliculus is important for spatial orientation.