Developmental Neuropsychology
Developmental Neuropsychology: Executive function and social cognition in the adolescent brain
Executive function and social cognition in the adolescent brain
Adolescence is a period of development characterized by intense fluctuations in both physical and hormonal change. Research has been sparse in this area, empirical research on neural and cognitive development is still lacking. For such a period that reflects the growth of cognitive flexibility, self-consciousness and changes in identity, the need for further research is apparent.
Beginning to experiment on adolescent brains
Through studies on animals, we have seen that specific sensory regions of the brain go through sensitive periods starting after birth, in which environmental stimulation seems to be crucial fro the normal development of the brain and perceptual capabilities. Experiments suggest that while this is true for animals, it may also be true for humans. During the 1970’s and 1970’s, it was demonstrated that certain brain areas, especially the prefrontal cortex develop far beyond early childhood. Further studies in the decades that followed showed that during the period of puberty and adolescence, the structure of the prefrontal cortex goes through substantial changes. Two important changes highlighted are:
- Myelination (increasing the transmission speed of neural information)
- Synaptogenesis (regularly used connections are strengthened and seldom used connections are removed)
Synaptogenesis was first found in 1975 within experiments using cats and was further researched using rhesus monkeys. Synaptic pruning (a period of synaptic destruction) and synaptogenesis in the brain area the prefrontal cortex exist on a differential time line. Proliferation of synapses occurs in the prefrontal cortex during childhood and once again during puberty, however this is followed by a stagnant period and elimination and reorganization of prefrontal synaptic connections following puberty.
An overall decrease in synaptic density as a result of synaptic pruning in the frontal lobes ensues during adolescence. This process is thought to be essential for refining the effectiveness of neural networks. This is especially true when given the sensitive case of sound recognition development.
The adolescent brain seen through MRI
Through the implementation of modern technology, it has become possible to view the brain of living specimens. With the introduction of magnetic resonance imaging, we can non-invasively view the human brain in a detailed three-dimensional visual. This has been instrumental in the furthering of research on the maturation of the frontal cortex of adolescence and onwards into adulthood.
Linear increases in white matter during adolescence
Over the past years, researchers have found one consistent outcome across MRI studies: during adolescence and childhood there is a steady increase in white matter located in certain brain regions. One study found a significant between the white and grey matter levels between two age groups. This increase in white matter has been suggested to be the result of developmental changes by some studies. One study found an increase in white matter in the right internal capsule and left arcuate fasciculus. Both regions are associated with speech so it was hypothesized that the increase in white matter was due to speech development.
Non-linear decreases in grey matter during adolescence
Grey
.....read more Developmental Neuropsychology: Cognitive control and motivational systems
Cognitive control and motivational systems in developmental neurobiology
Cognitive control is an executive process which can be vital to the maintenance and monitoring of long term goal oriented behavior. In the past, the development of cognitive control has been explained as a growth from infancy to adulthood. The role of context on cognitive control impacts one’s behavioral regulation abilities, for example in a stressful context one may experience diminished control. Recent studies suggest that cognitive control capacity is impacted by specific periods of development whereby one is more susceptible to incentive based modulation.
When examining studies on cognitive control performed in a controlled laboratory settings, we see a relatively stable improvement in cognitive control capacity from infants progressing to adults. However, outside of the laboratory setting, this is often not the case. This is particularly true for adolescence, who experience a reduced capacity for cognitive control when exposure to potentially risky behavior is at its peak. These fluctuations in behavior give evidence of dynamic maturation of the brain mechanisms responsible for motivation and cognitive processes. Two areas of the brain are highlighted for their importance in cognitive and motivational processes: the prefrontal cortex (essential for cognitive control) and the striatum (important for identifying interesting cues in an environment).
Examining the role of motivational modulation of cognitive control across development
Recently, research on the development of adolescences has focused on comparing cognitive capacity in neutral settings as opposed to motivational contexts. This research has implied that there exists a unique influence of motivation on cognition during the adolescent period, and that sensitivity to environmental cues (in particular incentive cues) changes at various points in development.
The behavior of adolescents has been shown to be differentially biased in motivational contexts. Studies have shown that motivational cues of potential reward are especially salient and potentially lead to the engagement in risky behavior and the further weakening of goal-orientated behavior.
Corticosubcortical control and its developmental neurobiology
This has led to the development of a neurobiological model of motivational and cognitive processes which aims to explain the behavior of adolescents outside of a laboratory context. Working with this model leads to the suggestion linear development of top down prefrontal regions relative to a n-shaped function for the development of bottom-up striatal regions involved in detecting particularly interesting cues in the environment.
The findings of Pasupathy and Miller indicate that the interactions between brain areas (especially within frontostriatal circuitry) is essential in the development of a model of motivational and cognitive control. Further studies have highlighted the important role of signaling inside corticostriatal circuitry for supporting the capacity to employ effective cognitive control. Further evidence for the theory that cognitive maturation occurs in the connectivity of structures rather than in unitary structures was found by examining the interactions between a frontoparietal network and cingulate-lateral prefrontal network. Works by Ernst, Galvan, Luna and Crone have been seminal in response to incentives, striatal responses follow an inverted U function across development. In contexts where a reward or incentive
.....read more Developmental Neuropsychology: Mild spastic cerebral palsy
Mild spastic cerebral palsy: An event-related brain potential study of error detection and response adjustment
The term cerebral palsy is used to define a number of disorders which affect posture and movement. Cerebral palsy is attributed to damage or abnormal development in the developing brain of fetus’s or infants. Though there are many contributing factors to cerebral palsy, one of particular note are infarcts (tissue death as a result of lack of oxygen) which lead to lesions in white and grey matter tracts. These lesions to white matter tracts are detrimental to executive functioning which has been proven when testing youths with cerebral palsy against a control group without.
The question posed by this study is whether youths with mild spastic cerebral palsy are aware of their errors when carrying out tasks requiring executive functioning.
It has often been the findings of neurocognitive research that poor motor preparation precedes error making. Error detection and the adjustments which follow are measured in this case using response-locked error-related negativity. The brain potential for incorrect responses is markedly higher than those following correct responses.
Method
Participants consisted of 11 patients, with a mean age of 14 years, diagnosed with mild cerebral palsy and a control group of 12 youths without cerebral palsy, with a mean age of 14, recruited from the same city. Though the intelligent quotient of some of the youths with mild cerebral palsy was within the range of learning disabilities, none were classified as being mentally retarded, as their daily lives were intact.
A computer based stimulus recognition task and electroencephalograms were used to record brain activity in the study. The task used was comparable to the Sternberg short-term memory paradigm. Participants were presented with 2 letters to be memorized, then were subsequently shown 4 letters, one or none of which were the letters which were memorized. Using two response buttons, participants would indicate either yes or no whether they identified there target letters in the new set. Reaction time was measured starting when the new set appeared until a button was pressed. Total time for the experiment was about 15 minutes.
Results
The results of the experiment find that the control group made more correct responses and less error responses than the experiment group. In addition, the patient group reacted slower on average than the control group. Error responses were also shown to be slower than correct responses, which proved true for both groups.
Discussion and conclusion
Given the findings of the study, poor motor preparation appears to be associated with error responses. This is especially salient in the patient group. Error detection was found in both groups and went on to predict better performance. A key point given by the authors is that error monitoring and the adjustment in performance which follows are essential to learning. The research conducted here implies that this process is thankfully present in the patient group. Additionally, it is proposed that the patient group was more sensitive to their error making
.....read more Developmental Neuropsychology: Treatments and guidelines for phenylketonuria
Revision of treatments and guidelines for phenylketonuria: evidence from neurocognition
An inborn, inherited, error of metabolism, phenylketonuria (PKU) is a rare, highly treatable disease. In contrast to its relatively treatable nature, when left untreated, PKU can result in seizures, intellectual disability, and further medical issues. The most common method for prevention is the early introduction of a strict diet highlighting the restriction of phenylalanine (Phe). Despite its treatability, patients with PKU show an average 8-10 points lower than normal in addition to underperforming in neuropsychological tests.
Cognitive impairments have been associated with concurrent blood Phe levels, and even more so with lifetime blood Phe levels. The question being raised by the authors concerns whether or not the recommend Phe level for patients with PKU is too high. Currently, from the age of 0-10, this level varies between 240 and 360 micromolars/L. The drawback is that this range is not the result of studies comparing outcomes at Phe levels <240 micromolar/L, between 240 and 360 micromolar/L, and >360 micromolar/L.
Current treatment advises an absolute upper target level for Phe levels, not taking into account possible fluctuations of Phe values and the phenylalanine:tyrosine ratio (Phe:Tyr). This feeds into the second area of question the authors pose: what are the effects of lifetime Phe, concurrent Phe, variation in lifetime Phe levels, and lifetime and concurrent Phe:Tyr in predicting cognitive outcome in early and continuously treated children and adolescents with PKU
Methods
Participants consisted of 67 patients with PKU with a mean age of 10.8, and a control group of 73 participants with a mean age of 10.9 recruited from friends and families of patients and also local newspaper advertisements. Of the patients with PKU, 27 had pretreatment Phe levels of > 1200, 18 had pretreatment Phe levels between 600-1200, and 18 had pretreatment Phe levels <600, and 12 patients had pretreatment Phe levels <360. To test for concurrent Phe and Tyr levels, a blood sample was taken in the morning following an overnight fast. This was also used to test for lifetime Phe level. Through a series of computer based neuropsychological tests, executive functions inhibitory control and motor control were measured.
Results and discussion
Patients with Phe levels >360 differed in 2 of 3 inhibition tasks, motor control, and cognitive flexibility. Controls performed notably more accurately than patients with Phe levels between 240-360. A key finding was that patients with Phe levels <240 performed no different to the control group. Additionally, patients with Phe levels <240 outperformed those with Phe levels between 240-360.
It was found that Phe variation, lifetime and concurrent Phe, and lifetime and concurrent Phe:Tyr were significantly related to speed and accuracy on numerous cognitive tests and also to each other.
The two major findings of this study are:
- Youths from the age of 6-15 with mean Phe levels of <240 since birth outperformed their peers with Phe levels between 240-360 on cognitive tests measuring motor control, inhibition, and cognitive flexibility.
- Phe:Tyr and Phe variation may have predictive value
Developmental Neuropsychology: ADHD
Examining a pilot study of methylphenidate, interstimulus interval, and reaction time performance of children with attention deficit/hyperactivity disorder
Attention deficit/hyperactivity disorder (ADHD) is one of the most common found disorders in children. Characterized by inattention and hyperactivity, ADHD is primarily an inherited disorder. Treatment usually involves methylphenidate (MPH), which works by regulating the dopamine system. This often results in a decrease in impulsivity, hyperactivity, and inattention. Despite this, children on MPH still experience deficits in cognitive faculties. Other studies have shown that children with ADHD perform differently depending on the length of a stimulus they are exposed to. If the stimulus is a short interstimulus interval (ISI), the children experience a more positive performance in comparison to a long ISI. A short ISI has been shown to improve memory recognition, vigilance, motor timing, and more. It is thought that both MPH and a short ISI act upon the dopamine levels in the brain. It is proposed that having both of these working at the same time may over-activate the system and lead to detrimental effects. The goal of this study was to measure the isolated and combined effect of MPH and ISI on mean response times and errors of commission.
Procedure
For this study the sample consisted of 13 children (10 boys; 3 girls) diagnosed with ADHD. During the time of the study, MPH was gradually introduced for a two week period, either adjusting to a higher or lower dosage. After the children had been on MPH for 4 weeks, they were tested twice, once after the administration of MPH, and once after a placebo. The task which the participants engaged in was a computer based reaction time test known as Go/No-Go test. This involved pressing a button when the letter Q appeared onscreen and press nothing when the letter O appeared. If the participant responded to the letter O, this was recorded as an error of commission.
Looking at results
The results from the study indicate:
Children respond faster on MPH than not on MPH
Children respond faster in a condition with a short ISI rather than a long ISI
The interaction of ISI and MPH was not significant
Children on MPH made more errors of commission during the condition with a short ISI
Children on the placebo made fewer errors of commission during the condition with a short ISI
For discussion
This study set out to ascertain whether children with ADHD were able to inhibit responses when they are in a state of over-activation. The findings of the study indicate that children perform better with either MPH or in the presence of a short ISI in comparison with a long ISI. When MPH and a short ISI are combined, they have detrimental effects. The stimulus shift hypothesis is supported by this outcome. This outcome calls into question the optimal dosage levels when treating ADHD.
State regulation and motivation in ADHD
Throughout past research, weak motivation
.....read more Developmental Neuropsychology: Autism
Investigating the role of the frontal cortex in autism
Little is known about the underlying neural developmental defects which result in the emergence of autistic behavior during the early years of life. The frontal lobe has been identified as the most likely region to be involved, and yet little is known about it. The frontal lobe plays a key role in higher order language, cognitive, emotional, and social faculties, each one affected by autism. This has provided support for the frontal lobe hypothesis of autism. Through the collection of data from MRI and postmortem anatomical, and also already existing neurofunctional, postmortem, and MRI results from more mature autistic patients, the authors find two suggestions:
In patients with autism, the frontal cortex is poor at interacting with other cortical regions
During early development, the frontal cortex appears to be irregularly over-connected with itself
Examining macroscopic evidence of early frontal maldevelopment
Brain growth of patients with autism is normal at birth, ranging from average to slightly smaller than average. However this is followed by a period of excessive growth which results in an enlarged brain volume at the toddler age. Investigations into which brain regions cause this growth indicate the frontal lobes to be at the site of peak growth. Grey and white matter in the frontal lobes are both disparately deviant in regards to other cortical regions. While several studies have shown that primary sensory cortices in autism function normally, the same cannot be said for the frontal lobes. The deficient functionality found in the frontal lobe is hypothesized as being a factor which disrupts the frontal lobes interaction with other areas of the brain. It is unknown whether autism is to be classified as disorder of overconnectivity, underconnectivity, or a combination of the two.
Examining microscopic evidence of frontal maldevelopment
There remains a lack of knowledge on the microstructural abnormalities that disrupt frontal neural circuit development, facilitate the macroscopic overgrowth of frontal white and grey matter, and facilitate abnormal frontal mediated behavior. Where the link exists between abnormal neuroinflammatory response and initial brain overgrowth in autism is a mystery. It has been suggested that activated glia could be a reflection of a fetal state or development. When it comes to cerebral cortical information processing, a cortical minicolumn is an essential component. Studies have shown that minicolumns and their surrounding neuropil space are unusually small in children with autism throughout the frontal cortex, but not the occipital cortex. One older study found an increased neuron density and reduced neuron size within the frontal cingulate cortex in a number of autistic cases. It is suggested that these glial and neuronal abnormalities are in some way connected.
Drawing a conclusion
This is the first time that studies have shown early developmental abnormalities at the microscopic level and the macroscopic level and the presence of the frontal lobes as the peak of cerebral pathology. The work of abnormal processes (neuroinflammation, migration defects, excess cerebral neurogenesis) cause deformity,
.....read more
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