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:

1. Myelination (increasing the transmission speed of neural information)
2. 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 matter, in contrast to white matter, does not seem to follow a linear pattern. Rather it seems to follow a region specific, non-linear pattern. Research has shown that grey matter follows an inverted-U shape in certain brain regions. Studies suggest that grey matter volume in the frontal lobe reaches its peak during puberty, which is then followed by a stagnation period and a steady decline until though adolescence until early adulthood. MRI research on the decline of grey matter throughout adolescence has two explanations:

1. Axonal myelination both facilitates an increase in white matter and a decrease in grey matter
2. The decrease in grey matter is a reflection of the reorganization which occurs during puberty

Examining the role of gender differences in the development of brain structure

Research has shown that there exists a significant difference in the amount of grey and white matter found in males and females. This was particularly true in the area known as the inferior frontal gyrus (IFG). Researchers corrected for the possible confound of total cerebral volume but still they found that males had a significantly higher amount of grey matter. Some researchers believe this difference is a result of differential steroid levels, others believe it may be due to greater hemispheric specialization found in males. As of current research, more study is needed to examine variability in frontal cortex anatomy.

The continuing brain changes after adolescence

Recent research using MRI suggests that the period at which the brain reaches maturity could be later than the end of adolescence. This change is seen especially in the frontal and parietal cortices following post-mortem cellular examinations of human brains which support an extended period of development. The adolescent brain appears to develop in a dynamic nature when looking at the growth of white and grey matter.

Examining the development of executive function

Executive functioning refers to a set of cognitive mechanisms which govern how well we control and coordinate our behavior and cognition. Lesion studies have shown that many of the skills involved in executive functioning are dependent on the frontal lobes. Given that changes occur in the frontal cortex during adolescence, executive functioning might be expected to improve during this period. Behavioral studies have shown that inhibitory control, processing speed, working memory, and decision making continue to develop throughout adolescence.

Examining the development of social cognition

Evidence suggests that the prefrontal cortex is associated with many high level cognitive functions, such as self-awareness and theory of the mind. In tandem with this neural development, puberty marks a great period of change for hormonal development. These two factors combined mark a significant change in social cognition. Research concerning the effects of puberty on social cognition capacitates has been limited.

Looking at perspective taking

The act of perspective taking refers to putting oneself in another’s shoes and it is an essential component for effective social communication. This act of estimating what another individual is thinking is  related to the first-order theory of mind. It is an area of much theoretical debate. One view surmises that we understand others by simulating their actions and cognitions. In support of this, research has shown that similar brain areas are activated when we perform an action and when we see another perform the same action. This is in line with the function of mirror neurons.  

The adolescent brain in action seen through fMRI

With the introduction of fMRI, we have a safe and effective way to examine brain regions.  As an example, through the use of fMRI we were able to examine how response inhibition has developed in addition to examining the structure supporting its development. During studies involving inhibition of a normal response, it was shown that activation of certain brain regions was for the most part the same across age groups.  An exception was a significantly larger volume of activation in the prefrontal of children. In contrast to this, adults showed higher activation in the ventral region of the prefrontal cortex. One suggested reason for this is that children have a heftier dependence on this area than adults.

The confounds on task performance

An issue which has been found when conducting fMRI is that of confounding effect of task performance. If there is a difference in the performance between two groups i.e. one outperforms the other, the results are often difficult to interpret.

Examining the development of social cognition within the brain

In terms of brain regions, the amygdala is an area which has repeatedly been shown to have a high functioning role in facial recognition and emotional processing. Though research in this area has been investigated with adolescents, still little is known about the continued development of facial recognition. Studies have shown that children may identify neutral faces as being more ambiguous than fearful faces, resulting in an increase in activation in the amygdala.

Effect on teenagers

puberty marks a period of synaptic reorganization, and as a result can be more susceptible to experiential input, in particular regarding social cognition and executive functioning. Further studies are needed to examine the further development of brain maturation beyond childhood and the implications this has on cognition. The authors specify questions into which skills undergo perturbation, sensitive periods for improvement and how does the environmental quality impact the changes which occur in the brain.