Development Review

Introduction 

In this article, the researchers are concerned with the development of the adolescent brain and the implications it has on executive function and social cognition (Blakemore and Choudhury, 2006). There is no doubt that adolescence marks a period of transition and significant development in a young adult’s life. Development factors affect levels of behavior, cognition and the physical brain. The researchers have reviewed histological and brain imaging studies that reveal certain changes in the neural networks of the brain during puberty and in adolescence. They are able to outline the path of grey and white matter development; thus shedding light on the implications of brain development for executive function and social cognition during this time.

Review

The first studies on the brains of young adults showered that the young brain has a sensitive period that affects the person’s sensory capacity. During the 1970’s, it was revealed that the prefrontal cortex undergoes changes well into adolescence. At this point, it was clear that there were two main changes in the brain before and after puberty; first, that as neurons grow, a layer of myelin is formed and acts to insulate the neuron and increasing its transmission speed. This showed that sensory and motor brain regions were able to become fully myelinated by age one. The second difference that was evident (based on the brains of pre-pubescent children and those of young adults) was related to the synaptic density in the pre-frontal cortex. For instance, an adult brain has approximately 100 billion neurons, while during development, many changes are occurring in the brain. The most intricate change involves the increase in complexity of the neural networks in the brain. Synaptic density refers to the number of synapses per unit volume of brain tissue. In postnatal development, synaptic density is greater than in adults. This is done by a process referred to as synaptic proliferation, or synaptogenesis, and can take up to several months to complete. The early peaks in synaptogenesis are shortly followed by synaptic termination, also called pruning. This results in the decline in synaptic density, and is precisely why adults show less synaptic density than their younger years. It is important to note that synaptic pruning is essential for the fire-tuning of functional neural networks. It allows the brain to optimize its performance, and make neighboring neural circuits more efficient. In addition to these changes, studies have also suggested that cognitive processes continue to develop throughout adolescence, especially in the frontal lobes.

Non-invasive brain imaging techniques include MRI (magnetic resonance imaging). MRI can produce a highly-detailed 3D image of the human living brain. MRI has been used to support the observation that the frontal cortex continues to mature well into adulthood. MRI has also been used to show that there is an increase in white matter in certain brain regions throughout childhood and adulthood. Typically, older children have a higher volume of white matter in the frontal cortex and parietal cortex compared to their younger counterparts. The younger group has a higher contrast of grey matter in the same regions. This change in increase of white matter and decrease in grey matter is associated with increased axonal myelination in the frontal and parietal cortices. This observations has been replicated in many other large research studies. They have also found that increase in white matter in Broca’s area and Wenicke’s area are associated with increased connections in the speech regions of the brain. Moreover, the corpus callosum (region connecting the two hemispheres) has also been implicated in the developmental changes that occur in the brain during adolescence and mid-twenties. Brain imaging studies have also been able to reveal gender related differences in grey and white matter between girls and boys. More specifically, the white matter in the inferior frontal gyrus (IFG) of boys increased sharply in volume with age, and no such change was observed in girls. This has been suggested to result from a difference in steroid levels between the two genders. However, it is important to note that most gender differences that have been observed are still under speculation and are the subject of much debate.