Article summary with Neural substrates of early executive function development by Fiske & Holmboe - 2019

What are the three most commonly used neuroimaging techniques?

• Electroencephalography (EEG) is a non-invasive method of measuring direct electrical activity in the brain. It is useful in researching the neural underpinnings of cognitive functions. It provides information about the size and frequency of a neuronal signal with high resolution, providing information about functional connectivity between brain regions during cognitive tasks.
• fMRI records the blood-oxygen-level-dependent response that occurs during neuronal firing. It has very good spatial resolution that enables researchers to produce images of spatially localized brain activation.
• fNIRS involves placing a cap containing sources and detectors of near-infrared light on the participant´s head. The system then records changes in light absorption levels between sources and detectors. fNIRS measures changes in concentration of oxygenated and deoxygenated haemoglobin in localized brain regions in response to a stimulus. Neuronal activity is signaled by an increase in oxygenated haemoglobin and a decrease in deoxyhaemoglobin.

What different models of executive function exist?

• According to the unity-diversity model, executive function is a hierarchical construct consisting of a domain-general unitary entity and three dissociable components, namely updating working memory, shifting, and inhibition. These elements are thought to be mediated by a fronto-parietal network.
• According to factor analytic work, there is a common executive function factor that represents the unity of the three core executive function abilities. The diversity of executive function exists in the factors that are specific to each component.
• The adaptive neural coding framework claims that executive function is a unitary, domain-general construct. This function recruits the same specialized frontal pathway in different ways depending on the task demands for a range of challenging cognitive tasks.
• The developmental model of executive function looks at changes observed in the structure of executive function and the underlying neural correlates during childhood and adolescence. The structure of executive function changes across development, from one that is largely unified and recruits a common neural network in early-to-middle childhood, to one that involves more diverse components that each recreuit specific neural networks.

What is working memory?

Working memory refers to a limited capacity cognitive system that enables the temporary storage and manipulation of information. The dorsolaterale prefrontal cortex (DL-PFC) is important in working memory development. Recent research has shown that also regions beyond the DL-PFC are important, as well as the connections between these areas.

What is inhibitory control?

Inhibitory control refers to the process of preventing an automatic response in order to achieve a behavioral or cognitive goal. Inhibitory control begins to develop towards the end of the first year of life, rapidly improving during the toddler and preschool years. It increases steadily throughout middle childhood, and reaches adult levels in early adolescence. The DL-PFC is activated in inhibitory control, but activation of the DL-PFC reduces with development. Also, during inhibitory control, there is parietal activation and striatum involvement.

How does the prefrontal cortex develop?

The prefrontal cortex is one of the last brain regions to fully mature. PFC maturation consists of both progressive (myelination, synaptogenesis, neuron proliferation) and regressive (cell death, loss in grey matter, synaptic pruning) changes. Increases in cortical volume, synapses and dendritic trees serve to facilitate information processing by forming connections between the prefrontal cortex and other cortical areas. Cortical thickness and volume develop following an inverted U-shaped trajectory, increasing in childhood and then declining during early adulthood.

What developmental changes in the prefrontal cortex underlie cognitive performance?

Research on direct links between structural prefrontal cortex development and cognitive performance is still limited, but the following results have been found:

• The structural integrity of the corpus callosum and white matter tracts that connect the prefrontal cortex to other brain regions during infancy are predictive of executive function in later childhood.
• Myelination allows for rapid and syncronised information processing that is required for many cognitive functions, including aspects of executive function.
• Performance improvements on executive function tasks indirectly parallel the structural changes in grey matter and are directly associated with the structural changes in white matter that occur in the same fronto-parietal cortices that are recruited during executive function task performance.
• Structural maturation of fronto-parietal regions is directly telated to the development of working memory (improvement in working memory was related to cortical volume reduction in the lateral prefrontal cortex and in the posterior parietal cortex).

What is the role of prefrontal dopamine during executive function development?

Prefrontal dopamine plays an important role in the development of executive functioning. Research has shown that the lack of prefrontal dopamine in children resulted in a selective deficit in tasks involving working memory and inhibition. Gene methylation, the silencing of a gene, also reflect the potential impact of dopaminergic genes on the prefrontal cortex during development. Gene mythelation falls under the umbrella-term of epigenetics. Epigenetic encompass changes in gene expression caused by biological mechanisms other than the DNA sequence itself. Environmental influences during post-natal development have an impact on methylation patterns, which can cause differences in psychological and behavioral outcomes.

What is frontal connectivity?

The prefrontal cortex develops rich connections within itself and with other cortical, subcortical and limbic brain regions, which together form a system that sub-serves executive function. Functional connectivity between the frontal and parietal cortices mediates early executive function development. Developmental improvements in executive function performance are partially due to functional integration via connectivity.

What is the refinement process of the prefrontal cortex?

Research indicated that with age there is a shift from global to local activation in the prefrontal cortex during executive function tasks. This shift may reflect the increased efficiency of the developing brain and the growing functionality of sub-regions of the prefrontal cortex for executive function. Mature executive function depends on the focalization of activity to brain regions directly linked to the related cognitive function, as well as decreased activity in supplementary brain regions.