The cerebral cortex can be divided into three main parts: the sensory areas, the motor areas, and the association areas. The association cortex is a complex distributed network, receiving information from the primary and secondary sensory and motor areas, as well as the brainstem and the thalamus, processing it, and sending it across multiple pathways to the hippocampus, the basal ganglia, and the cerebellum (Yeo et al., 2011).
It is commonly described as comprising of three areas, which also send information to each other: the posterior/parietal association area, the limbic/temporal association area, and the anterior/frontal association area (Purves et al., 2014).
The anterior/frontal association cortex is thought to play an essential role in cognitive processes, and by extension, in mental health (Gao et al., 2012). These cognitive processes in the anterior/frontal frontal cortex are called executive functions; they are particularly important in adapting an individual’s behaviour to a particular situation.
Some examples include the inhibition of socially unacceptable behaviours, and the projection of future consequences from current actions (Buchsbaum, 2004). More generally, executive functions can be thought as a set of self-regulation skills depending on highly interrelated brain functions such as mental flexibility, working memory, and self-control (Takeuchi et al., 2013).
What information and processes did researchers use to infer the central role of the anterior/frontal association cortex in these higher-order cognitive functions? There are two main lines of evidence: neuropsychological cases of acquired frontal lobe damage, and neuroimaging.
Years before neuroimaging was to be invented, scientists noticed cases of previously healthy people who after a stroke or a head injury damaging their frontal lobes would showcase a deterioration in their executive functions.
A famous case is Phineas Gage, a man who was the victim of an accident in 1848, where much of his brain’s left frontal lobe was destroyed by an iron rod driven completely through his head (Bigelow, 1850; Harlow, 1868; Barker, 1995). While his general intelligence seemed unimpaired, Phineas Gage started displaying socially unacceptable behaviours and lack of restraint, to a point where his friends and family said he was “no longer Gage” (Harlow, 1868).
While some of the symptoms may have been misreported at the time (Griggs, 2015), and recent research has hypothesized that the damage done by the iron rod was not entirely located in the frontal cortex (Van Horn et al., 2012), this remarkable case first demonstrated the importance of the anterior/frontal association cortex in many executive functions such as planning, generating, monitoring and inhibiting behaviours.
A second line of evidence available to us today is neuroimaging. Several structured tasks have been designed to test cognitive functions in individuals. For example, the Wisconsin Card Sorting Test requires individuals to classify cards according to different criteria, such as number, colour, or symbol; the test measures how well individuals adapt when the rules are changed (Grant & Berg, 1948).
Another test is the Tower of Hanoi, where individuals are asked to transfer a pyramid of discs from a peg to another without ever placing a larger disc onto a smaller one (Kotovsky et al., 1985). Two other such structured tests include the stroop colour word test, testing an individual’s inhibitory control (Golden & Freshwater, 1978), and the N-back task, used to measure attention and working memory (Kirchner, 1958).
Neuroimaging studies show that anterior/frontal association areas are most active when individuals engage in such tasks engaging their executive functions. For example, a meta-analysis of neuroimaging studies where individuals were placed in a fMRI machine and asked to engage in the Wisconsin Card Sorting Test revealed high activity in the prefrontal cortex, especially in the case of task-switching and response-suppression tasks (Buchsbaum et al., 2005).
This higher level of activity resorbs as the individual becomes more comfortable with the tasks (Hampshire, 2016). Neurodevelopmental conditions have also been studied using neuroimaging and cognitive tests. For example, the Sustained Attention to Response Task was administered to individuals with autistic spectrum disorders (Hendry et al., 2006), showing a dysfunction of the frontoparietal attentional network (Manly et al., 2003).
While these two lines of evidence explain why the anterior/frontal association cortex may play a central role in executive functions, and many neurodevelopmental conditions are thought to involve impaired functions in the anterior/frontal association cortex, it is important to note that the tests mentioned above do not always yield perfect results (Nyhus & Barceló, 2009), and better tests need to be designed to understand how exactly the anterior/frontal association areas interact with other areas of the brain.
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