Prefrontal Cortex - Executive Functions

Executive Functions

The original studies of Fuster and of Goldman-Rakic emphasized the fundamental ability of the prefrontal cortex to represent information not currently in the environment, and the central role of this function in creating the "mental sketch pad". Goldman-Rakic spoke of how this representational knowledge was used to intelligently guide thought, action, and emotion, including the inhibition of inappropriate thoughts, distractions, actions, and feelings. In this way, working memory can be seen as fundamental to attention and behavioral inhibition. Fuster speaks of how this prefrontal ability allows the wedding of past to future, allowing both cross-temporal and cross-modal associations in the creation of goal-directed, perception-action cycles. This ability to represent underlies all other higher executive functions.

Shimamura proposed Dynamic Filtering Theory to describe the role of the prefrontal cortex in executive functions. The prefrontal cortex is presumed to act as a high-level gating or filtering mechanism that enhances goal-directed activations and inhibits irrelevant activations. This filtering mechanism enables executive control at various levels of processing, including selecting, maintaining, updating, and rerouting activations. It has also been used to explain emotional regulation.

Miller and Cohen proposed an Integrative Theory of Prefrontal Cortex Function, that arises from the original work of Goldman-Rakic and Fuster. The two theorize that “cognitive control stems from the active maintenance of patterns of activity in the prefrontal cortex that represents goals and means to achieve them. They provide bias signals to other brain structures whose net effect is to guide the flow of activity along neural pathways that establish the proper mappings between inputs, internal states, and outputs needed to perform a given task”. In essence, the two theorize that the prefrontal cortex guides the inputs and connections, which allows for cognitive control of our actions.

The prefrontal cortex is of significant importance when top-down processing is needed. Top-down processing by definition is when behavior is guided by internal states or intentions. According to the two, “The PFC is critical in situations when the mappings between sensory inputs, thoughts, and actions either are weakly established relative to other existing ones or are rapidly changing”. An example of this can be portrayed in the Wisconsin Card Sorting Test (WCST). Subjects engaging in this task are instructed to sort cards according to the shape, color, or number of symbols appearing on them. The thought is that any given card can be associated with a number of actions and no single stimulus-response mapping will work. Human subjects with PFC damage are able to sort the card in the initial simple tasks, but unable to do so as the rules of classification change.

Miller and Cohen conclude that the implications of their theory can explain how much of a role the PFC has in guiding control of cognitive actions. In the researchers' own words, they claim that, “depending on their target of influence, representations in the PFC can function variously as attentional templates, rules, or goals by providing top-down bias signals to other parts of the brain that guide the flow of activity along the pathways needed to perform a task”.

Experimental data indicate a role for the prefrontal cortex in mediating normal sleep physiology, dreaming and sleep-deprivation phenomena.

When analyzing and thinking about attributes of other individuals, the medial prefrontal cortex is activated. However, it is not activated when contemplating about the characteristics of inanimate objects. As of recent, researchers have used neuroimaging techniques to find that along with the basal ganglia, the prefrontal cortex is involved with learning exemplars, which is part of the exemplar theory, one of the three main ways our mind categorizes things. The exemplar theory states that we categorize judgements by comparing it to a similar past experience within our stored memories.

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