Prefrontal cortex


In mammalian brain anatomy, the prefrontal cortex covers the front part of the frontal lobe of the brain. It is the association cortex in the frontal lobe. This region is responsible for processing and adapting one's thinking in order to meet certain goals in different situations. These processes of thinking can include the brain allowing one to focus, control how they behave, and make different decisions.
The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA46, and BA47.
This brain region is involved in a wide range of higher-order cognitive functions, including speech formation, gaze, working memory, and risk processing. The basic activity of this brain region is considered to be orchestration of thoughts and actions in accordance with internal goals. Many authors have indicated an integral link between a person's will to live, personality, and the functions of the prefrontal cortex.
This brain region has been implicated in executive functions, such as planning, decision making, working memory, personality expression, moderating social behavior and controlling certain aspects of speech and language. Executive function relates to abilities to differentiate among conflicting thoughts, determine good and bad, better and best, same and different, future consequences of current activities, working toward a defined goal, prediction of outcomes, expectation based on actions, and social "control".
The frontal cortex supports concrete rule learning, with more anterior regions supporting rule learning at higher levels of abstraction.

Structure

Definition

There are three possible ways to define the prefrontal cortex:
  • as the granular frontal cortex
  • as the projection zone of the medial dorsal nucleus of the thalamus
  • as that part of the frontal cortex whose electrical stimulation does not evoke movements

    Granular frontal cortex

The prefrontal cortex has been defined based on cytoarchitectonics by the presence of a cortical granular layer IV. It is not entirely clear who first used this criterion. Many of the early cytoarchitectonic researchers restricted the use of the term prefrontal to a much smaller region of cortex including the gyrus rectus and the gyrus rostralis. In 1935, however, Jacobsen used the term prefrontal to distinguish granular prefrontal areas from agranular motor and premotor areas. In terms of Brodmann areas, the prefrontal cortex traditionally includes areas 8, 9, 10, 11, 12, 13, 14, 24, 25, 32, 44, 45, 46, and 47, however, not all of these areas are strictly granular – 44 is dysgranular, caudal 11 and orbital 47 are agranular. The main problem with this definition is that it works well only in primates but not in nonprimates, as the latter lack a granular layer IV. In a recent research study, scientists stimulated the amygdala area in monkeys, where they found that the auditory and prefrontal cortices got activated, which is similar to what happens in humans.

Projection zone

To define the prefrontal cortex as the projection zone of the mediodorsal nucleus of the thalamus builds on the work of Rose and Woolsey, who showed that this nucleus projects to anterior and ventral parts of the brain in nonprimates, however, Rose and Woolsey termed this projection zone "orbitofrontal." It seems to have been Akert, who, for the first time in 1964, explicitly suggested that this criterion could be used to define homologues of the prefrontal cortex in primates and nonprimates. This allowed the establishment of homologies despite the lack of a granular frontal cortex in nonprimates.
The projection zone definition is still widely accepted today, although its usefulness has been questioned. Modern tract tracing studies have shown that projections of the mediodorsal nucleus of the thalamus are not restricted to the granular frontal cortex in primates. As a result, it was suggested to define the prefrontal cortex as the region of cortex that has stronger reciprocal connections with the mediodorsal nucleus than with any other thalamic nucleus. Uylings et al. acknowledge, however, that even with the application of this criterion, it might be rather difficult to define the prefrontal cortex unequivocally.

Electrically silent area of frontal cortex

A third definition of the prefrontal cortex is the area of frontal cortex whose electrical stimulation does not lead to observable movements. For example, in 1890, David Ferrier used the term in this sense. One complication with this definition is that the electrically "silent" frontal cortex includes both granular and non-granular areas.

Subdivisions

According to Striedter, the PFC of humans can be delineated into two functionally, morphologically, and evolutionarily different regions: the ventromedial PFC consisting of:
  1. the ventral prefrontal cortex
  2. the medial prefrontal cortex present in all mammals
and the lateral prefrontal cortex, consisting of:
  1. the dorsolateral prefrontal cortex
  2. the ventrolateral prefrontal cortex present only in primates.
The LPFC contains the Brodmann areas BA8, BA9, BA10, BA45, BA46, and BA47. Some researchers also include BA44. The vmPFC contains the Brodmann areas BA12, BA25, BA32, BA33, BA24, BA11, BA13, and BA14.
The table below shows different ways to subdivide parts of the human prefrontal cortex based upon Brodmann areas.
  • The dorsolateral prefrontal cortex is composed of the BA8, BA9, BA10, and BA46.
  • The ventrolateral prefrontal cortex is composed of areas BA45, BA47, and BA44.
  • The medial prefrontal cortex is composed of BA12, BA25, and anterior cingulate cortex: BA32, BA33, BA24. Within that area is the dorsal nexus, which interconnects many parts of the brain.
  • The ventral prefrontal cortex is composed of areas BA11, BA13, and BA14.
  • The dorsolateral prefrontal cortex contains BA8, including the frontal eye fields.
  • The ventrolateral prefrontal cortex contains BA45 which is part of Broca's area. Some researchers also include BA44 the other part of Broca's area.

    Interconnections

The prefrontal cortex is highly interconnected with much of the brain, including extensive connections with other cortical, subcortical and brain stem sites. The dorsal prefrontal cortex is especially interconnected with brain regions involved with attention, cognition and action, while the ventral prefrontal cortex interconnects with brain regions involved with emotion. The prefrontal cortex also receives inputs from the brainstem arousal systems, and its function is particularly dependent on its neurochemical environment. Thus, there is coordination between one's state of arousal and mental state. The interplay between the prefrontal cortex and socioemotional system of the brain is relevant for adolescent development, as proposed by the Dual Systems Model.
The medial prefrontal cortex has been implicated in the generation of slow-wave sleep, and prefrontal atrophy has been linked to decreases in SWS. Prefrontal atrophy occurs naturally as individuals age, and it has been demonstrated that older adults experience impairments in memory consolidation as their medial prefrontal cortices degrade. In older adults, instead of being transferred and stored in the neocortex during SWS, memories start to remain in the hippocampus where they were encoded, as evidenced by increased hippocampal activation compared to younger adults during recall tasks, when subjects learned word associations, slept, and then were asked to recall the learned words.
The ventrolateral prefrontal cortex has been implicated in various aspects of speech production and language comprehension. The VLPFC is richly connected to various regions of the brain including the lateral and medial temporal lobe, the superior temporal cortex, the infertemporal cortex, the perirhinal cortex, and the parahippoccampal cortex. These brain areas are implicated in memory retrieval and consolidation, language processing, and association of emotions. These connections allow the VLPFC to mediate explicit and implicit memory retrieval and integrate it with language stimulus to help plan coherent speech. In other words, choosing the correct words and staying "on topic" during conversation come from the VLPFC.

Function

Executive function

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 integration of past and 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. The prefrontal cortex is responsible for many executive functions, such as remembering information, cognitive flexibility, and inhibitory control. Although the prefrontal cortex does this, it does not carry out these responsibilities alone as other brain regions also play important roles during this. For example, the amygdala sends signals to the prefrontal cortex area known as 46d in order for one to comprehend the situation and to determine how to react.
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. 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 the characteristics of inanimate objects.
Studies using fMRI have shown that the medial prefrontal cortex, specifically the anterior medial prefrontal cortex, may modulate mimicry behavior. Neuroscientists are suggesting that social priming influences activity and processing in the amPFC, and that this area of the prefrontal cortex modulates mimicry responses and behavior.
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.
A 2014 meta-analysis by Professor Nicole P.Yuan from the University of Arizona found that larger prefrontal cortex volume and greater PFC cortical thickness were associated with better executive performance.