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In cognitive science and neuropsychology, executive functions (collectively referred to as executive function and cognitive control) are a set of cognitive processes that are necessary for the cognitive control of behavior:
selecting and successfully monitoring behaviors that facilitate the
attainment of chosen goals. Executive functions include basic cognitive
processes such as attentional control, cognitive inhibition, inhibitory control, working memory, and cognitive flexibility. Higher-order executive functions require the simultaneous use of multiple basic executive functions and include planning and fluid intelligence (e.g., reasoning and problem-solving).
Executive functions gradually develop and change across the
lifespan of an individual and can be improved at any time over the
course of a person's life. Similarly, these cognitive processes can be adversely affected by a variety of events which affect an individual. Both neuropsychological tests (e.g., the Stroop test) and rating scales (e.g., the Behavior Rating Inventory of Executive Function) are used to measure executive functions. They are usually performed as part of a more comprehensive assessment to diagnose neurological and psychiatric disorders.
Cognitive control and stimulus control, which is associated with operant and classical conditioning,
represent opposite processes (internal vs external or environmental,
respectively) that compete over the control of an individual's elicited
behaviors;
in particular, inhibitory control is necessary for overriding
stimulus-driven behavioral responses (stimulus control of behavior). The prefrontal cortex is necessary but not solely sufficient for executive functions; for example, the caudate nucleus and subthalamic nucleus also have a role in mediating inhibitory control.
Cognitive control is impaired in addiction, attention deficit hyperactivity disorder, autism, and a number of other central nervous system disorders. Stimulus-driven behavioral responses that are associated with a particular rewarding stimulus tend to dominate one's behavior in an addiction.
Neuroanatomy
Historically, the executive functions have been seen as regulated by the prefrontal regions of the frontal lobes, but it is still a matter of ongoing debate if that really is the case.
Even though articles on prefrontal lobe lesions commonly refer to
disturbances of executive functions and vice versa, a review found
indications for the sensitivity but not for the specificity
of executive function measures to frontal lobe functioning. This means
that both frontal and non-frontal brain regions are necessary for intact
executive functions. Probably the frontal lobes need to participate in
basically all of the executive functions, but they are not the only
brain structure involved.
Neuroimaging and lesion
studies have identified the functions which are most often associated
with the particular regions of the prefrontal cortex and associated
areas.
- The dorsolateral prefrontal cortex
(DLPFC) is involved with "on-line" processing of information such as
integrating different dimensions of cognition and behavior. As such, this area has been found to be associated with verbal and design fluency, ability to maintain and shift set, planning, response inhibition, working memory, organisational skills, reasoning, problem-solving, and abstract thinking.
Side view of the brain, illustrating dorsolateral prefrontal and orbitofrontal cortex
- The anterior cingulate cortex (ACC) is involved in emotional drives, experience and integration.
Associated cognitive functions include inhibition of inappropriate
responses, decision making and motivated behaviors. Lesions in this area
can lead to low drive states such as apathy, abulia or akinetic mutism
and may also result in low drive states for such basic needs as food or
drink and possibly decreased interest in social or vocational
activities and sex.
- The orbitofrontal cortex (OFC) plays a key role in impulse control, maintenance of set, monitoring ongoing behavior and socially appropriate behaviors.
The orbitofrontal cortex also has roles in representing the value of
rewards based on sensory stimuli and evaluating subjective emotional
experiences. Lesions can cause disinhibition, impulsivity, aggressive outbursts, sexual promiscuity and antisocial behavior.
Furthermore, in their review, Alvarez and Emory state that:
The
frontal lobes have multiple connections to cortical, subcortical and
brain stem sites. The basis of "higher-level" cognitive functions such
as inhibition, flexibility of thinking, problem solving, planning,
impulse control, concept formation, abstract thinking, and creativity
often arise from much simpler, "lower-level" forms of cognition and
behavior. Thus, the concept of executive function must be broad enough
to include anatomical structures that represent a diverse and diffuse
portion of the central nervous system.
The cerebellum also appears to be involved in mediating certain executive functions, as do the ventral tegmental area and the substantia nigra.
Hypothesized role
The
executive system is thought to be heavily involved in handling novel
situations outside the domain of some of our 'automatic' psychological
processes that could be explained by the reproduction of learned schemas or set behaviors. Psychologists Don Norman and Tim Shallice have outlined five types of situations in which routine activation of behavior would not be sufficient for optimal performance:
- Those that involve planning or decision-making
- Those that involve error correction or troubleshooting
- Situations where responses are not well-rehearsed or contain novel sequences of actions
- Dangerous or technically difficult situations
- Situations that require the overcoming of a strong habitual response or resisting temptation.
A prepotent response is a response for which immediate reinforcement (positive or negative) is available or has been previously associated with that response.
Executive functions are often invoked when it is necessary to
override prepotent responses that might otherwise be automatically
elicited by stimuli in the external environment. For example, on being
presented with a potentially rewarding stimulus, such as a tasty piece
of chocolate cake,
a person might have the automatic response to take a bite. However,
where such behavior conflicts with internal plans (such as having
decided not to eat chocolate cake while on a diet), the executive
functions might be engaged to inhibit that response.
Although suppression of these prepotent responses is ordinarily
considered adaptive, problems for the development of the individual and
the culture arise when feelings of right and wrong are overridden by
cultural expectations or when creative impulses are overridden by
executive inhibitions.
Historical perspective
Although
research into the executive functions and their neural basis has
increased markedly over recent years, the theoretical framework in which
it is situated is not new. In the 1940s, the British psychologist Donald Broadbent drew a distinction between "automatic" and "controlled" processes (a distinction characterized more fully by Shiffrin and Schneider in 1977), and introduced the notion of selective attention, to which executive functions are closely allied. In 1975, the US psychologist Michael Posner used the term "cognitive control" in his book chapter entitled "Attention and cognitive control".
The work of influential researchers such as Michael Posner, Joaquin Fuster, Tim Shallice, and their colleagues in the 1980s (and later Trevor Robbins, Bob Knight, Don Stuss,
and others) laid much of the groundwork for recent research into
executive functions. For example, Posner proposed that there is a
separate "executive" branch of the attentional system, which is
responsible for focusing attention on selected aspects of the environment. The British neuropsychologist
Tim Shallice similarly suggested that attention is regulated by a
"supervisory system", which can override automatic responses in favour
of scheduling behaviour on the basis of plans or intentions. Throughout this period, a consensus emerged that this control system is housed in the most anterior portion of the brain, the prefrontal cortex (PFC).
Psychologist Alan Baddeley had proposed a similar system as part of his model of working memory
and argued that there must be a component (which he named the "central
executive") that allows information to be manipulated in short-term memory (for example, when doing mental arithmetic).
Development
The executive functions are among the last mental functions to reach maturity. This is due to the delayed maturation of the prefrontal cortex, which is not completely myelinated
until well into a person's third decade of life. Development of
executive functions tends to occur in spurts, when new skills,
strategies, and forms of awareness emerge. These spurts are thought to
reflect maturational events in the frontal areas of the brain.
Attentional control appears to emerge in infancy and develop rapidly in
early childhood. Cognitive flexibility, goal setting, and information
processing usually develop rapidly during ages 7–9 and mature by age 12.
Executive control typically emerges shortly after a transition period
at the beginning of adolescence.
It is not yet clear whether there is a single sequence of stages in
which executive functions appear, or whether different environments and
early life experiences can lead people to develop them in different
sequences.
Early childhood
Inhibitory control and working memory act as basic executive functions that make it possible for more complex executive functions like problem-solving to develop.
Inhibitory control and working memory are among the earliest executive
functions to appear, with initial signs observed in infants, 7 to 12
months old. Then in the preschool years, children display a spurt in performance on
tasks of inhibition and working memory, usually between the ages of 3
and 5 years. Also during this time, cognitive flexibility, goal-directed behavior, and planning begin to develop.
Nevertheless, preschool children do not have fully mature executive
functions and continue to make errors related to these emerging
abilities – often not due to the absence of the abilities, but rather
because they lack the awareness to know when and how to use particular
strategies in particular contexts.
Preadolescence
Preadolescent
children continue to exhibit certain growth spurts in executive
functions, suggesting that this development does not necessarily occur
in a linear manner, along with the preliminary maturing of particular
functions as well. During preadolescence, children display major increases in verbal working memory; goal-directed behavior (with a potential spurt around 12 years of age); response inhibition and selective attention; and strategic planning and organizational skills. Additionally, between the ages of 8 and 10, cognitive flexibility in particular begins to match adult levels.
However, similar to patterns in childhood development, executive
functioning in preadolescents is limited because they do not reliably
apply these executive functions across multiple contexts as a result of
ongoing development of inhibitory control.
Adolescence
Many
executive functions may begin in childhood and preadolescence, such as
inhibitory control. Yet, it is during adolescence when the different
brain systems become better integrated. At this time, youth implement
executive functions, such as inhibitory control, more efficiently and
effectively and improve throughout this time period.
Just as inhibitory control emerges in childhood and improves over time,
planning and goal-directed behavior also demonstrate an extended time
course with ongoing growth over adolescence. Likewise, functions such as attentional control, with a potential spurt at age 15, along with working memory, continue developing at this stage.
Adulthood
The major change that occurs in the brain in adulthood is the constant myelination of neurons in the prefrontal cortex.
At age 20–29, executive functioning skills are at their peak, which
allows people of this age to participate in some of the most challenging
mental tasks. These skills begin to decline in later adulthood. Working
memory and spatial span are areas where decline is most readily noted.
Cognitive flexibility, however, has a late onset of impairment and does
not usually start declining until around age 70 in normally functioning
adults. Impaired executive functioning has been found to be the best predictor of functional decline in the elderly.
Models
Top-down inhibitory control
Aside from facilitatory or amplificatory mechanisms of control, many authors have argued for inhibitory mechanisms in the domain of response control, memory, selective attention, theory of mind, emotion regulation, as well as social emotions such as empathy. A recent review on this topic argues that active inhibition is a valid concept in some domains of psychology/cognitive control.
Working memory model
One
influential model is Baddeley's multicomponent model of working memory,
which is composed of a central executive system that regulates three
subsystems: the phonological loop, which maintains verbal information;
the visuospatial sketchpad, which maintains visual and spatial
information; and the more recently developed episodic buffer that
integrates short-term and long-term memory, holding and manipulating a
limited amount of information from multiple domains in temporal and
spatially sequenced episodes.
Researchers have found significant positive effects of biofeedback-enhanced relaxation on memory and inhibition in children.
Biofeedback is a mind-body tool where people can learn to control and
regulate their body to improve and control their executive functioning
skills. To measure one's processes, researchers use their heart rate and
or respiratory rates. Biofeedback-relaxation includes music therapy, art, and other mindfulness activities.
Executive functioning skills are important for many reasons,
including children's academic success and social emotional development.
According to the study "The Efficacy of Different Interventions to
Foster Children's Executive Function Skills: A Series of Meta-Analyses",
researchers found that it is possible to train executive functioning
skills.
Researchers conducted a meta-analytic study that looked at the combined
effects of prior studies in order to find the overarching effectiveness
of different interventions that promote the development of executive
functioning skills in children. The interventions included computerized
and non-computerized training, physical exercise, art, and mindfulness
exercises. However, researchers could not conclude that art activities or physical activities could improve executive functioning skills.
Supervisory attentional system (SAS)
Another conceptual model is the supervisory attentional system (SAS).
In this model, contention scheduling is the process where an
individual's well-established schemas automatically respond to routine
situations while executive functions are used when faced with novel
situations. In these new situations, attentional control will be a
crucial element to help generate new schema, implement these schema, and
then assess their accuracy.
Self-regulatory model
Russell Barkley proposed a widely known model of executive functioning that is based on self-regulation. Primarily derived from work examining behavioral inhibition, it views executive functions as composed of four main abilities. One element is working memory that allows individuals to resist interfering information. A second component is the management of emotional responses in order to
achieve goal-directed behaviors. Thirdly, internalization of
self-directed speech is used to control and sustain rule-governed
behavior and to generate plans for problem-solving. Lastly, information
is analyzed and synthesized into new behavioral responses to meet one's
goals. Changing one's behavioral response to meet a new goal or modify
an objective is a higher level skill that requires a fusion of executive
functions including self-regulation, and accessing prior knowledge and
experiences.
According to this model, the executive system of the human brain
provides for the cross-temporal organization of behavior towards goals
and the future and coordinates actions and strategies for everyday
goal-directed tasks. Essentially, this system permits humans to
self-regulate their behavior so as to sustain action and problem-solving
toward goals specifically and the future more generally. Thus,
executive function deficits pose serious problems for a person's ability
to engage in self-regulation over time to attain their goals and
anticipate and prepare for the future.
Teaching children self-regulation strategies is a way to improve
their inhibitory control and their cognitive flexibility. These skills
allow children to manage their emotional responses. These interventions
include teaching children executive function-related skills that provide
the steps necessary to implement them during classroom activities and
educating children on how to plan their actions before acting upon them. Executive functioning skills are how the brain plans and reacts to situations.
Offering new self-regulation strategies allow children to improve their
executive functioning skills by practicing something new. It is also
concluded that mindfulness practices are shown to be a significantly
effective intervention for children to self-regulate. This includes
biofeedback-enhanced relaxation. These strategies support the growth of
children's executive functioning skills.
Problem-solving model
Yet
another model of executive functions is a problem-solving framework
where executive functions are considered a macroconstruct composed of
subfunctions working in different phases to (a) represent a problem, (b)
plan for a solution by selecting and ordering strategies, (c) maintain
the strategies in short-term memory in order to perform them by certain
rules, and then (d) evaluate the results with error detection and error
correction.
Lezak's conceptual model
One of the most widespread conceptual models on executive functions is Lezak's model.
This framework proposes four broad domains of volition, planning,
purposive action, and effective performance as working together to
accomplish global executive functioning needs. While this model may
broadly appeal to clinicians and researchers to help identify and assess
certain executive functioning components, it lacks a distinct
theoretical basis and relatively few attempts at validation.
Miller and Cohen's model
In
2001, Earl Miller and Jonathan Cohen published their article "An
integrative theory of prefrontal cortex function", in which they argue
that cognitive control is the primary function of the prefrontal cortex
(PFC), and that control is implemented by increasing the gain of sensory or motor neurons that are engaged by task- or goal-relevant elements of the external environment. In a key paragraph, they argue:
We assume that the PFC serves a
specific function in cognitive control: the active maintenance of
patterns of activity that represent goals and the means to achieve them.
They provide bias signals throughout much of the rest of the brain,
affecting not only visual processes but also other sensory modalities,
as well as systems responsible for response execution, memory retrieval,
emotional evaluation, etc. The aggregate effect of these bias signals
is to guide the flow of neural activity along pathways that establish
the proper mappings between inputs, internal states, and outputs needed
to perform a given task.
Miller and Cohen draw explicitly upon an earlier theory of visual
attention that conceptualises perception of visual scenes in terms of
competition among multiple representations – such as colors,
individuals, or objects. Selective visual attention
acts to 'bias' this competition in favour of certain selected features
or representations. For example, imagine that you are waiting at a busy
train station for a friend who is wearing a red coat. You are able to
selectively narrow the focus of your attention to search for red
objects, in the hope of identifying your friend. Desimone and Duncan
argue that the brain achieves this by selectively increasing the gain of
neurons responsive to the color red, such that output from these
neurons is more likely to reach a downstream processing stage, and, as a consequence, to guide behaviour. According to Miller and Cohen, this selective attention
mechanism is in fact just a special case of cognitive control – one in
which the biasing occurs in the sensory domain. According to Miller and
Cohen's model, the PFC can exert control over input (sensory) or output
(response) neurons, as well as over assemblies involved in memory, or emotion. Cognitive control is mediated by reciprocal PFC connectivity with the sensory and motor cortices, and with the limbic system.
Within their approach, thus, the term "cognitive control" is applied to
any situation where a biasing signal is used to promote
task-appropriate responding, and control thus becomes a crucial
component of a wide range of psychological constructs such as selective attention, error monitoring, decision-making, memory inhibition, and response inhibition.
Miyake and Friedman's model
Miyake
and Friedman's theory of executive functions proposes that there are
three aspects of executive functions: updating, inhibition, and
shifting.
A cornerstone of this theoretical framework is the understanding that
individual differences in executive functions reflect both unity (i.e.,
common EF skills) and diversity of each component (e.g.,
shifting-specific). In other words, aspects of updating, inhibition, and
shifting are related, yet each remains a distinct entity. First,
updating is defined as the continuous monitoring and quick addition or
deletion of contents within one's working memory. Second, inhibition is
one's capacity to supersede responses that are prepotent in a given
situation. Third, shifting is one's cognitive flexibility to switch
between different tasks or mental states.
Miyake and Friedman also suggest that the current body of
research in executive functions suggest four general conclusions about
these skills. The first conclusion is the unity and diversity aspects of
executive functions. Second, recent studies suggest that much of one's EF skills are inherited genetically, as demonstrated in twin studies.
Third, clean measures of executive functions can differentiate between
normal and clinical or regulatory behaviors, such as ADHD. Last, longitudinal studies demonstrate that EF skills are relatively stable throughout development.
Banich's "cascade of control" model
This
model from 2009 integrates theories from other models, and involves a
sequential cascade of brain regions involved in maintaining attentional
sets in order to arrive at a goal. In sequence, the model assumes the
involvement of the posterior dorsolateral prefrontal cortex (DLPFC), the mid-DLPFC, and the posterior and anterior dorsal anterior cingulate cortex (ACC).
The cognitive task used in the article is selecting a response in the Stroop task,
among conflicting color and word responses, specifically a stimulus
where the word "green" is printed in red ink. The posterior DLPFC
creates an appropriate attentional set, or rules for the brain to
accomplish the current goal. For the Stroop task, this involves
activating the areas of the brain involved in color perception, and not
those involved in word comprehension. It counteracts biases and
irrelevant information, like the fact that the semantic perception of
the word is more salient to most people than the color in which it is
printed.
Next, the mid-DLPFC selects the representation that will fulfill
the goal. The task-relevant information must be separated from other
sources of information in the task. In the example, this means focusing
on the ink color and not the word.
The posterior dorsal ACC is next in the cascade, and it is
responsible for response selection. This is where the decision is made
whether the Stroop task participant will say "green" (the written word
and the incorrect answer) or "red" (the font color and correct answer).
Following the response, the anterior dorsal ACC is involved in
response evaluation, deciding whether one's response were correct or
incorrect. Activity in this region increases when the probability of an
error is higher.
The activity of any of the areas involved in this model depends
on the efficiency of the areas that came before it. If the DLPFC imposes
a lot of control on the response, the ACC will require less activity.
Recent work using individual differences in cognitive style has
shown exciting support for this model. Researchers had participants
complete an auditory version of the Stroop task, in which either the
location or semantic meaning of a directional word had to be attended
to. Participants that either had a strong bias toward spatial or
semantic information (different cognitive styles) were then recruited to
participate in the task. As predicted, participants that had a strong
bias toward spatial information had more difficulty paying attention to
the semantic information and elicited increased electrophysiological
activity from the ACC. A similar activity pattern was also found for
participants that had a strong bias toward verbal information when they
tried to attend to spatial information.
Assessment
Assessment
of executive functions involves gathering data from several sources and
synthesizing the information to look for trends and patterns across
time and settings. Apart from standardized neuropsychological tests, other measures can and should be used, such as behaviour checklists, observations, interviews, and work samples. From these, conclusions may be drawn on the use of executive functions.
There are several different kinds of instruments (e.g.,
performance based, self-report) that measure executive functions across
development. These assessments can serve a diagnostic purpose for a
number of clinical populations.
Experimental evidence
The executive system has been traditionally quite hard to define, mainly due to what psychologist Paul W. Burgess calls a lack of "process-behaviour correspondence". That is, there is no single behavior that can in itself be tied to executive function, or indeed executive dysfunction.
For example, it is quite obvious what reading-impaired patients cannot
do, but it is not so obvious what exactly executive-impaired patients
might be incapable of.
This is largely due to the nature of the executive system itself.
It is mainly concerned with the dynamic, "online" co-ordination of
cognitive resources, and, hence, its effect can be observed only by
measuring other cognitive processes. In similar manner, it does not
always fully engage outside of real-world situations. As neurologist Antonio Damasio
has reported, a patient with severe day-to-day executive problems may
still pass paper-and-pencil or lab-based tests of executive function.
Theories of the executive system were largely driven by observations of patients with frontal lobe damage. They exhibited disorganized actions and strategies for everyday tasks (a group of behaviors now known as dysexecutive syndrome)
although they seemed to perform normally when clinical or lab-based
tests were used to assess more fundamental cognitive functions such as memory, learning, language, and reasoning.
It was hypothesized that, to explain this unusual behaviour, there must
be an overarching system that co-ordinates other cognitive resources.
Much of the experimental evidence for the neural structures
involved in executive functions comes from laboratory tasks such as the Stroop task or the Wisconsin Card Sorting Task
(WCST). In the Stroop task, for example, human subjects are asked to
name the color that color words are printed in when the ink color and
word meaning often conflict (for example, the word "RED" in green ink).
Executive functions are needed to perform this task, as the relatively
overlearned and automatic behaviour (word reading) has to be inhibited
in favour of a less practiced task – naming the ink color. Recent functional neuroimaging studies have shown that two parts of the PFC, the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC), are thought to be particularly important for performing this task.
Context-sensitivity of PFC neurons
Other evidence for the involvement of the PFC in executive functions comes from single-cell electrophysiology studies in non-human primates, such as the macaque
monkey, which have shown that (in contrast to cells in the posterior
brain) many PFC neurons are sensitive to a conjunction of a stimulus and
a context. For example, PFC cells might respond to a green cue in a
condition where that cue signals that a leftwards fast movement of the
eyes and the head should be made, but not to a green cue in another
experimental context. This is important, because the optimal deployment
of executive functions is invariably context-dependent.
One example from Miller & Cohen involves a pedestrian crossing the street. In the United States, where cars drive on the right side of the road, an American learns to look left
when crossing the street. However, if that American visits a country
where cars drive on the left, such as the United Kingdom, then the opposite behavior would be required (looking to the right).
In this case, the automatic response needs to be suppressed (or
augmented) and executive functions must make the American look to the
right while in the UK.
Neurologically, this behavioural repertoire clearly requires a
neural system that is able to integrate the stimulus (the road) with a
context (US or UK) to cue a behaviour (look left or look right). Current
evidence suggests that neurons in the PFC appear to represent precisely
this sort of information. Other evidence from single-cell electrophysiology
in monkeys implicates ventrolateral PFC (inferior prefrontal convexity)
in the control of motor responses. For example, cells that increase
their firing rate to NoGo signals as well as a signal that says "don't look there!" have been identified.
Attentional biasing in sensory regions
Electrophysiology and functional neuroimaging studies involving human
subjects have been used to describe the neural mechanisms underlying
attentional biasing. Most studies have looked for activation at the
'sites' of biasing, such as in the visual or auditory cortices. Early studies employed event-related potentials
to reveal that electrical brain responses recorded over left and right
visual cortex are enhanced when the subject is instructed to attend to
the appropriate (contralateral) side of space.
The advent of bloodflow-based neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) has more recently permitted the demonstration that neural activity in a number of sensory regions, including color-, motion-, and face-responsive regions of visual cortex, is enhanced when subjects are directed to attend to that dimension of a stimulus, suggestive of gain control in sensory neocortex. For example, in a typical study, Liu and coworkers
presented subjects with arrays of dots moving to the left or right,
presented in either red or green. Preceding each stimulus, an
instruction cue indicated whether subjects should respond on the basis
of the colour or the direction of the dots. Even though colour and
motion were present in all stimulus arrays, fMRI activity in colour-sensitive regions (V4) was enhanced when subjects were instructed to attend to the colour, and activity in motion-sensitive regions
was increased when subjects were cued to attend to the direction of
motion. Several studies have also reported evidence for the biasing
signal prior to stimulus onset, with the observation that regions of the
frontal cortex tend to come active prior to the onset of an expected
stimulus.
Connectivity between the PFC and sensory regions
Despite
the growing currency of the 'biasing' model of executive functions,
direct evidence for functional connectivity between the PFC and sensory
regions when executive functions are used, is to date rather sparse.
Indeed, the only direct evidence comes from studies in which a portion
of frontal cortex is damaged, and a corresponding effect is observed far
from the lesion site, in the responses of sensory neurons. However, few studies have explored whether this effect is specific to
situations where executive functions are required. Other methods for
measuring connectivity between distant brain regions, such as
correlation in the fMRI response, have yielded indirect evidence that
the frontal cortex and sensory regions communicate during a variety of
processes thought to engage executive functions, such as working memory,
but more research is required to establish how information flows
between the PFC and the rest of the brain when executive functions are
used. As an early step in this direction, an fMRI study on the flow of
information processing during visuospatial reasoning has provided
evidence for causal associations (inferred from the temporal order of
activity) between sensory-related activity in occipital and parietal
cortices and activity in posterior and anterior PFC.
Such approaches can further elucidate the distribution of processing
between executive functions in PFC and the rest of the brain.
Bilingualism and executive functions
A growing body of research demonstrates that bilinguals might show
advantages in executive functions, specifically inhibitory control and
task switching.
A possible explanation for this is that speaking two languages requires
controlling one's attention and choosing the correct language to speak.
Across development, bilingual infants, children, and elderly show a bilingual advantage when it comes to executive functioning. The advantage does not seem to manifest in younger adults.
Bimodal bilinguals, or people who speak one oral language and one sign
language, do not demonstrate this bilingual advantage in executive
functioning tasks.
This may be because one is not required to actively inhibit one
language in order to speak the other.
Bilingual individuals also seem to have an advantage in an area known as
conflict processing, which occurs when there are multiple
representations of one particular response (for example, a word in one
language and its translation in the individual's other language). Specifically, the lateral prefrontal cortex
has been shown to be involved with conflict processing. However, there
are still some doubts. In a meta-analytic review, researchers concluded
that bilingualism did not enhance executive functioning in adults.
In disease or disorder
The study of executive function in Parkinson's disease suggests subcortical areas such as the amygdala, hippocampus and basal ganglia are important in these processes. Dopamine
modulation of the prefrontal cortex is responsible for the efficacy of
dopaminergic drugs on executive function, and gives rise to the Yerkes–Dodson Curve.
The inverted U represents decreased executive functioning with
excessive arousal (or increased catecholamine release during stress),
and decreased executive functioning with insufficient arousal. The low activity polymorphism of catechol-O-methyltransferase is associated with slight increase in performance on executive function tasks in healthy persons. Executive functions are impaired in multiple disorders including anxiety disorder, major depressive disorder, bipolar disorder, attention deficit hyperactivity disorder, schizophrenia and autism. Lesions to the prefrontal cortex, such as in the case of Phineas Gage,
may also result in deficits of executive function. Damage to these
areas may also manifest in deficits of other areas of function, such as motivation, and social functioning.
Future directions
Other
important evidence for executive functions processes in the prefrontal
cortex have been described. One widely cited review article
emphasizes the role of the medial part of the PFC in situations where
executive functions are likely to be engaged – for example, where it is
important to detect errors, identify situations where stimulus conflict
may arise, make decisions under uncertainty, or when a reduced
probability of obtaining favourable performance outcomes is detected.
This review, like many others, highlights interactions between medial and lateral PFC,
whereby posterior medial frontal cortex signals the need for increased
executive functions and sends this signal on to areas in dorsolateral
prefrontal cortex that actually implement control. Yet there has been no
compelling evidence at all that this view is correct, and, indeed, one
article showed that patients with lateral PFC damage had reduced ERNs (a
putative sign of dorsomedial monitoring/error-feedback)
– suggesting, if anything, that the direction of flow of the control
could be in the reverse direction. Another prominent theory
emphasises that interactions along the perpendicular axis of the
frontal cortex, arguing that a 'cascade' of interactions between
anterior PFC, dorsolateral PFC, and premotor cortex guides behaviour in accordance with past context, present context, and current sensorimotor associations, respectively.
Advances in neuroimaging
techniques have allowed studies of genetic links to executive
functions, with the goal of using the imaging techniques as potential endophenotypes for discovering the genetic causes of executive function.
More research is required to develop interventions that can
improve executive functions and help people generalize those skills to
daily activities and settings
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