The anterior cingulate cortex is the frontal part of the cingulate cortex
that resembles a "collar" surrounding the frontal part of the corpus callosum.
It consists of Brodmann areas 24, 32, and 33.
It appears to play a role in a wide variety of autonomic functions, such as
regulating blood pressure and heart rate.
It is also involved in rational cognitive functions, such as reward
anticipation, decision-making, empathy, impulse control, and emotion.
Anatomy The anterior cingulate cortex can be
divided anatomically based on cognitive, and emotional components. The dorsal
part of the ACC is connected with the prefrontal cortex and parietal cortex as
well as the motor system and the frontal eye fields making it a central station
for processing top-down and bottom-up stimuli and assigning appropriate
control to other areas in the brain. By contrast, the ventral part of the ACC is
connected with the amygdala, nucleus accumbens, hypothalamus, and anterior
insula, and is involved in assessing the salience of emotion and motivational
information. The ACC seems to be especially involved when effort is
needed to carry out a task such as in early learning and problem-solving.
On a cellular level, the ACC is unique in its abundance of specialized neurons
called spindle cells. These cells are a relatively recent occurrence in
evolutionary terms and contribute to this brain region's emphasis on
addressing difficult problems, as well as the pathologies related to the ACC.
Tasks A typical task that activates the ACC
involves eliciting some form of conflict within the participant that can
potentially result in an error. One such task is called the Eriksen flanker task
and consists of an arrow pointing to the left or right, which is flanked by two
distractor arrows creating either compatible trials. Another very common
conflict-inducing stimulus is the Stroop task, which involves naming the color
ink of words that are either congruent or incongruent. Conflict occurs because
people’s reading abilities interfere with their attempt to correctly name the
word’s ink color. A variation of this task is the Counting-Stroop, during
which people count either neutral stimuli or interfering stimuli by
pressing a button. Another version of the Stroop task named the Emotional
Counting Stroop is identical to the Counting Stroop test, except that it
also uses segmented or repeated emotional words such as "murder" during
the interference part of the task. Functions
Many studies attribute specific functions such as error detection,
anticipation of tasks, attention, motivation, and modulation of emotional
responses to the ACC. = Error detection and conflict
monitoring= The most basic form of ACC theory states
that the ACC is involved with error detection. Evidence has been derived
from studies involving a Stroop task. However, ACC is also active during
correct response, and this has been shown using a letter task, whereby
participants had to respond to the letter X after an A was presented and
ignore all other letter combinations with some letters more competitive than
others. They found that for more competitive stimuli ACC activation was
greater. A similar theory poses that the ACC’s
primary function is the monitoring of conflict. In Eriksen flanker task,
incompatible trials produce the most conflict and the most activation by the
ACC. Upon detection of a conflict, the ACC then provides cues to other areas in
the brain to cope with the conflicting control systems.
Evidence from electrical studies Evidence for ACC as having an error
detection function comes from observations of error-related negativity
uniquely generated within the ACC upon error occurrences. A distinction has
been made between an ERP following incorrect responses and a signal after
subjects receive feedback after erroneous responses.
No-one has clearly demonstrated that the ERN comes from the ACC., but patients
with lateral PFC damage do show reduced ERNs.
Reinforcement learning ERN theory poses that there is a mismatch between actual
response execution and appropriate response execution, which results in an
ERN discharge. Furthermore, this theory predicts that, when the ACC receives
conflicting input from control areas in the brain, it determines and allocates
which area should be given control over the motor system. Varying levels of
dopamine are believed to influence the optimization of this filter system by
providing expectations about the outcomes of an event. The ERN, then,
serves as a beacon to highlight the violation of an expectation. Research on
the occurrence of the feedback ERN shows evidence that this potential has larger
amplitudes when violations of expectancy are large. In other words, if an event
is not likely to happen, the feedback ERN will be larger if no error is
detected. Other studies have examined whether the ERN is elicited by varying
the cost of an error and the evaluation of a response.
In these trials, feedback is given about whether the participant has gained or
lost money after a response. Amplitudes of ERN responses with small gains and
small losses were similar. No ERN was elicited for any losses as opposed to an
ERN for no wins, even though both outcomes are the same. The finding in
this paradigm suggests that monitoring for wins and losses is based on the
relative expected gains and losses. If you get a different outcome than
expected, the ERN will be larger than for expected outcomes. ERN studies have
also localized specific functions of the ACC.
The rostral ACC seems to be active after an error commission, indicating an error
response function, whereas the dorsal ACC is active after both an error and
feedback, suggesting a more evaluative function. This evaluation is emotional
in nature and highlights the amount of distress associated with a certain
error. Summarizing the evidence found by ERN studies, it appears to be the case
that ACC receives information about a stimulus, selects an appropriate
response, monitors the action, and adapts behavior if there is a violation
of expectancy. Evidence against error detection and
conflict monitoring theory Studies examining task performance
related to error and conflict processes in patients with ACC damage cast doubt
on the necessity of this region for these functions. The error detection and
conflict monitoring theories cannot explain some evidence obtained by
electrical studies that demonstrate the effects of giving feedback after
responses because the theory describes the ACC as strictly monitoring conflict,
not as having evaluative properties. It has been stated that "The cognitive
consequences of anterior cingulate lesions remain rather equivocal, with a
number of case reports of intact general neuropsychological and executive
function in the presence of large anterior dorsal cingulate lesions. For
an alternative view of anterior cingulate, see Rushworth's review.
= Reward-based learning theory= A more comprehensive and recent theory
describes the ACC as a more active component and poses that it detects and
monitors errors, evaluates the degree of the error, and then suggests an
appropriate form of action to be implemented by the motor system. Earlier
evidence from electrical studies indicate the ACC has an evaluative
component, which is indeed confirmed by fMRI studies. The dorsal and rostral
areas of the ACC both seem to be affected by rewards and losses
associated with errors. During one study, participants received monetary
rewards and losses for correct and incorrect responses, respectively.
Largest activation in the dACC was shown during loss trials. This stimulus did
not elicit any errors, and, thus, error detection and monitoring theories cannot
fully explain why this ACC activation would occur. The dorsal part of the ACC
seems to play a key role in reward-based decision-making and learning. The
rostral part of the ACC, on the other hand, is believed to be involved more
with affective responses to errors. In an interesting expansion of the
previously described experiment, the effects of rewards and costs on ACC’s
activation during error commission was examined. Participants performed a
version of the Eriksen flanker task using a set of letters assigned to each
response button instead of arrows. Targets were flanked by either a
congruent or an incongruent set of letters. Using an image of a thumb,
participants received feedback on how much money they gained or lost. The
researchers found greater rostral ACC activation when participants lost money
during the trials. The participants reported being frustrated when making
mistakes. Because the ACC is intricately involved with error detection and
affective responses, it may very well be that this area forms the bases of
self-confidence. Taken together, these findings indicate that both the dorsal
and rostral areas are involved in evaluating the extent of the error and
optimizing subsequent responses. A study confirming this notion explored the
functions of both the dorsal and rostral areas of the ACC involved using a
saccade task. Participants were shown a cue that
indicated whether they had to make either a pro-saccade or an anti-saccade.
An anti-saccade requires suppression of a distracting cue because the target
appears in the opposite location causing the conflict. Results showed differing
activation for the rostral and dorsal ACC areas. Early correct anti-saccade
performance was associated with rostral activation. The dorsal area, on the
other hand, was activated when errors were committed, but also for correct
responses. Whenever the dorsal area was active,
fewer errors were committed providing more evidence that the ACC is involved
with effortful performance. The second finding showed that, during error
trials, the ACC activated later than for correct responses, clearly indicating a
kind of evaluative function. = Role in consciousness=
The ACC area in the brain is associated with many functions that are correlated
with conscious experience. Greater ACC activation levels were present in more
emotionally aware female participants when shown short ‘emotional’ video
clips. Better emotional awareness is associated with improved recognition of
emotional cues or targets, which is reflected by ACC activation.
The idea of awareness being associated with the ACC has some evidence with it,
in that it seems to be the case that, when subjects' responses are not
congruent with actual responses, a larger error-related negativity is
produced. One study found an ERN even when
subjects were not aware of their error. Awareness may not be necessary to elicit
an ERN, but it could influence the effect of the amplitude of the feedback
ERN. Relating back to the reward-based learning theory, awareness could
modulate expectancy violations. Increased awareness could result in
decreased violations of expectancies and decreased awareness could achieve the
opposite effect. Further research is needed to completely understand the
effects of awareness on ACC activation. In The Astonishing Hypothesis, Francis
Crick identifies the anterior cingulate, to be specific the anterior cingulate
sulcus, as a likely candidate for the center of free will in humans. Crick
bases this suggestion on scans of patients with specific lesions that seem
to interfere with their sense of independent will, such as alien hand
syndrome. = Role in registering pain=
The ACC registers physical pain as shown in functional MRI studies that showed an
increase in signal intensity, typically in the posterior part of area 24 of the
ACC, that was correlated with pain intensity. When this pain-related
activation was accompanied by attention-demanding cognitive tasks, the
attention-demanding tasks increased signal intensity in a region of the ACC
anterior and/or superior to the pain-related activation region. The ACC
is the cortical area that has been most frequently linked to the experience of
pain. It appears to be involved in the emotional reaction to pain rather than
to the perception of pain itself. Pathology
Studying the effects of damage to the ACC provides insights into the type of
functions it serves in the intact brain. Behavior that is associated with lesions
in the ACC includes: inability to detect errors, severe difficulty with resolving
stimulus conflict in a Stroop task, emotional instability, inattention, and
akinetic mutism. There is evidence that damage to ACC is present in patients
with schizophrenia, where studies have shown patients have difficulty in
dealing with conflicting spatial locations in a Stroop-like task and
having abnormal ERNs. Participants with ADHD were found to have reduced
activation in the dorsal area of the ACC when performing the Stroop task.
Together, these findings corroborate results from imaging and electrical
studies about the variety of functions attributed to the ACC.
There is evidence that this area may have a role in obsessive–compulsive
disorder due to the fact that what appears to be an unnaturally low level
of glutamate activity in this region has been observed in patients with the
disorder, in contrast to many other brain regions that are thought to have
excessive glutamate activity in OCD. Recent SDM meta-analyses of voxel-based
morphometry studies comparing people with OCD and healthy controls has found
people with OCD to have increased grey matter volumes in bilateral lenticular
nuclei, extending to the caudate nuclei, while decreased grey matter volumes in
bilateral dorsal medial frontal/anterior cingulate cortex. These findings
contrast with those in people with other anxiety disorders, who evince decreased
grey matter volumes in bilateral lenticular / caudate nuclei, while also
decreased grey matter volumes in bilateral dorsal medial frontal /
anterior cingulate gyri. The ACC has been suggested to have
possible links with Social Anxiety, along with the amygdala part of the
brain, but this research is still in its early stages. A more recent study, by
the Wake Forest Baptist Medical Centre, confirms the relationship between the
ACC and anxiety regulation, by revealing mindfulness practice as a mediator for
anxiety precisely through the ACC The adjacent subcallosal cingulate gyrus
has been implicated in major depression and research indicates that deep-brain
stimulation of the region could act to alleviate depressive symptoms.
A study of brain MRIs taken on adults that had previously participated in the
Cincinnati Lead Study found that people that had suffered higher levels of lead
exposure as children had decreased brain size as adults. This effect was most
pronounced in the ACC and is thought to relate to the cognitive and behavioral
deficits of affected individuals. Impairments in the development of the
anterior cingulate, together with impairments in the dorsal medial-frontal
cortex, may constitute a neural substrate for socio-cognitive deficits
in autism, such as social orienting and joint attention.
Additional images See also
Cingulate cortex Cingulate gyrus
Cingulate sulcus Reward system
References