Anterior cingulate cortex
In human brains, 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 is involved in certain higher-level functions, such as attention allocation, reward anticipation, decision-making, 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, hippocampus, 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, or von Economo neurons. 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 or incompatible trials. Another very common conflict-inducing stimulus that activates the ACC 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. Thus, ACC affects decision making of a 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.Patients with lateral prefrontal cingulate damage 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.
Social evaluation
Activity in the dorsal anterior cingulate cortex has been implicated in processing both the detection and appraisal of social processes, including social exclusion. When exposed to repeated personal social evaluative tasks, non-depressed women showed reduced fMRI BOLD activation in the dACC on the second exposure, while women with a history of depression exhibited enhanced BOLD activation. This differential activity may reflect enhanced rumination about social evaluation or enhanced arousal associated with repeated social evaluation.The anterior cingulate cortex gyrus is involved in effort to help others.
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 basis 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 is supported by some evidence, 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 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.