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The cingulate cortex is located in the medial wall of the cerebral hemispheres and has extensive reciprocal connections with various limbic structures as well as motor and premotor areas. In studies with rodents and studies of brain activation in humans, the cingulate cortex is implicated in the processes of selective attention and response selection. This research paper explores the essential areas of convergence of the two bodies of literature, and offers a common mnemonic associative interpretation of cingulate cortical function.
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1. Anatomy Of The Cingulate Cortex
The cingulate cortex is a part of the limbic cortex, a term referring to the cortical areas that receive axonal fibers from neurons in the anterior group of thalamic nuclei. By modern convention, cingulate cortex constitutes Brodman’s areas 24 and 29 in small animals such as rabbits and rats, and also an additional area, 23, in primates and humans. Brodman’s areas 24 and 29 are often referred to, respectively, as anterior and posterior cingulate cortex. A cytoarchitectural map illustrating these areas is shown in Fig. 1.
Both the anterior and posterior cingulate cortices receive afferent fibers from the anterior medial (AM), midline, and intralaminar thalamic nuclei. However, the anterior cingulate cortex is also innervated by the medial dorsal (MD) and parafasicular thalamic nuclei, while the posterior cingulate cortex receives projections from the remaining members of the anterior group of thalamic nuclei: the anterior ventral (AV), anterior dorsal (AD), and lateral dorsal (LD) thalamic nuclei. In addition, neurons of the lateral dorsal and ventral anterior thalamic nuclei project to the posterior cingulate cortex.
Cingulate cortical neurons are richly innervated by pontine and midbrain fibers that fairly uniformly distribute norepinephrine and serotonin but restrictively distribute dopamine to the anterior cingulate cortex. Many additional afferent systems project to the cingulate cortex, including fibers from the visual cortex, hippocampus, subiculum, entorhinal cortex, and amygdala.
Cingulate cortical neurons send efferent fibers to most of the aforementioned thalamic areas, the subiculum, entorhinal cortex, pons, and many areas of the striatal motor system including the caudate nucleus, nucleus accumbens, and zona incerta. Cingulate cortical neurons have also been found to project to multiple areas of the motor and premotor cortex, suggesting that numerous parallel pathways exist whereby cingulate neurons can modulate motor output systems of the brain. In primates there exist direct reciprocal projections of cingulate neurons to lateral prefrontal and parietal cortex, areas believed to be critical for higher-order perceptual and mnemonic functions (Goldman-Rakic 1988).
2. The Role Of The Cingulate Cortex In Attention
2.1 Associative Attention: Cingulate Cortical Neuronal Activity
Research in behavioral neuroscience with rabbits and rats indicates that the cingulate cortex mediates selective attention, or attention focused on particular stimuli. The stimuli that are selectively processed by cingulate cortical neurons are associatively significant stimuli, i.e., stimuli that signal important events such as reward or aversion, and call for action on the part of the subject. Since the selective processing of the significant stimuli is a learned, associative process, an apt characterization of the cingulate cortex is that it mediates associative attention to significant stimuli.
Support for the hypothesis that the cingulate cortex mediates associative attention to significant stimuli is shown by results of numerous studies on the activity of cingulate cortical neurons during Pavlovian and instrumental conditioning in animals. For example, extensive research has documented the responses of cingulate cortical neurons during discriminative instrumental learning in rabbits (Gabriel 1993, 2001). In these studies, rabbits occupying a large rotating wheel apparatus learned to step in response to a tone cue (CS +) to prevent a foot-shock delivered five seconds later, and they learned to ignore a different tone (CS +) not followed by shock. Neuronal activity in multiple areas of the cingulate cortex exhibited the development, during training, of massive discriminative neuronal activity, defined as significantly greater firing frequencies in response to the CS+ than to the CS – (see Fig. 2). Discriminative activity also developed during acquisition of a discriminative approach response, in which rabbits learned to make oral contact with a drinking spout to obtain a water reward following the CS+ and to inhibit contact following the CS- , which did not predict a water reward (Freeman et al. 1996).
Discriminative neuronal activity in the cingulate cortex has also been reported during classical Pavlovian conditioning of heart rate and eyeblink responses in rabbits (Powell et al. 1990). In addition, studies in rats have demonstrated the occurrence of neuronal responses in the anterior and posterior cingulate cortex that are specific to stimuli that predict reinforcement during appetitive conditioning (Segal and Olds 1972, Takenouchi et al. 1999). The activity of cingulate cortical neurons in all these studies may be viewed as a neuronal code for the associative significance of cues since the neurons developed selective responses to stimuli that predicted the occurrence of significant events.
Cingulate cortical neurons have also been shown to exhibit salience compensation, a phenomenon that is supportive of a cingulate cortical involvement in mediating associative attention (Gabriel 1993, Gabriel and Taylor 1998). When rabbits are trained with nonsalient cues such as a brief duration (200 ms) CS + and CS- , greater brief-latency cingulate cortical discriminative neuronal responses are observed than when training is carried out with more enduring (e.g., 500 ms) CSs. The enhanced neuronal encoding of the brief stimuli, or salience compensation, has been interpreted as an attentional process that amplifies the neural representation of nonsalient yet associatively significant stimuli in order to maximize the resources available for processing those cues.
The importance of the attentional processing in the cingulate cortex is demonstrated by studies showing that bilateral, combined lesions of the anterior and posterior cingulate cortices severely impair discriminative avoidance and approach learning in rabbits (Gabriel 1993). In addition, adult rabbits exposed to cocaine in utero, an exposure which induced morphological and biochemical abnormalities in the anterior cingulate cortex, exhibited attenuated anterior cingulate discriminative neuronal activity and learning deficits when nonsalient CSs were used during discriminative avoidance learning and Pavlovian conditioning of eyeblink responses (Gabriel and Taylor 1998, Romano and Harvey 1998). The results of these studies illustrate that deficits due to cingulate cortical damage emerge when a high demand is placed on attentional processing.
2.2 Executive Attention: Anterior Cingulate Cortex
The application of neuroimaging and electrical recording techniques, such as positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and high-density electroencephalography (EEG), has led to a large volume of data that implicate the cingulate cortex in cognitive processing. In convergence with results from animal studies, many cognitive experiments in humans have supported the idea that the anterior cingulate cortex is involved in processes subserving attention. These studies show that the anterior cingulate cortex is engaged during tasks in which routine or automatic processing is insufficient, as when novel or conflict-laden situations are encountered. This type of attentional processing has been termed executive attention (Posner and DiGirolamo 1998). Situations likely to require executive attention are: a) planning and decision making; b) error detection; c) novel and early stages of learning; d) difficult and threatening situations; and e) overcoming habitual behavior.
The role of the anterior cingulate cortex in executive attention is supported by numerous imaging studies that have shown activation in the anterior cingulate cortex during tasks that engender conflict (Posner and DiGirolamo 1998). An example would be a task that requires the selection of a particular response from multiple competing responses. Most of these studies employ a subtraction technique whereby the brain activation found in a neutral or control condition is subtracted from the activation produced by an experimental condition. For example, activation has been found in the anterior cingulate cortex during the generate-uses task, which requires subjects to state uses commonly associated with visually or acoustically presented words (e.g., generating the response ‘drive’ to the stimulus word ‘car’). The subtracted control condition for this task involves merely reading and pronouncing the words. It is argued that executive attention (and thus anterior cingulate cortical activation) is brought into play as a result of the conflict created by the multiple uses that are potentially relevant to a given stimulus. The activation in the anterior cingulate cortex in this task declines as the subjects are repeatedly exposed to the same words and the generation of uses becomes more routine and less dependent on executive control.
Additional evidence for the contribution of the anterior cingulate cortex to executive attention comes from the results of multiple experiments using the Stroop task, a task that requires subjects to name the ink color of visually presented words in a congruent condition (e.g., the word ‘red’ printed in red ink), an incongruent condition (e.g., the word ‘green’ printed in red ink), or a neutral condition (e.g., the word ‘door’ printed in red ink) (Posner and DiGirolamo 1998). The anterior cingulate cortex has been found active in both the incongruent and congruent conditions when compared to the neutral condition. It has been suggested that both the congruent and the incongruent conditions involve conflict because subjects must respond to the ink color while inhibiting a response to the word’s meaning. Some studies have found more activation in the incongruent condition than in the congruent condition, in line with the expectation that the incongruent condition creates more conflict and thus recruits more executive attention in the anterior cingulate cortex.
Studies employing high-density scalp recordings of EEG have pointed to an involvement of the anterior cingulate cortex in error detection, another aspect of executive attention (Dahaene et al. 1994, Holroyd et al. 1998, Gehring et al. 1993). These experiments have demonstrated a marked electrical negativity at mid frontal regions of the scalp. The negativity peaks about 100 milliseconds after subjects make an incorrect response, such as an erroneous key press in a reaction time task. Brain electrical source analyses (BESA) carried out independently by separate investigators consistently localize the error-related negativity (ERN) to the anterior cingulate cortex. These results support the role of the anterior cingulate cortex in executive attention invoked during error-related processing (see Fig. 3).
3. Movement-Related Processing: Response Selection By The Cingulate Cortex
Considering the ample connections of the cingulate cortex with motor and premotor cortex as well as areas of the striatal motor system, it is not surprising that the cingulate cortex has been linked to processes such as response selection. Several studies have documented the existence of a topographic organization of the cingulate cortex with respect to particular response modalities. For example, different areas of the anterior cingulate cortex are active depending on whether subjects perform in tasks involving oculomotor, manual, or spoken responses (Paus et al. 1993).
A case study of patient D.L., who sustained a circumscribed right hemisphere lesion of the anterior cingulate cortex after surgery to remove a tumor, adds further support for cingulate cortical involvement in movement-related processing (Turken and Swick 1999). Interestingly, D.L. exhibited entirely normal performance in Stroop-like and divided attention tasks when responses were orally reported; however, D.L. showed a dramatic deficit in the same tasks when manual responses were required. These results were interpreted as favoring the idea that command signals are sent to motor output areas through the anterior cingulate cortex. The authors characterize the role of the anterior cingulate cortex as confirming the appropriateness of the selected response, thus to facilitate correct responding while suppressing incorrect responding.
Premotor neuronal activity in cingulate cortex has been demonstrated in several studies of the single-unit correlates of learning and performance. In one such study, approximately half of all single neurons in anterior and posterior cingulate cortex in rabbits exhibited premotor firing ramps that consisted of progressive increases in firing frequency preceding the onset of the behavioral (locomotory) avoidance responses (Kubota et al. 1996). Also, neuronal firing in ventral portions of the anterior cingulate cortex was correlated with the onset of licking behavior during appetititve conditioning of rats (Takenouchi et al. 1999). Thus, cingulate cortical neurons become active preceding the initiation of learned motor responses.
The involvement of the anterior cingulate cortex in response selection does not negate the role of the cingulate cortex in associative and executive attention. Appropriate response selection for a given situation can only occur if attention is devoted to the significant associative stimuli and if conflict among competing motor responses is resolved.
4. Emotion And The Affective Dimension Of Pain
Traditionally, the cingulate cortex has been viewed as part of a brain circuit that is involved in the experience and expression of emotion (Papez 1937, Maclean 1975). Although more recent evidence has suggested an important role for the cingulate cortex in processes such as attention and response selection, the role of the cingulate cortex in emotion remains unchallenged.
Activations of the anterior cingulate cortex, in particular, have been found to accompany the experience of emotion in numerous neuroimaging studies. For example, when cerebral blood flow (CBF) was measured using PET while subjects viewed emotional film clips and recalled emotional situations, the anterior cingulate cortex was the only structure to exhibit CBF changes correlated with subjects’ scores on the Levels of Emotional Awareness Scale (LEAS), a test that measures the capacity to perceive and differentiate complex emotions in oneself and others (Lane et al. 1998). The results suggested that individual differences in emotional awareness could be related to the degree of activation in the anterior cingulate cortex.
Evidence has also been presented in support of a role for the anterior cingulate cortex in mediating the emotional response to pain in humans (Price 2000). Pain is thought to involve two components, a sensory component and an affective component. The affective component reflects the unpleasantness associated with pain and its long-term consequences. The anterior cingulate cortex receives direct input from spinal pain pathways and other input from areas (e.g., the prefrontal cortex) that are involved in cognitive aspects of pain processing, such as evaluating the immediate threat of the pain and its potential interference with daily activities. The anterior cingulate cortex is thus positioned to integrate these two types of pain-related information in order to select appropriate coping responses such as escape or avoidance.
5. Learning And Memory
5.1 Distinct Roles Of The Anterior And Posterior Cingulate Cortex
Compelling evidence suggests an important role for the cingulate cortex in the mediation of learning and memory processes. Available data indicate that the contributions of the anterior and posterior cingulate cortices are functionally distinct. A contribution of the anterior and posterior cingulate cortices to early and late stages of learning, respectively, has been documented in discriminative avoidance learning in rabbits as well as in conditioned visual discrimination in rats (Gabriel 1993, Bussey et al. 1997). In rabbits, discriminative neuronal activity (see Sect. 2.1) in the anterior cingulate cortex develops after fewer training trials than in the posterior cingulate cortex. The observations of neuronal activity coincide nicely with restricted lesion studies showing that lesions confined to the anterior cingulate cortex result in a deficit of behavioral performance in the early stages of learning, whereas lesions confined to the posterior cingulate cortex result in a loss of performance at later stages of learning.
5.2 Context-Specific Retrieval Patterns And Spatial Processing: Posterior Cingulate Cortex And The Anterior Thalamus
Although the evidence clearly indicates an involvement of the cingulate cortex in the learning-based coding of associatively significant stimuli, further evidence indicates that this coding subserves the retrieval of learned, context-appropriate responses. Evidence in support of this idea is shown by the existence of unique topographical distributions of CS -related neuronal activity in different cell layers of the posterior cingulate cortex and in various anterior thalamic nuclei in rabbits, during discriminative instrumental learning. Some layers are activated maximally by the CS in the initial stages of training, others in intermediate training stages, and others as the rabbits attain asymptotic discriminative performance. The distribution of the activations changed not only across time (the stage in training) but also with respect to the spatial context. For instance, the same set of cues elicited different patterns of activation depending on whether the rabbits were engaged in a moderately learned discriminative avoidance task or (in a separate training apparatus) a well-learned discriminative approach task. These context-specific patterns of CS -elicited activity could be associated with the learned responses that are appropriate to a given situation or context. Thus, when a given context specific pattern is elicited on cue presentation, the learned response is retrieved. This mechanism could subserve pattern separation, i.e., the ability to defeat proactive and retroactive interference when multiple similar cues are associated with different memories or responses, as when one tries to recall the names of several recently met individuals. The retrieval hypothesis is consistent with the finding that the brief latency cue-elicited context-specific patterns in the posterior cingulate cortex are followed by premotor firing, i.e., firing which precedes the onset of the learned behavioral response (see Sect. 2.3).
Additional evidence suggests that the context-specific patterns in the posterior cingulate cortex depend on the integrity of hippocampal connections, which may supply information concerning the operative spatial context to the cingulate cortex. Fornix lesions which disconnect the hippocampal formation from the anterior thalamus disrupt the training-stage related patterns in the posterior cingulate cortex and these lesions impair concurrent performance in two different discriminative learning tasks that employ very similar cues (Smith et al. 2000).
Posterior cingulate cortical neurons also have functional properties that are similar to those found in neurons of the hippocampus and parietal cortex during spatial processing. For instance, rodent hippocampal neurons are selectively active when subjects occupy a particular location in space, while other neurons code information about directional heading, independently of spatial location or ongoing behavior. Direction-coding neurons have also been documented in the posterior cingulate cortex and related thalamic nuclei, and these neurons, together with those of the hippocampal formation, are thought to contribute to the sense of direction and place in spatial learning situations. Interestingly, in primates, hippocampal neurons are selectively active when the subject is viewing (rather than occupying) a particular space in the environment. The posterior cingulate cortex of primates contains neurons that discharge with eye movement and eye position, a tuning property also found among neurons in the frontal and parietal cortical areas (Olson et al. 1996). The eye direction-coding neurons in primate cingulate cortex are hypothesized to participate in the spatial interpretation of retinal images.
6. Concluding Comment
Studies carried out by behavioral neuroscientists using rats and rabbits as subjects have shown that the cingulate cortex is a critical substrate of learned responses to predictive stimuli. Cingulate cortical neurons in these animals code associatively significant stimuli and exhibit context-specific topographic patterns that could mediate cued retrieval of context-appropriate learned behavior. These functions occur as a result of intimate interactions of the hippocampal and cingulothalamic brain regions. Studies of cognitive neuroscientists concerning brain activation during cognitive task performance in human subjects have yielded results that are fundamentally in agreement with the studies with rats and rabbits. For example, there is clear agreement that the anterior cingulate cortex subserves an attentional role as its neurons are recruited in situations of high cognitive conflict, e.g., when stimuli acquire new meanings at the outset of learning, or when a decision among multiple-response alternatives must be reached. The involvement in behavioral learning and the associative and memory-bearing characteristics of cingulate cortical neuronal activity have also led behavioral neuroscientists to speak of cingulate cortical attention as associative in character, i.e., a learned form of attention. Cognitive neuroscientists have, on the other hand, discussed the cingulate cortex as involved in attentional processes without reference to memory. Given the findings of behavioral neuroscience and the very close neuroanatomical association of the cingulate cortex with other structures (e.g., the hippocampus) that are acknowledged components of the brain’s memory system, it is very likely that early in the twenty-first century there will be an even greater convergence of behavioral and cognitive neuroscience upon a common mnemonic interpretation of cingulate cortical function.
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