The caudate nucleus is one of the structures that make up the corpus striatum, which is part of the basal ganglia in the human brain. Although the caudate nucleus has long been associated with motor processes because of its relation to Parkinson's disease and Huntington's disease, it also plays important roles in nonmotor functions, such as procedural learning, associative learning, and inhibitory control of action. The caudate is also one of the brain structures that compose the reward system, and it functions as part of the cortico-basal ganglia-thalamo-cortical loop. In a study of the cingulate gyrus in monkeys, it was found that the two cingulates form a network including the caudate nucleus, the claustrum, the thalamic nuclei (AM AV LD), the lateral prefrontal cortex and the posterior parietal cortex.
Structure
Along with the putamen, the caudate forms the dorsal striatum, which is considered a single functional structure; anatomically, it is separated by a large white-matter tract, the internal capsule, so it is sometimes also described as two structures—the medial dorsal striatum (the caudate) and the lateral dorsal striatum (the putamen). In this vein, the two are functionally distinct not because of structural differences, but merely because of the topographical distribution of function.
The caudate nuclei are near the center of the brain, sitting astride the thalamus. There is a caudate nucleus in each hemisphere of the brain. Each nucleus is C-shaped, with a wider "head" (caput in Latin) at the front, tapering to a "body" (corpus) and a "tail" (cauda). Sometimes a part of the caudate nucleus is called the "knee" (genu). The caudate head receives its blood supply from the lenticulostriate artery; the tail of the caudate receives its blood supply from the anterior choroidal artery.
The head and body of the caudate nucleus form part of the floor of the anterior horn of the lateral ventricle. The body travels briefly towards the back of the head; the tail then curves back toward the anterior, forming the roof of the inferior horn of the lateral ventricle. This means that a coronal section (on a plane parallel to the face) that cuts through the tail will also cross the body and head of the caudate nucleus.
Neurochemistry The caudate is highly innervated by dopaminergic neurons that originate from the substantia nigra pars compacta (SNc). The SNc is in the midbrain and contains cell projections to the caudate and putamen, using the neurotransmitter dopamine. There are also inputs from various association cortices.
Motor functions
Spatial mnemonic processing The caudate nucleus integrates spatial information with motor behavior formulation. Selective impairment of spatial working memory in subjects with Parkinson's disease and the knowledge of the disease's impact on the amount of dopamine supplied to the striatum have linked the caudate nucleus to spatial and nonspatial mnemonic processing. Spatially dependent motor preparation has been linked to the caudate nucleus through event-related fMRI analysis techniques. Activity in the caudate nucleus was demonstrated to be greater during tasks featuring spatial and motoric memory demands than those that involved nonspatial tasks. Specifically, spatial working memory activity has been observed, via fMRI studies of delayed recognition, to be greater in the caudate nucleus when the activity immediately preceded a motor response. These results indicate that the caudate nucleus could be involved in coding a motor response. With this in mind, the caudate nucleus could be involved in the recruitment of the motor system to support working memory performance by the mediation of sensory-motor transformations.
Directed movements The caudate nucleus contributes importantly to body and limbs posture and the speed and accuracy of directed movements. Deficits in posture and accuracy during paw-usage tasks were observed after the removal of caudate nuclei in cats. A delay in initiating performance and the need to shift body position constantly were both observed in cats after partial removal of the nuclei.
In monkeys, after the application of cocaine to the caudate nucleus and the resulting lesions produced, there was a "leaping or forward movement". Because of its association with damage to the caudate, this movement demonstrates the inhibitory nature of the caudate nucleus. The "motor release" caused by this procedure indicates that the caudate nucleus inhibits the tendency for an animal to move forward without resistance.
Cognitive functions
Goal-directed action A two-pronged approach of neuroimaging (including PET and fMRI) and anatomical studies expose a strong relationship between the caudate and cortical areas associated with executive functioning: "non-invasive measures of anatomical and functional connectivity in humans demonstrate a clear link between the caudate and executive frontal areas."
Meanwhile, behavioral studies provide another layer to the argument: recent studies suggest that the caudate is fundamental to goal-directed action, that is, "the selection of behavior based on the changing values of goals and a knowledge of which actions lead to what outcomes." One such study presented rats with levers that triggered the release of a cinnamon flavored solution. After the rats learned to press the lever, the researchers changed the value of the outcome (the rats were taught to dislike the flavor either by being given too much of the flavor, or by making the rats ill after drinking the solution) and the effects were observed. Normal rats pressed the lever less frequently, while rats with lesions in the caudate did not suppress the behavior as effectively. In this way, the study demonstrates the link between the caudate and goal-directed behavior; rats with damaged caudate nuclei had difficulty assessing the changing value of the outcome. In a 2003-human behavioral study, a similar process was repeated, but the decision this time was whether or not to trust another person when money was at stake. While here the choice was far more complex—the subjects were not simply asked to press a lever, but had to weigh a host of different factors—at the crux of the study was still behavioral selection based on changing values of outcomes.
Memory The dorsal-prefrontal cortex subcortical loop involving the caudate nucleus has been linked to deficits in working memory, specifically in schizophrenic patients. Functional imaging has shown activation of this subcortical loop during working memory tasks in primates and healthy human subjects. The caudate may be affiliated with deficits involving working memory from before illness onset as well. Caudate nucleus volume has been found to be inversely associated with perseverative errors on spatial working memory tasks.
The amygdala sends direct projections to the caudate nucleus. Both the amygdala and the caudate nucleus have direct and indirect projections to the hippocampus. The influence of the amygdala on memory processing in the caudate nucleus has been demonstrated with the finding that lesions involving the connections between these two structures "block the memory-enhancing effects of oxotremorine infused into the caudate nucleus". In a study involving rats given water-maze training, the caudate nucleus was discovered to enhance memory of visually cued training after amphetamine was infused post-training into the caudate.
Learning In a 2005 study, subjects were asked to learn to categorize visual stimuli by classifying images and receiving feedback on their responses. Activity associated with successful classification learning (correct categorization) was concentrated to the body and tail of the caudate, while activity associated with feedback processing (the result of incorrect categorization) was concentrated to the head of the caudate.
Sleep Bilateral lesions in the head of the caudate nucleus in cats were correlated with a decrease in the duration of deep slow wave sleep during the sleep-wakefulness cycle. With a decrease in total volume of deep slow wave sleep, the transition of short-term memory to long-term memory may also be affected negatively.
However, the effects of caudate nuclei removal on the sleep–wakefulness pattern of cats have not been permanent. Normalization occurs by three months after caudate nuclei ablation. This discovery may be due to the inter-related nature of the roles of the caudate nucleus and the frontal cortex in controlling levels of central nervous system activation. The cats with caudate removal, although permanently hyperactive, had a significant decrease in rapid eye movement sleep (REMS) time, which lasted about two months. However, afrontal cats had a permanent decrease in REMS time and only a temporary period of hyperactivity.
Contrasting with associations between "deep", REM sleep and the caudate nucleus, a study involving EEG and fMRI measures during human sleep cycles has indicated that the caudate nucleus demonstrates reduced activity during non-REM sleep across all sleep stages. Additionally, studies of human caudate nuclei volume in congenital central hypoventilation syndrome (CCHS) subjects established a correlation between CCHS and a significant reduction in left and right caudate volume. CCHS is a genetic disorder that affects the sleep cycle due to a reduced drive to breathe. Therefore, the caudate nucleus has been suggested to play a role in human sleep cycles.
Emotion
The caudate nucleus has been implicated in responses to visual beauty, and has been suggested as one of the "neural correlates of romantic love".
Approach-attachment behavior and affect are also controlled by the caudate nucleus. Cats with bilateral removal of the caudate nuclei persistently approached and followed objects, attempting to contact the target, while exhibiting a friendly disposition by the elicitation of treading of the forelimbs and purring. The magnitude of the behavioral responses was correlated to the extent of the removal of the nuclei. Reports of human patients with selective damage to the caudate nucleus show unilateral caudate damage resulting in loss of drive, obsessive–compulsive disorder, stimulus-bound perseverative behavior, and hyperactivity. Most of these deficits can be classified as relating to approach-attachment behaviors, from approaching a target to romantic love.
Language Neuroimaging studies reveal that people who can communicate in multiple languages activate exactly the same brain regions regardless of the language. A 2006 publication studies this phenomenon and identifies the caudate as a center for language control. In perhaps the most illustrative case, a trilingual subject with a lesion to the caudate was observed. The patient maintained language comprehension in her three languages, but when asked to produce language, she involuntarily switched between the three languages. In short, "these and other findings with bilingual patients suggest that the left caudate is required to monitor and control lexical and language alternatives in production tasks."
Local shape deformations of the medial surface of the caudate have been correlated with verbal learning capacity for females and the number of perseverance errors on spatial and verbal fluency working memory tasks for males. Specifically, a larger caudate nucleus volume has been linked with better verbal fluency performance.
A neurological study of glossolalia showed a significant reduction in activity in the left caudate nucleus during glossolalia compared to singing in English.
Threshold control