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IMPORTANT: So as not to have to "double edit," this sandbox is not identical to the caudate nucleus stub. While this sandbox has a bulk of the work, please see the stub for the most recent version.

The caudate nucleus is one of three basic structures that make up the basal ganglia. Along with the putamen and globus pallidus, as well in conjunction with the thalamus and two related structures (the substantia nigra and subthalamic nucleus), the caudate nucleus constitutes a system that is responsible largely for voluntary movement. While this system has long been associated with motor processes due primarily to the basal ganglia’s role in Parkinson’s disease, there is mounting evidence that the structures that make up the basal ganglia –– the caudate nucleus included –– play important roles in various other nonmotor functions as well. Organizationally, the interplay between the basal ganglia and various regions in the brain is best described as a series of cortico-striatal loops, in which the striatum receives axons from the cortex (along with the putamen, the caudate nuclei are the main input regions for the basal ganglia), processes the information, then relays this back to distinct cortical regions. The caudate plays an important role in three of these loops: the oculomotor, dorsolateral, and ventral/orbital circuits. In a related way then, the caudate nucleus has been implicated with voluntary movement, learning, memory, sleep, and social behavior.

Anatomy
Together with the putamen, the caudate forms the dorsal striatum. The dorsal striatum is considered a single functional structure, yet because it is separated by a large white matter tract –– the internal capsule –– it is often referred to 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 as a result of structural differences, but merely due to the topographical distribution of function.

Goal-Directed Action
A review of neuroimaging studies, anatomical studies of caudate connectivity, and behavioral studies reveals a surprising role for the caudate in executive functioning. A study of Parkinson's patients (see below) may also contribute to a growing body of evidence.

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 direction 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.

In short, neuroimagery and anatomical studies support the assertion that the caudate plays a role in executive functioning, while behavioral studies deepen our understanding of the ways in which the caudate guides some of our decision-making processes.

Learning
The caudate nucleus contributes to learning in two distinct ways. In a 2005 study, subjects were asked to learn to categorize visual stimuli by first classifying images and then receiving feedback on their responses. Activity associated with successful classification learning (correct categorization) was concentrated to the body and and tail of the caudate, while activity associated with feedback processing (the result of incorrect categorization) was concentrated to the head of the caudate.

Language
Neuroimaging studies reveal that people who can communicate in multiple languages activate the exact 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 from one language to another. 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."

Parkinson's Disease
Parkinson's Disease is likely the most studied basal ganglia disorder. Patients with this progressive neurodegenerative disorder often first experience movement related symptoms (the three most common being tremors at rest, muscular rigidity, and akathisia) which are later combined with various cognitive deficiencies, including dementia. Parkinson's disease depletes dopaminergic neurons in the nigrostriatal tract, a dopamine pathway that is connected to the head of the caudate. As such, many studies have correlated the loss of dopaminergic neurons that send axons to the caudate nucleus and the degree of dementia in Parkinson's patients. And while a relationship has been drawn between the caudate and Parkinson's motor deficiencies, the caudate has also been associated with Parkinson's concomitant cognitive impairments. One review contrasts the performance of patients with Parkinson's and patients that strictly suffered from frontal-lobe damage in the Tower of London test. The differences in performance between the two types of patients (in a test that, in short, requires subjects to select appropriate intermediate goals with a larger goal in mind) draws a link between the caudate and goal-directed action. However, the studies are not conclusive. While the caudate has been associated with executive function (see "Goal-Directed Action"), it remains "entirely unclear whether executive deficits in [Parkinson's patients], reflect pre-dominantly their cortical or subcortical damage."

Alzheimer's Disease
A 2013 study has suggested a link between Alzheimer's patients and the caudate nucleus. MR images were used to estimate the volume of caudate nuclei in patients with Alzheimer's and normal volunteers. The study found a "significant reduction in the caudate volume" in Alzheimer's patients when compared to the normal volunteers. While the correlation does not indicate causation, the finding may have implications for early diagnosis.

Attention-Deficit Hyperactivity Disorder
A 2002 study draws a relationship between caudate asymmetry and symptoms related to ADHD. The authors used MR images to compare the relative volumes of the caudate nuclei (as the caudate is a bilateral structure), and drew a connection between any asymmetries and symptoms of ADHD: "The degree of caudate asymmetry significantly predicted cumulative severity ratings of inattentive behaviors." This correlation is congruent with previous associations of the caudate with attentional functioning.

Schizophrenia
The volume of white matter in the caudate nucleus has been linked with patients diagnosed with Schizophrenia. A 2004 study used magnetic resonance imaging to compare the relative volume of white matter in the caudate among Schizophrenia patients. Those patients who suffer from the disorder have "smaller absolute and relative volumes of white matter in the caudate nucleus than healthy subjects."