The Basal Ganglia

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Perhaps the most frequently identified structure in neuroimaging studies of the SRT task is the subcortical collection of nuclei called basal ganglia. Increases in the basal ganglia's activity have been observed during both implicit and explicit sequence learning, although researchers have tended to emphasize automatic processing when characterizing its computational role. They have done so in part because several lines of evidence indicate that the basal ganglia are a key component of a habit learning system, especially in relationship to the production of sequential movements.

Neurological disorders have also emphasized the role of the basal ganglia in motor control, particularly in the production of sequential movements. Two prominent degenerative disorders, Parkinson's disease and Huntington's disease, involve a loss of tissue in the basal ganglia. For the former, the disorder primarily involves a loss of dopaminergic neurons arising in the substantia nigra. For the latter, the genetic abnormalities result in extensive atrophy of the striatum, at least in the initial stages of the disease. Patients with either Parkinson's or Huntington's disease exhibit a learning deficit on the SRT task. In both disorders, the coordination problems are most pronounced during the production of sequential actions.

Kent Berridge and colleagues have taken a neuroe-thological approach to the study of sequential behavior, exploring the neural basis of grooming in the rat. This behavior is highly stereotyped, consisting of three distinct arm strokes arranged in an essentially fixed order. Although lesions of the cerebral cortex and cerebellum produce only transient disruptions in this behavior, extensive striatal lesions can produce a chronic impairment in grooming. Interestingly, the deficit is not manifest as a loss of particular component elements, nor are the elements produced in a shuffled order. Rather, following the lesions, the animals frequently fail to complete the production of the complete sequence.

Because the grooming sequence performed by the rat is thought to be innate, the basal ganglia's function in this example is likely unrelated to learning per se. Thus, although many researchers have emphasized that the basal ganglia are necessary for learning new associations between stimuli and responses (S-R associations), it appears that the structure is critical for performing behaviors that are well established in the organism's repertoire. Given that the component elements are still performed after the basal ganglia damage, one possible contribution of the basal ganglia may be scheduling to elements in the appropriate order. That the structure is critical for both the acquisition of new sequences and the performance of old ones provides an intriguing clue to its computational role.

Finally, neurophysiological studies have revealed that neurons within the monkey striatum are sensitive not only to which response in a sequence is being performed but also to the context of that response. In other words, these neurons show increases in activity that are dependent on the particular response occurring in a particular sequence. For example, a monkey can be trained to perform two movement sequences, one consisting of response sequence ABC and the other consisting of response sequence CBA. A context-sensitive neuron may show increases in activity before response B in the sequence ABC but not in the sequence CBA. This pattern is consistent with the hypothesis that the basal ganglia have chunked a series of discrete responses into a sequence, a process that is essential for the automatization of skills. However, similar context dependency is also observed in the supplementary motor area (SMA), a cortical area that is reciprocally connected with the basal ganglia. These results suggest a variety of computational interpretations. One hypothesis is that as skill develops, the basal ganglia form a representation of the sequence, enabling a shift in the locus of control from the cortex to the subcortex. In contrast, it may be that the representation of the sequence remains cortical and that the basal ganglia provide a mechanism to rapidly progress through the series of gestures as the sequence unfolds. The latter hypothesis would be consistent with the grooming studies showing that the striatal lesions result in a failure to complete the sequence.

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