Striatal dopamine release in sequential learning.

Sequential learning is an important aspect of cognitive processing. Neuropharmacological evidence acquired in laboratory animals suggests that striatal dopaminergic mechanisms may be important for processing of this form of learning. However, because experiments conducted on dopamine deficient patients have reported contradictory evidence, the role of dopamine and the striatum remains unclear in human sequential learning. We used a newly developed dynamic molecular imaging technique to determine whether striatal dopamine is released during performance of a sequential learning task. In this study we localized striatal regions where dopamine receptor ligand (11C-raclopride) was displaced from receptor sites, during performance of a motor sequence learning (serial reaction time) task. The results suggest that the task induces release of endogenous dopamine in the posterior two-third of dorsomedial aspect of left putamen and the anterior part of the body of caudate bilaterally. The activations of the left putamen and the right caudate coincided with the activations observed earlier during performance of a motor planning task. Since these activations are associated with the selection and execution of a response, the activation in the left caudate, which was not observed in motor planning, is probably associated with the detection of a change in the 'context', and in the formulation of a new 'rule'. Thus, the results suggest that sequential learning involves two striatal dopaminergic mechanisms, one for the detection of a change in context, and the other for selection and execution of the response.