Although a critical involvement of dopamine has been implicated in the functions of the basal ganglia, especially in learning, little is known about its mechanisms. In two macaque monkeys, the ...nigrostriate dopamine system was unilaterally depleted by neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We asked the monkeys to learn sequential button press tasks. In addition to the slowness of movement, they showed specific deficits in learning action strategy and reward prediction when using contralateral arm to the dopamine depletion, but with ipsilateral arm, they learned the tasks efficiently. Activity of single, midbrain dopamine neurons was recorded from two monkeys in a task in which they chose one correct, rewarding button among three alternatives in a trial and error basis. It was found that the magnitudes of responses to beep sound informing correct and incorrect choices precisely reward prediction error signals. We present a hypothetical scheme for the mechanisms of action learning in the basal ganglia, in which the nigrostriate dopamine system provides the striatum, locus of learning, with reward prediction errors as a teaching signal for learning, and excitability of the striate projection neurons to specific cortical inputs is modified by the teaching signal so as to achieve goal-directed action selection.
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