Takafumi Minamimoto

Monitoring and switching of cortico-basal ganglia loop functions by the thalamo-striatal system.

Recent physiological and tract tracing studies revealed tight coupling of the centre médian and parafascicular nuclei (the CM-Pf complex), which are posterior intralaminar nuclei (ILN) of the thalamus, with basal ganglia circuits. These nuclei have previously been classified as part of the ascending reticulo-thalamo-cortical activating system, with studies of single neuron activity and of interruption of neuronal activity suggested that they participate in the processes of sensory event-driven attention and arousal, particularly in the context of unpredicted events or events contrary to predictions. In this article, we propose a hypothetical model that envisions that the CM-Pf complex functions in two different modes depending on the predictability of external events, i.e., one for monitoring top-down biased control through the cortico-basal ganglia loop system for selecting signals for action and cognition and the other for switching from biased control to bottom-up control based on the signals of salient external events. This model provides a new insight into the function of the CM-Pf complex and should lead to a better understanding of this important brain system.

Dopamine neurons encode teaching signals for learning reward-based decision strategy

Abstract 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.