A02Motor control and behaviors Roles of the dopamine system in reward and attentional processing
Dopamine neurons in the midbrain are well known for its crucial roles in reward processing. These neurons are excited by rewards when the rewards are better than expected, and inhibited by rewards when the rewards are worse than expected. Such reward-related dopamine signals have been implicated in motivation and learning to obtain rewards. Clinical studies, on the other hand, have established that the degeneration of dopamine neurons causes multiple dysfunctions including not only reward-related motivational dysfunctions but also reward-unrelated motor and cognitive dysfunctions as seen in Parkinson’s disease. The reward theory of dopamine could account for the reward-related dysfunction but it seems difficult to explain how the reward-unrelated dysfunctions are caused by the loss of dopamine reward signals.
A hint to resolve this contradiction came from our recent electrophysiological studies in monkeys reporting that dopamine neurons transmit more diverse signals than previously thought. We found that dopamine neurons are divided into at least two functional subgroups, one signaling “motivational value” and the other signaling “salience” (Matsumoto & Hikosaka, Nature, 2009; Matsumoto & Takada, Neuron, 2013; Matsumoto, Mov Disord, 2015). Dopamine neurons signaling motivational values are located in the ventral tegmental area (VTA) and the ventromedial part of the substantia nigra pars compacta (SNc) in which dopamine neurons mainly project to limbic territories of the striatum and prefrontal cortex. Dopamine neurons signaling salience are located in the dorsolateral part of the SNc in which dopamine neurons mainly project to motor and cognitive territories of the striatum and prefrontal cortex.
In the present study, we investigate the “causal relationships” between dopamine value signals and reward-related motivational functions and between dopamine salience signals and reward-unrelated motor and cognitive functions. We apply our optogenetic technique in monkeys (Inoue et al., Nat Commun, 2015) to manipulate one of the two dopamine signals and examine the effects on monkey’s behavior. This study will provide more mechanistic accounts of dysfunctions caused by the degeneration of dopamine neurons.