The ventral midbrain encodes the strength of subsequent force generations even without external rewards

The ventral midbrain (VM), which contains dopaminergic neurons, plays a role in motivation and learning. Our recent study showed that VM pre-movement activity is associated with subsequent peak grip force under incentive motivations. On the other hand, electrical stimulation on the VM in anesthetized monkeys induced muscle activity. These findings raise the question of whether the association between VM activity and peak grip force is independent of external rewards. To investigate this question, we measured the VM activity using functional magnetic resonance imaging. Thirty healthy volunteers underwent a ready-set-go task in a 3T-MRI scanner. In this task, participants prepared at the display of the ready cue and gripped the force device as fast as possible at the display of the go cue. Reaction time and peak grip force varied across trials but were not correlated with each other. Consistent with our previous finding in the context of external rewards, VM pre-movement activity was correlated only with subsequent peak grip force but not with reaction time, even without any external rewards. Furthermore, the multivoxel pattern analysis indicated that subsequent peak grip force could be decoded from pre-movement activity in VM as well as in the contralateral primary motor cortex Taken together, we concluded that the VM pre-movement activity encodes the strength of subsequent force generation even without external rewards.