The human primary motor cortex (M1) participates in motor learning and response selection, functions that rely on feedback on the success of behavior (i.e. reward). To investigate the possibility that behavioral contingencies alter M1 activity in humans, we tested intracortical inhibition with single and paired (subthreshold/suprathreshold) transcranial magnetic stimulation during a slot machine simulation that delivered variable money rewards for three-way matches and required no movement. A two-way match before the third barrel had stopped (increased reward expectation) was associated with more paired-pulse inhibition than no match. Receiving a large reward on the preceding trial augmented this effect. A control task that manipulated attention to the same stimuli produced no changes in excitability. The origin of this reward-related activity is not clear, although dopaminergic ventral tegmental area neurons project to M1, where they are thought to inhibit output neurons and could be the source of the finding. Transcranial magnetic stimulation of M1 may be useful as a quantitative measure of reward-related activity.

Objective There has been no modern effort to replicate, further characterize, or quantify the dramatic effects on affect described in controlled studies from the 1960s using bilateral frontal electrodes with an extra-cephalic reference in a mixed group composed primarily of mildly depressed individuals. We performed a comprehensive, quantitative assessment of the effects of bifrontal TDCS on emotion in 21 healthy subjects. Methods In a double-blind crossover study, we administered tests of emotional state, affect, emotional decision-making, arousal, and psychomotor functions during sham, anodal, and cathodal TDCS. Results We found no systematic effects on any measure, despite two subjects who had pronounced mood effects in the predicted direction. There were no adverse events. Conclusions In line with some other studies, we found no consistent effects of bifrontal TDCS on measures of emotional function of psychomotor performance. Significance These results demonstrate the safety of bilateral anterior frontal TDCS with an extra-cephalic reference, but raise questions about its effectiveness as a modulator of mood and emotional cognition, at least in healthy subjects.

Background-The primary motor cortex is important for motor learning and response selection, functions that require information on the expected and actual outcomes of behavior. Therefore, it should receive signals related to reward and pathways from reward centers to motor cortex exist in primates. Previously, we showed that gamma aminobutyric acid-A(GABA A )-mediated inhibition in motor cortex, measured by paired transcranial magnetic stimulation (TMS), changes with expectation and uncertainty of money rewards generated by a slot machine simulation.