Momentary reward induce changes in excitability of primary motor cortex.

Objective: To investigate the human primary motor cortex (M1) excitability changes induced by momentary reward.

Methods: To test the changes in excitatory and inhibitory functions of M1, motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI) were tested in the abductor pollicis brevis (APB) muscle of non-dominant hand in 14 healthy volunteers by transcranial magnetic stimulation (TMS) during a behavioral task in which subjects were pseudorandomly received either reward target or non-target stimuli in response to a cue. To control sensorimotor and attention effects, a sensorimotor control task was done replacing the reward target with non-reward target.

Results: The SICI was increased, and the SAI was decreased significantly during the presentation of the reward target stimuli. Those changes were not evident during non-reward target stimuli in the sensorimotor control task, indicating that this change is specific to momentary reward.

Conclusions: Momentary rewarding is associated with change in intracortical inhibitory circuits of M1.

Significance: TMS may be a useful probe to study the reward system in health and in many diseases in which its dysfunction is suspected.