Reward does not modulate corticospinal excitability in anticipation of a Stroop trial.

Action preparation is associated with a transient decrease of corticospinal excitability just before target onset. We have previously shown that the prospect of reward modulates preparatory corticospinal excitability in a Simon task. While the conflict in the Simon task strongly implicates the motor system, it is unknown whether reward prospect modulates preparatory corticospinal excitability in tasks that implicate the motor system less directly. To that effect, we examined reward-modulated preparatory corticospinal excitability in the Stroop task. We administered a rewarded cue-target delay paradigm using Stroop stimuli that afforded a left or right index finger response. Single-pulse transcranial magnetic stimulation was administered over the left primary motor cortex and electromyography was obtained from the right first dorsal interosseous muscle. In line with previous findings, there was a preparatory decrease in corticospinal excitability during the delay period. In contrast to our previous study using the Simon task, preparatory corticospinal excitability was not modulated by reward. Our results indicate that reward-modulated changes in the motor system depend on specific task-demands, possibly related to varying degrees of motor conflict.