Objective: The aim of this study was to further characterize surround inhibition (SI) in the primary motor cortex (M1) by comparing its magnitude and time course during a simple reaction time task (SRT) and a choice reaction time task (CRT). Methods: In both the SRT and the CRT, subjects performed the same right index finger flexion in response to an acoustic signal. For CRT, the alternative choice was a similar movement using the left index finger, as distinguished by a different tone. In both tasks, single pulse transcranial magnetic stimulation (TMS) was applied at rest, 75 ms (T1) and 25 ms before EMG onset (T2), and during the first peak of EMG (T3) in the right first dorsal interosseous muscle (FDI). Motor evoked potentials (MEPs) were recorded from both FDIs, which act as synergists in the task, and the right surrounding, relaxed abductor pollicis brevis muscle (APB). Results: For right hand movement, SI started earlier and was more pronounced for CRT compared to SRT. For left hand movement in the CRT, SI was similar to that of right hand movement. Conclusions: We conclude that SI occurs earlier and stronger with increasing task difficulty. Significance: The timing as well as the bilateral effect of the inhibition suggests that motor areas involved in motor planning, proximate to the motor cortex, contribute to the genesis of surround inhibition. Published by Elsevier Ltd. on behalf of International Federation of Clinical Neurophysiology.
