Corticospinal excitability is reduced in a simple reaction time task requiring complex timing.

Increasing the complexity of a movement has been shown to result in longer simple reaction time (RT), which has been attributed to sequencing or timing requirements following the go-signal. However, RT differences may also be due to differences in corticospinal excitability (CE) as previous studies have found an enhanced excitatory state of corticospinal neurons in complex tasks. Transcranial magnetic stimulation (TMS) was used in the present study to probe the excitability of the motor pathway during the simple RT interval for single (simple) versus multiple (complex) key press responses. Premotor RT data indicated that participants responded significantly (p<.001) faster in the simple task compared to the complex task, confirming response complexity was manipulated appropriately. Analysis of the CE data indicated that motor evoked potential (MEP) amplitudes increased with time following the go-signal in both conditions and that MEP amplitudes in the simple task were significantly larger than those in the complex task when evoked within 75ms of movement onset (p=.009). These findings suggest that the rate of increase for initiation-related neural activation is reduced for complex as compared to simple movements, which may partially explain differences in RT.