Subcortical motor circuit excitability during simple and choice reaction time.

The purpose of the current study was to examine the relationship between movement preparation and excitability of subcortical motor circuits, as measured by the reflexive response to a startling acoustic stimulus. We compared the size and incidence of activation in the sternocleidomastoid (startle indicator) from participants completing either a simple or choice reaction time (RT) task. Consistent with predictions, results indicated that the startle reflex habituated after several presentations of the SAS for the choice RT group but not for the simple RT group, which we attributed to advance motor preparatory processes involved in a simple RT task. Additionally, when participants from the choice RT group were put into a simple RT condition, the startle reflex response returned to nonhabituated levels. We conclude that the increased corticospinal activation associated with advance preparation may also result in increased subcortical activation, accounting for the observed lack of habituation to a startling stimulus in simple RT.