Running head: Differential effects of startle on RT Differential effects of startle on reaction time for finger and arm movements

Recent studies employing a reaction time (RT) task have reported that a preprogrammed response could be triggered directly by a startling acoustic stimulus (115-124 dB) presented along with the usual “go” signal. It has been suggested that details of the upcoming response could be stored subcortically and are accessible by the startle volley, directly eliciting the correct movement. However, certain muscles (e.g. intrinsic hand) are heavily dependent on corticomotoneuronal connections, and thus would not be directly subject to the subcortical startle volley in a similar way to muscles whose innervations include extensive reticular connections. In the present study fourteen participants performed 75 trials in each of two tasks within a RT paradigm: an arm extension task, and an index finger abduction task. In 12 trials within each task, the regular “go” stimulus (82 dB) was replaced with a 115 dB startling stimulus. Results showed that in the arm task, the presence of a startle reaction led to significantly shorter latency arm movements compared to the effect of the increased stimulus intensity alone. In contrast, for the finger task, no additional decrease in RT due to startle was observed. Taken together these results suggest that only movements that involve muscles more strongly innervated by subcortical pathways are susceptible to response advancement by startle.