Reaction time and acoustic startle in normal human subjects.

We studied the effects of collision between a voluntary command and the startle response by interrupting a simple visual reaction time task with an acoustic startle. We observed two main effects. First, the reaction time was markedly shortened when the startle was delivered at intervals of 0-75 ms after the 'go' signal. Second, the startle response elicited when the subject's attention was focused onto reacting to the visual 'go' signal involved more muscles and induced larger EMG responses than when the startling stimulus was delivered unexpectedly. Startle response and voluntary commands may share a number of structures along the motor path. Reaction time can be shortened up to the latency of a startle response if the voluntary command is issued a short time after the startling stimulus. Facilitation of the startle responses during motor preparation may be due to the excitability enhancement that occurs in various neural structures along the motor path before the onset of EMG activity.