Motor preparation in an anticipation-timing task.

Previous findings from experiments involving anticipation-timing tasks have indicated that a point in time may exist after which a participant is committed to producing a pre-programmed movement. For example, if a "stop" signal is given too long after a "go" signal but prior to movement initiation, the response is often still produced. It has been suggested that a startling stimulus may act to elicit a pre-programmed response in reaction time (RT) tasks without involvement of the cerebral cortex (Valls-Solé et al. 1999). The present experiment employed a startling stimulus to investigate the temporal course of motor preparation during a stop-signal anticipation-timing task. Participants timed a key release coincident with the sweep of a clock hand reaching a target. On some trials, the clock hand stopped prior to reaching the target (meaning participants were to refrain from responding), which was accompanied by either a startling acoustic stimulus (124 dB) or control stimulus (82 dB). Results from startle trials indicate that while some advance preparation of motor circuits was evident, subcortical pre-programming and storage of the motor command in circuits common to the voluntary and startle response pathways was not completed well in advance of response production.