Startle reveals response preparation can take up to 400ms in an instructed-delay paradigm


Response preparation in simple reaction time (RT) tasks has been modelled as an increase in neural activation following the warning signal to a level below initiation threshold, which is maintained until the go-signal. However, the minimum amount of time needed for the nervous system to attain a fully prepared state remains unclear as tasks typically provide long foreperiods to ensure maximal preparation. The purpose of the current experiment was to investigate the time course of response preparation using an instructed-delay task with a short (500ms) fixed foreperiod. Participants completed wrist flexion or extension movements in response to an auditory go-signal, with the required movement being indicated by the warning-signal. To probe response preparation, a startling acoustic stimulus (SAS), which involuntarily triggers the release of sufficiently prepared responses, was randomly presented at six different time points during the 500ms foreperiod: 0, 100, 200, 300, 400, or 500ms prior to the go-signal. Results showed that few responses were elicited by startle at the earliest time point and the proportion of startle-triggered movements significantly increased up until 100ms prior to the go-signal. As increased response triggering by a SAS is associated with an increased preparatory state, these data indicate that response preparation can take up to 400ms to reach a stable elevated state following the warning-signal in an instructed-delay task.

Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation and Science