Abstract
The startle reflex is a defensive physiological response to an unexpected and intense stimulus, resulting in a generalized flexion response that typically includes activation in the sternocleidomastoid (SCM) muscle. A startling acoustic stimulus (SAS) can also involuntarily trigger the release of a pre-planned movement with decreased latency, a phenomenon termed the StartReact effect (Carlsen et al., 2004). It is generally accepted that only an unexpected, intense stimulus leads to an overt startle reflex. However, because startle habituation is attenuated during reaction (RT) time tasks, it is unclear whether foreknowledge of an impending SAS would have any effect on the startle reflex or the RT speeding effect of startle. To test this, sixteen participants completed a simple RT task consisting of two sequential blocks. In one block, the SAS, which replaced the usual go-signal, was randomly presented in 20% of trials without the knowledge of the participants. In the other, participants were warned of the upcoming SAS. One group completed the random block first, while a second completed the warned block first. Results showed that advance knowledge of an upcoming SAS had no effect on the incidence of observing a startle reaction (p = .971), but led to significant RT savings in both control and startle trials (p = .019). These data suggest that when used in the context of a RT task, a SAS does not need to be unexpected in order to elicit a startle reaction and speeded RTs, and can benefit advance preparation.Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation