Abstract
The "StartReact" effect refers to the rapid involuntary triggering of a prepared movement in response to a loud startling acoustic stimulus (SAS). This effect is typically confirmed by the presence of short latency EMG activity in startle reflex-related muscles such as the sternocleidomastoid (SCM). However, there is debate regarding the specific neural pathways involved in the StartReact effect. Some research has implicated a subcortically-mediated pathway, which would predict different response latencies depending on the presence of a startle reflex. Alternatively, other research has suggested that this effect involves the same pathways responsible for voluntary response initiation and simply reflects higher preparatory activation levels, and thus faster voluntary initiation. To distinguish between these competing hypotheses, the present study assessed preparation level during a simple reaction time (RT) task involving wrist extension in response to a control tone or a SAS. Premotor RT and startle circuitry engagement (as measured by SCM activation) were determined for each control and SAS trial. Additionally, preparation level on each trial was measured using motor evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS). Results showed that SAS trial RTs were significantly shorter (p=.004) in the presence of startle-related SCM activity, and longer in its absence. Nevertheless, preparation levels (as indexed by MEP amplitude) did not differ between trials with and without SCM activation (p=.72). These results indicate that the StartReact effect relates to involvement of the startle reflex circuitry rather than simply being a result of an increased preparatory level.Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation and Science