Startle reflex activation is related to early response triggering and is indicative of an alternate response initiation pathway


The presentation of a loud, startling acoustic stimulus in a simple reaction time (RT) task has been shown to substantially decrease RT. This so called "StartReact" effect has been attributed to early and involuntary response triggering due to activation of the startle reflex pathway. However, the relationship between the startle reflex, response triggering, and latency of the response is not fully understood. The purpose of the current study was to examine the conditions under which a prepared response can be triggered by an auditory stimulus. Participants performed a simple RT wrist extension task in response to a visual go-signal. On selected trials, an auditory stimulus of different intensities (80dB, 100dB, 110dB, 120dB) was presented either 300ms or 1000ms prior to the visual go. The presence/absence of a startle reflex was determined by activation of the sternocleidomastoid (SCM+/SCM-), and response triggering was defined as EMG onset occurring within 300ms of the auditory stimulus. As expected, the incidence of SCM+ trials was higher with increasing intensity and later presentation of the auditory stimulus. When SCM activation was observed, the incidence of response triggering was much higher across all intensities ?100dB, and these responses exhibited shorter RT latency than those triggered in the absence of SCM activation. Conversely, when responses occurred in response to the visual go, RT latencies were similar across all conditions. Together, these results suggest that the presence of SCM activation is an important marker for response triggering and is indicative of a separate and faster response initiation pathway.

Acknowledgments: This research was supported by the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of Research, Innovation and Science.