Foreknowledge of an impending startling stimulus does not affect the proportion of startle reflexes or latency of StartReact responses

Abstract

During a simple reaction time (RT) task, movements can be initiated early and involuntarily through presentation of a loud startling acoustic stimulus (SAS), a phenomenon termed the StartReact effect. In order to infer that activity in startle-related structures led to the early response triggering, it is important to observe a concurrent startle reflex in sternocleidomastoid. It is generally accepted that to consistently elicit a startle reflex, the SAS must be both intense and unpredictable. However, it remains unclear what effect explicit foreknowledge of an impending SAS has on the effectiveness of a SAS to elicit a startle reflex when preparing a motor response. To test this, participants completed two separate blocks of a simple RT task (counterbalanced order), where the control auditory go-signal was replaced with a SAS on 20 % of trials. In an unwarned block, knowledge of the trial type (SAS vs. control) was not provided in advance, while in a warned block, the trial type was forewarned. Results revealed that while foreknowledge of an impending SAS reduced the magnitude of the startle reflex, it did not affect the proportion of startle reflexes elicited or the magnitude of the StartReact effect. An increase in control trial RT was observed during the unwarned block, but only when it was performed first. These results indicate that preparation of a motor response leads to sufficiently increased activation in startle-related neural structures such that even with explicit knowledge of an upcoming SAS, participants are unable to proactively gate the upcoming sensory input.

Differential effects of intensity and response preparation components of acoustic warning signals

“…Carlsen and collaborators studied the influence of startle in motor readiness and concluded that a startling stimulus represents the trigger for a faster release of previously prepared movements (Carlsen et al, 2011). Furthermore, a more recent study from his laboratory supported the physiological independence of startle reflex from the mechanism of response preparation (Drummond, Leguerrier, & Carlsen, 2016). Nonetheless, also in this study the direction of the startle RT effect was in line to the classic intensity effect (i.e., fastening RT) and opposite to our results.…”