Abstract
In a simple reaction time task, the presentation of a startling acoustic stimulus has been shown to trigger the prepared response at short latency, known as the StartReact effect. However, it is unclear under what conditions it can be assumed that the loud stimulus results in response triggering. The purpose of the present study was to examine how auditory stimulus intensity and preparation level affect the probability of involuntary response triggering and the incidence of activation in the startle reflex indicator of sternocleidomastoid (SCM). In two reaction time experiments, participants were presented with an irrelevant auditory stimulus of varying intensities at various time points prior to the visual go-signal. Responses were independently categorized as responding to either the auditory or visual stimulus and those with or without SCM activation (i.e., SCM+/−). Both the incidence of response triggering and proportion of SCM+ trials increased with stimulus intensity and presentation closer to the go-signal. Data also showed that participants reacted to the auditory stimulus at a much higher rate on trials where the auditory stimulus elicited SCM activity versus those that did not, and a logistic regression analysis confirmed that SCM activation is a reliable predictor of response triggering for all conditions.
Highlights
In a simple reaction time task, the presentation of a startling acoustic stimulus has been shown to trigger the prepared response at short latency, known as the StartReact effect
The protocols used in such experiments vary widely, including the intensity of the auditory stimulus and whether activation in a startle reflex indicator such as the SCM is assessed
Independent of whether the response was triggered by the auditory stimulus, trials were considered to either be SCM+ or SCM−, depending on whether there was short latency activation observed in the startle reflex indicator muscle
Summary
In a simple reaction time task, the presentation of a startling acoustic stimulus has been shown to trigger the prepared response at short latency, known as the StartReact effect. This SCM+/− comparison can be used to control for stimulus intensity effects associated with the louder go-signal and used to confirm whether a greater reduction in RT is present on SCM+ versus SCM− trials, indicative of an effect due to activation of the startle reflex circuitry, rather than the intensity of the stimulus[20,24] Consistent with this viewpoint, a recent meta-analysis has shown that for most responses SCM+ trials are performed at significantly shorter RT latency than SCM− trials[25], which has been taken as evidence that faster initiation processes involving subcortical pathways occur when a startle reflex is observed. Whether or not confirmation of SCM activation is required may affect the intensity of the SAS employed in a given study, as it is well known that a louder go-signal increases the likelihood of eliciting a startle reflex[21,29]
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