What Is the StartReact Effect?

The use of a startling acoustic stimulus during a simple reaction time task results in the rapid initiation of a prepared response at extremely short latencies (< 80 ms). This so-called "StartReact effect" has been increasingly employed to probe subcortical contributions to response preparation, as it is thought to occur due to increased activation in reticulospinal pathways associated with engagement of the startle reflex. However, the lack of an agreed-upon definition of what exactly constitutes a StartReact effect, combined with differences in methodological protocols, has resulted in inconsistent interpretation of experimental results. Based on a comprehensive review of the literature, including evidence for the physiological mechanism underlying the effect, we propose that the clearest definition of the StartReact effect is "the early and involuntary triggering of a prepared movement in the presence of a startle reflex". Reflexive startle activity has been shown to be strongly associated with involuntary response initiation and avoids other potential confounding variables that have been shown to speed reaction time. Here we argue that classification of trials based on startle-related activation in sternocleidomastoid is the most robust method to confirm a StartReact effect. Special situations, such as pre-pulse inhibition, movements involving musculature that require additional considerations, and lowered response preparation levels, are also considered with regards to how to confirm the presence of a StartReact effect. Future directions, including the use of a StartReact protocol as a potential adjuvant therapy for movement disorders, are discussed.