Startle and the StartReact Effect: Physiological Mechanisms.

It has been well documented that a prepared response can be triggered at short latency following the presentation of a loud acoustic stimulus that evokes a reflexive startle response. Different hypotheses have been proposed for this so-called "StartReact" effect, although there is still much debate surrounding the physiological mechanisms involved in the observed reduction in reaction time (RT). In this review, we outline the various neurophysiological explanations underlying the StartReact effect and summarize the data supporting, and at times opposing, each possibility. Collectively, the experimental results do not unequivocally support a single explanation and we suggest the most parsimonious mechanism may involve a hybrid framework involving a distribution of neural pathways. Specifically, we propose that multiple node networks at the cortical, brainstem, and spinal levels are involved in response preparation and initiation, and the relative contributions of these structures depends on the type of stimulus delivered and the type of movement required. This approach may lead to greater understanding of the pathways involved in response preparation, initiation, and execution for both healthy and motor disordered populations.