Abstract
The reflexive startle response can be used as a tool to study the neurophysiology and pathophysiology of neural circuits. Higher intensity stimuli are generally more likely to elicit the startle response. There is currently debate as to whether the startle reflex consists of a single response or multiple components subserved by separate neural pathways. The purpose of this study was to examine how the startle response was affected by stimulus intensity during a reaction time task. Startle response data from two experiments (Carlsen et al, 2007; Carlsen, in review) that required participants to react to an auditory stimulus of varying intensity (82-124 dB) were analyzed. We integrated EMG from sternocleidomastoid (SCM, a robust indicator of startle), in three time frames following the stimulus (0-50 ms, 50-100 ms, 100-150 ms). SCM activity was analyzed via a 3 (time bin) X 5 (stimulus intensity) repeated measures ANOVA. In both experiments a significant interaction was found (p < .05) whereby an early startle component (50-100 ms) was observed only at higher intensities (≥ 116 dB) followed by a later startle component (100-150 ms) in response to lower stimulus intensities (103 dB and above). These results suggest that the early component involves a fast, high threshold pathway, whereas the later component uses a slower and lower threshold circuit. This threshold-dependent two-component model provides a novel description of the neurophysiology underlying the startle reflex circuitry.
Supported by NSERC
