An intense electrical stimulus elicits a startle reflex and a StartReact effect in unimanual movements irrespective of stimulus laterality


An intense tactile-electrical stimulus can elicit a startle reflex and a StartReact effect in simple reaction time (RT) tasks. Electrical stimulation has typically been applied through electrodes placed on the non-moving limb, with recent data showing that biceps stimulation resulted in relatively few startle responses (<20% of trials; Daher et al., 2022). The purpose of this study was to evaluate whether stimulus laterality affects the incidence, latency, and magnitude of the startle reflex. Participants (n=10) lifted the right hand off a switch as quickly as possible following a visual Go-signal, with a startling electrical stimulus (SES) presented to the dorsum of either the moving or the non-moving hand in 33% of trials. Results showed no significant effect of stimulus laterality on the proportion of SES trials where a startle reflex in the sternocleidomastoid (SCM) was observed (moving hand: 48.6%; non-moving hand: 42.6%; p=.371). Furthermore, the hand on which the SES was presented had no effect on either the magnitude (p-values>.247) or the latency (p-values>.494) of the startle reflex. Finally, for trials where a startle reflex was observed, premotor RT was not different between presentation sides (p=.696) but was significantly shorter compared to non-startle trials (p-values<.001). These findings suggest that a SES can be delivered on the moving or non-moving hand without a difference in the observed startle reflex and StartReact effect, and that this may be a superior stimulation location relative to the biceps.