StartReact effects are dependent on engagement of startle reflex circuits: support for a subcortically mediated initiation pathway.

The "StartReact" effect refers to the rapid involuntary triggering of a prepared movement in response to a loud startling acoustic stimulus (SAS). This effect is typically confirmed by the presence of short-latency electromyographic activity in startle reflex-related muscles such as the sternocleidomastoid (SCM); however, there is debate regarding the specific neural pathways involved in the StartReact effect. Some research has implicated a subcortically mediated pathway, which would predict different response latencies depending on the presence of a startle reflex. Alternatively, other research has suggested that this effect involves the same pathways responsible for voluntary response initiation and simply reflects higher preparatory activation levels, and thus faster voluntary initiation. To distinguish between these competing hypotheses, the present study assessed preparation level during a simple reaction time (RT) task involving wrist extension in response to a control tone or a SAS. Premotor RT and startle circuitry engagement (as measured by SCM activation) were determined for each trial. Additionally, preparation level at the go signal on each trial was measured using motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS). Results showed that SAS trial RTs were significantly shorter (P = 0.009) in the presence of startle-related SCM activity. Nevertheless, preparation levels (as indexed by MEP amplitude) were statistically equivalent between trials with and without SCM activation. These results indicate that the StartReact effect relates to engagement of the startle reflex circuitry rather than simply being a result of an increased level of preparatory activation.NEW & NOTEWORTHY The neural mechanism underlying the early triggering of goal-directed actions by a startling acoustic stimulus (SAS) is unclear. We show that although significant reaction time differences were evident depending on whether the SAS elicited a startle reflex, motor preparatory activation was the same. Thus, in a highly prepared state, the short-latency responses associated with the StartReact effect appear to be related to engagement of startle reflex circuitry, not differences in motor preparatory level.