Startle-triggered responses indicate reticulospinal drive is larger for voluntary shoulder versus finger movements.

Recent primate studies have implicated a substantial role of reticulospinal pathways in the production of various voluntary movements. A novel way to assess the relative reticulospinal contributions in humans is through the use of a "StartReact" paradigm where a startling acoustic stimulus (SAS) is presented during a simple reaction time (RT) task. The StartReact response is characterized by short-latency triggering of a prepared response, which is attributed to increased reticulospinal drive associated with startle reflex activation. The current study used a StartReact protocol to examine differences in reticulospinal contributions between proximal and distal effectors by examining EMG onset latencies in lateral deltoid and first dorsal interosseous during bilateral shoulder or finger abduction. The magnitude of the StartReact effect, and thus relative reticulospinal drive, was quantified as the difference in RT between startle trials in which startle-reflex related EMG activation in the sternocleidomastoid (SCM) was present (SCM +) versus absent (SCM -). A significantly larger StartReact effect was observed for bilateral shoulder abduction versus bimanual finger abduction and a higher incidence of SCM + trials occurred in the proximal task. Additionally, both startle reflex and response-related EMG measures were larger on SCM + trials for the shoulder versus finger task. These results provide compelling novel evidence for increased reticulospinal activation in bilateral proximal upper-limb movements.

Retrospective composite analysis of StartReact data indicates sex differences in simple reaction time are not attributable to response preparation.

Simple reaction time (RT) can vary by sex, with males generally displaying faster RTs than females. Although several explanations have been offered, the possibility that response preparation differences may underlie the effect of sex on simple RT has not yet been explored. A startling acoustic stimulus (SAS) can involuntarily trigger a prepared motor response (i.e., StartReact effect), and as such, RT latencies on SAS trials and the proportion of these trials demonstrating startle-reflex EMG in the sternocleidomastoid (SCM) muscle are used as indirect measures of response preparation. The present study employed a retrospective analysis of composite individual participant data (IPD) from 25 datasets published between 2006 and 2019 to examine sex differences in response preparation. Linear mixed effects models assessed the effect of sex on control and SAS RT as well as the proportion of SAS trials with SCM activation while controlling for study design. Results indicated significantly longer control RT in female participants as compared to males (p = .017); however, there were no significant sex differences in SAS RT (p = .441) or the proportion of trials with startle reflex activity (p = .242). These results suggest that sex differences in simple RT are not explained by variations in levels of response preparation but instead may be the result of differences in perceptual processing and/or response initiation processes.