AbstractPre-programmed responses can be triggered early and involuntarily when a loud startling acoustic stimulus (SAS) is presented. This "StartReact" effect has been attributed to additional drive from reticulospinal centres associated with the startle reflex. However, movements that are mainly innervated by cortico-motoneural pathways, such as intrinsic hand movements, are resistant to startle-triggering. This absence of StartReact implies that only movements with substantial reticulospinal drive can be triggered early by a SAS. Recent evidence suggests increased involvement of the reticulospinal tract in bilateral compared to unilateral actions. Therefore, this study investigated the susceptibility of unimanual versus bimanual finger movements to early release by a SAS, with the prediction that only bimanual movements would be subject to the StartReact effect. Participants (n=10) performed 80 simple reaction time (RT) trials for both unimanual and bimanual index finger abduction tasks in response to a control tone (82dB) or SAS (115dB). Results showed that RTs were significantly shorter for both unimanual and bimanual movements on SAS trials (p<.001), yet no RT differences existed between SAS trials with or without an associated startle reflex in the sternocleidomastoid (p=.394). These results suggest that RT speeding on SAS trials was simply due to the well-known stimulus intensity effect rather than to early triggering by startle. These data support previous findings that imply movements with stronger cortical innervation are less susceptible to the StartReact effect. Furthermore, any increased involvement of reticular formation in bilateral finger movements is not sufficient to enable early response triggering by startle.
Acknowledgments: Supported by NSERC and the Ontario Ministry of Research, Innovation and Science