The StartReact effect is larger for bimanual shoulder versus bimanual finger responses: Evidence for increased reticulospinal drive for proximal versus distal responses


The reticulospinal tract is known to control voluntary movement of proximal muscles, however, recent studies have provided evidence that voluntary movement of distal muscles can also be driven by this pathway. The StartReact effect can be used to examine reticulospinal involvement in the production of voluntary movements. The StartReact effect occurs when a startling acoustic stimulus (SAS) elicits a startle reflex and involuntarily triggers the release of a pre-planned motor response, resulting in decreased reaction time (RT). These studies have provided evidence for increased reticulospinal drive in flexion versus extension movements, gross versus fine hand movements, and bimanual versus unimanual movements. The present study used a StartReact protocol to determine if reticulospinal contributions differed between bilateral proximal and distal tasks. The magnitude of the StartReact effect, and thus relative reticulospinal contributions, were quantified by the difference in RT between startle trials in which reflexive activation was present versus absent in the sternocleidomastoid (SCM) startle indicator (i.e., SCM+ versus SCM-). Twenty-five participants performed a simple RT task requiring either bilateral finger abduction or bilateral arm abduction where the auditory go-signal was occasionally replaced with a SAS. Results showed a significantly larger StartReact effect for arm abduction versus finger abduction, as well as a higher incidence of SCM+ trials in the more proximal task. Additionally, both startle reflex-related and response-related EMG measures were larger on SCM+ trials for the shoulder versus finger task. These results suggest that in the upper limb, proximal movements receive greater reticulospinal drive than distal movements.