Abstract
Voluntary muscle commands underlying human movements are traditionally thought to primarily involve corticospinal pathways, although some movements (e.g. proximal effectors) also utilize brainstem (reticulospinal) pathways. Additionally, studies using a startling acoustic stimulus (SAS) have implicated involvement of subcortical structures in the production of substantially shortened reaction time (RT). The present experiment aimed to determine whether proximal and startle-triggered movements show increased contributions of reticulospinal drive. EMG-EMG coherence analysis was used to elucidate reticular contributions to movement production based on the strength of correlated oscillations in the 10-20 Hz band, which has been shown to be associated with these neural inputs. Participants completed a simple RT task involving either a bilateral shoulder (proximal) or index finger (distal) abduction movement following an auditory go-signal or a SAS. As expected, RT was significantly shorter on startle trials for both movements. On control trials EMG-EMG coherence was significantly greater in the reticulospinal range for the proximal movement. Startle trials resulted in a further significant increase in 10-20 Hz coherence for the proximal movement, while this difference was absent for the distal movement. In other words, reticulospinal input is greater for proximal than distal movements and a SAS further increases reticulospinal activation for proximal effectors. These results provide novel evidence for differential contributions of brainstem structures to movement production that are dependent upon effector location and role.Acknowledgments: Supported by NSERC and the Ontario Ministry of Research and Innovation and Science