Abstract
When required, humans can generate very short latency reaches towards visual targets, like catching a cellphone falling off a desk. During such rapid reaches, express arm responses are the first wave of upper limb muscle recruitment, occurring ~80-100 ms after target appearance. There is accumulating evidence that express arm responses arise from signaling along the tecto-reticulo-spinal tract, but the involvement of the reticulo-spinal tract has not been well-studied. Since the reticulospinal tract projects bilaterally, we studied whether express arm responses would be generated bilaterally. Human participants (n = 14; 7 female) performed visually guided reaches in a modified emerging target paradigm where either arm could intercept a target. We recorded electromyographic activity bilaterally from the pectoralis major muscle. Our analysis focused on target locations where participants reached with the right arm on some trials, and the left arm on others. In support of the involvement of the reticulospinal tract, express arm responses persisted bilaterally regardless of which arm reached to the target. The latency of the express arm response did not depend on whether the arm was chosen to reach or not, however the magnitude of the express arm response was correlated to the level of anticipatory activity. The bilateral generation of express arm responses supports the involvement of the reticulo-spinal tract. We surmise that the correlation between anticipatory activity and the magnitude of express arm responses on the reaching arm arises from convergence of cortically derived signals with a parallel subcortical pathway mediating the express arm response.
New and Noteworthy Express arm responses have been proposed to arise from the tecto-reticulo-spinal tract originating within the superior colliculus, but the involvement of the reticulo-spinal tract has not been well studied. Here, we show these responses appear bilaterally in a task where either arm can reach to a newly appearing stimulus. Our results suggest that the most rapid visuomotor transformations for reaching are performed by a subcortical pathway that lies in parallel with corticospinal circuits.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Conflicts of interest: The authors declare no competing financial interests
This version of the manuscript has been revised to reflect changes in the express arm response detection method, and subsequent analyses affected by this change. Further the introduction and discussion have been revised to address the potential that the response is mediated by the cortico-spinal tract.
