PT  - JOURNAL ARTICLE
AU  - Sarah L. Kearsley
AU  - Aaron L. Cecala
AU  - Rebecca A. Kozak
AU  - Brian D. Corneil
TI  - Express Arm Responses Appear Bilaterally on Upper-limb Muscles in Arm Choice Reaching Task
AID  - 10.1101/2021.09.24.461726
DP  - 2021 Jan 01
TA  - bioRxiv
PG  - 2021.09.24.461726
4099  - http://biorxiv.org/content/early/2021/09/25/2021.09.24.461726.short
4100  - http://biorxiv.org/content/early/2021/09/25/2021.09.24.461726.full
AB  - When required, humans can generate very short latency reaches towards a visual target, like catching a phone falling off a desk. During such rapid reaches, express arm responses are the first wave of upper limb muscle recruitment, occurring within ~80-100 ms of 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 expressed bilaterally. Human participants (n = 14; 7 female) performed visually guided reaches in a modified emerging target paradigm where either arm could be used to intercept a target once it emerged below a barrier. 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, the express arm response persisted bilaterally regardless of which arm reached to the target. While the latency of the express arm response was the same on the reaching vs non-reaching arm, the response magnitude was slightly larger on the reaching arm, in part due to anticipatory muscle recruitment related to arm choice. Our results support the involvement of the reticulo-spinal tract in mediating the express arm response, and we surmise that the increased magnitude on the arm chosen to move arises from convergence of cortically derived signals with the largely independent express arm response.New and Noteworthy Express arm responses have been proposed to arise from the tecto-reticulo-spinal tract. These responses have been linked to the superior colliculus, but the involvement of the reticulo-spinal tract has not been well studied. Here we show these responses appear bilaterally regardless of arm choice when either arm can be used to intercept a newly appearing stimulus, supporting involvement of the reticulo-spinal tract. We propose this response is mediated largely independent of the volitional cortical related activity.Competing Interest StatementThe authors have declared no competing interest.