Express arm responses appear bilaterally on upper-limb muscles in an arm choice reaching task

Abstract

When required, humans can generate very short latency reaches towards visual targets, like catching a falling cellphone. 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 the 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 and magnitude of the express arm response did not depend on whether the arm was chosen to reach or not. However, on the reaching arm, 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.

Shoulder muscle recruitment of small amplitude motor units during the delay period encodes a reach movement plan which is sensitive to task context

“…The rather early activity observed in the absence of overt movement further suggests that this activity is likely to reflect the activation of small motor units, which may explain why their contribution in whole muscle EMG activity may have gone unnoticed. Although speculative, based on the bilateral activation of the stimulus linked responses recently observed by (Kearsley et al 2021), we propose that such early activity may reflect the input from the reticular spinal tract that could be the source of gamma unit activation that is being assessed by the high-density surface EMG electrodes used in this study. Alternatively such early kinematic activity may also reflect contributions of the parietal cortex that have been recently shown to project to neurons in ventral spinal cord (Rathelot et al 2017).…”

“…We extended these ideas to study flow of information into the peripheral musculature. Previous efforts have tried to address this issue in the case of immediate movements and have shown evidence of a stimulus linked response (SLR) in proximal arm and neck muscles (Corneil and Munoz 2014; Pruszynski et al 2010) that may be associated with neural activity associated with the reticulospinal tract (Kearsley et al 2021). However, since the SLR is mostly associated with reflexive movements, dissociating early and late components of movement planning from execution are challenging.…”

“…Second, the use of spatial filtering techniques for high-density surface array EMG recordings is known to selectively enhance local voltage changes which would be more likely to reflect the recruitment patterns of motor units (Gazzoni et al 2004; Luca et al 2006; Mambrito and De Luca 1984; Masuda and De Luca 1991; Merletti and Farina 2009; Rau and Disselhorst-Klug 1997) rather than global muscle activity; and our use of template matching and the measurement of the propagation of the signal along the muscle fibres suggests this to be the case. Third, unlike many previous reports, our study focussed on proximal shoulder muscles that may be preferentially targeted by the reticulospinal system that may be responsible for the observed delayed activity since these muscles also robustly express a stimulus locked response (Kearsley et al 2021). Finally, since this early activity was correlated with reaction time, had spatially selective response fields (see Fig S4 ) and was only observed for delayed movements but not for cursor movements generated by isometric muscle force, it cannot be an artifact.…”

“…In addition to suppressing reflex gains, delay period activity that are routinely observed in motor cortical cells (Requin et al 1990; Tanji et al 1988), as well as in oculomotor areas such as the Frontal Eye Fields (FEF) and the Superior Colliculus (SC) (Dorris and Munoz 1998; Rungta et al 2021), are also correlated with benefits in reaction time, raising the possibility that central preparatory activity may have also direct facilitatory effects on agonist muscle activity as well, possibly via more direct connections from the reticulospinal tracts on spinal/cervical motor neurons (Kearsley et al 2021). Consistent with this notion, Mellah et al 1990 have shown the presence of delay period motor unit activity in axial and proximal skeletal muscles during a delayed movement task in monkeys.…”

Cue-driven motor planning facilitates express visuomotor responses in human arm muscles

“…Interestingly, previous EMG work shown that humans can produce extremely fast stimulus-driven arm muscles responses that are inflexibly locked in space and time to visual stimuli (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), which were originally termed stimulus-locked responses (SLRs; 5). Specifically, the SLRs consistently encode the location of a visual stimulus within ~100ms from its presentation, irrespective of the mechanical RT.…”