Symbolic cues enhance express visuomotor responses in human arm muscles at the motor planning rather than the visuospatial processing stage

Abstract

Humans can produce "express" (~100ms) arm muscle responses that are inflexibly locked in time and space to visual target presentations, consistent with subcortical visuomotor transformations via the tecto-reticulo-spinal pathway. These express visuomotor responses are sensitive to explicit cue-driven expectations, but it is unclear at what stage of sensory-to-motor transformation such modulation occurs. Here, we recorded electromyographic activity from shoulder muscles as participants reached toward one of four virtual targets whose physical location was partially predictable from a symbolic cue. In an experiment in which targets could be veridically reached, express responses were inclusive of the biomechanical requirements for reaching the cued locations and not systematically modulated by cue validity. In a second experiment, movements were restricted to the horizontal plane so that the participants could perform only rightward or leftward reaches, irrespective of target position on the vertical axis. Express muscle responses were almost identical for targets that were validly cued in the horizontal direction, regardless of cue validity in the vertical dimension. Together, these findings suggest that the cue-induced enhancements of express responses are dominated by effects at the level of motor plans and not solely via facilitation of early visuospatial target processing. Notably, direct cortico-tectal and cortico-reticular projections exist that are well-placed to modulate pre-stimulus motor preparation state in subcortical circuits. Our results could reflect a neural mechanism by which contextually relevant motor responses to compatible visual inputs are rapidly released via subcortical circuits that are sufficiently along the sensory- to-motor continuum.

Express visuomotor responses reflect knowledge of both target location and conscious intent during reaches of different amplitudes

“…Consistently with our previous work (Contemori et al, 2022), we defined the mechanical reaction time by indexing the first time point at which the radial hand velocity exceeded the baseline mean velocity (i.e. average velocity recorded in the 100 ms preceding the target onset time) by more than five standard deviations (see Figure 2B for exemplar results).…”

“…In both experiments, the target was presented via an emerging moving target paradigm (Figure 1B; Kozak et al, 2020; Kearsley et al, 2022; Contemori et al, 2022). To start the trial the participants had to align the cursor and gaze at a ‘home’ position (a blue ring of ~2 cm in diameter) located at the centre of the monitor and aligned with the mid-body line.…”

“…To determine the response correctness, we measured the initial reach direction by adopting a procedure previously described by Contemori et al, 2022. Briefly, we compared the initial hand-trajectory direction (i.e.…”

“…This conception, however, is challenged by converging evidence showing that humans muscles can receive express target-driven motor signals at latencies (~90 ms) that leave little time for cortical visuomotor transformation (Goonetilleke et al, 2015; Gu et al, 2019; Selen et al, 2021; Billen 2022). Notably, the onset time of express visuomotor responses is far less variable than the mechanical reaction time (RT; Contemori et al, 2022), which depends mostly on longer latency (plausibly cortical) muscle response components. Express visuomotor responses are also inflexibly tuned to the physical stimulus location even in circumstances requiring volitional generation of non-veridical responses (e.g.…”

“…Previous work showed that the requirement to avoid rapid target-directed responses impaired the generation of express visuomotor responses (Pruszynski et al, 2010; Wood et al, 2015; Atsma et al, 2018). More recent work showed that express visuomotor responses are modulated by explicit cues about the temporal (Contemori et al, 2021a) and spatial (Contemori et al, 2021b) presentation of visual stimuli, and that express responses can incorporate contextual pre-stimulus expectations about the required movement to reach the target (Gu et al, 2018; Contemori et al, 2022). In all, these findings appear to reflect context-dependent modulation of the express circuit, potentially conveyed top-down via direct cortico-tectal (see for review Boehnke and Munoz, 2008) and cortico-reticular projections (Keizer and Kuypers, 1984, 1989; Fregosi et al, 2017; Darling et al, 2018; Fisher at al., 2021).…”