Trial-by-trial modulation of express visuomotor responses induced by symbolic or barely detectable cues

Abstract

Human cerebral cortex can produce visuomotor responses that are modulated by contextual and task-specific constraints. However, the distributed cortical network for visuomotor transformations limits the minimal response time of that pathway. Notably, humans can generate express visuomotor responses in arm muscles that are inflexibly tuned to the target location and occur 80-120ms from stimulus presentation (stimulus-locked responses, SLRs). This suggests a subcortical pathway for visuomotor transformations that might involve the superior colliculus and its downstream reticulo-spinal projections. Here we investigated whether cognitive expectations can modulate the SLR. In one experiment, we recorded surface EMG from shoulder muscles as participants reached toward a visual target whose location was unpredictable in control conditions, and partially predictable in cue conditions by interpreting a symbolic cue (75% validity). Valid symbolic cues led to earlier and larger SLRs than control conditions; invalid symbolic cues produced later and smaller SLRs than control conditions. This is consistent with a cortical top-down modulation of the putative subcortical SLR-network. In a second experiment, we presented high-contrast targets in isolation (control) or ~24ms after low-contrast stimuli, which could appear at the same (valid cue) or opposite (invalid cue) location as the target, and with equal probability (50% cue validity). We observed earlier SLRs than control with the valid low-contrast cues, whereas the invalid cues led to the opposite results. These findings may reflect bottom-up attentional mechanisms, potentially evolving subcortically via the superior colliculus. Overall, our results support both top-down and bottom-up modulations of the putative subcortical SLR network in humans.

Express Arm Responses Appear Bilaterally on Upper-limb Muscles in an Arm Choice Reaching Task

“…If similar mechanisms govern express arm responses in our paradigm, then cortical inputs related to implied motion and the timing of target appearance would increase the pre-target activity within the superior colliculus. Express arm responses have been potentiated in other behavioural paradigms that presumably engage different top-down inputs into the superior colliculus (3, 23, 24, 29).…”

“…The emerging target paradigm (23) has emerged as an efficient means to elicit express arm responses, increasing the prevalence and magnitude of the response (5,12,24). Past work has investigated how certainty about the time of target emergence (5), cueing (24), or the properties of the emerging target (5,12) influence the express arm response. All such work using the emerging target task, as well as all past studies of the express arm response (1,4,9) investigated reaches made with one arm.…”

“…Previous work has shown an emerging target paradigm, wherein a moving target transiently disappears and then emerges from behind a barrier, elicits robust express arm responses in the reaching arm in almost every participant (5, 12, 23, 24). Here, we modified this paradigm by increasing the number of potential locations of target emergence and allowing the subject to reach toward the emerging target with either arm.…”

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.…”

“…Indeed, the neural pathway for SLRs has been hypothesized to parallel that of express saccades (19), which involves the superior colliculus and its downstream projection to the reticular formation (20)(21)(22)(23)(24). Consequently, we and others have referred to short-latency EMG responses observed within the established "SLR" time window as "express" arm muscles responses (16)(17)(18), and will use this terminology here.…”