Trial-by-trial modulation of express visuomotor responses induced by symbolic or barely detectable cues
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 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 involving 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 extrapolating a symbolic cue (75% validity). Valid symbolic cues led to faster and larger SLRs than control conditions; invalid symbolic cues produced slower 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 faster SLRs than control with the valid low-contrast cues, whereas the invalid cues led to the opposite results. These findings may reflect exogenous priming mechanisms of the SLR network, 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. NEW & NOTEWORTHYExpress visuomotor responses in humans appear to reflect subcortical sensorimotor transformation of visual inputs, potentially conveyed via the tecto-reticulo-spinal pathway. Here we show that the express responses are influenced both by symbolic and barely detectable spatial cues about stimulus location. The symbolic cue-induced effects suggest cortical top-down modulation of the putative subcortical visuomotor network. The effects of barely detectable cues may reflect exogenous priming mechanisms of the tecto-reticulo-spinal pathway.