Rapid motor responses quickly integrate visuospatial task constraints

Abstract

We have recently shown that subjects can appropriately modulate their rapid motor responses (traditionally termed reflexes) to move their hand to a spatial target when the target is displayed ~2 s before a mechanical perturbation (Pruszynski et al. in J Neurophysiol 100:224-238, 2008). The goal of this study was to investigate how quickly visual information can be used to modulate rapid motor responses to an impending mechanical perturbation. Following a 2 s to 10 ms target preview delay (PD), a perturbation either displaced the subject's hand into or out of the previewed target. We also included a condition, where the target appeared after perturbation onset (target PD = +90 ms). In all cases, subjects were instructed to react as quickly as possible to the perturbation by reaching into the displayed target. Our results indicate that subjects began to incorporate visual information into their rapid motor responses with PDs as small as 70 ms. Interestingly, subjects reacted faster when the target was presented ~150 ms before the perturbation than when they had 2 s to prepare a response. Using receiver operative characteristic (ROC) analysis, we examined modulation of muscle activity as a function of preview delay in three predefined epochs. No modulation was found in the short-latency epoch (R1; 20-45 ms). In contrast, both the long-latency (45-105 ms) and voluntary (120-180 ms) epochs were modulated at essentially the same time, 140 ms from visual presentation of the target to the beginning of each respective epoch.