The Nature of Reflexes in Online Planning and Control
Abstract
Contexts and intentions shape our movement goals, which evolve with new information, often requiring us to replan and modify our actions online. Although it is well-established that stretch reflexes are tuned to movement goals, the timescale at which they update when the goal changes during ongoing movement is largely unknown. Addressing this gap, the current study investigated how quickly and to what extent stretch reflexes are modulated when a new spatial target appeared as participants performed a default reaching action. Mechanical perturbations were applied to the arm randomly after one of four target preview delays (ranging from 0 ms to 200 ms), or not at all, serving as a cue to quickly reach toward the new spatial target. If no perturbation occurred, participants completed the default reach. The results indicate that stretch reflexes in the long-latency epoch (R3; 75-100 ms) are modulated ~120 ms after target onset, nearly matching voluntary response times and faster than previously reported in a postural analog of this study (Yang et al. in Exp Brain Res 211:231–242 2011). Interestingly, small but reliable modulation of reflexes in the short-latency epoch (R1; 20-45 ms) was also observed after both 90 ms and 200 ms of preview delay, suggesting that even the earliest stretch responses can be influenced by new goal information during movement. These findings suggest a close coupling between reflexive and voluntary control during dynamic motor tasks.