Do pointing responses account for proprioceptive drift?


While performing multiple back and forth pointing movements when vision of the hand is unavailable, movements can gradually drift away from their intended target (Brown et al., 2003). Although imperceptible to the participant, these movements can drift substantially soon after the initiation of the trial (e.g., within 5-10 reaches). The purpose of this study was to determine the level of "awareness" that people have of this drift in limb position. For example, if presented with a new target after a movement has drifted, do we rely on our actual limb position ("drifted off target") or our believed position ("on target") to reach the new target? To explore this question, participants performed back and forth pointing movements with vision of the targets and fingertip for the first 10 seconds of every trial, followed by a period (~45 s) of continuous pointing with or without vision of the hand (cursor). During this latter period, the original targets disappeared and a new target appeared at -5 cm, 0 cm, or +5 cm relative to the original target locations. We expected that if participants were able to base their pointing on their actual unseen (and off target) limb position, they would point accurately to the new location. However, if participants remained unaware that their limb position had drifted, their pointing would be reflective of locations defined relative to the original targets. Our results suggest that participants pointed to the new targets based on the perception that there was no drift in limb position.

Acknowledgments: Supported by the Natural Sciences and Engineering Research Council of Canada