Imprecise perception of hand position during early motor adaptation

Localizing ones body parts is important for movement control and motor learning. Recent studies have shown that the precision with which people localize their hand places constraints on motor adaptation. While these studies have assumed that hand localization remains equally precise across learning, we show that precision decreases rapidly during early motor learning. In three experiments, healthy young participants (n=92) repeatedly adapted to a 45{degrees} visuomotor rotation for a cycle of two to four reaches, followed by a cycle of two to four reaches with veridical feedback. Participants either used an aiming strategy that fully compensated for the rotation (experiment 1), or always aimed directly at the target, so that adaptation was implicit (experiment 2). We omitted visual feedback for the last reach of each cycle, after which participants localized their unseen hand. We observed an increase in the variability of angular localization errors when subjects used a strategy to counter the visuomotor rotation (experiment 1). This decrease in precision was less pronounced in the absence of re-aiming (experiment 2), and when subjects knew that they would have to localize their hand on the upcoming trial, and could thus fully allocate attention to hand position (experiment 3). We propose that attention to vision during strategic re-aiming decreases the precision of perceived hand position. We discuss how these dynamics in precision during early motor learning could impact on motor control, and shape the interplay between implicit and strategy-based motor adaptation. NEW & NOTEWORTHYRecent studies indicate that the precision with which people localize their hand limits implicit visuomotor learning. We found that localization precision is not static, but decreases early during learning. This decrease is pronounced when people apply a re-aiming strategy to compensate for a visuomotor perturbation, and diminishes when the hand is fully attended. We propose that these attention-dependent dynamics in position sense during learning may influence how implicit and strategy-based motor adaption interact.