Error Size Shape Relationships between Motor Variability and Implicit Motor Adaptation

SIMPLE SUMMARY: This study focused on two characteristics of human motor control regarding an acquired skill. First, when performing a movement and generating errors, our movement is corrected based on the errors even if we do not have the intention to correct them. Second, our movement outcomes are always varied even when trying to perform the same movement repeatedly. Although previous studies have examined the relationship between the magnitude of movement correction and motor variability, their findings have been inconsistent. We hypothesized that the impact of motor variability might depend on the error size driving the movement correction and investigated the relationship between different error sizes. Data showed that larger motor variability led to a greater correction of movement when a large, not small, error occurred. The results provide further knowledge regarding the role that motor variability plays in motor learning. ABSTRACT: Previous studies have demonstrated the effects of motor variability on motor adaptation. However, their findings have been inconsistent, suggesting that various factors affect the relationship between motor variability and adaptation. This study focused on the size of errors driving motor adaptation as one of the factors and examined the relationship between different error sizes. Thirty-one healthy young adults participated in a visuomotor task in which they made fast-reaching movements toward a target. Motor variability was measured in the baseline phase when a veridical feedback cursor was presented. In the adaptation phase, the feedback cursor was sometimes not reflected in the hand position and deviated from the target by 0°, 3°, 6°, or 12° counterclockwise or clockwise (i.e., error-clamp feedback). Movements during trials following trials with error-clamp feedback were measured to quantify implicit adaptation. Implicit adaptation was driven by errors presented through error-clamp feedback. Moreover, motor variability significantly correlated with implicit adaptation driven by a 12° error. The results suggested that motor variability accelerates implicit adaptation when a larger error occurs. As such a trend was not observed when smaller errors occurred, the relationship between motor variability and motor adaptation might have been affected by the error size driving implicit adaptation.