Movement speed effects on limb position drift

Abstract

Previous research has shown that even when limb position drifts considerably during continuous blind performance, the topological and metrical properties of generated hand paths remain remarkably invariant. We tested two possible accounts of this intriguing effect. According to one hypothesis, position drift is due to degradation of limb-position information. This hypothesis predicted that drift of static hand positions at movement reversals should not depend on movement speed. According to the other hypothesis, position drift is due to degradation of movement information. This hypothesis predicted that drift of static hand positions at movement reversals should vary with movement speed. We tested these two hypotheses by varying the required movement speed when normal human adults performed back-and-forth manual positioning movements in the absence of visual feedback. Movement distance and direction were well preserved even though hand positions between movements drifted considerably. In accord with the movement error hypothesis, but not in accord with the position hypothesis, the rate at which hand positions drifted depended on movement speed. The data are consistent with the idea that hand position, which defines the origin of the trajectory control coordinate system, and movement trajectory are controlled by distinct neural mechanisms.

Fig. 1

A, B The experimental setup. A A back-projection screen was suspended above a one-way mirror that was in turn suspended above a glass-covered table surface. This arrangement provided the impression that the display was in the same depth plane as the table surface. B The forearm was secured to a custom-made air-jet sled. Flock of Bird sensors (open squares) were fixed to the sled and the upper arm

Fig. 2

A, B Kinematic analysis. A Calculation of cumulative (dashed lines) and instantaneous drift (solid lines). Cumulative drift was defined as the Euclidean distance between the start location adopted on the initial trial and each successive start location. Instantaneous drift was defined as the Euclidean distance between each start location and the previously-adopted start location. B Calculation of mean drift slope and plateau level

Fig. 3

A series of 70 hand paths produced by one participant in each speed condition and movement direction. The start position and target are indicated by open and closed circles, respectively. Progression of trials over time is represented by the gray shade of the path, where early trials are darker than late trials. Trials with fingertip position feedback are drawn in black

Fig. 4

A, B Mean fingertip velocity and instantaneous hand position drift. A Mean velocity (m/s) as a function of speed condition (slow, medium, fast). B Mean instantaneous drift (m) as a function of speed condition. Instantaneous drift was greater for the high-speed condition than for the low-speed condition. Error bars represent the standard error of the mean

Fig. 5

A, B Cumulative drift analysis. A Mean cumulative hand drift (m) as a function of speed (fast, slow) and movement number. This plot shows that drift accumulated more quickly in the fast condition than in the slow condition. B Left panel shows mean cumulative drift slope (mm/movement) as a function of speed condition. Slope was greater in the fast condition than in the slow condition. Right panel shows mean plateau (m) as a function of speed condition. There was no significant difference between speed conditions. Error bars represent the standard error of the mean

Fig. 6

Instantaneous drift analysis. Mean instantaneous drift perpendicular and parallel to the axis of movement (m) as a function of speed condition (slow, fast). Error bars represent the standard error of the mean. Perpendicular drift was greater in the fast condition

Fig. 7

A, B Drift relative to the axes of movement. A Mean cumulative drift perpendicular to the axis of movement (m) as a function of speed condition (slow, fast) and movement number. Perpendicular drift accumulated more quickly under the fast movement condition than under the slow movement condition. Left inset: mean drift slope (mm/movement) is greater in the fast condition than in the slow condition. Right inset: drift plateau (m) does not vary with speed condition. B Mean cumulative drift parallel to the axis of movement (m) as a function of speed condition and movement number. Parallel drift accumulation did not vary with speed condition. Neither mean drift slope (left inset) nor drift plateau (right inset) varied with speed condition. Error bars represent the standard error of the mean