Limb position drift: implications for control of posture and movement

Abstract

In the absence of visual feedback, subject reports of hand location tend to drift over time. Such drift has been attributed to a gradual reduction in the usefulness of proprioception to signal limb position. If this account is correct, drift should degrade the accuracy of movement distance and direction over a series of movements made without visual feedback. To test this hypothesis, we asked participants to perform six series of 75 repetitive movements from a visible start location to a visible target, in time with a regular, audible tone. Fingertip position feedback was given by a cursor during the first five trials in the series. Feedback was then removed, and participants were to continue on pace for the next 70 trials. Movements were made in two directions (30 degrees and 120 degrees ) from each of three start locations (initial shoulder angles of 30 degrees, 40 degrees, 50 degrees, and initial elbow angles of 90 degrees ). Over the 70 trials, the start location of each movement drifted, on average, 8 cm away from the initial start location. This drift varied systematically with movement direction, indicating that drift is related to movement production. However, despite these dramatic changes in hand position and joint configuration, movement distance and direction remained relatively constant. Inverse dynamics analysis revealed that movement preservation was accompanied by substantial modification of joint muscle torque. These results suggest that proprioception continues to be a reliable source of limb position information after prolonged time without vision, but that this information is used differently for maintaining limb position and for specifying movement trajectory.

FIG. 1.

The experimental setup. A: a back projection screen was suspended above a 1-way mirror 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: forearm was secured to a custom-made air-jet sled. Flock of Bird sensors (shown as open squares) were fixed to the sled and the upper arm.

FIG. 2.

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.

FIG. 3.

Series of 70 hand paths produced by 1 participant (S5) in each start location and movement direction. Top: 30° movement paths. Bottom: 120° movement paths. In each plot, the prescribed start location is shown as an open circle, and the target location is shown as a closed circle. Progression of trials over time is represented by the gray shade of the path, where early trials are darker than late trials. Movements with visual feedback about fingertip position are drawn in black.

FIG. 4.

Mean instantaneous and cumulative hand position drift. A: mean instantaneous drift (m) as a function of trial number, start location, and movement direction. Error bars represent inter-subject SE. Shaded area represents the portion of the block during which drift accumulated rapidly. B: mean instantaneous drift collapsed across trial number and start location. Mean instantaneous drift was greater for 120° than for 30° movements. C: mean cumulative drift as a function of trial number, start location, and movement direction. Error bars represent inter-subject SE. Drift accumulated rapidly over the 1st 40 trials and then reached a plateau. Shaded area in both A and C highlights the rapid accumulation portion of the block. D: drift accumulation rate for 30° and 120° movements. Early bars reflect drift accumulation rates between trials 6 and 40, and late bars reflect drift accumulation rates between trials 41 and 70. Early drift accumulation rate was greater for 120° movements than for 30° movements.

FIG. 5.

Vectors representing mean drift distance and direction for each participant. Outer scale shows the relative positions of the 3 start locations. Inner scale reflects the distances of the drift vectors. These drift vectors show that, although participants did not drift toward their body, there was no one direction toward which drift was directed. Top right: mean within-trial drift direction variance, as a function of trial number, start location, and movement direction. This plot indicates that participants drifted consistently after vision was removed.

FIG. 6.

A: paths produced by plotting the series of movement start positions adopted by 1 participant (S4) in each condition. For clarity, we plotted start positions from vision-absent trials only and only every 3rd start location is plotted. Initial starting position is represented by crosshairs (+). B: instantaneous drift direction variance as a function of movement number. Only vision absent trials were included in this analysis.

FIG. 7.

Hand movement distance (A) and direction (B) as a function of trial number, start location, and movement direction. Results are shown for the trials over which drift accumulated most rapidly. Error bars represent inter-subject SE.

FIG. 8.

A: representative hand paths from a single participant (S3). Start location and target are indicated by an open and closed circle, respectively. The early-block hand path is the 1st no-feedback movement, and the late-block path is one performed after the participant had reached the drift plateau. B: comparisons of early and late cumulative drift, movement distance and direction, and peak velocity. The hand drifted significantly but movement distance, movement direction, and peak velocity were preserved. Error bars represent inter-movement SE. C: shoulder (left) and elbow (right) muscle torque profiles during the 1st 250 ms of the same early- and late-block movements. Values greater and less than 0 reflect flexor and extensor muscle torque, respectively. D: comparisons of early- and late-block shoulder (left) and elbow (right) muscle torque impulse. Error bars represent inter-movement SE.

FIG. 9.

A: simulation results for representative 120° (left) and 30° (right) movement paths. Observed early (black) and late (gray) paths are shown next to paths predicted by the simulation (light gray). Predicted paths represent the movement path that would have been performed had participants not modified muscle torque profiles as the hand drifted. B: observed early (black) and late (gray) shoulder and elbow muscle torques for the 120° (left) and 30° (right) movements depicted above. Simulated movements show the movement path that would have been performed if the early muscle torque profiles had been used at the late movement start location.

FIG. 10.

A: mean angular excursion at each joint for 120° movements. B: mean angular excursion at each joint for 30° movements. C: mean angular drift at each joint for 120° movements. D: mean angular drift at each joint for 30° movements. In all panels, error bars represent inter-subject SE.