The direction of walking--but not throwing or kicking--is adapted by optic flow

Abstract

Optic flow is known to adapt the direction of walking, but the locus of adaptation remains unknown. The effect could be due to realignment of anatomical eye, head, trunk, and leg coordinate frames or to recalibration of a functional mapping from the visual direction of the target to the direction of locomotion. We tested whether adaptation of walking to a target, with optic flow displaced by 10 degrees , transfers to facing, throwing, and kicking a ball to the target. A negative aftereffect for initial walking direction failed to transfer to head orientation or throwing or kicking direction. Thus, participants effectively threw or kicked the ball to the target, and then walked in another direction to retrieve it. These findings are consistent with recalibration of a task-specific visuo-locomotor mapping, revealing a functional level of organization in perception and action.

Fig. 1

Adaptation to displaced optic flow and the aftereffect in subsequent walking direction among participants asked to walk toward a target in a textured virtual environment (adaptation) and in an otherwise dark field environment (posttest).The illustration shows a plan view of the mean walking paths on the first (solid blue curve) and last (dashed red curve) of 38 adaptation trials and the first posttest trial (solid black curve).The corresponding vectors starting at the origin of the graph represent the initial walking direction. The path and vector data are taken from Bruggeman, Zosh, and Warren (2007).

Fig. 2

Results for the adaptation phase: (a) plan view of mean walking paths and (b) mean virtual heading error as a function of walked distance. Results for the first adaptation trial are shown in the solid blue curves, and average results for the last three adaptation trials are shown in the dashed red curves. In (a), the dotted black curve corresponds to the prediction of the egocentric-direction strategy, and the y-axis corresponds to the prediction of the optic-flow strategy. In (b), the dotted black line corresponds to the egocentric-direction prediction of a 10° heading error, and the x-axis corresponds to the optic-flow prediction of a 0° error.

Fig. 3

Direction error on each trial. Results are shown separately for initial walking direction and for kicking, throwing, and facing directions. Participants experienced normal optic flow in the pretest and posttest trials, and displaced (all data converted to +10°) optic flow during adaptation. The solid curve represents the exponential fit of the decay in heading error during adaptation, Y(t), with t = the number of the adaptation trial.

Fig. 4

Shift in direction error between pretest and posttest as a function of task. The mean shift in direction error from pretest to the first posttest trial is a measure of the negative aftereffect. Error bars indicate I SE, based on between-subjects variance.