Movements of the eyes and hands are coordinated by a common predictive strategy.

Although attempts to intercept a ball in flight are often preceded by predictive gaze behavior, the relationship between the predictive control of gaze and the effector is largely unexplored. The present study was designed to investigate the influence of the spatiotemporal demands of the task on a switch to the predictive control. Ten subjects immersed in a virtual environment attempted to intercept a ball that disappeared for 500 ms of its parabolic approach. The timing of the blank was varied through manipulation of the post-blank duration prior to the ball's arrival, and the shape of the trajectory was manipulated through variation of the pre-blank duration. Results reveal that the gaze movement trajectory during the blank was curvilinear, appropriately scaled to the curvature of the invisible moving ball, and the gaze vector was within 4° of the ball upon reappearance, despite 10° to 13° of ball movement. The timing of the blank did not influence the accuracy of predictive positioning of the paddle at the time of ball reappearance, indicated by the distance of the paddle relative to the ball's eventual passing location. However, analysis of trial-by-trial covariations revealed that, when the gaze vector more accurately predicted the ball's trajectory at reappearance, the paddle was also held closer to the ball's eventual passing location. This suggests that predictive strategies for paddle placement are more strongly mediated by the accuracy of gaze behavior than by the observed range of trajectories, or the timing of the blank.

Assessment of an augmented reality apparatus for the study of visually guided walking and obstacle crossing.

To walk through the cluttered natural environment requires visually guided and anticipatory adjustments to gait in advance of upcoming obstacles. However, scientific investigation of visual contributions to obstacle crossing have historically been limited by the practical issues involved with the repeated presentation of multiple obstacles upon a ground plane. This study evaluates an approach in which the perception of a 3D obstacle is generated from 2D projection onto the ground plane with perspective correction based on the subject's motion-tracked head position. The perception of depth is further reinforced with the use of stereoscopic goggles. To evaluate the validity of this approach, behavior was compared between approaches to two types of obstacles in a blocked design: physical obstacles, and the augmented reality (AR) obstacles projected upon the ground plane. In addition, obstacle height, defined in units of leg length (LL), was varied on each trial (0.15, 0.25, 0.35 LL). Approaches to ended with collision on 0.8% of trials with physical obstacles per subject, and on 1.4% trials with AR obstacles. Collisions were signaled by auditory feedback. Linear changes in the height of both AR and physical obstacles produced linear changes in maximum step height, preserving a constant clearance magnitude across changes in obstacle height. However, for AR obstacles, approach speed was slower, the crossing step peaked higher above the obstacle, and there was greater clearance between the lead toe and the obstacle. These results suggest that subjects were more cautious when approaching and stepping over AR obstacles.