The effects of constraining eye movements on visually evoked steering responses during walking in a virtual environment.

We have previously shown that participants who step in place while viewing a moving scene that simulates walking towards and turning a corner demonstrate anticipatory sequential reorientation of axial body segments with timing characteristics similar to those seen during real turning. We propose that the coordination of axial body segments during steering represents a robust pre-programmed postural synergy triggered by gaze realignment in the desired direction of travel. The primary aim of the current study was to test this hypothesis by studying the effects of constraining eye movement on visually evoked steering responses exhibited by participants stepping in place in a virtual environment. We predicted that preventing participants from generating anticipatory gaze shifts would significantly attenuate or eliminate visually evoked postural responses. A secondary aim was to investigate the nature of the visual cues that trigger the coordinated eye and whole body response by testing whether spatial (distance from the corner) or temporal (time to contact with corner) parameters modulated with the speed of the visual scene (normal, half speed and double speed). Six university graduate student (27.8 +/- 5.0 years) participants were asked to step in place at a self-selected comfortable pace while immersed in a virtual environment which simulated walking down a hallway and turning a corner. In half of the trials participants were required to maintain gaze direction on a static target placed in the middle of the viewing screen. Whole body kinematics and gaze behaviour were recorded. In support of our hypothesis, gaze fixation on a stationary target resulted in the suppression of anticipatory steering responses. Although postural adjustments were still observed during constrained gaze trials, they were reactive rather than anticipatory in nature and were significantly smaller than trials in which gaze was unconstrained. Our results further suggest that the time of eye and body reorientation is dependent on the temporal rather than spatial visual cues, i.e. visually specified estimation time to contact with the virtual corner. These results indicate that gaze redirection is a prerequisite for the initiation of a pre-programmed steering synergy and suggest that these robust postural responses are intimately linked to the oculomotor control processes within the central nervous system.