The role of vision in maintaining heading direction: effects of changing gaze and optic flow on human gait.
How is heading direction maintained in human gait? This question was investigated with respect to the role of optic flow and in the context of different movement strategies. While walking on a treadmill the deviation from the ideal straight path was measured in terms of lateral sway induced by a lateral gaze shift (by looking at a moving visual target). The role of the focus of expansion (FOE) within a radially expanding optic flow pattern was investigated by varying its relative velocity of expansion from 0- to 4-fold (the equivalent of walking speed), thus increasing the perceptibility of FOE. If FOE was a relevant cue for maintaining heading direction, a reduction of lateral sway amplitude was expected with increasing flow velocity. The presence of a radially expanding flow pattern did not reduce lateral sway. Lateral sway was least when the visual background remained stable without any flow pattern. Increasing the velocity of the flow pattern resulted in an increase in lateral sway. If the relative velocity of the flow pattern was raised beyond that corresponding to walking speed, lateral sway amplitude approached the maximal values observed in the dark. In all experiments, sway amplitude increased linearly with the increasing excursion of the visual target. Different strategies to perform the gaze shift (eye or head turns) only resulted in minor differences in lateral sway amplitude. The results show that gaze shifts during locomotion induce lateral sway, which depends upon the presence, and characteristics, of background optic flow. Under the present conditions, the FOE within the flow field seems not to be a dominant cue to control heading. However, the systematic increase in lateral sway induced by high flow velocities indicates that motion parallax has an effect on heading during locomotion.