Retinal optic flow during natural locomotion

Abstract

We examine the structure of visual motion on the retina during natural locomotion in real world environments. Natural locomotion generates a rhythmic translation and rotation profile of the head in space, which means that visually specified heading varies throughout the gait cycle. This presents a challenge if optic flow is to be used to control heading towards a goal. The complex, phasic head movements that occur through the gait cycle create a highly unstable pattern of flow relative to the head. In contrast, vestibular-ocular-reflex mediated fixation simplifies patterns of optic flow on the retinae, resulting in regular features that may be valuable for the control of locomotion. In particular, the sign and magnitude of foveal curl in retinal flow fields specifies the body’s trajectory relative to the fixation point. In addition, the peak in the divergence of the retinal flow field specifies the walker’s instantaneous overground velocity/momentum vector in retinotopic coordinates. Assuming that walkers can determine the body position relative to fixation, this time-varying retinotopic cue for the body’s momentum could provide a visual control signal for foot placement over complex terrain. In contrast, the temporal variation of heading is large enough to be problematic for use in steering towards a goal. Consideration of optic flow in the context of natural locomotion therefore suggests a re-evaluation of the role of optic flow in the visual control of locomotion.