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Jonathan Matthis, Karl Muller, Mary Hayhoe; The dynamics of optic flow during natural locomotion. Journal of Vision 2018;18(10):336. doi: 10.1167/18.10.336.
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© ARVO (1962-2015); The Authors (2016-present)
A long history of research on the visual control of locomotion has explored the role of optic flow in the regulation and guidance of human walking, but the optic flow stimulus experienced during natural locomotion has never been recorded. To this end, we used optic flow estimation algorithms to measure head-centered optic flow recorded from the head-mounted camera of a mobile eye tracker. The traditional view of optic flow holds that the Focus of Expansion (FoE) lies in a stable location in the walker's direction of travel. In contrast, our analysis shows that the acceleration patterns of the head during gait cycle cause the FoE to move constantly at very high velocities within the walker's field of. Thus it is unlikely to be useful for controlling heading. In contrast, when we recalculated optic flow in a retinal reference frame, we found flow patterns that were far more regular than those seen in the head-centered reference frame. Thus it seems unnecessary to "correct" for the effects of eye movements on retinal optic flow in order to recover the FoE in the head-centered optic flow, as has generally been thought. Rather, the gaze stabilization reflexes that allow for fixation during locomotion simplify the visual motion patterns on the retina. Fixation nulls motion at the fovea, resulting in regular patterns of outward flow. This regularization should increase a walker's sensitivity to the subtle variation of flow velocity and orientation that specify 3D structure-from-motion information. These results therefore cast doubt on the idea that walkers use the FoE to control heading, but suggest a critical role of visual motion information for the perception of the 3D scene structure and a possible role in the control of posture during locomotion.
Meeting abstract presented at VSS 2018
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