Reduction of perceived visual speed during walking: Effect dependent upon stimulus similarity to the visual consequences of locomotion
A.E.I Thurrell1, 3 and A. Pelah2, 3
1 MRC Human Movement and Balance Unit, Institute of Neurology, National Hospital for Neurology and Neurosurgery, Queen Square, London, WC1N 3BG, United Kingdom
2 Department of Electronics, University of York, Heslington, York, YO10 5DD United Kingdom
3 Department of Physiology, University of Cambridge, Downing Street, Cambridge, CB2 3EG, United Kingdom
PURPOSE We reported previously that perceived optic flow speed is reduced linearly with walking speed (Thurrell et al. 1998 Perception 27 pp 147-148), a mechanism we postulate would preserve world constancy during normal self-motion. We consider here whether the reduction also occurs with visual scenes that are progressively less characteristic of upright locomotion.
METHODS Subjects viewed an optic flow reference stimulus comprising either an expanding tunnel of concentric rectangles, horizontally or vertically translating gratings, a rotating spoked wheel or, as a control condition, flashing stationary rectangles, for 15 s while walking at an instructed rate (i.e. very slow, slow... very fast). Subjects then ceased walking and attempted to match the reference optical velocity (or frequency for flashing stimulus) by adjusting a similar visual scene with a hand-held knob. Notionally, human walking corresponds principally to expanding flow, with the translating grating providing a lesser degree of correspondence and the rotating wheel still less.
RESULTS Confirming previous findings, subjects made speed settings that indicated a perceived underestimation of optic flow during walking. A weaker reduction also occurred for the gratings and one weaker still, but not significant, occurred for the rotating wheel. There was no effect of walking velocity on the perceived flashing frequency of the stationary tunnel.
CONCLUSIONS The presence of a weaker effect for the translating grating than for the expanding tunnel (and the absence of one for the rotating wheel) suggests that the influence of walking on the perception of optic flow may depend on the degree of similarity of the visual scene to that experienced during everyday, upright locomotion. The lack of an effect for the flashing tunnel suggests the mechanism acts on velocity perception per se rather than being a time-distance construct.