Perceived visual speed has been reported to be reduced during walking. This effect has been attributed to subtraction of part of the walking speed from the visual speed (Durgin et al., 2005; 2007). Previously, we have shown that this subtraction only occurs in a restricted range of low visual speeds. For higher speeds, visual speed is judged to be faster during walking than during standing (Souman et al., 2008). Here, we tested whether the effect of walking on perceived visual speed depends on the functional relationship between the two (Exp. 1). We measured visual speed perception for contracting optic flow during forwards walking, in effect reversing the direction of the optic flow relative to the walking direction. Participants compared the visual speed of a ground plane, presented through an HMD, in two intervals: walking or standing. The speed match for three standard speeds (1, 2, 3 m/s) of contracting optic flow was determined in a constant stimuli paradigm. Similar to expanding optic flow, walking caused slow visual speeds to appear slower and fast visual speeds to appear faster relative to standing still. In a second experiment, with expanding optic flow, we measured visual speed discrimination performance to test whether the visual system still has access to the retinal speed signal during walking. In half the trials, walking speed was equal in both intervals (homogeneous), while it differed in the other half (heterogeneous). If the visual system has direct access to the retinal speed, differences in walking speed should not affect discrimination performance. However, we found that discrimination thresholds were significantly higher in the heterogeneous trials, suggesting that the retinal speed signal is not available for speed discrimination.


Durgin, F.H., Gigone, K., & Scott, R. (2005) JEP:HPP 31:339-353
Durgin, F.H., & Gigone, K. (2007) Perception 36:1465-1475
Souman, J. L., Frissen, I, & Ernst, M.O. (2008) Journal of Vision, 8:1146