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Abstract:
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.
