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Abstract

Perceived visual speed has been reported to be reduced during walking compared to standing still. This so-called ‘subtraction effect’ has been attributed to an automatic subtraction of part of the walking speed from the visual speed (Durgin et al., 2005). We investigated how general this subtraction effect is, by varying both visual speed and walking speed in a series of experiments. Observers judged the visual speed of a simulated ground plane (presented through a HMD) in a 2IFC task. In one interval, they walked in place on a treadmill, in the other they stood still. In different experiments, the interval with the visual standard speed, the order of the intervals, and the walking speed were varied. In all experiments, observers consistently reported the perceived visual speed for the lowest standard speed to be lower during walking than during standing still. However, most observers also perceived the highest standard speed as faster during walking than during standing still, which is clearly incompatible with the subtraction effect. We tested the apparent interaction between visual speed and walking in another experiment by presenting the exact same visual speed in the two intervals and asking the observers again to judge which of the two appeared to be faster. As in the previous experiments, the visual speed was reported to be faster during standing for slow visual speeds; this gradually changed into the opposite for faster visual speeds. Taken together, the results question the generality of the subtraction effect and raise doubts regarding the hypothesized functional role of this effect.