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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 [1]. In this study, we investigated how general this
subtraction effect is, by varying visual speed, walking speed and the order of the intervals in
which observers walked or stood still. Five observers judged the visual speed of a simulated
ground plane that was presented on a HMD in a 2IFC task. In one interval, they judged the
visual speed while walking in place on a treadmill (0.6, 1.0, or 1.4 m/s), and they did the same
while standing still in the other interval. Simulated visual standard speed, presented during
walking, was 1.0, 2.0, or 3.0 m/s. All observers compared the three visual standard speeds
during the three walking speeds against a range of visual test speeds during standing still and
indicated in which of the two intervals the visual speed appeared to be higher. For three of the
observers the order of the intervals was standing—walking, while it was reversed for the other
two observers. From the speed judgments, the PSE’s in the nine conditions were estimated
by fitting psychometric functions. Surprisingly, the PSE’s were hardly affected by walking
speed. Visual standard speed strongly affected visual speed judgments for the observers who
first stood still and then walked. The lowest standard speed was reported to be perceived as
slower during walking than during standing still, while the opposite was true for the highest
standard speed. When observers first walked and then stood still, this effect did not occur.
Taken together, the results question the generality of the subtraction effect and raise doubts
regarding the hypothesized functional role of this effect.