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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). 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. We conducted a series of five experiments, which all followed the same basic paradigm. Observers (four to six per experiment) 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), while they did the same while standing still in the other interval. Simulated visual standard speed was 1.0, 2.0, or 3.0 m/s. In different experiments, the interval in which the visual standard speed was presented,
the order of the standing and walking intervals and the number of different walking speeds were varied. From the speed judgments, the PSE’s for the visual standard speeds were estimated by fitting psychometric functions. Surprisingly, the PSE’s were hardly affected by walking speed. Consistently in all four experiments, observers reported the perceived visual speed for the lowest standard speed to be lower during walking than during standing still, which would be compatible with a subtraction effect. 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. Taken together, the results question the generality of the subtraction effect and raise doubts
regarding the hypothesized functional role of this effect.
