Treadmill locomotion captures visual perception of apparent motion

A visual illusion of perceived motion direction induced by treadmill locomotion is reported. Directionally ambiguous motions of shifting frames of sinusoidal horizontal gratings are perceived moving downward more frequently when the stimuli are shown in front of the observers' feet while walking on a treadmill. To confirm this effect quantitatively, we asked naive observers to answer whether the direction of the motion of the grating pattern was perceived as upward or downward while they were walking or standing on a treadmill. The frequency of the "downward" response was significantly higher under the walking condition. This effect reveals that treadmill locomotion captures perceived direction of ambiguous motion in accordance with the direction of the optic flow during natural walking. This finding suggests that the effect reflects a perceptual mechanism to compensate for the absence of inputs in the action-perception cycle during locomotion.