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.

Although many of our actions are triggered by sensory events, almost nothing is known about our perception of the timing of those sensory events. Here we show that, when people react to a sudden visual stimulus that triggers an action, that stimulus is perceived to occur later than an identical stimulus that does not trigger an action. In our experiments, participants fixated the center of a clock face with a rotating second hand. When the clock changed color, they were required to make a motor response and then to report the position of the second hand at the moment the clock changed color. In Experiment 1, in which participants made a target-directed saccade, the color change was perceived to occur 59 ms later than when they maintained fixation. In Experiment 2, in which we used a go/no-go paradigm, this temporal distortion was observed even when participants were required to cancel a prepared saccade. Finally, in Experiment 3, the same distortion in perceived time was observed for both go and no-go trials in a manual task in which no eye movements were required. These results suggest that, when a visual stimulus triggers an action, it is perceived to occur significantly later than an identical stimulus unrelated to action. Moreover, this temporal distortion appears to be related not to the execution of the action (or its effect) but rather to the programming of the action. In short, there seems to be a temporal binding between a triggering event and the triggered action.

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