The Temporal Structure of Visual Perception: Insights From an Illusion of Reversed Motion
Does the visual system process incoming information continuously, or in discrete epochs or "snapshots", like a video camera? To address this question, we made use of the fact that temporal subsampling of a visual stream causes illusory reversals of the motion direction. This “Wagon Wheel Illusion”, most apparent in movies or on television, can also be observed under continuous illumination, suggesting that our visual systems too might sample motion in a sequence of discrete epochs. This phenomenon is bistable by nature, with the actual motion direction generally dominating perception. The illusion disappears when attention is distracted away from the moving stimulus, indicating that it is the attentional capture of motion information that seems to operate discretely. A motion energy model subsampling visual inputs at a rate between 10 and 20 Hz can quantitatively predict the strength of this illusion. EEG recordings confirm that the only visible correlate of illusory motion perception falls into the 10-20Hz frequency range, and is most apparent over right parietal regions, known for their involvement in attending to temporal events. Altogether, the data strongly supports the notion that attention samples motion information in a sequence of discrete snapshots of duration 50-100ms.