Select occluder type:
What to observe:
- When there are no occluders, one has the impression of a red square moving back and forth. This is a standard demonstration of apparent motion. In reality, two red squares flash on and off at two different positions: One appears when the other disappears, and vice versa.
- With the real occluders, the impression of a moving red square is lost. Instead, a larger green square is perceived to move back and forth in front of two stationary red squares. In reality, the red squares flash on and off just like before. The green squares flash on and off, too, but in counterphase. That is, whenever the red square is absent, the green square is present.
- With the virtual occluders, the impression of a moving red square is also lost. The gray square that is instead perceived to move is not really there: The only evidence for it in each of the two alternating pictures are the missing quadrants of the black disks (and the disappearance of the red squares). Although the moving gray square is just a contruction created by your brain, it still ruins the impression of a moving red square. This virtual gray square is a so-called Kanizsa figure.
The fact that we experience motion from two stationary objects flashing on and off is sometimes explained by saying that they stimulate an elementary motion detector in much the same way as a moving object. However, this can not explain why the motion of the red square is lost in the conditions with additional occluders. It is particularly difficult to see why an elementary motion detector should react differently to the version with the virtual occluders, because here the stimulus is absolutely identical in the regions close to the red squares. An alternative, more convincing explanation is that the brain seeks an interpretation that explains the events in the stimulus. In the standard condition, the motion interpretation explains the appearance and disappearance of the red square at different positions. In the two conditions with occluders, the motion of the occluders provides an explanation for the disappearance and apparence of the occluders, and at the same time for the disappearance and appearance of the red square. Hence, the interpretation that the red squares are moving is not necessary.
- Sigman, E., & Rock, I. (1974). Stroboscopic movement based on perceptual intelligence. Perception, 3(1), 9-28.
- Ekroll, V., & Borzikowsky, C. (2010). The role of occlusion cues in apparent motion. Perception, 39(12), 1606-1623. doi:10.1068/p6646