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Perceiving the present causes illusions

Jul 2008
Hank Hogan

A tenth of a second doesn’t sound like much. However, in that time a jogger will cover more than a foot and a major league fastball will travel 13 times as far. The fact that joggers don’t run into things and that it is possible for a bat to make contact with the ball is a testament to the human visual system. It also helps explain optical illusions, say researchers.

A group from Pasadena-based California Institute of Technology and from the University of Sussex in the UK recently examined the hypothesis of perceiving the present as a justification for visual illusions.

The “perceiving the present” theory explains our ability to hit baseballs, to run across a crowded room or even to snag an object drifting by in the wind.

There is a lag of about a tenth of a second between the time that photons stimulate the retina and the time that an object is perceived. In those ~100 ms, however, a moving object will have traveled to a new location. Because people can successfully intercept moving objects and because they don’t run into things when moving, there have to be mechanisms to compensate for this delay.

Those mechanisms aren’t foolproof, however, and the researchers contend that that is what lies behind optical illusions. Tricks fake out the visual system by, for example, using converging radial lines to create a vanishing point. In the real world, convergence implies forward movement and an outwardly pointing optic smear on the retina, with nearby objects flowing past. In compensating for this, the visual system will make straight lines that are superimposed on converging radial ones appear curved. After all, that is how the straight lines would appear in the next instant if the observer were moving toward the vanishing point.

Invoking these and similar perceiving-the-present mechanisms, the researchers explained more than 50 optical illusions, categorizing these known ones using a 7 × 4 matrix of 28 classes. With predictions derived from the matrix, they even created some new illusions.

Cognitive Science, April 2008, pp. 459-503.

1. The photosensitive membrane on the inside of the human eye. 2. A scanning mechanism in optical character generation.
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