Plato's Cave: Motion Phenomena

Puzzling Phenomena in Motion Perception

The visual system exhibits an amazing ability to generate a percept of three-dimensional form from the simplest moving stimuli.

When a circle is displayed on an oscilloscope, and it's width or height are modulated sinusoidally, it produces a strong percept of a circle rotating in depth, although the direction of rotation is unspecified by the stimulus, and thus the percept is apt to reverse its direction of rotation in a bistable manner like a Necker cube.

This phenomenon is not confined to regular geometrical forms, but extends to irregular forms too. For example when an irregular piece of bent wire is rotated in front of a light source, subjects viewing the two-dimensional shadow of the object can perceive its three-dimensional shape as long as it continues to rotate, but perceive only a flat shape when the rotation stops.

These phenomena have been explored extensively with computer simulations which display the two-dimensional projection of a three-dimensional pattern of points rotating about some axis in three-space. Even an irregular cloud of such points, like a star cluster, creates a very compelling percept of three-dimensional space when that cluster is rotated about an axis, but freezes into a two-dimensional pattern as soon as the rotation stops.

Another example known as "biological motion" perception was demonstrated by Johansson [Johansson 1976] who attached small lights at key points on people's bodies, and took movies of them walking about in a dark room, so that only the moving lights were seen.

Subjects viewing the films could immediately recognize the moving points of light as moving human forms, and could even distinguish males from females, or recognize familiar individuals from their gait.

These phenomena have been deeply mysterious to modelers of visual perception, and have never been explained to any satisfaction by any model.

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