Plato's Cave: Motion in the Bubble World

Motion in the Bubble World

The Bubble World model addresses the problem of motion perception in the same manner as it addresses all problems, by reifying all possible interpretations of the sensed phenomenon, and then choosing from that multitude of percepts the one which represents the simplest percept.

Consider a single visible point rotating about an invisible axis, so as to produce the two-dimensional sensation of a point travelling around an ellipse. The first step is to inverse-project this ellipse onto the face of a block of perceptual tissue, as shown below. This will produce a stimulation of motion perception throughout the surface of an elliptical cylinder through the depth of the perceptual block, as suggested below right.

This motion signal is also consistent with a number of alternative cyclic motions around the surface of the elliptical cylinder which are not parallel to the ellipses on the front and rear surfaces of the block, as suggested below (left). Among these multiple possible cyclic motions are two special cycles which happen to form perfect circles in depth, whose two-dimensional projection coincides exactly with the original ellipse, as shown below (right). The visual system seems to have a preference for perceiving rigid rotation, and since these two motions are the only ones consistent with rigid rotation, they constitute the most likely, or simplest three-dimensional perceptual hypothesis of the two-dimensional elliptical rotation.

The special circular symmetry of these two cycles can be best detected and recognized by a cell located at the three-dimensional center of these circles within the volume of the perceptual tissue. I cannot say exactly how such a cell would recognize this circular motion except to suggest some form of harmonic resonance, which might define the center of symmetry of the two rotations.

The above explanation applies not only to full circular motion viewed in perspective, but also to any circular motion through an arc, which will be likewise perceived as a circular rotation about a central axis. If more points are seen rotating about the same axis, the percept of the rotation will become even more pronounced, and if some points are seen to disappear by occlusion behind the rotating body, this will eliminate one of the two alternative rotation axes, resulting in a stable percept.

Although the details this explanation remain somewhat vague, this model does propose certain specific principles of perception:

It will be seen that these are the same general principles seen throughout this approach to perception.

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