Gibson’s most detailed analysis of the KSD problem came from work on the perception of dynamic occlusion (viewing one surface become progressively hidden behind another as they move; Gibson, Kaplan, Reynolds & Wheeler, 1969; Kaplan, 1969). As one surface goes behind another, the sensations coming from the rear surface stop hitting the retina; they disappear. However, was is perceived is the progressive occlusion of a persisting surface; it is not disappearing, it is going out of view. Gibson and his students identified the kinematic pattern of transformation of the optic array that was specific to occlusion and distinguished it from the pattern specific to a surface actually going out of existence. In the former case, optical texture from the rear surface is progressively deleted over time from the optic array at an edge as it goes in behind the closer surface, and that texture progressively accretes as it comes back into view. In the latter case, there are a variety of transformations depending on how the surface is disappearing (melting vs being eaten, etc). Each event creates a specific optical pattern, but these patterns are not identical to the underlying dynamics. Observers, however, readily and easily perceive and report the underlying dynamics, not the optical patterns. Additional evidence that people are perceiving the dynamics comes from work in multiple object tracking (Scholl & Pylyshyn, 1999). People can track multiple moving targets over time, and can continue to do so even if the objects move in and out of view, but only if they do so in an occlusion event. If the objects go out of view by imploding, tracking goes to chance. In the occlusion case, the visual attention system continues to perceive a persisting object and can often pick it back up when it returns to view. In the imploding case, this system perceives that the object has ceased to exist, and it no longer tracks it.
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