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Megan L. Meyer, Thomas F. Shipley; Effect of knowledge on apparent motion paths. Journal of Vision 2004;4(8):560. doi: 10.1167/4.8.560.
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In two-frame apparent motion displays the moving object typically moves along the shortest path; when an object does not change orientation a straight path is generally seen. Here we consider the effects of object solidity and observer knowledge on the perceived path of motion for a familiar object, a basketball. We investigated the effect of varying the locations of a basketball that appeared on either side of a barrier wall positioned at the top of a display. When the locations of the basketball were near the top of the display, and the necessary distance for the ball to pass under the barrier wall was long, subjects generally reported the basketball as moving along a straight path that passed through the barrier wall. In contrast, when the locations of the basketball were closest to the bottom of the barrier wall, and a short path could be constructed, subjects saw either curved paths that passed under the barrier wall or angled paths that avoided the barrier wall and appeared to bounce off of the floor. The greatest number of nonlinear paths was reported for the locations of the basketball that were closest to the base of the barrier wall. When these displays were inverted, reports of bouncing paths decreased. When black geometric shapes were substituted for the identifiable objects in the displays, reports of both curved and bouncing paths decreased. The visual system appears to follow a limited solidity constraint such that when objects move they do not pass though one another. An observer's knowledge or expectations about the movement of an object, for example, that basketballs bounce, can influence the perceived path in apparent motion displays. When paths are short, knowledge may allow construction of paths such that one solid object will not pass through another solid object.
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