Amodal representation depends on the object seen before partial occlusion.

Abstract

We demonstrate history-dependent effects in the amodal representation of partially occluded objects. The experience of seeing the fully visible objects before partial occlusion is shown to be influential in the way the objects are represented after occlusion has occurred. Using the method of ambiguous apparent motion correspondence to probe the extent of amodal continuation, bars of variable length were partly occluded by a moving rectangle. After a variable delay period, the part of the bars that remained visible underwent an apparent motion sequence. Subjects reported whether the perceived motion was horizontal or vertical. With a 1-s delay after occlusion, each of the five subjects tested showed a bias favoring motion in the direction of the elongated bars. These results indicate greater amodal continuation in the case of long bars after they have been occluded for an appreciable period of time. A control experiment varying the stereoscopic disparity of the occluder ruled out explanations based on two-dimensional effects, reinforcing the conclusion that the past history has a specific effect on amodal representations. With a delay of 2 s following occlusion, four of six subjects tested showed the history effect, indicating potentially longer durations for the effect, with individual differences in the duration. We conclude that the amodal representation of an object depends on the object seen before partial occlusion.