Contextual influences in amodal completion: Evidence from magnetic brain responses

Abstract

In the process of perceiving partly occluded objects as whole objects, so-called amodal completion, there are two influences that can lead to qualitatively different sorts of completions. Global influences originate from the overall figural aspects of the shape, while local influences originate from parts of the shape. Here we measured the influence of these contextual aspects of partly occluded shapes using MEG recordings. Subjects were presented with a partly occluded shape, and after 1 s the occluder was removed, revealing a physical (possible) completion. There were two sorts of partly occluded shapes: one in which global and local influences result in different perceived completions, so-called ambiguous shapes, and another in which local and global influences result in the same perceived completion, so-called convergent shapes. We found an early left-occipital component, which was sensitive to both the figural aspects of the physical completion and the congruence between the physical completion and the perceived completion. In contrast, an early right-occipital component and a later anterior temporal component were sensitive only to the congruence between the physical completion and the perceived completion. We conclude that the left early component may reflect the figural aspects of the physical completion, modulated by the contextual aspects, whereas the early right-occipital component and the later component mainly reflect the violation of the expectancy of the perceived completion that was built up by the context.