Cortical responses to contextual influences in amodal completion.

Amodal completion refers to the process in the visual system that enables us to perceive partly occluded objects as whole objects. Both the overall shape of a visual object (global aspect) and the region immediately surrounding the occluder (local aspect) are known to determine the process of completion. We investigated the influence of overall shape context in completion on human brain activity using MEG recordings. Subjects were presented with two different types of shapes that were partly occluded by a rectangle. In the so-called convergent shapes, the local and global contexts trigger the same completion, and in the so-called divergent shapes, local and global contexts trigger different completions. The occluder was removed after 1 s, revealing the whole (completed) shape where expectancies based on the local and global context could either be fulfilled or violated. We found an early (from 200 ms on) left occipital component, which was predominantly sensitive to the figural aspects of the uncovered continuation, but was also modulated by the congruency between the physical shape and the perceived shape (based on the contextual cues). In contrast, an early right occipital component and a later (400 ms) anterior temporal component were sensitive only to the congruency between the physical and the perceived shape, showing the relevance of the global context in amodal completion.