Object-completion effects in the human lateral occipital complex.

Abstract

The ability of the human visual system to recognize partially occluded objects is a striking feat, which has received extensive psychophysical documentation. Here we studied the manifestation of completion effects in the functional magnetic resonance imaging (fMRI) activation of high-order object areas (the lateral occipital complex - LOC). Subjects were presented with three types of images: (i) whole line drawings of animal or unfamiliar shapes ('whole'); (ii) the same shapes, occluded by parallel stripes which occupied roughly half of the surface area of the images ('grid'); and (iii) the same stripes, 'scrambled' so that the relative position of the regions between the stripes was changed while the local feature structure remained intact. Behavioral measurements showed a high degree of object completion in the 'grid' condition, but not in the 'scrambled' condition. The fMRI results show a significantly higher activation to the 'grid' images compared to the 'scrambled' images. This enhanced activation indicates the operation of non-local completion effects, since the local features in both sets of images were the same. The cortical regions showing the highest 'completion' effects co-localized with regions in the LOC which showed the highest activation to the 'whole' images compared to the 'scrambled' images. Activation in early retinotopic areas was similar in both the 'grid' and the 'scrambled' conditions. Our results point to the LOC as a central site in which object completion effects are manifested.