Selectivity of Macaque Inferior Temporal Neurons for Partially Occluded Shapes

Humans are able to recognize shapes even when they are partially occluded by another pattern. We determined whether inferior temporal (IT) units of awake rhesus mon-key remain selective for shapes that are partially occluded by other patterns. In order to link the shape selectivity to shape discrimination, we established that the monkey, like humans, can discriminate partially occluded shapes. We found that IT units are selective for shapes partially oc-cluded by static or moving patterns. The responsiveness, however, decreased with the degree of occlusion. The shape-selective responses to the occluded shapes re-quired longer stimulus durations compared to nonocclud-ed shapes, matching the better shape discrimination performance at short durations in the no-occlusion com-pared to the occlusion conditions. Occluder visibility had no effect for shape outlines, indicating that (amodal) shape completion, which is absent when the occluder is invisible, is not necessary for shape selectivity. These data indicate that IT neurons can, indeed, contribute to the invariance of shape perception for changes in retinal input caused by partial occlusion of the shape. [Key words: inferior temporal cortex, macaque monkey, shape perception, shape selectivity, shape occlusion, com-pletion, visual cortex] Single neurons in the inferior temporal (IT) cortex of the ma-caque monkey respond selectively to patterns differing in shape (Gross et al., 1972; Desimone et al., 1984; Tanaka et al., 1991). This shape selectivity is thought to underlie shape perception and recognition. Indeed, the properties of shape selectivity in-vestigated so far match characteristics of higher shape percep-tion. Shape recognition is relatively position (Biederman and Cooper, 1991), size (Biederman and Cooper, 1992), and visual cue invariant (Regan, 1991; Regan and Hamstra, 1991), and the shape selectivity of IT neurons generally is also invariant with changes in the position (Gross and Mishkin, 1977; Schwartz e