The perception of a briefly presented shape is strongly impaired when it is followed by another pattern, a phenomenon called backward masking. We found that the vast majority of a sample of shape-selective neurons in the macaque inferior temporal cortex respond selectively to backward-masked shapes, although these shapes could not be discriminated by human and monkey subjects. However, this selective response was brief, since it was either interrupted by the mask or overridden by a response to the mask itself We show that reliable discrimination of briefly presented shapes by single neurons depends on the temporal integration of the response. Presentation of the mask, however, reduces the number of spikes available for integration, explaining backward masking. These results also provide direct neurophysiological evidence for the "interruption theory" of backward masking.The detection and identification of a stimulus (target) can be impaired by the presentation of another stimulus (the mask). It is well known that this masking also occurs when the mask is presented after the target (1). Several types of this backward masking exist (reviewed in ref. 2), depending on the kind of stimuli used (light flashes versus patterns), the spatial overlap of target and mask, and the function relating the degree of masking to the temporal interval separating target and mask onset [stimulus onset asynchrony (SOA)]. These counterintuitive effects have given rise to considerable philosophical interest (3), and several theories (see ref.2) have been proposed concerning their origin, ranging from neurophysiological models (4) to more cognitive ones (5).In the present experiments, we studied one type of backward masking in which the targets and the mask were patterned stimuli (shapes), presented at the same retinal location, and in which the target identification decreases monotonically with decreasing SOA. This pattern backward-masking paradigm has become a widely used psychophysical paradigm in the investigation of human visual perceptual processes (e.g., see refs. 6-8) (10)(11)(12)]. Both of these types of masking differ from the one studied here. Masking by mere luminance stimuli is largely monocular, presumably retinal in origin (9). In contrast, the dependence of pattern backward masking on the similarity of target shape and mask (13,14), as well as its interocular transfer (1, 15), suggests a cortical origin. Metacontrast implies that target and mask do not overlap spatially and frequently shows nonmonotonic functions relating SOA and target inter-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. ference (2), unlike the pattern backward masking we investigated.To understand the cortical mechanisms underlying pattern backward masking, we recorded the activity of single inferior temporal cortex (IT) neurons stimulated with briefly presented shapes, followed or no...

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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 monkey 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 occluded by static or moving patterns. The responsiveness, however, decreased with the degree of occlusion. The shape-selective responses to the occluded shapes required longer stimulus durations compared to nonoccluded shapes, matching the better shape discrimination performance at short durations in the no-occlusion compared 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.

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