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Abstract

Partial occlusion of objects is common in natural environments, and in most cases does not affect the identification of the occluded object. However, under certain conditions identification fails. The distinction between the two cases lies in which object parts remain visible through the occluder. We have recently shown for Rhesus monkeys performing a discrimination task on sets of natural images that the occlusion of specific image regions systematically influences behavioral performance. Here, we report on how these differences between image regions influence the responses of area IT neurons to partially occluded images.
For four natural images, we determined three partially occluded variants that were reliably identified by the monkeys, and three variants that the monkeys failed to identify correctly. Images and their occluded variants were specific for each monkey observer. The three variants of each category differed in the image portion visible through the occluder (10, 30 or 50). We recorded from 341 IT neurons in two monkeys. 102 neurons were selected because their preferred non-occluded image evoked responses significantly different from baseline (p<.01). To assess the influences of occlusion category for these neurons, we compared the responses to all six occluded variants of the preferred image using a two-way ANOVA (factors occlusion category, stimulus size). 33 neurons reliably distinguished between the occlusion categories at least at one size of visible image portion (p<.01). In 27 of the 33 neurons, firing rates were elevated when identifiable image portions were shown to the monkeys.
These findings indicate that image portions that monkeys preferentially relied on to guide their behavior had a greater impact on the response of IT neurons than image portions that were determined to be less relevant for behavior.