Computation of Object Size in Visual Cortical Area V4 as a Neural Basis for Size Constancy

Even when we view an object from different distances, so that the size of its projection onto the retina varies, we perceive its size to be relatively unchanged. In this perceptual phenomenon known as size constancy, the brain uses both distance and retinal image size to estimate the size of an object. Given that binocular disparity, the small positional difference between the retinal images in the two eyes, is a powerful visual cue for distance, we examined how it affects neuronal tuning to retinal image size in visual cortical area V4 of macaque monkeys. Depending on the imposed binocular disparity of a circular patch embedded in random dot stereograms, most neurons adjusted their preferred size in a manner consistent with size constancy. They preferred larger retinal image sizes when stimuli were stereoscopically presented nearer and preferred smaller retinal image sizes when stimuli were presented farther away. This disparity-dependent shift of preferred image size was not affected by the vergence angle, a cue for the fixation distance, suggesting that different V4 neurons compute object size for different fixation distances rather than that individual neurons adjust the shift based on vergence. This interpretation was supported by a simple circuit model, which could simulate the shift of preferred image size without any information about the fixation distance. We suggest that a population of V4 neurons encodes the actual size of objects, rather than simply the size of their retinal images, and that these neurons thereby contribute to size constancy.