Fujita, I. Tanaka, S.
Interaction between stimulus size and binocular disparity in cortical area V4: Possible neural basis for size constancy.
Size constancy describes our ability to perceive the size of an object relatively unchanged when it is viewed from different distances, which causes the size of its retinal image to change. The retinal image size of an object varies inversely to viewing distance from the observer. The brain compensates this change in retinal image size by using visual cues for depth, and then calibrates the perceived size. One such visual depth cue is binocular disparity. An earlier study by Ungerleider et al. (1977) demonstrated that lesions in the prestriate cortex, but not in the inferior temporal cortex or the pulvinar of monkeys, make size discrimination solely dependent on retinal image size, suggesting that the prestriate cortex contributes to size constancy. We hypothesized that prestriate cortical area V4 may be involved in this perceptual phenomenon, given that many V4 neurons are tuned to size and binocular disparity. We examined the manner in which V4 neurons integrate binocular disparity with retinal image size information, and addressed if they could provide a neural basis for size constancy.
We searched for neurons that encode the size of objects, but not retinal image size, in area V4 of two macaque monkeys (Macaca fuscata and M. mulatta). Hypothetical neurons that uniquely encode the size of objects irrespective of viewing distances should systematically change their preferred retinal image size depending on the binocular disparity. While the monkeys were engaged in a simple fixation task, a disk of binocularly correlated random dot stereograms (RDSs) was presented over the receptive field of V4 neurons. We examined V4 neuron responses to various combinations of binocular disparities and cyclopean image sizes of the disk. The background around the disk was filled with binocularly uncorrelated RDSs, such that changes in binocular disparity or size of the disk did not accompany any change in the monocular images.
In the majority of V4 neurons that were selective for both binocular disparity and cyclopean image size, binocular disparity and size showed significant interactions (P < 0.01, two-way ANOVA). These cells shifted their preferred size towards larger stimulus disks with larger crossed disparities, i.e., disks presented nearer to the monkey. The direction of the shifts was consistent with that expected for size constancy. The magnitude of the shifts varied from cell to cell, suggesting that each of the object-size coding cells contributes to the scaling of the retinal image size at specific physical distances. We suggest that V4 neurons contribute to size constancy by systematically changing their size tuning depending on the viewing distance from the objects.