Shingo Tanaka and Ichiro Fujita
Cyclopean image size tuning in macaque area V4 is dependent on binocular disparity in a manner consistent with size constancy
The perceived size of an object remains relatively stable despite changes in the retinal image size that accompany changes in distance from the observer. This perceptual phenomenon is called size constancy. The brain uses both distance information and retinal image size to estimate the size of an object. To explore the neural basis for size constancy, we examined the interaction between binocular disparity and size information in macaque area V4, a visual cortex known to be involved in stereopsis and size discrimination. We recorded cyclopean image size tunings of 96 V4 neurons in two macaque monkeys performing a fixation task at various binocular disparities. Random dot stereograms were to eliminate monocularcues for distance and size. Eighty-five neurons were found to be selective for both binocular disparity and cyclopean image size. In some cells, tuning curves to cyclopean image size were scaled depending on the binocular disparity while the preferred size became larger as the stimulus was presented nearer. This was consistent with psychophysical tested with the same stimuli in humans. From these results, we suggest that V4 neurons contribute to size constancy.