Failure of vergence size constancy challenges our understanding of visual scale

The closer an object is, the more the eyes have to rotate to fixate on it. This degree of eye rotation (or vergence) is thought to play an essential role in size constancy, the process of perceiving an object as having a constant physical size despite changes in distance. But vergence size constancy has never been tested divorced from confounding cues such as changes in the retinal image. We control for these confounding cues and find no evidence of vergence size constancy. This has three important implications. First, we need a new explanation for binocular vision’s contribution to visual scale. Second, the vergence modulation of neurons in V1 can no longer be responsible for size constancy. Third, given the role attributed to vergence in multisensory integration, multisensory integration appears to be more reliant on cognitive factors than previous thought.