





Comment: This striking illusion helps us understand the role of the neuronal brain mechanism for size constancy in visual perception. I don’t know what the creators of the illusion would say, but this is how it seems to me. The two images are each composed of two facial expressions, namely * calm * and * angry * . However, in the face on the left the angry features are composed of high spatial-frequency components, and the calm features are composed of low spatial-frequency components. For the face on the right, calm is expressed by high spatial-frequency features and angry is low frequency. If we view the two faces from a distance of 1 foot, the high-frequency features dominate so the left face is angry while the right face is calm. If we view the faces from a distance of 10 feet, the low-frequency features dominate so the left face is calm while the right face is angry. This all happens because the size-constancy mechanism in the visual system expands the brain’s representation of the faces to compensate as their retinal size contracts tenfold when we move back from a 1-foot view to a 10-foot view. As this happens, visual resolution is degraded with the increase in viewing distance, and the low-frequency features become dominant. Hence the switch in facial expression. For a detailed neuronal model of the brain’s putative size-constancy mechanism see * The Cognitive Brain * , pp. 89-93. If there is another explanation for this illusion, I would very much like to see it. —Arnold Trehub