The Site of Orientational Constancy

The visual world remains perceptually quite stable during rotation of the head. There is some physiological evidence that in the cat this phenomenon of orientational constancy might result from changes in the orientation selectivity of neurones in the visual cortex during head tilt. In humans adaptation to a grating causes elevation of the threshold contrast for detecting gratings of similar orientation, and it can be argued that this aftereffect depends on neurones in the human visual cortex. If the head is tilted during such adaptation, but is vertical when the threshold is tested with a vertical grating, the maximum aftereffect occurs for an adapting grating that was vertical on retinal coordinates. We conclude that in humans the mechanism for orientational constancy occurs after the visual cortex.