Orientation constancy in neurons of monkey visual cortex

We studied the effect of body tilt on the orientation selectivity of single neurons in the visual cortex of an alert monkey. The monkey performed a vi sual fixation task either in the upright position or with its whole body ti lted about the naso-occipital (roll) axis by +/- 25 degrees or +/- 30 degre es. We determined the preferred stimulus orientation for 51 of 117 neurons in two or, if possible, three body positions (i.e. with the whole body upri ght, and tilted either left ear or right ear down). In striate cortex, most of the neurons were of a non-compensatory type, showing a change in the pr eferred orientation according to the body tilt and the estimated counter-ro lling of the eye. By contrast, about 40% of the neurons in prestriate corte x were of a compensatory type, preferring similar orientations in all body positions. This suggests that mechanisms which produce orientation constanc y with respect to the direction of gravity are implemented at an early stag e of cortical processing.