The effect of head tilt on meridional differences in acuity: Implications for orientation constancy

Horn and Hill (1969) and others have reported that a small number of units in the cat visual cortex undergo changes in receptive field orientation associated with body tilt. Such units reportedly compensate for tilt and may represent a mechanism for human orientation constancy. To test this, we measured meridional differences in visual acuity for head-vertical and head-tilted viewing conditions. The results of Experiment 1 did not directly support or refute the involvement of tilt-compensatory units. The results of Experiment 2, in which we controlled for countertorsion of the eyes, showed that meridional acuity differences correspond to the retinal and not the spatial orientation of the stimulus. We conclude that tilt-compensatory cortical units are not involved in human orientation constancy. The physiological evidence indicating the existence of tilt-compensatory units in the visual cortex is also reexamined.