Phoria, Hering's laws, and monocular perception of direction.

By recording eye movements while subjects fixated a stationary stimulus and reported on its apparent position, four experiments tested whether Hering's laws of visual direction describe monocular perception of egocentric direction. Experiments 1, 2, and 3 alternately covered each eye. Experiments 1 and 2 showed that the stimulus appeared to displace leftward when the left eye was occluded, and vice versa. The apparent displacement was correlated with lateral heterophoria in the occluded eye, and phoria was necessary for the illusory displacement. As a descriptive model, these results expand the range covered by Hering's laws, but they leave open several process models, including ones based on retinal image displacements without reference to extraretinal information. Experiment 4 eliminated retinal image displacements and found apparent target displacements that imply processing of extraretinal information.