Evaluation of retinal orientation and gaze direction in the perception of the vertical.

The orientation of the median plane of the eye with respect to the head varies with gaze direction according to Listing's Law. The subjective vertical (SV), however, is known to be only partially affected by these involuntary variations of eye orientation. In order to learn more about the compensatory process underlying this finding, six normal-sighted young subjects were tested monocularly and binocularly in eight directions of gaze. The results show that: (1) the SVs, determined monocularly for both eyes, fall on corresponding retinal meridians, the binocular SV-settings generally lie between the monocular ones; (2) the tilt of the SV is not linearly related to the tilt angle of the median plane of the eye, as hitherto assumed. On theoretical considerations, the dependence of ocular tilt on gaze direction may be decomposed into three components, which are all treated differently in the compensation process. We interpret these results in the following way: the SV is determined from the sensorially fused image of both eyes and the tilts of the eyes are accounted for by an extra-retinal signal which is common to both eyes. The characteristics of the compensation mechanism may be explained by an extra-retinal signal which relies on information about gaze direction and Listing's Law. Such a signal might be derived from an efference copy of gaze direction commands.