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Abstract
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.