Fixation disparity, accommodation, dark vergence and dark focus during inclined gaze.

We investigated several oculomotor functions at different angles of vertical inclination of the gaze direction from 15 deg upwards to 45 deg downwards. The mean accommodative resting state (measured in a dark visual field) increased when the eyes or the head were declined from 0 to 45 deg. Fixation disparity (the vergence error in minutes of arc relative to the principle visual directions) became more eso when a fusion target at a viewing distance of 40 cm was lowered: declining the gaze by 45 deg changed mean fixation disparity by 1.8 min arc with eye inclination (keeping the head upright), and by 0.9 min arc with head inclination (with eye position unchanged relative to the head). When the eyes were lowered, the individual rate of eso change in fixation disparity was correlated with the amount of the subjects' near shifts in the resting position of vergence, measured in darkness. Significant test-retest correlations between repeated measurements showed that the effects of eye inclination on vergence varied in a reproducible way among individuals with good binocular vision.