PURPOSE. Previous studies reported that vergence fluctuates systematically during voluntary head movement with a peak-to-peak amplitude of approximately 0.5 deg. Despite these considerable vergence errors, stereopsis was found to be relatively unaffected during head motion. The purpose of this study was to investigate vergence fluctuations during head motion, using two different psychophysical approaches.
METHOD. Estimates of instantaneous horizontal fixation disparity (FD) were obtained from the perceived horizontal separation between two polarized vertical nonius lines, presented simultaneously. The lines were flashed for 20 ms at random times during voluntary horizontal head motion (24 deg p-p; 1.0 and 1.5 Hz), synchronised to a metronome. In a second experiment, small blue flashes were presented separately to the two eyes on a yellow BACKGROUND: The flashes were above (LE) or below (RE) a binocular fixation target. Perimetric profiles indicated the horizontal location of the foveal blue-blind scotoma in each eye during 4 different phases of head movement. Comparison of the locations of the left- and right-eye scotomas provided estimates of the vergence changes during 0.5, 1.0, and 1.5 Hz head motion. Two authors and one naive subject participated.
RESULTS. During voluntary head movements, FD data indicate peak-to-peak changes in vergence eye position of 5 min arc or less for the 3 subjects. Estimated vergence changes from the relative locations of the foveal blue-blind scotomas are more variable, but indicate peak-to-peak amplitudes between 5 and 20 min arc in 2 subjects. No consistent phase relationship exists between the vergence change and head velocity across temporal frequencies of head motion. Stereothresholds determined for vertical line targets increase by about 40% during voluntary head motion, but are not related systematically to the temporal frequency.
CONCLUSIONS. Psychophysical estimates of the amplitude of vergence change during voluntary head movement are systematically smaller than literature values based on objective eye movement recordings. Nevertheless, the resulting disjunctive eye motion is sufficient to impair stereopsis.