Influence of sustained vergence and accommodative convergence on disparity detection

The ability to detect absolute disparity can be assessed by studying the depth perception or vergence eye movement responses to briefly flashed (200 msec) disparity stimuli. It has been previously shown that sustained convergence influences the vergence dynamics to subsequently flashed convergent disparities leading to an increase in peak velocity of these movements when vergence stimuli were presented from the sustained vergence position. Another factor that has been suggested to have an influence on divergence dynamics, to step disparity targets, is accommodation. Changes in vergence dynamics could have resulted from modifications at the level of the disparity detectors, vergence burst cells or at the level of the extraocular muscles. Two experiments were conducted to test the hypothesis that sustained vergence and accommodative convergence could have a potential influence on the response behavior of disparity detectors, which results in changes to perceived depth from convergent and divergent horizontal disparities.;Experiment 1. Experiment 1 was performed to study the effect of sustained vergence on depth perception from horizontal disparity targets. Twenty subjects were recruited. Depth perception to briefly flashed (200 msec) convergent, divergent or zero disparity targets were assessed before and after a period of sustained vergence either at a +4 degree, -4 degree or a 0 degree demand with respect to a fixation distance of 50 cm. The flashed disparity step targets for depth perception were presented with respect to the fixation target. The same sequence of disparity steps were tested before and after sustained vergence. Repeated measures ANOVA was performed on data from 19 out of the 20 subjects. Results indicate that the ability to detect depth from convergent disparities was significantly influence by sustained vergence demand (p=0.04). Depth perception from convergent disparities was increased following sustained divergence and decreased following sustained convergence. There was no significant effect of sustained vergence on depth perception from the divergent or zero disparity step targets. Phoria adaptation following sustained vergence was not found to have a statistically significant effect on depth perception from convergent or divergent disparity targets (p=0.08).;Experiment 2. Twenty one subjects were recruited for the study. Subjects fixated on target at different fixation distances (45, 60 and 100 cm) for 6 sec, following which briefly flashed (200 msec) disparity targets were presented either with a convergent, divergent or zero demand with respect to the fixation. Target for fixation was either an accommodative target (letter X) or a Gaussian blob target presented in a randomized order. Repeated measures ANOVA was used to analyze the data. Results showed that the accuracy of depth perception decreased with closer fixation distances (p < 0.001) for divergent disparity targets. There was no significant effect of fixation distance on depth perception from convergent disparity targets. Accommodation did not influence the accuracy of depth perception either in the convergent or divergent directions.;The results obtained for depth perception from crossed disparities is congruent with the results obtained previously for vergence dynamics to crossed disparities following sustained vergence. An increase in the effort required to sustain a convergence posture leads to improvements in convergent disparity detection. Even though previous work has shown that influence of sustained vergence on divergence dynamics, depth perception results for divergent disparity were not in agreement with those results. Accommodation did not have a significant influence on depth perception from large horizontal disparity stimulus. Thus it was concluded that there is a common pool of disparity detectors for depth perception and vergence eye movements at least for the crossed disparities whose response characteristics are modified following sustained vergence.