Facilitation of vergence changes by saccades: influences of misfocused images and of disparity stimuli in man.

When confronted with randomly presented targets demanding change in mean visual direction (version of 4.5-7 deg) as well as a change in vergence (1.5-2.5 deg) both naive and experienced subjects can make short-latency saccades which differ markedly and appropriately in the excursion of the two eyes, and which thereby achieve a large fraction of the required change in vergence. Mean values for the portion of vergence change occurring during the saccades, over all target positions, ranged from 41 to 70% for the five subjects tested, but subject-specific directional biases in performance were also conspicuous. When such targets were presented randomly during monocular viewing, so that only accommodation cues could induce vergence change, a consistent fraction (13-48%) of the resulting vergence movement also occurred during the saccades. These data indicate that on average about one-quarter of the intra-saccadic vergence change achieved during binocular viewing could have been due to accommodation stimuli perceived by one eye. The monocular results indicate that the direction of image misfocus (nearer vs. farther), which guides accommodation-vergence, can be correctly distinguished for targets which are about 6 deg from the centre of the fovea, and that this assessment can be made within the 200-300 ms latency for saccadic eye movements, and hence without trial-and-error refocusing. The binocular inequality of saccades during monocular viewing involved conspicuous subject-specific right-left asymmetries, as well as consistently positive within-subject correlations between intra-saccadic vergence change and finally realized accommodation-vergence movement, and consistent correlations between phoria before the saccade and intra-saccadic vergence change.