Noncongruent stimuli and tonic adaptation.

The purpose of this investigation was to determine the influence of noncongruent stimuli on tonic accommodation and tonic vergence. Eight visually normal subjects were tested in a within-subjects design. Tonic states were measured using a haploscope-optometer immediately before and after viewing a reduced Snellen chart binocularly with suppression control in a synoptophore for 8 min. One stimulus was set at either 4 D or 4 MA, with the other set at either the maximally high (e.g., greater than 4 D or 4 MA) or low setting (e.g., less than 4 D or 4 MA) that still permitted clear and single binocular vision. Significant changes in tonic vergence but not tonic accommodation were found. The results suggest that when the oculomotor system is challenged, or stressed, under such noncongruent binocular viewing conditions, it is tonic vergence and not tonic accommodation which adapts rapidly.