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Purpose : What limits vergence abilities? Current models propose that vergence movements are controlled by a phasic component, which responds to image disparities, and a slower tonic component that adapts based on the output of the phasic component. Adaptation in the tonic component frees the phasic component to compensate for further image disparities. Limits of vergence should arise when the tonic component can no longer adapt, but this failure has yet to be observed empirically. We tested the hypothesis that vergence limits would arise when there was evidence of a weakening adaptation in the tonic component.

Methods : We adapted the vergence system of 6 subjects using a Wheatstone stereoscope. Binocular eye position was collected with an Eyelink 1000 eyetracker. Subjects binocularly viewed a detailed image of a natural scene. The image was initially presented at zero disparity, and moved laterally in one eye at a rate of 0.5°/s to reach 4° of eccentricity, where it remained for 5 m. Then the image moved in blocks, where each block comprised 1° of movement followed by 5 m of viewing at the new eccentricity. Blocks repeated until subjects experienced diplopia for 75% of a block. The eyetracker was used to calculate the angle between the eyes optical axes, which we term divergence. Phoria was measured every 30 s by calculating divergence when the image in one eye was replaced with a gray field for 5 s. As a control, subjects also viewed the display with one image moving continuously without the 5 m adaptation periods.

Results : Subjects were able to fuse the images until eye divergence reached 7.14°, on average. We used phoria to estimate adaptation in the tonic component, with larger phorias indicating less adaptation. Adaptation, as measured by phoria, decreased over successive blocks (linear trend, p < 0.01). In addition, subjects reached a significantly greater divergence with adaptation than in the control condition (5.17°, p = 0.02).

Conclusions : Our results support the hypothesis that the amount of vergence normal viewers can produce is limited by adaptation in the tonic component. In earlier blocks, when the tonic component adapted more fully, subjects were able to fuse the images, but in later blocks adaptation of the tonic component lagged behind, and diplopia was experienced. Future work can explore how the tonic component could adapt further, allowing the eyes to experience vergence angles unreachable in normal experience.