Abstract
Purpose::
For many individuals, after a period of sustained vergence, the eyes return very slowly to the phoria (fusion-free) position upon dissociation. This phenomenon, termed phoria adaptation (PA), results from a build-up of neural innervation to the extra-ocular muscles. The neurological underpinnings of PA are not well understood. It is not clear whether sustained fusion is necessary to develop PA, or whether other forms of vergence (eg. accommodative convergence) can result in PA. The purpose of this experiment is to determine whether sustained accommodative convergence can inducePA.
Methods::
20 subjects, 21 to 33 yrs of age, were enrolled in the study. Subjects had normal binocular vision, 14 were exophoric (0.5 Δ to 10 Δ), 4 were esophoric (0.5 Δ to 1.5 Δ) and 2 were orthophoric. Dissociated phoria was measured using the modified Thorington technique at 40cm. Subject’s were on a chin rest and their phoria was measured before (pre-phoria) and after (post-phoria) adapting to either a 6 Δ BO or a negative lens for 5 minutes, on different days. Negative lens power was calculated based on the individual’s stimulus AC/A ratio in order to equate the amount of convergence to 6 Δ. Subjects fixated on a word target at 40cm during the adaptation time. While adapting through the negative lens, subject’s left eye was patched in order to purely elicit accommodative vergence.
Results::
The amount of adaptation was calculated by subtracting the pre from the post phoria measurement. Mean adaptation to the 6 Δ BO stimulus was 2.15 ± 1.18 Δ, and to the negative lens was 1.55 ± 1.88 Δ. Student’s t-test was used to analyse the data. PA either with prism or lens was significantly different from 0 (p<0.0001) but there was no significant difference in the magnitude of adaptation due to prism or negative lens (p=0.243).
Conclusions::
PA was induced with both the fused target and the monocular accommodative target. The mean amplitude of PA was similar in the two conditions. Therefore, accommodative convergence can induce PA, suggesting a common neural site for the production of PA resulting from different forms of convergence.
Keywords: vergence • eye movements