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P. Thiagarajan, V. Lakshminarayanan, W. R. Bobier; Convergence Training Alters Only the Speed of CA Response Reduction and Not the CA/C Ratio. Invest. Ophthalmol. Vis. Sci. 2008;49(13):132.
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It is well established that the degree to which individuals can comfortably converge their eyes while accommodation is held constant (positive fusional vergence, PFV) can be increased with orthoptics. While this training has been prescribed for sometime to reduce asthenopia, the basic mechanisms of its success are not fully understood. Increased PFV amplitude should require reduced convergence accommodation (CA). We hypothesized that the increased PFV following training could result from a faster adapting slow fusional vergence that reduces CA over time.
11 emmetropes with normal binocular vision served as subjects. Initially, baseline measures of distance & near phorias, stimulus AC/A & CA/C ratios, & near BO fusional range were taken. In addition, to monitor the speed of vergence adaptation, subjects fixated a DoG target at 40cm while the phoria and CA responses were monitored for 15 minutes at every 3rd minute of binocular viewing through 12Δ BO. Following the baseline testing, on a separate ‘vergence adaptation’ session, the cross-link ratios and the near BO fusional range were measured following 15 minutes of prism adaptation to 12Δ BO while fixation was held at a high-contrast target at 40cm. Following two weeks of positive fusional vergence training, the measures taken at the baseline were retested.The measures taken during the baseline, under vergence adaptation and following training were compared.
There was a significant increase (p < 0.01) in the AC/A ratio and a decrease (p<0.01) in the CA/C ratio under the vergence adapted state. Decay functions showed a significant decrease (p<0.01) in the time constant and increase (p<0.01) in the magnitude of phoria adaptation and CA response reduction after the training. No significant (p>0.05) difference in the cross-link ratios were found before and after the training.BO to blur value at 0.4m was significantly increased (p < 0.01) from the pre training value under both vergence adapted state and following the training.
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