Effect of vergence adaptation and positive fusional vergence training on oculomotor parameters
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Accommodation and vergence, the two important oculomotor systems, exhibit the property of adapation which maintains the response for comfortable prolonged viewing. Several mathematical models have been developed to describe the basic underlying mechanism of accommodation and vergence. Currently used models contradict with each other in the placement of critical elements in the model. This thesis addressed this controversy and empirically verified these models. The effect of vergence adaptation and its influence on certain critical oculomotor parameters have been evaluated in two studies. The specific aims, methods, results and conclusions of each chapter are as follows: Chapter 3 Aim To evaluate the effect of vergence adaptation on convergence accommodation (CA) response and the effect of CA stimulation on accommodative adaptation to determine the model that best fits human accommodation and vergence interaction. Methods This study investigated the effect of vergence adaptation on the convergence accommodation (CA) response as a function of vergence stimulus magnitude and duration in 10 emmetropes. Convergence was induced using no prism, 6, and 12 prism dioptres as stimuli at 0.4m, viewed for 5, 10 & 15 minutes of duration in randomized separate sessions. Phoria measures and CA responses were recorded at the baseline, immediately following prism insertion and following specific durations of viewing through the prism (post-task). Also the effect of CA cross-link on the tonic accommodation (TA) adaptation was investigated where TA response was measured before and after the convergence task. Results & conclusions Repeated measures of ANOVA showed no significant (p > 0.05) phoria adaptation or CA response change with no prism as the stimulus. For 6 and 12 prism dioptres, there were significant reductions (p< 0.01) in CA with phoria adaptation. No significant (p>0.05) phoria adaptation or reduction in the CA response between 5, 10 and 15 minutes of viewing showing no effect of duration. No significant difference (p = 0.85) between the pre and post task TA response while vergence was adapted. The results of the study show that vergence adaptation reduces CA response supporting models which predict the CA crosslink to reduce its output as tonic vergence adaptation progresses. However the convergence accommodation does not appear to lead to increased output of tonic accommodation. Chapter 4 Aim The purpose of this study was to evaluate certain critical parameters of vergence and accommodation under vergence adaptation (induced with a BO Δ), before and after positive fusional vergence training. Methods Eleven emmetropes with normal binocular vision participated in the study. Distance & near phoria, AC/A & CA/C ratios, & positive fusional amplitude at near were evaluated before and after two weeks of positive fusional vergence training. Phoria adaptation and CA responses were monitored every 3 minutes for 15 minutes while the subjects viewed through 12Δ BO under open-looped accommodation at 0.4m before and after training. On a separate vergence adaptation session (before training), phoria adaptation was induced under dual closed-loop condition using 12Δ BO at 0.4m. Cross-link ratios, BO fusional amplitude at 0.4m and near phoria were measured following 15 minutes of prism adaptation. Subjects underwent 2 weeks of positive fusional vergence training using variable tranaglyphs and aperture rule at 0.4m. Phoria adaptation and CA responses monitored over time were exponentially fit and were compared before and after training. AC/A & CA/ C ratios and BO to blur value at 0.4m taken before training, under the vergence adapted state and after training were analyzed using repeated measures ANOVA. Results & conclusions No significant difference (p > 0.05) in the cross-link ratios were found before and after training. However, there was a significant (p < 0.01) increase and decrease in the AC/A and CA/C ratios respectively under the vergence adapted state. BO to blur value at 0.4m was significantly increased (p < 0.01) from the pre training value under both vergence adapted condition and following training. Rate constants and magnitudes of phoria adaptation and CA response reduction were significantly (p < 0.01) different following training demonstrating robust and greater magnitude of vergence adaptation in the BO direction reducing the CA response faster. However, this improved vergence adaptability is not reflected in the static measures of AC/A and CA/C ratios. The increased BO to blur value following training is caused by the increased speed of prism adaptation reducing the CA response during BO fusional amplitude testing.