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W. R. Bobier, V. Sreenivasan, E. L. Irving, V. Lakshminarayanan; Model Simulations Predicting Vergence Adaptations To Near Adds. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3826.
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Previously, we reported dynamic changes in accommodation and vergence from their normal baseline measures when young adults1 viewed a high contrast target at 33cm through a near add (+2D). Phoria and accommodation (binocular viewing) showed a similar pattern, - an initial spike in exophoria and accommodation, which reduced significantly over the first 3 minutes. However, when accommodation was measured with vergence open loop (monocular viewing) the response remained steady over 20 minutes. Convergence accommodation (CA) was assumed to account for the differences in binocular and monocular measures. We sought to determine if these patterns of change in phoria and accommodation could be predicted by two current oculomotor models2,3 that have adapting elements.
The two oculomotor models2,3 differ in how they define adaptation and in how they predict the effect of adaptation on the reciprocal cross-links between vergence and accommodation (CA and AC). Both models were simulated (Matlab Simulink, Mathworks, USA) using parameters described in these models2,3 except for AC, CA, ABIAS and VBIAS which were obtained from our empirical measures. The stimulus values for accommodation and vergence were set at 1D and 4MA respectively, in order to simulate the reduction of accommodation and the increase in convergence induced by viewing a 33cm target through +2D lens. Simulations were run for 3 time points (0-180, 181-360, and 361-540 secs) in order to match the critical time points resulting from the empirical data.
Both models showed good agreement with empirical measures of vergence adaptation within 180sec. One model2 closely predicted the experimental time course for the reduction in binocular accommodation. This was achieved where the reduced output of the phasic controller following vergence adaptation led to a reduction in CA crosslink output. The other model3 did not predict any significant change in binocular accommodation as its vergence adapting element did not act to reduce CA.
Our empirical data was best described by Schor’s dynamic model wherein the wiring, arranges a vergence adapting element that reduces phasic output which in turn reduces crosslink output.(1) Sreenivasan, V. et al, Vision Res 2008;48;1262-69(2) Schor, C. M. Ophthal. Physiol. Opt. 1999.; 19; 134-150(3) Hung,G. K. Ophthal Physiol Opt, 1992; 12; 319-326
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