SENSITIVITY ANALYSIS OF RELATIVE ACCOMMODATION AND VERGENCE

A sensitivity analysis was performed to determine the variation in res ponse to changes in parameter values of a previously developed nonline ar static model of accommodation and vergence. To determine normal beh avior, model simulation responses were computed using previously obtai ned parameter values in 4 subjects under 2 conditions. In the first, r elative accommodation was evaluated by maintaining the vergence stimul us constant at 2.5 meter angles (MA) and varying the accommodative sti mulus from -2.5 to 2.5 diopters (D) in 0.25-D steps. In the second, re lative vergence was evaluated by maintaining the accommodative stimulu s constant at 2.5 D and varying the vergence stimulus from 25 prism di opters (PD) base-in to 25 PD base-out in 5-PD steps. Sensitivity of th e model parameters, consisting of controller gains for accommodation ( ACG) and vergence (VCG), crosslink gains for accommodation-to-vergence (AC) and vergence-to-accommodation (CA), deadspace operators for acco mmodation (AE +/- AD) and vergence (VE +/- VD); and the tonic levels f or accommodation (ABIAS) and vergence (VBIAS) were assessed by varying them at 50% and 150% of their normal valued. It was found that the ac commodation and vergence systems were most sensitive to variation in c rosslink gain, moderately sensitive to variation in controller gain an d tonic level, and least sensitive to variation in size of the deadspa ce. These results may provide a quantitative basis for the occurrence of ocular dysfunctions associated with abnormal crosslink gains, such as strabismus, in clinic patients.