QUANTITATIVE-ANALYSIS OF THE ACCOMMODATIVE CONVERGENCE TO ACCOMMODATION RATIO - LINEAR AND NONLINEAR STATIC MODELS

Ogle and his colleagues proposed two measures of oculomotor linkage ca lled the accommodative convergence to accommodation (AC/A) ratio, This ratio provided a clinically useful assessment of the drive of accommo dation, or focusing system, on vergence, or binocular fixation system, The phoria method measured the relatively large deviation in eye alig nment under the monocular condition, whereas the fixation disparity me thod measured the relatively small misalignment of the eyes under the binocular condition to obtain the AC/A ratio, Ogle et al, indicated th at these two measures should be equal, However, experimental results s howed a substantial difference between the AC/A ratios obtained by the two methods, To quantitatively assess the difference between the two methods, a linear static model was first evaluated, This model was bas ed on an earlier successful model of the accommodation and vergence sy stem, The linear-model solution showed that these two methods were equ ivalent and thus could not account for the differences found, Then, a nonlinear static model, containing the deadspace operators depth of fi eld and Panum's fusional area (PFA), was evaluated, Since two solution s were possible for each deadspace operator, there were four basic sol utions, However, there were two binocular-viewing paradigms, This resu lted in four prism-viewing and four lens-viewing solutions, Finally, a difference was taken between the prism- and lens-viewing measures, gi ving a total combination of 16 solutions, Only four of these solutions were equal to that using the phoria method, Some of the other solutio n lines were widely separated, thus providing a range of possible data values across different solution lines, Calculations showed that the variation in AC/A ratio values for data across difference solution lin es was comparable to that found experimentally, Thus, the deadspace op erators in the nonlinear model were able to account for the VE discrep ancy between the AC/A ratio determined by the phoria VR and fixation d isparity methods.