Linear model of accommodation and vergence can account for discrepancies between AC/A measures using the fixation disparity and phoria methods.

The AC/A ratios obtained previously by the fixation disparity method were in most cases greater than those by the phoria method. An earlier study analysed a simplified linear model of the accommodation and vergence system. It was determined that the equations corresponding to the two methods were equal, indicating that such a model could not account for the discrepancy found experimentally. However, two important components--tonic accommodation and tonic vergence--were omitted from this simplified model. In the present study, an analysis of the complete linear model showed that the equations corresponding to the two methods are different. In addition, calculation of the error between the cross-link gain and the overall accommodative input--convergence output values indicated that the AC/A should be higher using the fixation disparity than the phoria method. Therefore, the predictions of the complete linear model are consistent with the data obtained experimentally.