Proximal contribution to a linear static model of accommodation and vergence.

To determine the influence of target proximity on accommodation and vergence under both open- (OL) and closed-loop (CL) viewing conditions, a static interactive feedback model, which included proximal accommodation (PA) and proximal vergence (PV) inputs, was developed and analysed quantitatively. It was based on an earlier static dual-interactive feedback model of accommodation and vergence. The proximal inputs were added to both the accommodative and vergence loops at the output of the respective controllers. The values of the PA and PV gains were obtained from experimental dual-OL data. The model equations were analysed over a stimulus range of 1 to 6 D (or MA). It was found that under the dual-OL condition, the contribution of PA to the overall accommodative output ranged from 42.5 to 81.6%, whereas the contribution of PV to the overall vergence output ranged from 56.1 to 88.5%. In contrast, under all other stimulus conditions, with the exception of the PA contribution to the accommodative output under the Aol, Vcl condition (26.5-41.0%), the relative contributions were much smaller, ranging from 0.04 to approximately 7.0%. In particular, under the dual-CL condition, representing the normal binocular visual feedback condition, the relative contributions were only 4.0 and 0.04% for PA and PV, respectively. Thus, although the relative contributions of PA and PV were large under the dual-OL condition, they were generally very small under the various CL conditions that simulated more naturalistic viewing situations. Nevertheless, proximity may still play an important role by providing cues for attaining coordinated and harmonious motor responses under specific viewing conditions.