Sensitivity analysis of the stimulus-response function of a static nonlinear accommodation model.

The effect of parameter variation of a nonlinear static feedback control model of the accommodation system was investigated. Simulations of a MATLAB/SIMULINK model showed a nonlinear relationship between stimulus and response in which the response curve was above the 1:1 line in the region to the left of the crossover and below the 1:1 line in the region to the right of the crossover. At the crossover, the response curve exhibited an inflection that was constant and equal to the tonic accommodation value (ABIAS). Sensitivity analysis showed that increasing depth of focus (deadspace range between +/- DSP) increased the separation between the boundary lines of the deadspace region, with a larger separation associated with late-onset myopia and congenital nystagmus. Increasing accommodative controller gain (ACG) increased the slope of the function on either side of the deadspace, with lower ACG values corresponding to lower slopes that indicated an amblyopic deficit. Increasing ABIAS increased the accommodative level at the inflection region. In addition, the saturation level of the accommodative response decreased with increasing age, while the slope remained the same, which was consistent with the Hess-Gullstrand theory of presbyopia. The accuracy and relative simplicity of the model indicated that it could serve as a basis for further comprehensive investigation of the basic and clinical aspects of the accommodation system.