Model of tonic accommodation after sustained near focus

The effect of short-term near focus on tonic accommodation (TA) was previously investigated in 48 visually normal subjects. After sustained focus upon a near target for 10 min, all illumination was extinguished and the decay of accommodation was monitored for 20 min. We calculated from these results that accommodation decayed rapidly with a time constant of about 0.4 min, followed by a slower decay with a time constant of about 32 min. However, the asymptotic level remained slightly above the preadaptation level after 20 min in the dark. A feedback model with two parallel dynamic components (transient and decay), which summed with a "bias" element in the forward loop, was developed to account for the results. The gains of the dynamic elements were varied until the simulation curves matched the experimental adaptation and decay data. During the closed-loop adaptation phase, the outputs of the dynamic elements moved in opposite directions but combined to yield the overall accommodation step response. Moreover, when the model was subsequently open-looped in the dark, the transient component declined rapidly, whereas the decay component remained above the preadaptation level. When the 48 subjects were grouped equally into 4 refractive categories, somewhat different decay time courses could be simulated with changes in model parameter values.