On the linearity of accommodation dynamics

Available experimental data on the gain and phase lags in the closed-loop a ccommodation responses to stimuli whose dioptric vergence changes sinusoida lly with time are examined. It is shown that both phase lags and gain chang e almost linearly with the temporal frequency of the stimulus. This implies that the phase lags correspond to a frequency-independent time delay and t hat, if the system is linear, the impulse response should take a symmetrica l (sin x/x)(2) form, with a corresponding standard form of step response. C omparison of this derived step response with the quasi-exponential results found experimentally underlines the limitations of attempting to generalise dynamic accommodation performance from responses obtained with predictable sinusoidally-changing stimuli and the application of a linear model. Consi deration of the frequency response which would lead to an exponential step response supports the argument that, although the gain results obtained wit h sinusoidal stimuli are reasonably representative of those applying with o ther forms of stimulus, the predictable nature of sinusoidally-changing sti muli makes the derived phase lags inapplicable to non-predictable step or o ther stimuli. Other limitations in linear models of the response are discus sed. (C) 2000 Elsevier Science Ltd. All rights reserved.