Abstract
Abstract: :
Purpose:The neural input to the ciliary body has been assumed in dynamic models of accommodation to resemble a step change in optical vergence (Beers & Van Der Heijde 1996). Models predict that velocity of accommodation is reduced as lens viscosity increases with age. However the peak velocity of accommodation to small step stimuli is independent of age (Heron et al 2001). An adaptable pulse–step innervation applied to the ciliary muscle could overcome increased lens viscosity and maintain peak velocity with age. We tested the feasibility of this hypothesis with a dynamic model of accommodation, based on anatomical and physiological correlates, to illustrate how a neural process could slow the progression of presbyopia. Methods: We developed a dynamic feedback control systems model using MATLAB/SIMULINK®. An inverse plant model, representing velocity and position coding near–response cells (burst and tonic cells) in the supra–oculomotor nucleus, produces a pulse and step innervation needed to yield rapid step changes in lens power. The E–W nucleus and ciliary body transduce this innervation to force with a phasic–tonic controller. The muscle force acts on the passive plant to change the combined lengths of the serially arranged agonist (anterior zonule, lens capsule and matrix) and antagonist (choroid, posterior ciliary body and zonule) components. Age related changes of elastic modulus and time constants, for the passive agonists, antagonists and the ciliary muscle, were taken from the literature. The pulse height and width were varied to simulate age related changes of acceleration and velocity attributes of accommodation that we measured in humans (Bharadwaj & Schor 2003). Reduced amplitude of accommodation with age was modeled as increased minimum lens curvature resulting from lens growth. Results: We analyzed position, velocity and acceleration profiles of model simulations for small and larger step responses. The increased viscosity of the lens with age was overcome by increasing the pulse height to restore acceleration and the pulse width to restore velocity of small step responses. The lag of accommodation that results from the increased elastic modulus of passive components of the accommodation plant was reduced by increasing the gain of the phasic response of the ciliary muscle. Simulated slopes of peak velocity versus response magnitude (main sequence) and total acceleration duration versus response magnitude were in good agreement with empirical results. Conclusions: Model simulations demonstrate that an adaptable pulse–step innervation would shorten the response time and increase the velocity of small–step responses as the accommodative system ages.
Keywords: aging: visual performance • ciliary muscle • ocular motor control