A pulse-step model of accommodation dynamics

Abrupt step changes in human ocular accommodation have been traditionally modeled using a continuous feedback control system supplied by a step-position control signal. However, recent behavioral data show that, while the velocity of the step response increases proportionally with response magnitude, the peak acceleration remains constant. This argues against a step input control signal and suggests the existence of a dual-mode control of accommodation: an initial fixed innervation component related to the constant acceleration followed by an innervation component that increases with response amplitude. Specifically, we proposed a pulse-step that provides a velocity-coded input to the system that is integrated to form two position-input signals, that when combined produce high velocity responses. The pulse height controls the acceleration; the pulse width controls the velocity and the step height controls the position of the accommodation response. The pulse-step model simulations were similar to empirical observations and illustrated an enhancement of the peak velocity of accommodation when compared to when the pulse component was removed from the model. The main functional advantage of the pulse is to overcome the high viscosity of the crystalline lens and achieve rapid step responses.