Understanding accommodative control in the clinic: modeling latency and amplitude for uncorrected refractive error, presbyopia and cycloplegia

Abstract

Accommodation refers to the process of increasing the optical power of the eyes crystalline lens so as to focus on objects at different distances. Refractive error is a mismatch between the physical size of the eye and its optical power when accommodation is fully relaxed, while functional presbyopia is a decrease in the range of accommodation with age that results in the near point of accommodation lying beyond the near working distance. Both conditions mean that sharp focus may not be achievable for some distances, and observers will experience sustained defocus. A familiar example is an older person struggling to read text on their phone. Here, we identify a problem with current models of the neural control of accommodation: they predict excessive internal responses to stimuli outside the range of accommodation, leading to unrealistic adaptation effects. Specifically, after a prolonged period of viewing stimuli outside their accommodative range, current models predict that observers would show long latencies in the accommodative response to stimuli within range. These latencies are not observed empirically, indicating a problem with current models. We propose a simple solution, exploiting the predictive nature of accommodative control, and demonstrate that the new model performs correctly. We also model cycloplegia as a change in gain, and include a lower bound on the neural signal driving accommodation so as to model the additional relaxation of accommodation often seen with cycloplegia. We show that with these modifications, we can obtain plausible predictions for the accommodative response and accommodative convergence signal in a wide range of clinically relevant situations such as functional presbyopia, fogging lenses, corrected and uncorrected refractive error, and cycloplegia.