Abstract
When fixating on a stationary object, the power of the eye’s lens fluctuates. Studies have suggested that changes in these so-called microfluctuations in accommodation may be a factor in the onset and progression of short-sightedness. Like many physiological signals, the fluctuations in the power of the lens exhibit chaotic behaviour. A breakdown or reduction in chaos in physiological systems indicates stress to the system or pathology. The purpose of this study was to determine whether the chaos in fluctuations of the power of the lens changes with refractive error, i.e. how short-sighted a subject is, and/or accommodative demand, i.e. the effective distance of the object that is being viewed. Six emmetropes (EMMs, non-short-sighted), six early-onset myopes (EOMs, onset of short-sightedness before the age of 15), and six late-onset myopes (LOMs, onset of short-sightedness after the age of 15) took part in the study. Accommodative microfluctuations were measured at 22 Hz using an SRW-5000 autorefractor at accommodative demands of 1 D (dioptres), 2 D, and 3 D. Chaos theory analysis was used to determine the embedding lag, embedding dimension, limit of predictability, and Lyapunov exponent. Topological transitivity was also tested for. For comparison, the power spectrum and standard deviation were calculated for each time record. The EMMs had a statistically significant higher Lyapunov exponent than the LOMs (0.64pm 0.33 vs. 0.39pm 0.20~hbox {D}/hbox {s}) and a lower embedding dimension than the LOMs (3.28pm 0.46 vs. 3.67pm 0.49). There was insufficient evidence (non-significant p value) of a difference between EOMs and EMMs or EOMs and LOMs. The majority of time records were topologically transitive. There was insufficient evidence of accommodative demand having an effect. Power spectrum analysis and assessment of the standard deviation of the fluctuations failed to discern differences based on refractive error. Chaos differences in accommodation microfluctuations indicate that the control system for LOMs is under stress in comparison to EMMs. Chaos theory analysis is a more sensitive marker of changes in accommodation microfluctuations than traditional analysis methods.
Highlights
Accommodation is the change in the effective power of the eye’s lens to bring an object into focus on the retina
A mixed ANOVA looking at demand as “within-subjects effect”, with refractive error group as “between-subjects effect”, was calculated for the Lyapunov exponent (LE), embedding lag, embedding dimension, and limit of predictability
The analysis showed a statistically significant effect of refractive error on both the LE ( p = 0.038, F = 4.084) and embedding dimension ( p = 0.014, F = 5.735)
Summary
Accommodation is the change in the effective power of the eye’s lens to bring an object into focus on the retina. It is well known that even when fixating on a stationary object of interest, the eye exhibits microfluctuations in accommodation These fluctuations are a few tenths of a dioptre in magnitude and change at a rate of several Hertz (Charman and Heron 1988, 2015). It has been found that the magnitude of the fluctuations is correlated with the objective depth of focus (Yao et al 2010), and that when the depth of focus is increased, such as by a decrease in pupil size, there is a concomitant increase in the magnitude of the fluctuations (Yao et al 2010; Gray et al 1993a, b; Stark and Atchison 1997; Niwa and Tokoro 1998; Gambra et al 2009) This increase is often attributed to an increase in the magnitude of the low-frequency component (LFC) (
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
