Matthew P. Cufflin

Static accommodative responses following adaptation to differential levels of blur.

Purpose:  To investigate the effects of two levels of blur adaptation on visual resolution and steady‐state accommodation responses in emmetropes and myopes.

Effect of blur adaptation on human parafoveal vision.

PURPOSE This study was conducted to investigate whether neural compensation for induced defocus can alter visual resolution in other areas of the human retina beyond the fovea. In certain circumstances, the blur adaptation response may be influenced by refractive status. METHODS The effect of blur adaptation on the central 10° of the retina was investigated in 20 normally sighted observers (10 emmetropes and 10 myopes; median age, 21 years). Visual acuity (VA) was measured at the fovea and at five locations of the parafoveal nasal visual field (2°, 4°, 6°, 8°, and 10°) with best corrected distance vision. Myopic defocus of 1 D was introduced, and the same measurements were repeated immediately before and after a 30-minute adaptation. RESULTS VA declined with increasing eccentricity in the clear, blurred, and blur-adapted viewing conditions. The rate of decline was quantified by the parameter E2, which represents the amount of eccentricity dependence of the acuity task. Foveal and parafoveal VA decreased with the introduction of optical defocus and improved significantly after a period of blur adaptation. The consistent value of E2 in each condition indicated that these changes in VA were not eccentricity dependent. Changes in VA under blurred and blur-adapted conditions were of similar magnitudes in myopic and emmetropic observers. CONCLUSIONS Neural adaptation to blur improves VA under defocused conditions in the parafovea as well as the fovea, indicating that the underlying compensatory mechanism acts across a range of spatial scales and independently of retinal eccentricity. Foveal and parafoveal blur adaptation does not vary with refractive error.

The time course of blur adaptation in emmetropes and myopes.

This study examined the effect of myopic defocus on visual acuity (VA) over time, with attention being paid to the first point at which blur adaptation had a significant and measurable effect on defocused VA. Visual acuity was sampled at a higher rate than previous studies in order to assess the time course of blur adaptation processes in myopic and emmetropic observers.

Simultaneous measurement of objective refraction, accommodation response and axial length of the human eye.

Citation information: Alderson A, Mankowska A, Cufflin MP & Mallen EAH. Simultaneous measurement of objective refraction, accommodation response and axial length of the human eye. Ophthalmic Physiol Opt 2011, 31, 100–108. doi: 10.1111/j.1475‐1313.2010.00794.x

Dynamic accommodation responses following adaptation to defocus.

Purpose. Adaptation to defocus is known to influence the subjective sensitivity to blur in both emmetropes and myopes. Blur is a major contributing factor in the closed-loop dynamic accommodation response. Previous investigations have examined the magnitude of the accommodation response following blur adaptation. We have investigated whether a period of blur adaptation influences the dynamic accommodation response to step and sinusoidal changes in target vergence. Method. Eighteen subjects (six emmetropes, six early onset myopes, and six late onset myopes) underwent 30 min of adaptation to 0.00 D (control), +1.00 D or +3.00 D myopic defocus. Following this adaptation period, accommodation responses to a 2.00 D step change and 2.00 D sinusoidal change (0.2 Hz) in target vergence were recorded continuously using an autorefractor. Results. Adaptation to defocus failed to influence accommodation latency times, but did influence response times to a step change in target vergence. Adaptation to both +1.00 and +3.00 D induced significant increases in response times (p = 0.002 and p = 0.012, respectively) and adaptation to +3.00 D increased the change in accommodation response magnitude (p = 0.014) for a 2.00 D step change in demand. Blur adaptation also significantly increased the peak-to-peak phase lag for accommodation responses to a sinusoidally oscillating target, although failed to influence the accommodation gain. These changes in accommodative response were equivalent across all refractive groups. Conclusion. Adaptation to a degraded stimulus causes an increased level of accommodation for dynamic targets moving towards an observer and increases response times and phase lags. It is suggested that the contrast constancy theory may explain these changes in dynamic behavior.

Effect of correction of aberration dynamics on chaos in human ocular accommodation

We used adaptive optics to determine the effect of monochromatic aberration dynamics on the level of chaos in the accommodation control system. Four participants viewed a stationary target while the dynamics of their aberrations were either left uncorrected, defocus was corrected, or all aberrations except defocus were corrected. Chaos theory analysis was used to discern changes in the accommodative microfluctuations. We found a statistically significant reduction in the chaotic nature of the accommodation microfluctuations during correction of defocus, but not when all aberrations except defocus were corrected. The Lyapunov exponent decreased from 0.71 ± 0.07 D/s (baseline) to 0.55 ± 0.03 D/s (correction of defocus fluctuations). As the reduction of chaos in physiological signals is indicative of stress to the system, the results indicate that for the participants included in this study, fluctuations in defocus have a more profound effect than those of the other aberrations. There were no changes in the power spectrum between experimental conditions. Hence chaos theory analysis is a more subtle marker of changes in the accommodation control system and will be of value in the study of myopia onset and progression.

The effect of interrupted defocus on blur adaptation

Blur adaptation occurs when an observer is exposed to continuous defocus. However, it is unclear whether adaptation requires constant defocus, or whether the effect can still be achieved when the adaptation period is interrupted by short periods of clear vision.

Cognitive Demand and Accommodative Microfluctuations

Previous studies have shown cognition to have an influence on accommodation. Temporal variation in the accommodative response occurs during the fixation on a stationary target. This constantly shifting response has been called accommodative micro-fluctuations (AMFs). The aim of this study is to determine the effects of increasing task cognitive demand on the ocular accommodation response. AMFs for 12 myopes and 12 emmetropes were measured under three conditions of varying cognitive demand and comprising reading of numbers (Num), simple arithmetic (SA), and complex arithmetic (CA). Fast Fourier transforms were used to analyze the different frequency band components of the AMFs. Other aspects of AMFs including root mean square accommodation values and chaos analysis was applied. A repeated measures ANOVA revealed a significant main effect of cognition in the mean power of the high frequency component (HFC) (F2,44 = 10.03, p < 0.005). Pairwise analyses revealed that these differences exist between SA and CA tasks (p < 0.005) and the Num and CA (p < 0.005) tasks with the HFC power being the highest for the CA condition. It appears that the difficulty of a task does affect active accommodation but to a lesser extent than other factors affecting accommodation.