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Sangeetha Metlapally, Jianliang Tong, Humza Tahir, Clifton Schor; Do Higher Order Aberrations Affect the Magnitude of Odd-error Temporal Stimuli to Accommodation?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4265.
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Fincham proposed that microfluctuations of accommodation produced a temporal hunting cycle for detecting the direction to accommodation (odd-error signal) for steps upto 2.5 D (upper defocus limit). This notion assumes that accommodation is performing contrast discrimination that is affected by spatial frequency (SF), the magnitude of defocus (Green & Campbell, 1965) and higher order aberrations (HOAs), which influence retinal image quality. This study computes the impact of HOAs on signal strength produced by microfluctuations (Weber fraction) for responding to accommodative stimuli within the upper defocus limit.
We presented step-stimuli (1-3 D) for accommodation and disaccommodation, at baseline accommodation of 3 D. The stimulus was a maltese spoke. Wavefront aberrations were measured using a Shack-Hartmann wavefront sensor using a 4 mm artificial pupil. The direction of initial responses was noted and d-prime (d’) analysis used to evaluate the detectability of the correct direction to accommodate. We defined the upper defocus limit for each subject where d’ was 1. The Modulation Transfer Function was derived from Point Spread Functions computed from the wavefronts for 1 D and 2.5 D step-stimuli, at 2 and 4 cycles per degree (cpd), with and without HOAs. Microfluctuations were estimated from the standard deviation of the wavefronts at baseline. Weber fractions were derived from retinal image contrast changes due to microfluctuations.
As step defocus increased, accommodation only had medium (4 cpd) and low (2 cpd) SF available to discriminate contrast changes produced by microfluctuations. At 4 cpd, the Weber fraction is higher but the overall contrast is low, and vice versa at 2 cpd. HOAs generally reduced contrast changes from microfluctuations, and to a greater extent at higher SF or defocus.
Empirical measures demonstrated that microfluctuations of accommodation are potentially useful for defocus values less than 2.5 D, where Weber fractions at 2 cpd and 4 cpd exceed detection thresholds. HOAs perhaps serve to marginally diminish the effectiveness of microfluctuations in odd-error signal detection under these conditions.
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