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Effect of periodic defocus oscillations on contrast sensitivity


Purpose : To explore the resilience of the visual system to the blurring effect induced by defocus oscillations. This is determined by the loss in contrast sensitivity produced by sinusoidal oscillations in defocus with a range of amplitudes and temporal frequencies. The aim is to compare with natural conditions of accommodation microfluctuations.

Methods : Contrast sensitivity at 12 c/deg was measured monocularly in 5 young healthy emmetropic subjects through a tunable lens introducing sinusoidal oscillations of defocus around 0 D at 5, 15, and 25 Hz. Peak-to-peak oscillation amplitude varied from 0.15 D to 3 D. The visual stimulus was a Gabor patch at 3 m and a two-choice forced-choice protocol was used. Target presentations were randomized through temporal frequency, amplitude, and contrast (5 contrast values around and adjustment threshold determined in advance). The tunable lens was attached to an open-view Hartmann-Shack wave-front sensor (V. P. Ghoushchi et al., Biomed. Opt. Express 12, 3282-3295, 2021) measuring in real time, thus allowing a) verification of the subject’s accommodative state, and b) confirmation of the amplitude and frequency of defocus oscillations generated by the lens.

Results : The figure illustrates the average contrast threshold as a function of amplitude for all subjects. Although there was some variability between subjects, contrast sensitivity was fairly constant despite the defocus oscillations introduced. Only for the fastest oscillations (25 Hz) and highest amplitude (+/-1.5 D) contrast sensitivity appears to suffer a significant degradation.

Conclusions : Our results show that the visual system is resilient to fluctuations in defocus, at least concerning contrast sensitivity tasks. The fact that contrast sensitivity only appears to fall for fast and large oscillations in defocus, suggests that the eye does not average blur and a short time with the image in or around focus is enough to detect the target. These results may have some implications in myopia control and in visual simulators based on oscillating defocus.

This abstract was presented (poster) at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

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