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John Thomas Pirog, Antoine Barbot, Geunyoung Yoon; Impact of Long-Term Neural Adaptation to Ocular Aberrations on Phase Perception. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2495. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Optical aberrations degrade the amplitude and phase relationships among spatial frequencies (SFs) in the retinal image. Furthermore, long-term exposure to optical aberrations progressively alters visual processing through neural adaptation mechanisms [Sabesan & Yoon, 2009; Barbot & al., 2016]. The goal of this study is to test the hypothesis that neural compensation for phase shifts between SFs is one of the mechanisms underlying this adaptive process.
Three subjects with Keratoconus (KC)–a corneal disease that progressively degrades optical quality in normally-developed visual systems–were tested under full adaptive optics (AO) correction. Subjects reported the appearance of suprathreshold horizontal compound grating stimuli consisting of a fundamental frequency F (6 c/deg) and a secondary frequency 2F (12 c/deg). We manipulated the relative phase shift between the F and 2F components and asked subjects to judge whether the 2F component was phase shifted upwards or downwards relative to the F. Normal subjects (n=2) were tested under full AO correction and under AO-induced vertical coma (±0.33 and ±0.66 µm). Data were fitted with a Weibull psychometric function to estimate the point-of-subjective equality (PSE)–the phase shift at which subjects perceived the 2F to be upwards or downwards 50% of the time.
Under AO-correction, normal eyes accurately perceived the phase shift in the 2F frequency, with a PSE centered on 0° shift (-0.58±2.4°). Under induced vertical coma, normal eyes’ PSEs shifted either leftward or rightward with negative and positive vertical coma (±0.33 µm: ±11±4.5°; ±0.66 µm: ±24.6±2.4°), respectively. This shift was proportional to the magnitudes of induced coma suggesting that normal eyes are sensitive to alterations in phase relationships. Under full AO-correction, however, KC patients showed substantial alterations in perceived phase relationships, ranging from -16.94° to +26.25°, which may be explained by the habitual large-magnitude ocular aberrations.
We developed a method to accurately measure and assess sensitivity to phase shifts between SFs under various optical conditions. Our preliminary data additionally shows that under aberration-free conditions, KC eyes show measurable alterations in phase relationships between SFs. Altogether, our findings seem to suggest a neural compensatory mechanism for phase through long-term exposure to ocular aberrations.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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