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Ethan A. Rossi, Joseph Carroll, Austin Roorda; The relationship between the cone photoreceptor mosaic and visual acuity in normal observers and blue cone monochromat carriers. Journal of Vision 2008;8(17):20. doi: 10.1167/8.17.20.
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Purpose: Determine the relationship between the photoreceptor mosaic and visual acuity (VA) in normal observers and blue cone monochromat (BCM) carriers.
Methods: The adaptive optics scanning laser ophthalmoscope (AOSLO) was used to project an AO-corrected stimulus onto the retina of 9 observers (6 normal; 3 BCM carriers). High contrast photopic letter acuity was measured using a 4AFC tumbling E test at the preferred retinal locus (PRL) and temporal parafovea. Stimuli were presented at 840 or 658 nm with simultaneous photoreceptor imaging at 840 nm.
Results: Cones were well resolved at the PRL for all BCM carriers and two normal observers. BCM carriers had lower cone density and greater cone spacing than normal observers. Voronoi analysis revealed irregular cone packing and cone loss in BCM carriers. Power spectra of mosaic images confirmed irregular packing, with no Yellot's ring at test locations of BCM carriers. BCM carriers performed worse in the VA task than normal observers. VA was better than or matched the Nyquist limit for all normal observers and one BCM carrier at the PRL, but worse than the Nyquist limit beyond the PRL.
Conclusions: VA is not sampling limited (ie. limited by the spacing of rows of photoreceptors) beyond the PRL. Cone spacing closely matches VA outside the PRL, but there is a trend toward underperformance (VA drops faster than spacing). Although these BCM carriers had normal VA tested conventionally, their performance with AO-correction shows that AO-corrected tests may reveal cone loss before such loss is detectable with conventional methods. A large discrepancy between cone spacing and VA in two BCM carriers may indicate a post-receptoral deficit.
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