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Denise Valente, Mehdi Azimipour, Robert J Zawadzki, John S Werner, Ravi Jonnal; Investigating the morphology of possible S-cones using adaptive optics functional OCT. Invest. Ophthalmol. Vis. Sci. 2019;60(9):4595. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Histological studies of human retina have shown that S-cones have longer inner segments (IS) than neighbouring L/M cones, and plausibly shorter outer segments (OS). This study's purpose is to compare the functional response of individual cones in vivo with their OS morphology using adaptive optics (AO) - optical coherence tomography (OCT).
A 1.6MHz Fourier-domain mode-locked (FDML) swept-source OCT was used to image the retina, with aberrations corrected by a custom AO subsystem. Volumes subtending 1° were acquired at 30Hz. The high spatial resolution and volume rate permitted morphological changes to be quantified in single cones in response to visible stimuli.Three normal subjects were imaged at 2.5° temporal to the fovea for 6s. At t=2s, a 10ms flash (λ=550 ± 20nm, 80μW) was delivered, bleaching L and M cones equally by 13%; S cones are not responsive at this wavelength. The phase difference between IS/OS and COST was calculated for each cone. Increases in phase difference were attributed to OS elongation (Fig. 1).
In most cones, stimulus-evoked elongation was correlated positively with OS length (Fig. 2).Cones were divided into two categories - those with OS longer or shorter than 24 µm. Cones with longer OSs elongated 0.28µm on average, while cones with short OSs elongated just 0.12µm, with this difference being significant (p<<0.05).
The percentage of cones with short OSs is consistent with the ratio of S to L/M cones in this portion of the retina. The reduced elongation for this group shows a significant relation between OS length and response to middle-wave stimuli. Repeating the experiment with short-wave stimuli (e.g., λ=420 nm) may provide corroborating evidence that S-cone OSs are shorter than M/L cone OSs.With further measurements and statistical analysis this method may provide an important input to correlate cone morphology with its functional type, discriminating accordingly S from M/L cones in vivo.Funding: R00-EY-026068 (Jonnal); R01-EY-024239, P30-EY012576 (Werner); R01-EY-026556 (RJZ)
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
M-scans and OS length change as a function of time for cones with (A) long OS and (B) short OS.
Average OS elongation as a function of OS initial length. Since the population with short OS was significantly smaller (~2%), the standard error of the mean (SEM) is accordingly larger.
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