Qualitative and Quantitative Analysis of the Cone Mosaic as Oberved by Adaptive Optics

Abstract

Purpose: : To qualify and to analyse quantitatively the cone mosaic in normal subjects using a novel prototype of adaptive-optics (AO) retinal camera.

Methods: : Twenty-five healthy subjects aged 18-45 years were examined by AO retinal camera. The AO instrument was based on a 52-actuator electromagnetic deformable mirror and a 1024-lenslet Shack-Hartmann sensor (both Imagine Eyes, France). A super luminescent diode operating at 750 nm was focused at the retina to probe optical aberrations. A square 4x4 deg area at fundus was flood-illuminated by a pulsed infrared LED and imaged onto a low-noise CCD camera. AO images were taken at the eccentricities from 0 deg to 5 deg temporal from fovea of all subjects. Cone densities of 4x4 deg images at 4 deg were measured using a custom-made software. The variation of the individual cone reflectance over time was measured at 0 and 4 deg.

Results: : There were significant spatiotemporal variations of individual cone reflectance. No temporal or spatial cycle could be evidenced. The variations of the reflectivity of two neighboring cones were inversely correlated to their distance, that suggest a role for optical factors (e.g. blood flow or vitreous) in these variations. Accordingly, short-term time-lapse observation showed that vitreous opacities significantly interfered with cone reflectance. Within the fovea, the presence of reflective spots varying over time suggest that some foveal cones were highly reflective, and hence that circulating red cells were not the sole cause of the variations of cone reflectance. The mean cone density at 4 deg eccentricity was 22481 cones/mm2. On single 4 deg images, a central-peripheral density gradient could be evidenced in all cases.

Conclusions: : Normal cones show significant time and space variations that may be due in part to interference with vitreous and circulating red cells. Such variations have to be taken into account in the determination of cone density. Our data suggests the presence of highly reflective cones in the fovea. The central-peripheral gradient of cone density can be detected in a single 4deg image, which helps for spatial orientation of the image.

Keywords: photoreceptors • retina • imaging/image analysis: non-clinical