Distribution of mid-peripheral cones in emmetropic and myopic subjects using adaptive optics flood illumination camera.

PURPOSE: We measured in vivo cone photoreceptors up to 24° of eccentricity along the horizontal meridian of healthy human retina. We also investigated the impact on cone densities of axial eye length elongation occurring with myopia. METHODS: Using a flood illumination device coupled with an adaptive optics system, rtx1™, ( www.imagine-eyes.com), 55 right healthy retinas were imaged along the horizontal (i.e. nasal and temporal) meridian over a 48° field (i.e. from 3° to 24° each 3°). Then, cones were manually detected within 80 × 80 pixel regions of interest. Cone density and packing geometry (i.e. number of neighbours) were calculated (AOdetect software™). Subjects were divided into three groups: a group of 36 emmetropic (i.e. refractive error from -0.25D to +0.50D) subjects; a group of 10 low myopic subjects (i.e. refractive error from -0.50D to -2.50D); and a group of nine high myopic subjects (i.e. >-2.50D). RESULTS: Cone density decreased with eccentricity in both semi-meridians. The decrease in cone photoreceptors occurred mainly in the first 9°. The difference of cone density between the nasal and temporal semi-meridian increased with eccentricity from 0.6% at 3° to 26% at 24°. Average cone density of emmetropes (850 cones deg-2 or 11 087 cones mm-2 ), low myopes (830 cones deg-2 or 9731 cones mm-2 ), and high myopes (912 cones deg-2 or 9744 cones mm-2 ), suggested that the retinas of the high myopic subjects were more stretched than the low myopic subjects retinas and even more stretched than that of the emmetropes. The axial eyeball elongation (square of the ratio of the axial eye length of 9%) seems to explain the cone density (11%) difference between emmetropes and low myopes. However, while the eyeball elongation between low and high myopes is still important (i.e. 11%), cone density difference between both populations was negligible (i.e. 3%). The ratio of cone density varied from -17% to 22% as a function of eccentricity involving that the retinal stretching is not uniform along the horizontal meridian. CONCLUSION: The difference of cone density (i.e. cone mm-2 ) between groups supports the hypothesis that the retina is stretched with the eyeball elongation. However, this elongation does not seem to be uniform along the horizontal meridian favouring the hypothesis of a local elongation of the retina.

Distribution of cone density, spacing and arrangement in adult healthy retinas with adaptive optics flood illumination.

The aim of this article is to analyse cone density, spacing and arrangement using an adaptive optics flood illumination retina camera (rtx1™) on a healthy population. Cone density, cone spacing and packing arrangements were measured on the right retinas of 109 subjects at 2°, 3°, 4°, 5° and 6° of eccentricity along 4 meridians. The effects of eccentricity, meridian, axial length, spherical equivalent, gender and age were evaluated. Cone density decreased on average from 28 884 ± 3 692 cones/mm2, at 2° of eccentricity, to 15 843 ± 1 598 cones/mm2 at 6°. A strong inter-individual variation, especially at 2°, was observed. No important difference of cone density was observed between the nasal and temporal meridians or between the superior and inferior meridians. However, the horizontal and vertical meridians differed by around 14% (T-test, p<0.0001). Cone density, expressed in units of area, decreased as a function of axial length (r2 = 0.60), but remained constant (r2 = 0.05) when cone density is expressed in terms of visual angle supporting the hypothesis that the retina is stretched during the elongation of the eyeball. Gender did not modify the cone distribution. Cone density was slightly modified by age but only at 2°. The older group showed a smaller density (7%). Cone spacing increased from 6,49 ± 0,42 µm to 8,72 ± 0,45 µm respectively between 2° and 6° of eccentricity. The mosaic of the retina is mainly triangularly arranged (i.e. cells with 5 to 7 neighbors) from 2° to 6°. Around half of the cells had 6 neighbors.

Visual resolution and cone spacing in the nasal and inferior retina.

PURPOSE: To determine the retinal eccentricity at which cones are no longer an observable substitute for ganglion cells on nasal and inferior parafoveal visual acuity. METHOD: Visual acuities were measured on 12 healthy volunteers, under dynamic adaptive optic aberrations correction (crx1™) in white light, from 0° to 6°, every two degrees, along the nasal and inferior retinal meridians. Cone spacing was measured on images of the retina acquired using an adaptive optic flood illumination retina camera (rtx1™) at the same eccentricity, except at 0°. RESULTS: Cone spacing increased by around 0.13 min of arc per degree of eccentricity and a difference of 7% between both meridians was observed (higher cone spacing in the inferior retinal meridian). Visual resolution was higher in the nasal retinal meridian (difference of around 28% or 0.15 logMAR at 6°). Cone spacing can predict minimum angle of resolution (MAR) at 2° in both semi retinal meridians. In the inferior retinal meridian, MAR measurements are fairly well predicted by Watson's 50% mathematical model based on the midget retinal ganglion cell density. Along the nasal retinal meridian, the measured MAR lies between Watson's 50% and 100% models. CONCLUSIONS: At 2° of eccentricity, cone density accurately predicts visual resolution in both the nasal and inferior retina, supporting the idea that only 50% of the foveal midget retinal ganglion cells determine VA. The 50% model can also predict VA in the inferior retinal meridian at 4° and 6° of eccentricity. However, the 50% model underestimated visual acuity in the nasal retinal meridian at 4° and 6° of eccentricity consistent with the partially overlapping ON and OFF midget retinal ganglion cell receptive fields.

Effect of age, decentration, aberrations and pupil size on subjective image quality with concentric bifocal optics.

PURPOSE: We investigated the impact of lens centration, wearer aberrations, pupil size and age on the optics of two bifocal contact lenses using image simulation. METHOD: Fourteen conditions (i.e. two optical profiles with two and eight concentric zones; two conditions of centration: centred and 0.77 mm decentred; and three conditions of aberrations: 0, 0.15 and 0.35 µm RMS; three pupil sizes: 3, 4.5 and 6 mm) were tested on two populations (i.e. 20-40 and 40-60 years old) using a numerical simulation method. For each condition, images were calculated for proximities ranging from -4D to + 2D with steps of 0.25D. Subjects graded the quality of each simulated image (i.e. a target 'HEV' of 0.4 logMAR) on a continuous scale from 0 to 5. To limit the effect of the observer's own aberrations, subjects viewed the displayed images through a 3-mm pupil and their optimal correction. RESULTS: Both populations reported similar image quality (i.e. average absolute difference of 0.23) except for sharp and low contrast images, which obtained slightly higher grades with younger subjects, probably due to a better contrast sensitivity in this population. Typical decentration had no effect on bifocal contact lenses wearers' vision, as the ratio between areas dedicated to near and distance vision did not change. Aberrations (i.e. mainly 0.24 µm of spherical aberration on a 4.5-mm pupil) reduced the addition of the two radial zones bifocal optics and introduced a hyperopic shift (i.e. 0.50D) of the through-focus image quality for the eight radial zone bifocal lens. The combination of typical aberrations with typical decentration created the same effect as typical aberrations alone, meaning that aberration impact was stronger than decentration impact. The two radial zone bifocal lens was dependent on the pupil whereas the eight radial zone lens was not. CONCLUSIONS: When fitting new bifocal optics, the aberrations of the patients, as well as their pupil diameter, are the main subject dependent parameters influencing quality of vision. Typical contact lens decentration and lower cortical treatment efficiency of retinal images of older subjects had relatively little impact.