The application of nonconfocal split-detector to emerging human gene therapy trials will improve the potential of therapeutic success, by identifying patients with sufficient retained photoreceptor structure to benefit the most from intervention. Additionally, split-detector imaging may be useful for studies of other retinal degenerations such as AMD, retinitis pigmentosa, and choroideremia where the outer segment is lost before the remainder of the photoreceptor cell.

Using high-resolution adaptive-optics imaging combined with retinal densitometry, we characterized the arrangement of short-(S), middle-(M), and long-(L) wavelength-sensitive cones in eight human foveal mosaics. As suggested by previous studies, we found males with normal color vision that varied in the ratio of L to M cones (from 1.1:1 to 16.5:1). We also found a protan carrier with an even more extreme L:M ratio (0.37:1). All subjects had nearly identical S-cone densities, indicating independence of the developmental mechanism that governs the relative numerosity of L/M and S cones. L:M cone ratio estimates were correlated highly with those obtained in the same eyes using the flicker photometric electroretinogram (ERG), although the comparison indicates that the signal from each M cone makes a larger contribution to the ERG than each L cone. Although all subjects had highly disordered arrangements of L and M cones, three subjects showed evidence for departures from a strictly random rule for assigning the L and M cone photopigments. In two retinas, these departures corresponded to local clumping of cones of like type. In a third retina, the L:M cone ratio differed significantly at two retinal locations on opposite sides of the fovea. These results suggest that the assignment of L and M pigment, although highly irregular, is not a completely random process. Surprisingly, in the protan carrier, in which X-chromosome inactivation would favor L-or M-cone clumping, there was no evidence of clumping, perhaps as a result of cone migration during foveal development.

The rod photoreceptors are implicated in a number of devastating retinal diseases. However, routine imaging of these cells has remained elusive, even with the advent of adaptive optics imaging. Here, we present the first in vivo images of the contiguous rod photoreceptor mosaic in nine healthy human subjects. The images were collected with three different confocal adaptive optics scanning ophthalmoscopes at two different institutions, using 680 and 775 nm superluminescent diodes for illumination. Estimates of photoreceptor density and rod:cone ratios in the 5°–15° retinal eccentricity range are consistent with histological findings, confirming our ability to resolve the rod mosaic by averaging multiple registered images, without the need for additional image processing. In one subject, we were able to identify the emergence of the first rods at approximately 190 μm from the foveal center, in agreement with previous histological studies. The rod and cone photoreceptor mosaics appear in focus at different retinal depths, with the rod mosaic best focus (i.e., brightest and sharpest) being at least 10 μm shallower than the cones at retinal eccentricities larger than 8°. This study represents an important step in bringing high-resolution imaging to bear on the study of rod disorders.

Purpose To characterize retinal structure and function in achromatopsia (ACHM) in preparation for clinical trials of gene therapy. Design Cross-sectional study. Participants Forty subjects with ACHM. Methods All subjects underwent spectral domain optical coherence tomography (SD-OCT), microperimetry, and molecular genetic testing. Foveal structure on SD-OCT was graded into 5 distinct categories: (i) continuous inner segment ellipsoid (ISe), (ii) ISe disruption, (iii) ISe absence, (iv) presence of a hyporeflective zone (HRZ), and (v) outer retinal atrophy including retinal pigment epithelial (RPE) loss. Foveal and outer nuclear layer (ONL) thickness was measured, and presence of hypoplasia determined. Main Outcome Measures Photoreceptor appearance on SD-OCT imaging; foveal and ONL thickness; presence of foveal hypoplasia; retinal sensitivity and fixation stability; and association of these parameters with age and genotype. Results Forty subjects with mean age of 24.9 years (range 6 to 52) were included. Disease-causing variants were found in CNGA3 (n=18), CNGB3 (n=15), GNAT2 (n=4), and PDE6C (n=1). No variants were found in 2 individuals. 22.5% of subjects had a continuous ISe layer at the fovea; 27.5% had ISe disruption; 20% had an absent ISe layer; 22.5% had a HRZ; and 7.5% had outer retinal atrophy. No significant differences in age (p=0.77), mean retinal sensitivity (p=0.21) or fixation stability (p=0.34) across the 5 SD-OCT categories were evident. No significant correlation was found between age and foveal thickness (p=0.84), or between age and foveal ONL thickness (p=0.12). Conclusions The lack of clear association of disruption of retinal structure or function in ACHM with age suggests that the window of opportunity for intervention by gene therapy is wider in some individuals than previously indicated. Therefore the potential benefit for a given subject is likely to be better predicted by specific measurement of photoreceptor structure rather than simply by age. The ability to directly assess cone photoreceptor preservation with SD-OCT and/or adaptive optics imaging is likely to prove invaluable in selecting subjects for future trials and measuring their impact.

Previous studies have reported race- and sex-associated differences in macular thickness, and the inference has been that these differences represent similar anatomic features. However, the data on pit morphology collected in the present study reveal an important and significant variation. Between the sexes, the differences are due to global variability in retinal thickness, whereas the variation in thickness observed between the races appears to be driven by differences in foveal pit morphology. These differences have important implications for the use of SD-OCT in detecting and diagnosing retinal disease.

The authors demonstrated a novel method to distinguish HFL from true ONL. An accurate measurement of the ONL is critical to clinical studies measuring photoreceptor layer thickness using any SD-OCT system. Recognition of the optical properties of HFL can explain reflectivity changes imaged in this layer in association with macular pathology.

There is enormous variation in the X-linked L͞M (long͞middle wavelength sensitive) gene array underlying ''normal'' color vision in humans. This variability has been shown to underlie individual variation in color matching behavior. Recently, red-green color blindness has also been shown to be associated with distinctly different genotypes. This has opened the possibility that there may be important phenotypic differences within classically defined groups of color blind individuals. Here, adaptive optics retinal imaging has revealed a mechanism for producing dichromatic color vision in which the expression of a mutant cone photopigment gene leads to the loss of the entire corresponding class of cone photoreceptor cells. Previously, the theory that common forms of inherited color blindness could be caused by the loss of photoreceptor cells had been discounted. We confirm that remarkably, this loss of one-third of the cones does not impair any aspect of vision other than color.cone mosaic ͉ dichromacy ͉ retinal imaging H uman trichromacy relies on three different cone types in the retina; long-(L), middle-(M), and short-(S) wavelengthsensitive. Dichromatic color vision results from the functional loss of one cone class; however, one of the central questions has been whether individuals with this form of red-green colorblindness have lost one population of cones or whether they have normal numbers of cones filled with either of two instead of three pigments. Evidence has accumulated favoring the latter view, in which the photopigment in one class of cone is replaced, but the issue has not been resolved directly. Berendschot et al.(1) measured optical reflectance spectra of the fovea for normals and dichromats, and their analysis favored a replacement model. Psychophysical experiments, based on frequency of seeing curves, have also provided evidence that the packing of foveal cones in dichromats is comparable to that in trichromats (2, 3). Most recently, in comparing mean contrast gains derived from the electroretinogram (ERG) for dichromats to those of trichromats, Kremers et al. (4) concluded that complete replacement occurs in dichromacy.The L-and M-cone photopigments are encoded by genes that reside in a head-to-tail tandem array on the X chromosome (5). Two categories of mutations of these genes have been found to be associated with dichromacy. In one category of mutations, the gene(s) for a spectral class of pigment have been deleted or replaced with a functional gene for a different spectral class (6-10). In the other genetic category, a normal gene is replaced by a mutant one encoding a photopigment that does not function properly (11,12). The most frequently reported example of this latter cause is a mutation that substitutes the amino acid arginine for a cysteine at position 203 (C203R) of the pigment molecule. This cysteine is highly conserved among all G protein-coupled receptors, and is involved in forming an essential disulfide bond in the photopigment molecule (13). The mutation was originally discov...

Presence of a fovea centralis is directly linked to molecular specification of an avascular area in central retina, before the fovea (or `pit') begins to form. Modeling suggests that mechanical forces, generated within the eye, initiate formation of a pit within the avascular area, and its later remodeling in the postnatal period. Within the avascular area the retina is dominated by `midget' circuitry, in which signals are transferred from a single cone to a single bipolar cell, then a single ganglion cell. Thus in inner, central retina there are relatively few lateral connections between neurons. This renders the region adaptable to tangential forces, that translocate of ganglion cells laterally / centrifugally, to form the fovea. Optical coherence tomography enables live imaging of the retina, and shows that there is greater variation in the morphology of foveae in humans than previously thought. This variation is associated with differences in size of the avascular area and appears to be genetically based, but can be modified by environmental factors, including prematurity. Even when the fovea is absent (foveal hypoplasia), cones in central retina adopt an elongated and narrow morphology, enabling them to pack more densely to increase the sampling rate, and to act as more effective waveguides. Given these findings, what then is the adaptive advantage of a fovea? We suggest that the advantages of having a pit in central retina are relatively few, and minor, but together work to enhance acuity.