The lateral resolution of the AOSLO was sufficient for resolving the smallest foveal cones but not for all eyes, most likely because the amount of residual aberrations was subject dependent. One reason for the insufficient resolution is that the wavefront reconstructor in equation 1
was optimized for only a 6-mm diameter pupil. This size restriction poses a problem when imaging certain high myopes when the minifying effect of a high minus power lens is placed in front of the eye to bring the initial aberration magnitude to within a correctable range. In addition, the wavefront covariance matrix (C
), used as a prior in optimizing the reconstructor, is designed to approximate a particular power spectrum.53
AO performance depends on how well the aberration profile for a particular eye is approximated by this model. These limitations are currently being addressed by more accurate AO system modeling and control methods.54–56
In recent works, Chui et al.3,57
stated that diffraction may be the limiting factor for the AOSLO for resolving cones near the foveal center. Their resolution assessment was based on the Rayleigh criterion for a 6 mm diameter pupil (2.8 μm for an emmetropic eye). For the subjects imaged in this study, the Rayleigh criterion would predict the resolution limit to be from 2.65 to 3.55 μm, depending on eye length. With the exception of the two high density foveas reported by Curcio et al.,17
the smallest foveal cones are at least 2 μm in diameter which is approximately equal to cone spacing in the rod-free fovea.17,48,58,59
Cone size increased rapidly with eccentricity, so that by 150 μm away from the foveal center, the average cone spacing was greater than 3.5 μm, according to measurements plotted in . Since Chui et al.3,57
were able to resolve individual cones only at retinal eccentricities greater than 200 μm in emmetropes, it is unlikely that diffraction was the limiting factor. Fundamentally, the more accurate description of the resolution limit is the Sparrow criterion, which predicts a resolving power that is approximately 22% higher than the Rayleigh criterion for a circular aperture.60–62
In this study, the predicted lateral resolutions in the four eyes were 2.41, 2.43, and 2.55 μm, indicating that we were imaging close to the diffraction limit. Assuming that size is the only factor that makes foveal cones difficult to image, a diffraction limited AOSLO should be able to resolve the entire foveal cone mosaic in most eyes. Because this was not the case in the present study, a more robust AO system is needed to consistently achieve near diffraction limited image quality.
In measuring fixation, the AOSLO has the advantage over other modalities for being able to isolate precise locations on the retina used for fixation. Any potential alignment or timing error is eliminated, because the fixation target is generated as part of the image formation process.38
Nevertheless, our data are consistent with, albeit slightly lower than, those of Putnam et al.,28
who found that the PRL is displaced from the point of peak cone density. Our measurements serve to confirm that the PRL deviates significantly from the foveal center and reinforces the importance of clearly defining the location of 0 eccentricity whenever one is performing eccentricity-dependent measurements. Furthermore, when the angular distribution of foveal fixation is not approximately uniform, the horizontal and vertical standard deviation or the mean would not accurately describe fixation variability. Principal component analysis determines the orientation that accounts for the most variability in the data and thus provides a better overall metric for describing fixation variability.
One of the main purposes of this work was to provide baseline cone density measurements from 0 to 0.3 mm eccentricity, a region of the retina that is most important for spatial vision, but that has been rarely explored using in vivo imaging methods. With improved AO performance, most if not all cone photoreceptors in this central part of the fovea can be resolved. The cone density curves plotted in are in close agreement with six of the eight retinas presented in Curcio et al.17
It was mentioned as a possibility by the investigators that the two retinas with much higher foveal cone density than the rest may have been due to tissue shrinkage. It is nonetheless encouraging to find that in vivo density measurements are in close agreement with histologic data.
Inside the approximate foveola (0–0.2 mm eccentricity17,63
), the axial length induced retinal stretching could not be verified by cone density measurements alone because of high levels of intersubject variability. Although we were able to measure cone density only as close as 0.1 mm eccentricity if all subjects were to be included, we would expect intersubject variability to be even greater at the foveal center on the basis of histologic data.48
However, with increasing retinal eccentricity, the tendency for all cone mosaics to converge to a state that can be characterized by axial length becomes more apparent as observed at 0.3 mm eccentricity (c). Of interest, despite the amount of intersubject variability present near the foveal center, angular cone density actually increased significantly with axial length at any particular retinal eccentricity (d–f). In the interferometric acuity study conducted by Coletta and Watson,2
the investigators generated a 1° diameter circular grating patch to measure foveal acuity in a group of subjects with various axial lengths. According to their results, all the subjects performed similarly when acuity limits were specified in angular units of spatial frequency (cycles/deg). On the basis of the RMF estimates, the spatial frequency of the grating in retinal units (cycles/mm) for the longest eye would be at about half the rate of that for the shortest eye. On the basis of our measurements, if interferometric acuity at the fovea is indeed limited by cone spacing, then one would expect individuals with longer eyes to perform better than those with shorter eyes in terms of acuity in angular units (cycles/deg) and to perform similarly in terms of acuity in retinal units (cycles/mm). Since this was not the case according to two separate studies,2,23
we can rule out retinal stretching as a possible explanation for why foveal interferometric acuity does not improve with increasing level of axial myopia.
A rather extreme interpretation of our results is that the density of the foveolar cone mosaic is completely unaffected by myopia-related eye growth. This notion seems unlikely, because the retinal surface expands globally in myopia,64
and we have little reason to believe that retinal tissue at the foveola is somehow more durable than that in the rest of the retina. A more reasonable interpretation would be that retinal stretching affects the foveolar cone mosaic, but a number of other developmental factors primarily govern cone density distribution there. A thorough analysis of foveal cone density and packing structure in emmetropic retinas, in tandem with other structural measures (i.e., retinal thickness, size of the foveal avascular zone, and shape of the foveal depression65
) would be necessary to identify these potential factors. Nevertheless, since we were able to estimate the peak cone density in only four eyes, we still cannot rule out the possibility that peak cone density increases with eye growth, as seen in experimentally enlarged marmoset eyes.16