The Negative Cone Mosaic: A New Manifestation of the Optical Stiles-Crawford Effect in Normal Eyes

Abstract

PURPOSE. The purpose of this study was to describe a previously unreported manifestation of the optical Stiles-Crawford effect (oSCE) in normal eyes. METHODS.In a cohort of 50 normal subjects, the directional reflectance of cones in the retinal periphery was explored by flood-illuminated adaptive optics (FIAO) and optical coherence tomography (OCT). RESULTS.In 32 eyes (64%), off-axis FIAO images of the retinal periphery (~15-208 from the fovea) showed variably sized patches of hyporeflective dots (called here negative mosaic) coexisting with hyperreflective (positive) cones. In nine cases, shifting the entry pupil toward the optical axis restored the positive cone mosaic, with a point-by-point correspondence between positive and negative mosaics. Rods remained hyperreflective around negative and positive cones. These changes were paralleled by changes of the OCT reflectance of the cone outer segment tips and, to a lesser extent, of the inner/outer segment limit.CONCLUSIONS. By en face FIAO imaging of the retina, the contrast of cones over rods may be strongly dependent on the entry pupil to such an extent that their reflectance is lower than that of rods. We hypothesized that the negative cone mosaic aspect results from the differential Stiles-Crawford effect of cones and rods. Cone reflectance by en face FIAO parallels the reflectance from the cone outer segment tip line and to a lesser extent of the inner/outer segment limit by OCT. Taking this into account, the oSCE is of importance for the interpretation of high-resolution images of photoreceptors. (ClinicalTrials.gov number, NCT01546181.)

Understanding the changes of cone reflectance in adaptive optics flood illumination retinal images over three years

“…However, Vohnsen [30] has previously shown that this can only have an impact when the angle of incidence on the retina is greater than 7 degrees corresponding to a pupil entrance displacement of 2.7 mm. In previous work [24], the reflectance of the cones in AO flood-illuminated images of the retinal periphery has been shown to be greatly influenced by large displacements of the entry pupil. It is well known that the cones have relatively narrow angular reflectance functions, returning much more of the incident light towards a point near the center of the pupil than towards points farther away from the center.…”

“…The reflectance studies have been performed with all AO imaging modalities, including AO-SLO [11][12][13]16], AO-OCT [14] and flood-illuminated AO cameras [9,17,24]. The most common explanation so far for the cone reflectance variability of the order of an hour is the disc renewal process [9], which is thought to be the cause of both change in the outer segment length, observable through interference produced with a long-coherence light source AO-SLO [12], and in the refractive index at the interface between the inner and the outer segment, observable also with a short-coherence light source AO-SLO-OCT [14].…”

“…Nevertheless, the investigation of differences in cone reflectance between pathological and healthy retinas is still limited to a few isolated studies [20,21]. In addition, studies have demonstrated that cones in the central retina may vary their sensitivity or pointing direction in order to compensate for the eye's internal aberrations [22] or deterioration in the nearby cone's functionality [23] and that cones in the peripheral retina may vary their pointing direction depending on the position of the center of the illumination pupil [24]. The use of AO Optical Coherence Tomography (OCT) has also provided information on the dependence of the reflectance on depth in the inner cone interfaces, leading for the first time to a 3D characterisation of the cone reflectance [14].…”

“…Investigation of the cone reflectance in different retinal areas and over long time periods using an AO flood illumination camera is therefore lacking in the literature. It has already been observed that the position of the light source in an AO flood illumination system has an influence on the reflectance of the cone mosaic [9,24,[27][28][29], and this property has been used to study the angular tuning of single cones [15,28,30]. As the performance of the most commonly used cone detection algorithms depends on the quality of the images to which they are applied [31], it is also possible that the position of the light source itself affects not only the reflectance of the cones but also their detection and the subsequent estimation of the mosaic metrics.…”

Vision science and adaptive optics, the state of the field

“…AO is also useful to detect minute changes in vessel morphology (Paques et al 2015). Analysis of photoreceptor pointing on off-axis images are of interest in macular diseases, in which the visualization of the cone mosaic can be dramatically modified by multiangle observation (Miloudi et al 2015). In uveitis patients, we observed that paravascular inflammation (perivenous sheathing) may be more easily detected by AO than by other means (Errera et al 2014), providing us with a tool for mapping the extent of perivenous sheathing in cases of uveitis.…”