Author Response: Outer Retinal Bands

We thank Richard F. Spaide for his careful reading of our article, and we welcome this opportunity to address some of his concerns. Given the complexity of the relevant concepts, we have chosen to present a summary of our findings followed by a list of Spaide’s relevant criticisms (in italic) along with our rebuttals. In the interest of brevity, we have chosen not to respond to criticisms that do not bear on the scientific impact of our article (e.g., the availability of raw, linear data in the Spectralis [Heidelberg Engineering, Heidelberg, Germany] software). In cross-sectional optical coherence tomography (OCT) images (B-scans) of the retina, several bands are seen in the outer retina. Our article sought to investigate one of these bands—‘‘band 2’’—which originates from a structure confined to photoreceptor cells. We aimed to quantitatively characterize this structure by resolving it within single cones and measuring its average thickness and its position with respect to the adjacent bands 1 and 3. We hypothesized that the outer retinal band 2 would be thinner than the inner segment ellipsoid (16– 20 lm) and that it would lie near the inner and outer segment (IS/OS) junction, approximately half way between the external limiting membrane (ELM) and cone outer segment tips (COST). We imaged four healthy eyes at two retinal locations (foveal and parafoveal) using adaptive optics (AO)-OCT, which permits three-dimensional resolution of cone photoreceptors. From the resulting eight volumetric images, we segmented 9593 cones and measured the two parameters relevant to our hypotheses. We segmented each cone in three dimensions, using its lateral extent in en face projection and the locations of bands 1 and 3. Bands 1 and 3 were identified by identifying the A-lines comprising each cone and averaging them together to form the cone’s longitudinal reflectance profile (LRP). By quantifying cellular morphology on each of the 9593 cones, we found that band 2 has a thickness of 4.7 lm, corresponding to an object thickness of 3.5 lm or less, and that it lies approximately equidistant from the ELM and COST. These findings are consistent with the interpretation that band 2 arises from the junction between IS and OS, but inconsistent with the recent hypothesis that the band arises from the IS ellipsoid. Spaide’s letter raised seven significant criticisms of our methods and our interpretation of results. These are listed below, in order of perceived relative importance, along with our responses.