DOI: 10.1097/IAE.0000000000000452
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PMID: 25611880
Issn Print: 0275-004X
Publication Date: 2015/02/01
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Excerpt
We appreciate Dr. Curcio's interest in our article1 and her generous sentiment. Dr. Curcio highlighted two issues: the first is in reference to the use of the terms perifoveal and parafoveal, whereas the second issue addresses abnormalities in rod structure and function with age as it relates to rod density in the human retina.
In our article, we cited and referenced a previous publication by Curcio et al,2 where they stated that the greatest loss in rod density occurs in the “parafoveal region” (1–3 mm from the fovea or 3.5–10° from fixation). In addition, they noted that scotopic sensitivity loss in early-stage age-related macular degeneration occurred most severely within 9° (2.4 mm) of fixation, which is within 1 mm to 3 mm from the fovea, and thus within the parafoveal region as defined in their article. They also indicated that the location of age-related rod loss and sensitivity loss differ from the region where rod density is maximal, the latter being found 4 mm to 6 mm from the fovea.
In our article, we stated that within the parafoveal region (1–3 mm from the fovea), rod densities were “high.” Based on Dr. Curcio's clarification in her Letter to the Editor, we agree that this region may be more precisely referred to as “combined parafovea and perifovea region.” Although this region might well represent a zone where rod density is relatively high, what should have been emphasized was that while rod density loss and scotopic sensitivity loss are greatest in this region in macular degeneration, this region does not contain the area of maximal rod density.
The issue relating to the use of the terms perifoveal and parafoveal has an “Alice in Wonderland-like” quality. Various authors use these terms based on how they have chosen to define them, and thus their use has become ambiguous. We concur with Dr. Curcio's suggestion to reduce this ambiguity by reporting the distance from the center of the fovea (e.g., in millimeters) and to provide the scaling factor when converting the visual angle.
Dr. Curcio cites the classic work of Polyak.3 In his text, he defines seven concentrically organized retinal regions that include 1) the central fovea, 2) parafoveal region, 3) perifoveal region, and 4–7) peripheral regions. Although the retina can be divided into these subregions, it is of interest to recall the sentiments expressed by Polyak in his 1941 publication: “The purpose of the revised subdivisions of the primate retina into seven insular or circular and roughly concentrical regions is purely practical.” Polyak further states “It should be kept in mind, however, that even from a purely morphologic standpoint the boundaries between the contiguous regions are more or less arbitrary,” and that the change between regions is most often gradual, being almost imperceptible.
