Parametric representation of Stiles–Crawford functions: normal variation of peak location and directionality

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Abstract
Evidence suggests that the psychophysically determined Stiles–Crawford effect of the first kind (SCE) reflects waveguide properties of human photoreceptors. The peak of the SCE data set is assumed to reflect the principal alignment tendencies, and the spread (e.g., ρ value, the curvature or width at half-height) is assumed to reflect the directionality (i.e., interreceptor differences in alignment) of the population of photoreceptors being tested. As such, disruption of the normal SCE can be used and/or has been used (1) to document early natural history of retinal pathology involving the photoreceptors, (2) to provide a firm rationale for therapeutic intervention, and (3) to provide a method for monitoring therapies designed to alter the natural course of retinal-disease processes. We report large-sample norms for foveal SCE peak location and spread (horizontal peak location, nasal 0.51 ± 0.72, horizontal ρ value 0.047 ± 0.013, vertical peak location, superior 0.20 ± 0.64, vertical ρ value 0.053 ± 0.012), compare these norms with values determined in other laboratories, and discuss the various mathematical forms used for the empirical description of SCE data sets.