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Abstract
With the use of histologic techniques, it was shown that retinal receptors in several species are directed toward a point near the front of the eye lens. Because of inherent limitations in histologic methods, evidence concerning photoreceptor orientation obtained by a different technique would be of value. The psychophysically determined Stiles-Crawford effect (directional sensitivity of the retina) provides a valuable test of the anatomical findings. In this paper the theoretical basis for such a test is developed and working hypotheses have been proposed. One model calls for retinal receptors to be directed toward the center of the exit pupil of the eye (anterior pointing hypothesis), and a second requires the photoreceptors to point toward the center of the retinal sphere (center pointing hypothesis). The direction of the maximum of the photopic Stiles-Crawford function would be different for the two working hypotheses at extrafoveal points. Similarly, the integrated effective visual stimulus magnitude would be different for each of the two alternative proposals. The questions raised in this paper are significant because they can help clarify the "why" of retinal receptor optical properties.