When the stimulating light was incident on the vitreal surface, the intraretinal S-potential exhibited a high degree of directional sensitivity. Stimulation directed at the receptor surface produced a small degree of sensitivity only when the retina was devoid of the pigment epithelium. The elements proximal to the outer segments of the receptors may contribute to the differences that were observed to result from the two modes of stimulation.
E. L. PAUTLER, "Directional Sensitivity of Isolated Turtle Retinas," J. Opt. Soc. Am. 57, 1267-1269 (1967)