The relative contributions of pre-neural and neural factors to areal summation in the fovea

In order to determine the relative contributions of pre-neural and neural factors to areal summation in the fovea, measurements of Ricco's area were made. These were compared to the results of an ideal-observer analysis which incorporated only pre-neural factors, up to the level of the photoreceptor. The comparison indicated that Ricco's area in the fovea is largely (if not completely) accounted for by pre-neural factors. Thus, our results, in agreement with the recent analysis of contrast sensitivity by Banks, Geisler and Bennett (1987; Vision Research, 27, 1915-1924), are consistent with the hypothesis that a neural pathway exists which consists of units whose center mechanisms sum over only a single cone or a single row of cones. Our results also imply that the quantum efficiency for detection is constant for test areas up to 256 min2, once the effects of the optics and receptor aperture are factored out.