The purposes of this study were to measure areas of complete spatial summation (i.e., Ricco's area) for S- and L-cone mechanisms and to evaluate whether the sizes of Ricco 's area could be explained in terms of either the densities of photoreceptors or ganglion cells. Increment thresholds were measured at the fovea and at 1.5 degrees 4 degrees, 8 degrees and 20 degrees in the superior retina using a temporal two-alternative forced-choice procedure. Test stimuli ranging from -0.36 to 4.61 log area (min(2)) were presented on concentric 12.3 degrees adapting and auxiliary fields, which isolated either an S- or L-cone mechanism on the plateau of the the respective threshold vs. intensity function. The data indicate that from 0-20 degrees retinal eccentricity the size of Ricco's area is larger for the S-cone mechanism than the L-cone mechanism, increases monotonically for the L-cone mechanism, and for both cone mechanisms, increases between 8-20 degrees retinal eccentricity. This latter finding suggests that ganglion cell density rather than cone density defines the size of Ricco 's area in the parafoveal and peripheral retina. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S32-S35, 2001.