Ricco's areas for S- and L-cone mechanisms across the retina

The purposes of this study were to measure areas of complete spatial summat ion (i.e., Ricco's area) for S- and L-cone mechanisms and to evaluate wheth er the sizes of Ricco 's area could be explained in terms of either the den sities of photoreceptors or ganglion cells. Increment thresholds were measu red 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 procedur e. 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 e ither an S- or L-cone mechanism on the plateau of the the respective thresh old vs. intensity function. The data indicate that from 0-20 degrees retina l 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 ecce ntricity. This latter finding suggests that ganglion cell density rather th an cone density defines the size of Ricco 's area in the parafoveal and per ipheral retina. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S32-S35, 2001.