PURPOSE: The area of complete spatial summation (Ricco's area) for achromatic stimuli has previously been shown to decrease with increased background luminance. A popular hypothesis is that such a phenomenon reflects increased center-surround antagonism within the receptive field of the retinal ganglion cell. We wished to investigate if similar changes in Ricco's area occur with blue background luminance for the S-cone pathway, guided by the knowledge that the retinal ganglion cells with S-cone input do not display S-cone-mediated center-surround antagonism (S+/S-).METHODS: Spatial summation functions were measured for four young healthy observers under S-cone pathway isolation by presenting blue test stimuli on a background consisting of intense fixed yellow (600 cd/m) component in combination with a variable blue component (background range, 1.78 to 2.82 log S-Td). Ricco's area was estimated by two-phase regression analysis.RESULTS: All subjects demonstrated a notable decrease in Ricco's area with increasing blue background luminance. On average, Ricco's area decreased in size by 0.39 log units per log unit increase in blue background luminance.CONCLUSIONS: The change in Ricco's area with the blue background component is not what one would initially expect given the known organization of S-cone-driven cells at the retinal level. Spatial reorganization by the suppressive surround of the receptive fields at a cortical level and a reduction in the contribution from S-cones with the lowest weights in the retinal receptive field periphery are among the possible mechanisms of the summation changes observed. These findings have implications for the design of clinical tests of the S-cone pathway.