Resolution acuity and spatial summation of chromatic mechanisms in the peripheral retina.

Green stimuli are more difficult to detect than red stimuli in the retinal periphery, as reported previously. We examined the spatial characteristics of chromatic mechanisms using stimuli, modulated from an achromatic background to each pole of the “red–green” cardinal axis in DKL space at 20 deg eccentricity. The “blue–yellow” cardinal axis was also studied for comparison. By measuring both grating discrimination at the resolution limit (resolution acuity) and spatial summation, assessed by the Michaelis–Menten function, we demonstrated a marked “red–green” asymmetry. The resolution acuity was worse and spatial summation more extended for “green” compared to “red” stimuli, while showing significant individual variations. Ricco’s area was also measured, but not determined for “green” spots because of the poor small stimuli detection. These results cannot be explained by differences in L- and M-cone numerosity and/or spatial arrangement, but rather have postreceptoral origin, probably at the cortical level.