The ratio of L cones to M cones in the human parafoveal retina

We present psychophysically-based estimates of the relative numbers of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in the parafovea of three color-normal trichromats. Using methods previously applied to the fovea centralis, we obtained estimates of the relative numbers of L and M cones at retinal eccentricities of 1.0, 1.5, 2.0, 3.0, and 4.0 deg along the horizontal meridian of the temporal retina. Results for three observers indicate that the L to M cone ratio remains approximately invariant from the fovea to 4.0 deg eccentricity, with a mean ratio near 2:1.