L/M cone ratios in human trichromats assessed by psychophysics, electroretinography, and retinal densitometry

Abstract

Estimates of the relative numbers of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones vary considerably among normal trichromats and depend significantly on the nature of the experimental method employed. Here we estimate L/M cone ratios in a population of normal observers, using three psychophysical tasks-detection thresholds for cone-isolating stimuli at different temporal frequencies, heterochromatic flicker photometry, and cone contrast ratios at minimal flicker perception--as well as flicker electroretinography and retinal densitometry. The psychophysical tasks involving high temporal frequencies, specifically designed to tap into the luminance channel, provide average L/M cone ratios that significantly differ from unity with large interindividual variation. In contrast, the psychophysical tasks involving low temporal frequencies, chosen to tap into the red-green chromatic channel, provide L/M cone ratios that are always close to unity. L/M cone ratios determined from electroretinographic recordings or from retinal densitometry correlate with those determined from the high-temporal-frequency tasks. These findings suggest that the sensitivity of the luminance channel is directly related to the relative densities of the L and the M cones and that the red-green chromatic channel introduces a gain adjustment to compensate for differences in L and M cone signal strength.

The influence of retinal illuminance on L- and M-cone driven electroretinograms

“…From these data, L:M ratios can be calculated in trichromatic observers that are comparable with those obtained in psychophysical experiments (see Brainard et al, 2000;Kremers et al, 2000).…”

“…For chromatic-mediated detection, the sensitivities for L-and M-cone isolating stimuli are generally equal (Krauskopf, 2000;Kremers et al, 2000). In line with this finding, most subjects reported greater discomfort when observing the flickering L-cone stimulus at higher luminances.…”

Asymmetries in the contributions of On- and Off-mechanisms to the ERG signal.

“…The ratio between L-and M-cone driven responses at 30 Hz was found to be proportional to the ratio of L-to M-cone numbers in the retina (Brainard et al, 2000;Kremers et al, 2000). In four subjects (JK, BL, FH and RT) the measurements with 30 Hz modulation were performed (data not shown).…”

Organization of the Human Trichromatic Cone Mosaic

“…Whereas the short-wavelength-sensitive-(S) cone submosaic has been relatively well characterized (Williams et al, 1981;de Monasterio et al, 1985;Ahnelt et al, 1987;Curcio et al, 1991), the organization of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in the human retina has proven more elusive, because they exhibit no known morphological or histochemical differences and their pigments are 96% identical (Nathans et al, 1986). Several studies using indirect measurements have suggested that there are on average more L cones than M cones, with large intersubject variability (DeVries, 1946;Rushton and Baker, 1964;Cicerone and Nerger, 1989;Pokorny et al, 1991;Yamaguchi et al, 1997;Hagstrom et al, 1998;Brainard et al, 1999;Deeb et al, 2000;Kremers et al, 2000;Otake and Cicerone, 2000;Carroll et al, 2002). A limitation of these estimates is that they often involve the untested assumption that each M cone and each L cone contribute equally to the molecular, physiological, or psychophysical mechanisms being monitored to derive L:M ratio estimates.…”