Relative number of long- and middle-wavelength-sensitive cones in the human fovea.

Abstract

Flicker photometric measurements yield spectral sensitivity curves that are well fitted by sums of the spectral sensitivity curves of long-wavelength-sensitive (L) cones and middle-wavelength-sensitive (M) cones if the L cones are given twice the weight of the M cones. This result has been interpreted as implying that L cones are more numerous than M cones but is also consistent with a different numerical ratio, say, 1:1, and with the assignment of greater weight to the L cone input than to the M cone input by the mechanism subserving flicker photometry. Measurements of temporal sensitivity are presented for lights that modulate the inputs of either only the L cones or only the M cones. Sensitivity to modulation of the L cones is approximately twice that of modulation of the M cones at approximately 30 Hz, but that advantage disappears at approximately 2 Hz. Thus flicker sensitivity is equivocal with regard to cone numerosity. Electrophysiological, anatomical, and psychophysical evidence is reviewed, with particular weight placed on the statistics of color appearance of small, brief, monochromatic lights and on increment thresholds measured on the same observers. It is concluded that, in the central fovea, the ratio of L:M cone numbers is close to unity and may not be so variable as is usually supposed.