The mapping of the trichromatic cone mosaics by Austin Roorda and David Williams (1999) provides direct evidence for large individual differences in the ratio of L to M cones. What is the subjective impact of these differences on an individual's perception of color? Some basic theories of color vision have long proposed that a person's color perception is highly dictated by the ratio of L to M cones in their retina. This would imply that two subjects with drastically different L to M cone ratios would perceive a color of a given wavelength differently. Since adaptive optics has given us the capability to directly obtain measurements of a person's L to M cone ratio in living eyes, we can now perform psychophysical experiments on the same patches of retina that were imaged and mapped to investigate how this ratio determines our perception of color.

To test this hypothesis of color vision, former postdoc Yasuki Yamauchi measured unique yellow (the color which is neither reddish or greenish) at the same location in the two observers whose mosaics were imaged by Roorda and Williams. Unique yellow is the equilibrium point of red-green color perception, and is mediated by the excitation of L and M cones. If the relative number of L/M cones was the determinant of unique yellow, this theory would predict that subjects AN, with a ratio of 1.15, and JW, with a ratio of 3.79, would select a wavelength for unique yellow that would differ by approximately 80 nm, as illustrated by the yellow symbols on the Theory curve in the figure below. However the experimental results demonstrated that the settings for these two subjects were not substantially different (574.7 nm–red circle, and 576.8 nm–green circle, for subjects JW and AN with L/M ratios of 3.79 and 1.15, respectively). This indicates that the L/M cone ratio does not have an important influence on red-green color perception (Brainard et al., 2000).

As a result of this experiment, we believe that a person's experience may control their perception of color through a plastic mechanism in which the L and M cone strengths are adjusted to minimize the response to the average chromaticity in the environment. Yamauchi performed another experiment to test this possibility by conducting a long-term chromatic adaptation experiment, in which subjects were exposed to an altered chromatic environment (either red or green) for 4 hours per day. Subjects could choose to either spend four hours in a chromatically altered room (the infamous red and green rooms),

could wear tinted contact lenses,

or could wear a pair of filtered goggles, as chosen by former research associate, Nathan Doble.

As shown in the figure below, even after spending the rest of the day under normal illumination, the subject's unique yellow shown a significant shift after several days. Each yellow-orange circle represents a daily measurement obtained when subject YY was not exposed to a chromatically altered environment. During the first 15 days, his unique yellow measurements hovered around a baseline value of nearly 578 nm. After placing subject YY in the red room for a portion of each subsequent day (red circles), his unique yellow measurement increased by approximately 3 nm after 10 days of adaptation. Subject YY then spend the next 20 days in an ordinary environment that was not chromatically altered (2nd set of yellow-orange circles). You can see that his unique yellow slowly decreased during this time until it reached the initial, baseline value. It took more than a week for this aftereffect to decay. Subject YY was then placed in the green room for a portion of each of the next 10 days, and his unique yellow measurements shifted downwards by approximately 3.5 nm (green circles). However once he was again no longer exposed to the chromatically altered environment, his unique yellow measurements steadily increased and returned to their initial, baseline values (final set of yellow-orange circles). The longevity of these aftereffects indicates that color vision is mediated by a plastic mechanism that is responsive to the chromatic environment, and that it may continue to work even in adulthood, probably throughout one's entire life.

Roorda A., & Williams D.R. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520-522.

Brainard D., Roorda A., Yamauchi Y., Calderone J.B., Metha A., Neitz M., Neitz J., Williams D.R., & Jacobs G.H. (2000). Functional consequences of the relative numbers of L and M cones. Journal of the Optical Society of America A, 17(3), 607-614.

Neitz J., Carroll J., Yamauchi Y., Neitz M., Williams D.R. (2002). Color perception is mediated by a plastic neural mechanism that remains adjustable in adults. Neuron, 35, 783-792.

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