The most common form of colorblindness is an X-chromosome-linked variant form of color vision technically known as deuteranomaly. Colors are detected by humans through the combined action of three different types of so-called cone photoreceptors, each of which is optimally activated by different wavelengths of light. These sensitivities are altered in deuteranomalous colorblind individuals because they possess a variant form of one of the cone photoreceptors--the sensitivity of cones that should be "middle-wave" is shifted toward that of "long-wave" cones, resulting in decreased ability to differentiate between some colors that are easily distinguishable by those with normal color vision. In theory, however, it is possible that owing to the altered sensitivities of their cone photoreceptors, deuteranomalous individuals may be sensitive to color differences that are not apparent to those with normal color vision.
In the new work, researchers tested this idea by asking deuteranomalous and "color-normal" individuals to report whether they were able to distinguish between pairs of colors that were theoretically predicted to look different to deuteranomalous colorblind individuals but to appear the same to those with normal color vision. Indeed, the researchers found that some color pairs were only seen to be different by deuteranomalous individuals. The finding suggests that although these individuals may be blind to some colors accessible by color-normal individuals, they also have a sensitivity to a "color dimension" that is inaccessible to those with normal color vision. In their paper, the researchers remark that "[f]or a color-normal experimenter, it was striking to watch a deuteranamolous subject giving large difference ratings to apparently identical stimuli, and doing so without hesitation."
The researchers point out that because deuteranamolous colorblindness is caused by an X-chromosome-linked genetic alteration, and because women randomly inactivate one of their two X chromosomes in each of their cells, women who are carriers of the deuteranamoly trait (and hence have one normal X chromosome) are predicted to express four types of cone photoreceptor--the three normal cone types, as well as the cone with a shifted wavelength sensitivity. Thought it is unclear whether this would appreciably affect color vision in a way that allows expanded color sensitivities without the colorblindness experienced by deuteranamolous individuals, a test similar to that utilized in the present study could potentially be used to test this possibility.
The researchers include J.M. Bosten, J.D. Robinson and J.D. Mollon of the University of Cambridge in Cambridge, United Kingdom; G. Jordan of the University of Newcastle Upon Tyne in Upon Tyne, United Kingdom. This work was supported by the Levehulme Trust F/00125K.
Bosten et al.: "Multidimensional scaling reveals a color dimension unique to 'colordeficient' observers." Publishing in Current Biology, Vol. 15, R950-R952, December 6, 2005, www.current-biology.com