One woman tested as a true tetrachromat

Researchers at Newcastle University, the University of Cambridge and the University of Washington tested 24 women who carry a mild red-green color deficiency, whose retinas should hold a fourth cone type, to see whether any could use it.

Each woman completed a sensitive color-matching test designed to detect whether her color vision operated on three dimensions or four. Out of the 24 carriers tested, the large majority showed ordinary three-dimensional color vision, but one participant's results were consistent with genuine four-dimensional, tetrachromatic color vision.

Her results suggested she could distinguish shades that would look identical to someone with standard three-cone vision, using a fourth cone type most people's eyes do not carry in a functional form. The finding came from a single confirmed case among 24 carriers tested, so true functional tetrachromacy appears to be rare even among women who carry the right genetic setup for it, not a hidden trait in most carriers.

One carrier in 24 saw a fourth dimension of color.