Where Do the Unique Hues Come from?

Where are we to look for the unique hues? Out in the world? In the eye? In more central processing? 1. There are difficulties
looking for the structure of the unique hues in simple combinations of cone-response functions like (L − M) and (S − (L + M)): such functions may fit pretty well the early physiological processing, but they don’t correspond to the structure of unique
hues. It may seem more promising to look to, e.g., Hurvich & Jameson’s ‘chromatic response functions’; but these report on
psychophysical behaviour, not on underlying physiology. So ‘opponent processing’ isn’t any particular help on the unique hues—and
even physiology in general seems not to have come up with any good correlate or explanation. 2. Wright (Review of Philosophy and Psychology 2: 1–17, 2011) looks in a different place: to (a) magnitude of total visual response that a stimulus light provokes, the maxima and minima
of which, he thinks, give us the boundaries of the main hue categories (Wright connects these with Thornton’s ‘prime’ and ‘antiprime’ colours in an illuminant: Journal of the Optical Society of America 61 (1971): 1155–1163); and (b) the ratio of chromatic to achromatic response, the maxima and minima of which, he suggests, give us
the focal points of the unique hues. The suggestions are extremely interesting; but the desired correspondences have some counterexamples;
and where they hold, one could wonder how much they depend upon the particular choice of functions to measure (a) and (b);
and one could hope for more of an explanatory linkage between the sets of items in question. 3. Could the unique hues come
from, so to speak, the external world? White and black can easily be defined as particular kinds of reflectance. What of the
standard four unique hues? Variation in kinds of sunlight and skylight coincides well with variation along a line from unique
yellow to unique blue (cf. Shepard 1992, Mollon 2006). If we wanted something to calibrate our standards for unique yellow and blue as the lens of the eye changes with age, and
despite interpersonal cone differences, this would be a good basis—and there are several ways this can be extended to surface
colours. But is there any essential connection between these things: is there any rationale why the light of the sun and the
sky should be counted as unique hued? An answer may be: because in our environment, these illuminants are as close to white (or the natural illuminant colour) as you can get—to see things tinged with sunlight or skylight should be to see them minimally
tinged with any alien colour. Whereas other hues in an illuminant would be treated as tinging with a more alien colour the
thing seen.