Questions in Subjective Experience

Earlier today I was trying to generate an answer to "what would it mean for Neuroscience to succeed". This post is not about that. This post is about one of those dumb questions from philosophy "What makes red ... Red?".

Far from being a meaningless or rhetorical, this question takes on a concrete meaning in neuroscience, although we may lack tools and data to give a satisfying answer at the moment.

Light at a wavelength of 700 nanometers is not quite the answer, since color perception is relative and one can be prompted to experience illusory colors. But, even if we account for all these idiosyncrasies of perception, do we have a satisfying answer for why red feels like red and green feels like green and they seem obviously non-interchangeable ?

Perhaps. There are ( I simplify ) three channels of information sent from the eye to the brain. One is information on light/dark contrast. Another is yellow/blue contrast. Finally, we have red/green contrast. Simple black and white vision evolved first. Later, yellow/blue color vision appears to differentiate between different types of "white". Some completely red/green color blind individuals perceive color in this space. The Red/Green channel evolved most recently, in old world primates, and distinguishes between different types of 'yellow'.

So, trivially, red is "not green, not blue, bright"

Perhaps this is enough to satisfy you for a definition of red, but I am unconvinced. For instance, I feel completely satisfied with a definition of left as "a direction that is orthogonal to up and forward, and is not right". This is because, for most of my life, I have been largely unable to tell the difference between left and right. Now that I know how to read and have learned other skills with a left-right asymmetry, I'd notice if you flipped left and right, but I'm not convinced I would have noticed as a small child. However, I think that waking up one day to a world in which the red-green axis had been reversed would be quite alarming.

Red-green is something more than a mere sign convention within "not blue, bright". This axis represents some perceptual eigenfunction, and it needs to have more information than 'not green, not blue, bright'. If you accept the axiom that the experience of information is equivalent to the existence of said information, then the asymmetric quality of the red-green axis implies that either red or green, or both, contains some bits of information beyond simple color contrast.

One, somewhat boring answer is simply that, since the absorption spectra of green and blue cones overlap more, "green" necessarily contains some component in the "blueish" direction, more-so than red. So, red is simply the "less blueish" side of yellow.

Does this explanation also satisfy you for the yellow-blue axis ? It is possible that individuals without a red cone really do perceive color space as just having these two directions, like left(blue) right(yellow), and that the qualitative difference between them is slowly acquired once one is accustomed to the sky being blue and people being brown ? I'm not colorblind myself, so I'd need more data to answer this question.

Assume for a moment that this is not the case, the red, green, and blue, aren't just vectors in colorspace, but have some sort of intrinsically different representation in the brain. By this I mean that the extra bits of the 'red' vector are hard-wired into how the visual system works. This would require, at some point, 'redness' and 'greeness' to have representations in the brain that are more distinct than color opponancy. Speculation is that at some point "redness" is behaviorally relevant in ways that "greenness" is not, and so color perception is deeply related to some sort of behavioral cues. I stop short of speculating as to what those cues might be.

If neuroscience has any hope of mathematically describing experience, it will eventually address this question seriously. Eventually, we will be able to say yes my red is the same as your red, see, I proved it with math. Hopefully its just a matter of bits.


  1. Alright, Biffmotron asserts that Red/Green are precisely vectors in color space and nothing more, and that when we are born the colors are as symmetric as left-right.

    The proposed, totally doable experiment, is to construct augmented reality goggles that permute RGB colorspace, and see if or how a person adjusts to the permutation.

    The next question then is where to find someone who wants to walk around with AR goggles on their head for a month.

  2. er.. some sort of correction for the overlap of the absorption spectra of green and blue may be good ?

  3. To clarify, what I meant about red and green is not that they are opposite ends of the same perceptral axis, but rather that they are orthogonal coordinate that can be rotated/mirrored arbitrarily with no loss in information. In the case of colorblind people, the red and green axis have been 'folded' into a single color.

    Also, I think that AR gear is approaching a price point where it may be practical home experiments. Depending on the quality of gear you wanted, you could make a COTS rig for ~$1500.