The following was triggered by a piece on a friend’s blog (http://weymanwrites.blogspot.com/2018/06/writing-blind-for-those-who-can-see.html#more) saying ‘I’ve never been completely sure whether colour-blindness is a spectrum or a ‘you have it or you don’t’ kind of condition. I suppose this is hard to tell when you have unreliable participants for studies. How can you compare perception?’
The variability in the way people report colour blindness isn’t just about their subjectivity, but about the way colour vision works. The majority of the population carry genes coding for three separate versions of the iodopsin protein, which are expressed only (I think) in retinal cone cells. Each version absorbs a different range of light frequencies as shown here.
Each cone cell expresses only one version of the pigment, so an individual cone is therefore sensitive only to red, green, or blue light. If you look at a girl, for instance, her image will fall on some cones of each type on part of your retina. If her dress is reflecting green light, red and green sensitive cones in that area will send impulses off to the brain, but the blue cones won’t. What the brain interprets as green is signals from red and green cones only, with the green firing a bit more frequently than the red.
The even more interesting implication is that during infancy we must each learn, in total isolation, to distinguish these different patterns from each other. Only as we get older and become able to analyse what others say to us can we match the patterns we’ve learned with the words for colours. So that’s where some of the subjectivity of perception comes from. It isn’t just that no one else can ever really know what I’m experiencing, but that each person’s neurosensory system first learns to see colour in total isolation from everyone else’s. (The reason my first book is about telepathy is that I started wondering what it might do to you your head if you were the first person able to experience exactly what another person could: and the fact that each of our brains constructs its own particular version of experience is the reason the book I’m currently querying to agents is about reality, sanity, and delusion).
The X chromosome in each cell of a person with red-green colour blindness will carry an altered version of the gene for red sensitive iodopsin. The interesting thing is, in regard to the question of whether red-green colour blindness is a spectrum, that a number of mutations of each iodopsin gene exist. Some lead to a non functional iodopsin (so can’t see red and experience it as similar to grey), but several others lead to an iodopsin that is functional, but with a shifted spectral sensitivity (red curve on the graph shifted left to left or right). A person with it shifted a little to the left, for instance, would be completely unable to distinguish red from green). This means that there are several forms of red-green colour blindness, each differing subtly from the next in its effects, and the same is true of yellow green and blue blindness. The result is that colour vision ‘abnormalities’ do indeed form a spectrum, even before you factor in the subjectivity of reporting. Modern versions of the Ishihara colour vision test cards are designed to get around the subjectivity, and to some extent distinguish between different forms of the condition.
I’ve often wondered whether artists who use colour in interesting ways (for instance Ann Vastano, http://annvastano.com/portfolio.html) might not experience the world in a subtly different way to many other people.
Let no one ever say that human genetic diversity is anything but enriching.