To some people I am half colour blind even though I can see everything from blue to red like most people. For them it is odd that I can only see colours when they are directly presented to me. More than that, I can only see colours for which there are proper words. These people literally see black and white symbols in colour depending on which symbol it is. And the colours they see are sometimes those which I will never see in my life because they are invisible. Incredible? A journey through the visual brain shows how this feat is possible.
The first report of invisible colour perception came from Ramachandran and Hubbard (2001), then at San Diego. They briefly mentioned a man only known as S.S. who suffers from s-cone weakness, i.e. the cells in his eyes which are sensitive to blue are impaired. Therefore, he cannot see the full colour spectrum the way most people can.
S.S. also happens to be a grapheme-colour synaesthete, i.e. he literally sees a certain colour consistently when presented with, for example, a given number. However, these number-evoked colours were special. He described them as ‘weird’ or ‘extraspectral’. He had only ever seen them in his mind’s eye, never in the outside world.
Rather than being an isolated case, regular synaesthetes can see these so called ‘Martian’ colours as well (Ramachandran & Hubbard, 2003). Furthermore, Oliver Sacks writes about a musician he calls Michael whose synaesthesia links musical keys with colours. Sacks writes that ‘some keys seem to have a strange hue which he can hardly describe, and which he has almost never seen in the world about him’ (2007, p. 182). Pharell Williams could be yet another case.
How can we make sense of this? To understand how the human brain can give rise to Martian colour perception, a quick tour through the visual brain is necessary. When light hits the eye, it is transformed into the electrochemical information currency of the brain. The information travels from the eyes directly to the back of the brain within about 100 milliseconds (a tenth of a second). Because this area in the back of the head is so well understood to be the primary target for visual information, it is simply called the primary visual cortex or V1 for short.
After some low level information processing in V1, information is passed on to other visual areas which are specialized in certain jobs, e.g. V5 processes motion, V4 processes colour, and grapheme areas process numbers and letters.
When looking at where these different areas lie, one gets a sense for why motion-colour synaesthesia is not found while word/number-colour synaesthesia is so common. The human brain happens to be organised in such a way that V4 (colour) lies very close to the grapheme area. V5, on the other hand, is a lot farther. Research in the last decade (reviewed in Hubbard et al., 2011) has revealed that in synaesthetes colour and grapheme areas are unusually well connected and they show activation of the colour area just after the grapheme area responds when synaesthetes view graphemes.
Martian colours are thought to be so unusual because information has not taken the usual route (eye -> V1 -> V4) but instead went to a grapheme area first and then entered V4 (eye -> V1 -> grapheme area -> V4). Similarly, musically induced Martian colours may look so weird because auditory information recoded in V4 simply isn’t of the same quality as visual information which comes from V1.
So, colour perception appears not only determined by where information is processed but also by where it originates. A good lesson to remember whenever you try to localise function X in brain area Y: information origin matters.
Some people out there see things which are so unusual that there isn’t even a proper word for these experiences. To them I am not only half-colour blind (even though I do not suffer from s-cone weakness like S.S.). I am also unimaginative – as not just my eyes but also my imagination is limited to the colours of the rainbow, a subset of the colours which can be experienced. If you believe that the rainbow is complete, you may well be as colour blind as I am.
Hubbard, E.M., Brang, D., & Ramachandran, V.S. (2011). The cross-activation theory at 10 Journal of Neuropsychology, 5, 152-177 DOI: 10.1111/j.1748-6653.2011.02014.x
Ramachandran, V.S., & Hubbard, E.M. (2001). Psychophysical investigations into the neural basis of synaesthesia Proceedings of the Royal Society B, 268, 979-983 DOI: 10.1098/rspb.2000.1576
Ramachandran, V.S., Hubbard, E.M. (2003). The phenomenology of synaesthesia. Journal of Consciousness Studies, 10, 49-57.
Sacks, O. (2007). Musicophilia. New York: Vintage.