A man ended up in hospital as a result of an accident. He started complaining that the bed was too hard, but when they made it softer, it felt harder to him. Hotter felt colder, and so on. The doctors felt they had to operate on him to fix this peculiar condition where his senses were topsy turvy. This they duly did; when the man awoke afterwards, he sniffed around and demanded: “what smells purple?”
A May edition of New Scientist revealed the mechanics behind the very peculiar condition synaesthesia, whereby a small number of people experience senses spilling over into other realms. Some very common examples are people for whom words all have particular colours; or maybe each day of the week comes with a consistent colour, or particular words evoke particular tastes when seen.
Daniel Tammet, mentioned here a few times in the past, presents a particularly interesting case. He has Asperger's, aka high functioning autism. His memory is incredible, and he also experiences words as specific colours. Much more meaningful, however, is his relationship with numbers, which he loves. For him, each digit has a specific shape and evokes a particular feeling – indeed, he experiences a unique shape for every number up to 10,000. This greatly helps his ability with numbers: he has recited from memory pi to over 20,000 digit, by following the shaped landscape in his mind. He also sees the multiplication product of two numbers simply by the shape of the space between the two.
The New Scientist article says synaesthesia is involuntary, runs in families, and is thought to be due to incomplete pruning of neural pathways during the brain's development so that connections remain that are not there for most of us. For example, in a colour synaesthete's brain, the area processing colour does actually show activity when stimulated by the linked concept.
Why would one letter or word evoke a specific colour? The answer appears to be sometimes individualistic (based on the person's early experiences that forge consistent responses), but there are certainly some common crosswirings that have very simple explanations.
First words tend to evoke colour responses on the basis of the first letter – Tammet has himself commented on this. Also, the more common letters are typically associated with common colours, and less frequent letters with rarer colours. Next, colours are often associated with the first letter of the word – for example, B is frequently seen as blue or brown. Y is often yellow for English-speaking synaesthetes, but for Germans, it's G they often associate with yellow - which is gelb in German.
There is a lot more to it, but the above substantially demystifies a very odd condition. It doesn't speak directly to Tammet's memory ability, for example, but it does give a hint that links these related phenomena to brain abnormalities.
In the majority of people, congenital brain abnormalities can be overwhelmingly burdensome but for some, they open up new pathways.
13-Sept update:What does this mean in the context of the broad span of evolution? Possibly not much, because
a) The indications so far are that these neurological wirings happen at the level of the b0dy's physiology, not the gene, and so don’t inherit;
b) Evolutionary supremacy is the story of small genetic mutations dominating in the general population, on the basis of superior survival in a given environment. And the greatest implications of rewiring – superior numerical or memory ability – may not confer special advantage in a technological environment.
But the latter is open for debate.