Brain Pruning: the answer to autism?

"In adults [the human brain] has perhaps 100 billion neurons, each connected to its neighbours by 5000 synapses or so." New Scientist, 1 October 2008

"In the f[o]etal brain, all parts of the brain are interconnected, but as we age, the connections are pruned.  If the pruning genes get it wrong, the connections are off." - according to Vilayanur S Ramachandran, in New Scientist, 8 January 2011 (The fastest brain in the west, p26)

There is no disputing that the human brain undergoes a pruning process whereby the synapses, the connections between neurons, are culled while the brain is developing.

This is a normal part of the development process: meaningful connections are retained, and [at least some] unused ones are discarded.  This is a rationalisation that begins before the baby is born and continues for years afterwards.

And it's those synapses that enable brain functionality, particularly connections between different areas, which facilitate all manner of associative thought and reasoning.


The second quote above was made in the context of neuroscientist Ramachandran's study of synaesthesia, the leaking of one sense into another (the most common example being seeing letters or numbers as specific colours).

Occam's razor says to me that it need not take Ramachandran's concept of "pruning genes" for the pruning process to go awry. Development both before and after birth are affected by quite a range of factors.

Yet it struck me that disruption to that process may account for high-functioning autistics [so called "savants"] in particular.  The ability of Daniel Tammet, for example, to recall thousands of digits by visualising the string as a rolling landscape - that sounds like abnormal connections remaining in place between disparate parts of the brain.

Incorrect pruning may account for autism in a wider sense - not just those that are high functioning - but it seems easier to look to that process when describing aberrantly strong functionality than a weakening of capability, which can be due to a much wider variety of factors.

Synaesthesia unmasked

There's a science fiction story I remember (H L Gold's The Man With English) with a particularly memorable last line.
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.

The Amazing Brain 3: Daniel Tammet (again)

I've just finished reading Daniel Tammet's autobiography, Born On A Blue Day.

It's a very easy read, very engaging, and fascinating.

Tammet was born with high-functioning autism in England in 1979. It's a significant disability, beyond question, but there are compensations.

He has found it hard to manage many aspects of life, because of the way his brain is wired. It could be said that it has been difficult for him to apply an appropriate amount of focus to many day-to-day tasks. On some, he spends too much time because he is over-stimulated by the physical or numerical aspects of the task. On others, he doesn't understand enough of what is required, because it's something in which his mind simply doesn't engage.
For example, human interaction has been particularly difficult for him. His mind doesn't process the simple physical and verbal cues that most of us take for granted.

On the other hand, he has a special affinity for numbers - and languages. An overcompensating affinity, in some ways, yet something that enables him to appreciate beauty. Albeit of a different type to what most people understand.


He has eventually learnt mechanisms to deal with many of his obstacles, and to make the most of his abilities.

He first came to notice when he offered to memorise and recite pi for over 22,000 digits, as a fund-raiser for epilepsy, which he has suffered.

That illustrates two aspects about him: his ability and his humanity.

His abilities includes a prodigious memory, very very high maths calculation capacity, and synesthesia. The latter is a tendency for some senses to bleed into others. For example, he sees words as particular colours, and numbers as particular shapes.

He can calculate the product of two large numbers by visualising the two numbers as shapes, where the space between them is another shape, which is the answer.


And incredibly, despite his difficulty with human interaction, he is a very humble, honest, warm person. Something we should all be striving for.

More to come about his abilities. Meanwhile, I thoroughly recommend the book.

Tech: The tech personality 2: three mathematicians

I was struck by the profiles of three people who have a mathematical aptitude of one sort or another.

For some people, mathematics takes work. For some, it comes easy. For others, it's an art form.


Australian Terence Tao is the youngest ever winner of the Fields medal, the "Nobel of maths". His mathematical interests are wide-ranging, and he appears particularly well-adjusted.

Russian Grigory Perelman was also awarded a Fields. He declined it because they didn't understand his work - a mite unfair, because you don't need to understand some maths in full to appreciate its truth, beauty and relevance. The Fields was awarded to him anyway. He has apparently solved a very important puzzle - which will probably fetch him the million-dollar millenium prize. He apparently withdrew from mathematics in protest against a perceived lack of ethics in certain people in the upper echelons of maths. Currently unemployed.

Englishman Daniel Tammet has Asperger's Syndrome and, unusually, some insight into his condition. He also has an eidetic memory. He can recite pi to 22,000 places - "as beautiful as the Mona Lisa". He can multiply numbers in his head very easily - as does Tao - but interestingly, he can't do square roots (as some can), and can't do abstractions such as polynomials.

Three very different personalities. All very interesting. I can appreciate them from my love of mathematics, although they're leagues above me.

Here are their Wikipedia entries: Tao , Tammet, and Perelman.
And here's some interesting press about them: Tao, Tammet, and Perelman.

All six articles are worth a read.