In your telephone-line, it enables eavesdropping. In your brain, it could be the mechanism behind metaphors.

I was speaking to a friend on the phone the other day, when I suddenly heard the sound of buttons being pressed. You know — the multi-toned beeping sounds that come when you dial a number. It sometimes happens with new smartphone users who accidentally press numbers with their ears.

The trick for that is to let the phone touch your ear properly, so it can detect it and put the screen off automatically. If you try holding the phone millimetres away so as not to touch it, you will end up touching it, eventually.

In this case, however, my friend was on a landline. It’s a bit harder to accidentally dial numbers there.

The explanation, it turned out, was that her grandmother was dialling a number, from downstairs. My friend and her grandmother had different connections, but the wires touched each other somewhere, so the signals from one phone would sometimes get overheard on the other.

This kind of wire-crossing isn’t always a mistake. Sometimes, people do it deliberately, to eavesdrop on what other people are saying. It’s very simple: all you have to do is take your own wire, connect it to the person’s phone-line, and attach your own phone receiver at the other end.

(Mobile phone signals cross all the time. But they’re designed so that they don’t get mixed up).

Wire-crossing doesn’t just happen with phones. It can also happen with brains.

When you look at light, you see more than simple shades of grey. That’s because your eyes don’t just detect how bright the light is: they can also make out the frequency at which that light is vibrating. That information is sent forth to your brain, which gives you a colour-coded picture of the world.

But why should colour-coding be used only for light-frequencies? Why not for other things — like sounds, or notes of music?

Musician and singer Billy Joel would never ask that question. That’s because his brain does use colour-coding for music. When he hears a certain sound, it’s not just the sound: in his mind’s eye, he also sees a colour associated with it. Different sounds bring up different colours: slower and softer melodies, for example, bring out more shades of blues and greens.

Billy Joel’s ability is one version of what is known as ‘synæsthesia’ — the ability to connect one kind of sense with another. Some synæsthetes (as they’re called) can “see” sounds as colours playing out in their mind’s eye. Others can physically “feel” music touching on their skin, or “taste” the different alphabets, or even interpret smells as different colours.

Why do synæsthetes experience things the way the do?

One of the most common forms of synæsthesia is where people connect a number to a colour. A 7could look indigo, while a 6 seems green.

Psychologist V.S. Ramachandran once found an even stranger case. One person he met was partially colour-blind, so there were some colours he couldn’t see with his eyes. But he could still “see” them when he looked at certain numbers. “Martian numbers”, he called them: those were new colours that came only from numbers, that he had never experienced through his eyes.

That’s very interesting, because colours are processed in an area of the brain called the ‘fusiform gyrus’. And, right next to the colour-area, is the area related to how numbers look when written down! The colour-part and the number-part are almost touching. Just like the two telephone-lines in my friend’s house.

Brains run on electricity, just like phones. They have many small ‘neurons’ that send electric signals to each other.

Neurons can decide how enthusiastically they want to respond to signals from each other. Some neurons’ signals will be passed on as strongly as hey came in; other signals may be forwarded more weakly, or just ignored and not send on at all.

Somewhere out of this intricate web of Chinese Whispers, there arises thinking, and learning, and everything else a brain usually does.

Come to think of it, that shouldn’t be a surprise. If a versatile computer is nothing but a bunch of switches going on and off, then surely a web of neurons — so many more in number, and with so many options besides “zero” and “one” — can do much better?

The way neurons respond to other neurons isn’t fixed. When two connected neurons are “activated” at the same time, they become a bit more open to receiving each others’ signals. And if that happen often enough, they’ll reach a point where activating one neuron automatically activates the other.

That’s how learning happens. In one famous experiment, physiologist I.V. Pavlov rang a bell every time he fed a certain dog. Over time, the “bell ringing” neurons and the “food is ready” neurons got connected. So when Pavlov rang the bell, even when there was no food, the dog’s mouth would immediately start watering.

Of course, it’s actually a bit more complex than that. There are probably several stages between the “food” and “bell” neurons. But that’s the basic idea: one set of circuits starts to activate a different set of circuits. This can happen when two things that the circuits are about are somehow related — in that case, it’s called “learning”.

It can also happen by accident.

Brains are set up quite differently from phones. Telephone networks start out with nothing at all. It’s only later that people come, and dig trenches or put up poles, and finally fix up wires to get all the households connected. But when a new brain starts out, inside the head of a developing baby, all the neurons are already connected.

Too connected, in fact.

As the brain develops, it has to ‘prune’ its connections, removing all the extra ones that weren’t supposed to be there in the first place.

But what if some of those extra connections got missed? The ones from the fusiform gyrus, for instance?

Actually, I made a mistake when I said number-to-colours is the most common form of synæsthesia. There is another one, which is so common that almost everybody in the world has it. Those two senses are always connected, and their connection is taken so much for granted that we don’t think of it as “synæsthesia”.

Can you make out which two senses I’m talking about?

Actually, lots of senses are connected together to some extent. When you hear sounds like yel’oh or krimm-s’n or bloo, you probably think of certain colours even though the two are not directly related. (Web developers can even associate long numbers with colours — but only if they’re hexadecimals, six digits long, and preceded by a ‘#’).

Even so, those colours are only thought about, in your mind. You don’t actually see them, as if they’re actually in front of your eyes. For synæsthetes, the experience can be a lot more vivid.

Also, you didn’t always connect those sounds to those colours. It’s something you learnt slowly, over time. Memory might have a role to play in synæsthesia as well, but it seems like at least some of the connections have always been there.

For a synæsthete, those two senses automatically go together. They seem naturally linked, just like taste and smell.

Have you ever found yourself describing a smell as “sour”, or “sweet”, or even “sweet and fruity”? Why would you do that, if tastes and smell are not linked? Your nose is not your tongue, after all. But still, taste and smell are very closely linked. The smell of food can help to enhance its flavour. And if your nose is blocked, or for some other reason you lose your sense of smell, then food begins to taste much more bland as well.

As it turns out, smell and taste are linked in the brain as well. The places where they’re felt are not quite next to each other, but they’re closely interlinked.

Yes — that’s the synæsthesia I was talking about, the one that everybody has: the linking of taste and smell.

Not everything you taste is a taste. It can also be a texture, or a feeling.

What about the “smoothness” of honey, for example — or the “hotness” of a chilli, or the “sharpness” of a cheese?

How can honey feel smooth? Maybe because it’s on your tongue, which feels it like skin does as well as tasting its sweetness. But what about the chilli? It’s not hot at all.

I once tasted a chilli pickle. It was hot, but it was cold. Temperature-wise, it wasn’t hot at all. If you taste it, it could even feel cold. And yet, when you bite it, it makes your mouth burn.

Burn? But is it burning? I don’t see any flames coming out!

After a point, it seems like we’re taking words from one sense and using them for similar feelings in another. But why are they similar — and in what way? What does the taste of a good, strong cheese have to do with getting pricked by a thorn?

And it doesn’t stop there. People often use phrases like “loud colours” or “flashy pictures” or “rough, grating noises”. Do these metaphors — describing one thing as if it’s another thing — represent real, physical neuron connections in the brain?

Psychologist V.S. Ramachandran, among others, thinks they do. What’s more, he noticed that synæsthesia is more common among artists, musicians, novelists…in other words, those whose job it is to take two unrelated concepts and connect them together.

While this is yet to be tested, he thinks those metaphors are possibly because because of un-pruned neuron links in the brain. Connections that were made in the beginning, and never cleared away.

If that’s true, then synæsthesia could be just an extreme version of a metaphor. A connection which is always on, that is up and running before you even begin to hear the dial-tone.

This article was inspired by “Purple Numbers and Sharp Cheese”, a lecture by V.S. Ramachandran.

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