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Your Senses All Send the Same Signal

About 10 minutes

Your eyes do not see and your ears do not hear. They collect, and they send, and the seeing and the hearing happen in your brain. That is true, and it leaves an obvious hole in the middle of it.

Send what? Something leaves your eye and arrives in your brain. Whatever it is, it is not light, because light stops at the back of your eye and goes no further. So what makes the journey, and how does your brain know that this particular arrival means a red bus rather than a loud bang?

What actually travels

Nerves are not tubes. Nothing is poured down them. What runs along a nerve is a pulse — a fast little flick of electrical charge that travels the length of the nerve and arrives at the far end.

That is all. Not a picture, not a sound, not a colour. A pulse.

They move quickly. In the fastest nerves a pulse covers about a hundred metres in a second, which is why you can pull your hand off something hot before you have finished thinking about it.

Every pulse is exactly the same size

Here is the part that catches people out, and it is the whole answer.

A nerve pulse cannot be big or small. It cannot be strong or weak. It either happens or it does not, and when it happens it is the same size every single time. Your ear sends the same size of pulse for a whisper as for a shout.

So how does your brain know a shout from a whisper? By counting.

A quiet sound sends a few pulses a second. A loud one sends a great many. Nothing about any individual pulse changed — there are simply more of them, arriving faster. Brightness works the same way, and so does how hard something is pressing on your skin.

Follow one pulse all the way

Step through a single touch — a fingertip pressed against the edge of a table — and watch what happens to the signal at each stage.

Three different senses sending three identical signals to three different places Three separate tracks run left to right. Each one starts with a different collector: at the top a small burst of light, drawn as a dot with six short rays around it; in the middle a set of three curved lines opening rightwards, the way sound is usually drawn; at the bottom a short arrow pressing down onto a flat line, meaning a push on the skin. After the collector, all three tracks carry exactly the same square pulse pattern, tall and short in the same places, so the three signals are indistinguishable from one another. On the right the three tracks arrive at one tall panel divided into three separate compartments, each track entering a different compartment. The signal is the same in all three tracks. Only the compartment it arrives at is different.
  1. The table pushes on your fingertip. A nerve ending in the skin is squashed slightly out of shape, and that squashing is enough to set off a pulse.
  2. The pulse runs up the nerve in your finger. It is not fading and it is not growing. It is the same size at your wrist as it was at your fingertip.
  3. It passes through your arm and into your spinal cord, still unchanged. Press harder and you do not get a bigger pulse — you get more of them, one after another.
  4. It arrives at the touch part of your brain. Only now does it become a feeling, and the feeling is "something is pressing on my finger".

Look back at those four stages and notice what never changed. The pulse that arrived is the same pulse that set off. Nothing was added to it on the way to tell your brain it came from a finger, and nothing in it says "touch" — because a pulse from your eye would look identical.

So the address is the answer

If every pulse is the same, the difference cannot be in the signal. It has to be in where the signal goes.

Your brain has a separate patch for each sense. Pulses from your eyes end at one patch, pulses from your ears at another, pulses from your skin at a third. Each patch has one job and does that job with whatever arrives.

The touch patch does not check where a pulse came from. It cannot. It simply treats everything that reaches it as touch. The seeing patch treats everything that reaches it as sight.

That is the whole trick. Your senses are not different because they send different messages. They are different because their wires go to different places.

Imagine the wire from your ear could be re-routed so its pulses arrived at the seeing patch instead. Somebody claps. What happens?

Why this is worth knowing

It changes what your eyes and ears actually are. They are not windows. A window shows you the outside; your eye does not show your brain anything. Your eye is a converter, and everything it converts comes out looking the same as everything your ear converts.

Everything you have ever seen, heard or felt reached you as the same small pulse, repeated at different speeds, arriving at different addresses. The world you experience is assembled at the far end, out of a signal that carries none of it.

Your brain has never once touched the outside. It has only ever counted pulses, and worked out the rest.