Digital pain (rerun)

Today, another fun-for-your-Friday rerun from the past decade of FAIC.


When you bang your finger with a hammer or burn it on the stove, somehow the pain has to get from your finger to your brain via a nerve. That’s an immense distance on the cellular level. What possible mechanisms are there for that?

A nerve carries the pain signal from your finger to your brain like a wire carries electrical current. Perhaps zero “voltage” on that nerve would represent “no pain”, and then the “voltage” would vary smoothly up to some maximum that represented “extreme pain”. That’s a plausible mechanism.

Or, you could have a system where zero “voltage” meant “no pain”, 100% meant “severe pain”, and any lesser amount of pain is measured by the average power delivered over a period of time, say, a millisecond. If the nerve was on 100% for 250 microseconds, then off for 750 microseconds, that indicates a 25% level of pain. If it was then on for 220 microseconds, off for 780, then on for 200, off for 800, that would indicate that pain was decreasing from 25% to 22% to 20%. The granularity of a millisecond might not be quite right, but in principle this would work. [1. Lighting circuit dimmer switches work using these two mechanisms. Old-style dimmer switches work by adding a resistance to the line that decreases the voltage overall, and modern dimmer switches work by cutting out a certain percentage of the power signal.]

Both of those are analog systems: in an analog system the possible signals smoothly vary from 0% to 100%. But neither of them are how nervous systems actually work. If you measure the “signal strength” on pain nerves you see that they actually send groups of extremely short bursts, where the number of bursts per unit time indicates the level of pain. That is, the nervous system communicates pain by sending an integer from the source of the pain to the brain! It’s discrete, not analog.

Why is that?

You know what my favourite scene in the movie version of The Return of the King
is? It’s the one where Gandalf needs to send a message to Rohan in a hurry, so he has Pippin climb up the side of a mountain to light a signal beacon. We then see this great sequence as the beacon wardens set off the seven signal fires on Amon Din, Eilenach, Nardol, Erelas, Min-Rimmon, Calenhad and Halifirien, one after the other.

Listen carefully to the soundtrack at this point. It echoes Gandalf’s “White Rider” theme, which was established earlier in a visually parallel set of shots, as Gandalf and Pippin ride up the seven levels of Minas Tirith. But to give it some additional fire, they add these kick-ass violin arpeggios on top of the basso continuo. The whole thing works perfectly; it’s a triumph of cinematography!

But I digress.

The Gondorian war beacon system sends a single binary bit at extremely high speed over the huge distance from the White Tower in Gondor to Meduseld in Rohan. Suppose the Gondorians wanted to send more information than just “we need help!” — like, say, how big the opposing army was. They could come up with an analog convention. Perhaps a really big signal fire indicates a really big army, a small signal fire indicates a small army, a medium sized signal fire indicates a medium-sized army.

Obviously that doesn’t work. There are seven beacon fires. The beacon keepers would have to figure out how big the previous fire was and then set their fire accordingly. Error would accumulate along each part of the process, and the final result might bear little relation to the original input.

To send information accurately over long distances without error you need some kind of discrete system. You need signals that are clearly either ON or OFF. Once you’ve got ON and OFF, you can use them to transmit integers, letters, morse code, whatever you want.

That is why nerves are digital, not analog. Nerves need to transmit huge amounts of complex data over the vast distance from your toes to your brain without accruing any error along the way as the signal is picked up by new nerves and forwarded on. All the sensory nerves work this way: smell, touch, taste and so on, are digital. We are digital machines, we’re just digital machines made out of meat instead of silicon.


Next time on FAIC: We’ll resume the current series on uses and abuses of the static constructor.

16 thoughts on “Digital pain (rerun)

  1. I had a nerve conduction study done on my right forearm not too long ago. A doctor basically pokes you with a very short needle, much like a thumb tack, and the electrical impulses are transferred to a machine, which records the impulses for analysis and plays an audible crackling sound. It sounded a lot like a Geiger counter to me. It was quite interesting literally listening to the electrical impulses as they were traveling between my arm and brain. It was especially interesting to hear how the crackling became much more rapid as I increased movement in my fingers.

    • SCIENCE! in which you can hear the nerves firing in a cockroach leg, and conversely move the leg by feeding the nerves an audio source.

      • Wow…great link there!

        The doctors actually made my fingers jump as well as part of their tests. It was somewhat unnerving (haha!) having my fingers jerk uncontrollably as the pulses went through.

        There is also a rather interesting segment in the “What is the Secret of Life” episode of the BBC’s “History of Science” series where Michael Mosley (the narrator) is hooked up to electrodes which contort his face and tap his feet as music is played. You can watch the video here: http://video.idahoptv.org/video/2228034066. The relevant bit begins at roughly 33 minutes in, but I highly recommend the whole series. Lots of very interesting stuff.

  2. So if Aragorn burnt his finger on a stove, I guess that might lead to Gondor calling for Band-Aid.

    Yes, yes, no need to groan, I’ll see myself out, thank you.

  3. “…we’re just digital machines made out of meat instead of silicon.”
    Here’s a quotation from Wikipedia: “Each part of the brain shows a mixture of rhythmic and nonrhythmic activity, which may vary according to behavioral state. In mammals, the cerebral cortex tends to show large slow delta waves during sleep, faster alpha waves when the animal is awake but inattentive, and chaotic-looking irregular activity when the animal is actively engaged in a task.” Yes, the signals from the peripheral nervous system may be digital, but what about the rest of the brain activity? What about thinking, imagining, creating, fantasizing?
    OK, maybe I sound religious here, but I sincerely doubt that a mere digital machine could write like Tolkien: despite being a software developer for 20+ years (or maybe because of that) I have to say I still believe we have something in us that computers will never have (unless we lose it, too, and become just like them, making the Matrix real).
    And, Eric, you almost said it yourself back on the 10th of April, 2006, in your post titled “Writing Code Isn’t Rocket Science (It’s Worse Than That)”, where you reason that the rocket science at least deals with something knowable, whereas brain surgery works on human brain that remains a bit of enigma.
    Thanks for the great article, anyway!

  4. Even though the sensory signalling system is discrete, it is still analog rather than digital; the time between pulses is inversely proportional to the degree of pain. (Would you also argue that a rain gauge is digital because the number of water molecules it contains is an integer?) Similarly, we apparently perceive pitch because of neurons that fire with the frequency of the sound we’re hearing (see Daniel Levitin, This is Your Brain on Music). Finally, I haven’t seen the movie, nor do I know its soundtrack well, but I suspect that “basso ostinato” better describes the White Rider theme than “basso continuo.”

  5. An off-topic bugreport: When you include a footnote, the link contains an absolute URI (e.g. “href=”http://ericlippert.com/2013/02/08/digital-pain/#fn-434-1″”), but if I came here from an RSS feed, I am reading the article at something like “http://ericlippert.com/2013/02/08/digital-pain/?utm_source=rss&utm_medium=rss&utm_campaign=digital-pain” – which means clicking on the footnote triggers page reload (because of all those extra URI parameters). Couldn’t you use just “href=”#fn-434-1″“ for the footnote links?

  6. Geeky nitpick: the theme which plays over the “beacons” sequence is the *Gondor* theme, not attached to Gandalf. It’s originally introduced all the way back at the Council of Elrond, playing under Boromir’s speech about Minas Tirith. Dead on about the cinematography, though!

    (And I’m glad I wasn’t the only one who thought of “They’re Made out of Meat”, as noted by the friendly neighborhood Overlord above.)

  7. Note that “[The] White Rider” theme (OST LOTR2) is actually used when Gandalf rescues the beaten-up-by-Orks-at-Osgiliath Faramir and troops on the fields of Pelennor from the Ringwraiths. The piece used in the scene you describe is called “The White Tree” (OST LOTR3).

  8. > If you measure the “signal strength” on pain nerves you see that they actually send groups of extremely short bursts, where the number of bursts per unit time indicates the level of pain. That is, the nervous system communicates pain by sending an integer from the source of the pain to the brain! It’s discrete, not analog.

    Uh… frequencies aren’t discrete, there’s no reason you can’t have two and a half bursts per unit time.

  9. Great post Eric, and I think you got it mostly right. My only complaint is your conclusion: “That is why nerves are digital, not analog”. To make this conclusion is to assign intent and power to evolution. Nerves are (mostly) digital because they conferred fitness to certain organisms over many generations. Furthermore, there are many signal transduction systems in the body which remain analog, and yet they function perfectly well.

  10. Very interesting, specially because it is initially kind of counter-intuitive (actually, most of your posts are). Then if we were indeed analog machines, we would look absolutly crazy and volatile creatures. ;)
    Also, I don’t mean to annoy, but in the paragraph with the reference (1), you say first milliseconds, then microseconds and milliseconds again.

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