Out of Body Experiences

In one of those Hollywood coincidences, two recent movies, “Avatar” and “Surrogates” made use of the idea of remote-controlled bodies.   In “Surrogates”, people had robot bodies that were so much better than real ones that no one went out in public in the flesh any more.  Their robots were so much stronger and better-looking than they were  (E.g. Bruce Willis once again has hair!) that people preferred them.   They would lie on couches at home plugged into some kind of neural control system and walk their perfect selves around.

Bruce Willis zoning out in “Surrogates”

In “Avatar”, people use vat-grown bodies to walk around on an alien world.  The air is poisonous and the natives are hostile, so this is the only way to move about freely.   There too people lie on a couch to control their remote bodies.

The ideas are similar but the usages are not.  “Surrogates” uses its robots to make literal the behavioral masks that people wear in society.   Instead of having to learn to smile and make small talk, you let your robot be your persona.   That’s intriguing, but they didn’t do much with it.  The robots are mainly used to jump over cars and get shot in big chase scenes.  The surrogates are ultimately considered to be evil for some reason and are all destroyed.   Personally, I think it would be great to not have to risk your one and only body in some risky occupation like Willis’s policeman, but movies are much more conservative than SF fans.

In “Avatar”, the bodies are a way to literally see through another’s eyes, to empathize with people who are completely foreign.   They don’t live or think the way we do, and unfortunately they have something we want.  It becomes all the more important, then, to understand their perspective before they get steamrolled by modernity.

Anyway, these two movies got me wondering what would actually be involved in these neural controllers.   They would have to interface to our nervous systems somehow.  How much data would actually have to be sent and received?  What’s the bandwidth of the human IO system?

It turns out that no one knows.   That is, I was unable to find any reference that listed basic things like how many nerve fibers run from the spinal cord to the muscles and sensory organs.    The best I was able to find was this paper, “Measurements and mapping of 282,420 nerve fibers in the S1–5 nerve roots” from 2009, that described the nerves that run from the 5 sacral vertebrae, the ones at the very end.  The researchers had dissected several cadavers, taken pictures of the spinal cords, and counted them.   By hand!  They found ~280,000 nerve fibers total, of which the majority were sensory, followed by motor, and then “parasympathetic”, which control organs.

There are 31 pairs of spinal cord nerves total, one for each vertebra.

The other spinal nerves are unlikely to have similar numbers (E.g. the hands are likely to have far more nerves than the legs and feet), but I can’t find any references for them.  If they also had ~60K fibers/nerve, the whole spinal cord would have ~1.7M fibers.

The nerves that run directly from the brain to various parts of the skull are better understood, and are listed here.  There are 12 pairs of cranial nerves, of which the optic is by far the largest, with 1.2 million fibers to each eye.  The auditory only has 30,000, 40X fewer.   A high-quality video stream, such as the one from a Blu-ray player, needs 20 Mbits/sec (compressed, not raw pixels), while a high-quality compressed audio stream needs 200 to 500 Kbits/sec, so the 40X about jibes.

So let’s call it 1.2 * 2 + something ~= 3M fibers for cranial nerves.  With another 1.7M for the spine, we’ll round it up to 5M nerve fibers total that connect you to the outside world.  At what bit rate do these nerves run?  This study from U Penn in 2006 looked at the data rate in guinea pig optical nerves, and found that they averaged ~10 bits/second.   If that’s true for all fibers, that would mean that your total bandwidth is only ~50 Mbits/sec.  That’s about what a good wireless link can do.

So if you were floating in a tank inside the Matrix (and who’s to say you aren’t?), it would only take a medium-speed Ethernet to fool you into thinking you were in the real world.   The evil Matrix robots would have to spread that Ethernet into 5M fibers, most of which are only a micron across, but we’ll wave our hands vigorously and say “micro-machined silicon neural interface chips” and “superconducting quantum interference device magnetometers”, and leave it at that.

Of course, there are easier ways to take you out of your body.  Movies do it all the time.  You forget about the taste of the stale popcorn and the way your feet stick to the floor, and are lost in the story.   Digital cinema data rates are about 100 Mbits/second (again, that’s compressed with JPEG 2000, not raw), so that’s not far off of what it take to simulate everything.   The latest films double the data rate again for 3D, although it’s possible to share info between the two views.   That’s not easy, but it’s worth it if it lets theaters raise the price a couple of bucks, and so raise their margins from almost zero to something respectable.

However, there is a way to leave your body at a far lower data rate.  The technique takes a lot of practice, and usually has to be learned in childhood.   Many people, maybe even most, never got comfortable with it.  It can be physically damaging if done to excess, but it’s crucial for certain professions.

I’m referring, of course, to reading.   A typical novel contains about 100K words  (10 Mbit) of text, and takes maybe 3 hours (10K seconds) to read, for a data rate of 1000 bits/second.   Once immersed in it your own brain will create all the other sensations.   Your memory will provide smells, touches, sights, and sounds.   Your predictive and pattern-matching faculties (OK, call it your imagination) will invent the feeling of sailing with Long John Silver or playing quidditch.   It turns the brain against the body and disconnects it from the world.    No wonder some think it’s dangerous!

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