Bandwidth of your brain

How much bandwidth does it take to run a completely convincing virtual environment for one person? The answer is surprisingly small. In Consciousness Explained, Dennett claims that an "interactive smello-feelo television would have an astronomical bandwidth" (6); he didn't bother to estimate how astronomical. If you've ever studied physics, you'd be familiar with this kind of estimation: a Fermi problem. Let's play:

A Blu-ray disk outputs a maximum of 40 Mbit/s; it's the best quality video for the general consumer, and it's just about as good as our eyes can see. Blu-ray has surround-sound audio too, so the 40 Mbit/s covers two of our senses: sight and hearing.

Smell and taste are both slower and less sensitive than hearing, and way less than vision. We can distinguish something that's flashing once per second from something that's flashing once per half second. We can easily tell when a musician switches from one time signature to another. But have you ever smelled something that pulsates every second? 10 Mbit/s should cover both smell and taste, and that's being generous.

Proprioception is even simpler. The body has 200ish bones and a similar number of joints. At each moment we are aware of the angle of each of those joints. We are also aware of how tense each of  those skeletal muscles are. A virtuoso piano player can hit about 20 notes per second, and she must know exactly where her fingers are; that gives 20*200*16 = 64 kbit/s, with the 16 bits encoding the angle.

That just leaves touch: pressure, pain, and heat. Skin is the biggest sensory organ in the body, but is it more sensitive than sight? We can feel tiny vibrations and locate pressure with pin-prick precision, especially near our hands and lips. I think 50 Mbit/s would suffice for the entire body (including those extra sensitive parts :)

What about output? The only interesting way of sending information is through our muscles. Yes, I'm going to exclude any excretion of bodily fluids. We have 600ish skeletal muscles which can be moved at about 20 times per second, with varying levels of contraction. That's only about 1 Mbit/s. It's not surprising that our sensory input is 100 times greater than output. You'll probably find the same ratio if you look at your computer's network usage.

So that adds up to about 100 Mbit/s, give or take a factor of 10. In comparison, the current wireless LAN protocol (802.11n) has a max data rate of 600 Mbit/s: more than enough to saturate every one of our senses. All we need is a modem to convert digital signal to the right impulses, and splice it into our nerves.

 Yes, you can kick ass in the Matrix through an 802.11n wireless network.

What about Dennett's other objection, that interactive simulations will take more bandwidth? That is actually a mistake; I have already covered all the input and output channels of a human brain, and analyzed the total bandwidth. Interactivity with the environment is a result of the laws of physics: gravity, thermodynamics, light, solids and liquids, etc. Calculating the result of you throwing a rock into a pond will not demand any more bandwidth; this simulation only requires processing power. Today, we have graphics chips (from nVidia and ATI)  that can produce real-time, nearly realistic rendering of scenes; these chips use specialized circuits to compute the way light interacts with matter. Those same companies are also making specialized physics chips that can simulate how pieces of matter interact with each other.

What's the hold-up then? Why aren't we hooking our brains up to virtual worlds yet?

Oh, right.




Just found this: Each American consumed 34 gigabytes per day in 2008. That translates to only 3 Mbit/s, but the study does not include senses other than sight and sound. It's also analyzing average input; I'm looking at the maximum possible throughput.