[Epistemic status: Very speculative. I am not a neuroscientist and apologize for any misinterpretation of the papers involved. Thanks to the people who posted these papers in r/slatestarcodex. See also Mysticism and Pattern-Matching and Bayes For Schizophrenics]

Bayes’ Theorem is an equation for calculating certain kinds of conditional probabilities. For something so obscure, it’s attracted a surprisingly wide fanbase, including doctors, environmental scientists, economists, bodybuilders, fen-dwellers, and international smugglers. Eventually the hype reached the point where there was both a Bayesian cabaret and a Bayesian choir, popular books using Bayes’ Theorem to prove both the existence and the nonexistence of God, and even Bayesian dating advice. Eventually everyone agreed to dial down their exuberance a little, and accept that Bayes’ Theorem might not literally explain absolutely everything.

So – did you know that the neurotransmitters in the brain might represent different terms in Bayes’ Theorem?

First things first: Bayes’ Theorem is a mathematical framework for integrating new evidence with prior beliefs. For example, suppose you’re sitting in your quiet suburban home and you hear something that sounds like a lion roaring. You have some prior beliefs that lions are unlikely to be near your house, so you figure that it’s probably not a lion. Probably it’s some weird machine of your neighbor’s that just happens to sound like a lion, or some kids pranking you by playing lion noises, or something. You end up believing that there’s probably no lion nearby, but you do have a slightly higher probability of there being a lion nearby than you had before you heard the roaring noise. Bayes’ Theorem is just this kind of reasoning converted to math. You can find the long version here.

This is what the brain does too: integrate new evidence with prior beliefs. Here are some examples I’ve used on this blog before:

All three of these are examples of top-down processing. Bottom-up processing is when you build perceptions into a model of the the world. Top-down processing is when you let your models of the world influence your perceptions. In the first image, you view the center letter of the the first word as an H and the second as an A, even though they’re the the same character; your model of the world tells you that THE CAT is more likely than TAE CHT. In the second i...

Hypotheses and Hunches