Evolution doesn’t give a damn what you think a brain region is called
By: Mark Humphries
Tags: Neuroscience, Psychology, Artificial Intelligence, Machine Learning, Science

To evolution, the brain is just a gigantic bag of cells, wired together. The purpose of that gigantic bag of cells is to contribute to the survival of the organism in which it resides, to surviving long enough to reproduce. Those that reproduce, win; those that don’t, don’t.

If a random mutation causes or changes the wiring of some neurons to another group of neurons, and that mutation improves the chances of having offspring, it will likely spread through the population. If that random mutation adds connections from prefrontal cortex to visual cortex; from cortical interneurons to a structure outside cortex; from motor cortex to midbrain dopamine neurons, then it will happen. Evolution will not feel sorry that it’s just ruined another set of textbooks.

And it is just a giant bag of cells wired together. Our best evidence that it is not — that we can cling to our names of all the bits — come from studies where we cut a bit out or turn a bit off. When we cut out area X and we see a “deficit” in behaviour Y of an animal (like tying its shoelaces), then we think “aha! Area X is for tying shoelaces”. No. For starters, we never see a complete and permanent end to behaviour Y. We normally see that the animal is simply worse at doing or learning Y — not that it cannot do Y at all. The brain can carry on doing Y just fine, thanks, just not as well — there is massive redundancy in the brain. Like what you’d find if it was a giant bag of cells, wired together.

Moreover, seeing that behaviour Y gets worse logically tells us little about what area X is actually doing. It just tells us that damaging area X causes problems. Which on reflection isn’t surprising as you just ripped a chunk out of the brain. The logical fallacy is simple to demonstrate. I am right now going to make a new startling new prediction of how the mouse brain works: the ventrolateral medulla is necessary for mice to learn to associate pictures of Benedict Cumberbatch with food. Cut out the ventrolateral medulla, and a mouse will not learn to associate pictures of Benedict Cumberbatch with food.

Because it will be dead. The ventrolateral medulla contains the neurons which control the rhythm of breathing. Cut it out: no breathing. Ergo, no learning. Is the ventrolateral medulla a crucial brain area for learning? No. But by damaging it, we damage something vital to the process of learning. Thus, cutting a bit that we’ve given a name can have an effect on that named thing, and we learn nothing at all. Except that we have damaged a big bag of cells, wired together.

What’s more remarkable is when cutting bits out has no effect. If we cut some bits of brain out before learning we see an effect — learning is made slower or worse or both; but when we cut them out after learning, it has no apparent effect whatsoever. These bits of brain have become completely redundant. Again: giant bag of cells, wired together — there are many ways within that bag of cells to solve the problem at hand, enough for the brain to just stop using a bit of itself altogether.

And people get hyper-excited about finding odd signals where they were utterly unexpected. Like finding that reward changes activity in primary visual cortex. Or finding that sound is encoded in the hippocampus. These are great, interesting studies. But to be surprised by them is to fall into the naming fallacy. Evolution does not know nor care that we called this chunk of neurons the “hippocampus”. It is just another bag of neurons, connected to many other bags of neurons.