There was such a theory, developed in the 1960s and 1970s, I think, although I can remember neither the name of the linguist (I don't think it caught on broadly

There was such a theory, developed in the 1960s and 1970s, I think, although I can remember neither the name of the linguist (I don't think it caught on broadly, so it really was just one researcher) nor the name of the theory. I think it foundered for a number of reasons, one of which was that it had--so far as I know--no account of the creativity of humans "doing" language. That is, we can understand and speak any number of sentences we've never heard before, and it's hard (or impossible) to imagine that all those novel sentences were ever in the brain, or that new nerve connections grew so rapidly. For example, I can tell you that I've invented a fugelizer, and that it is useful for turning bryles into fugels. Now I've just used three words that you've never heard before, but you can sort of make sense of what I've said. But clearly there's no 'fugelizer' neuron in your brain, etc.