[gedankespieler]

If areas of the brain could have any purpose, why are the same purposes allocated to the same areas in each person? This doesn't make any sense to me. Of course, brains aren't totally uniform, but the fact that you can say "x area is used for y activity in general" seems pretty absurd. Or maybe there is a particular, most efficient arrangement of departments. But surely this can't be achieved in every individual. I wish studies didn't try to be as general sometimes - the anomalies are more interesting.

[aisofteng]

This is an excellent question, and an unanswered one. I want to add that functional areas of the brain are not 100% consistent and in fact vary not only by handedness but also by enough that a neurosurgeon cannot confidently operate on a specific location of the brain without first verifying what functionality it exhibits.

That said, the remarkable consistency of the location of, say, Broca's area, has always baffled me. Human DNA encodes signals that trigger the development of the neural tube and later the nervous system; that much is understandable - but how could it be possibly also encoding enough information to consistently result in similar functionality in similar areas?

As someone that has done an undergraduate fellowship in mathematical modeling of plasticity in large neural networks (and thus knows just enough to be confident about being wrong without realizing it), my personal mental model for this is that a functional area arises in an essentially deterministic manner based on its inputs - the auditory nerves, optic nerves, sensory input, and so on; the initial neural network must have no predetermined function and instead must have its functionality arise as a consequence of recurrent associations between input, output, and subsequent input, and so on.

That is, if a certain area receives visual input, it will adapt to discriminate details in visual sensory information, and similarly with auditory input, and so on - and eventually the only place that, say, Broca's area for language _could_arise is at the intersection of a particular set of sensory inputs.

As I said, this is essentially layman speculation. Furthermore, I try to imagine the computational complexity of demonstrating such determinism using a model, and it is staggering. There are on the order of 10^11 neurons, each of which is not only a very complex biochemical unit on its own but also can be modulated by or modulate up to something like 100,000 other neurons - let alone simulating the input to such a model.

It truly is staggering. We know so little, and it isn't even clear where to look to find out what we don't know we don't know.

[jonsen]

For one sensory input nerves and motor output nerves have distinct entry/exit points. I.e. visual preprocessing must ofcourse take place in the brain where the optical nerves enter.