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During the development of the human brain, specific areas come to perform specific functions. How (and when) does this differentiation come about?

Presumably, some areas of the brain naturally take on a function that is based on their physical proximity - for example, the visual cortex is the brain area directly connected to the optic pathway. Hence from a developmental perspective, it is natural for it to take on the role of processing visual information.

However, the production of language, for example, could potentially take place in any area of the brain adjacent to the auditory cortex. So how does it come about that it typically always gets processed in Wernicke's area?

  • Is it predetermined genetically? And if so, how? (pre-mapped synapses?)
  • Or is it determined later through some kind of learning process? And if so, what is the explanation for the similarity of brain maps between individuals?
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  • $\begingroup$ I think @what 's problem is the words "how does it 'know' to do this". The issue I have with your question is that it is unclear. I like the subject a lot, and I think this is a nice question, it just needs some clarification. $\endgroup$
    – AliceD ♦
    Aug 2 '15 at 10:39
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    $\begingroup$ Specifically: your title implies a neuro-anatomical question (in which case the question is too broad as is). Your first line in the body text implies a neurophysiological methods question (an easy question). The second line implies a functional neurophysiological question (a hard one to answer and likely too broad, even when focusing on Wernicke's area alone, as books can be filled on the topic). Your last line implies a teleological (almost philosophical) question, which may be interpreted as a genetic one, or an evolutionary question. $\endgroup$
    – AliceD ♦
    Aug 2 '15 at 10:39
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    $\begingroup$ I agree with AliceD that this question is both potentially interesting, and unfortunately unclear, as it could be interpreted in a variety of different ways (some of which are too broad to answer). I chose an interpretation that I'm interested in :-) and rewrote the question accordingly. @SydneyMaples, please feel free to back out or re-edit it if I've completely lost the original spirit, but maybe I can help keep an interesting question from being closed. $\endgroup$
    – Arnon Weinberg ♦
    Aug 2 '15 at 18:16
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    $\begingroup$ @ArnonWeinberg and AliceD Ah, thank you for clarifying for me where the confusion was. I'm the queen of asking broad / vague questions (maybe I tend to look for broad answers), and it tends to bite me back sometimes. ;) The edit is helpful and makes the question a bit more answerable. $\endgroup$
    – Sydney Maples
    Aug 2 '15 at 21:54
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Short answer
Neural wiring is governed by nature and nurture.

Background
I'm not a language person, but I will try to address the question on more familiar grounds to me, namely sensory systems.

A bunch of most intriguing studies have addressed your question directly. Among these is the study from Frost & Metin (1985), who severed the optic tracts of newborn hamsters and found that optic axons would project to the somatosensory thalamic (ventrobasal) nucleus instead. The experimentally induced retinal projection to the somatosensory nucleus occurred through the stabilization of an early, normally transient projection. Moreover, visual stimulation reliably evoked neural responses in the primary and secondary somatosensory cortices (SI and SII) in treated hamsters. Not only that - the representations of the visual field in SI and SII showed a partially retinotopic organization. In other words, the animals had effectively converted (part of) their somatosensory cortex into visual cortex.

Later, Sur et al. (1988) showed with a similar procedure that retinal cells could be induced to project to the medial geniculate nucleus (MGN), the principal auditory thalamic nucleus, in newborn ferrets. They showed that many MGN cells were then visually driven and that the auditory cortex became visually responsive. Some visual cells in auditory cortex were direction-selective or orientation-sensitive, resembling the complex cells in primary visual cortex.

Hence, functional visual projections can be routed into nonvisual structures in higher mammals, suggesting that the modality of a sensory thalamic nucleus or cortical area may be specified by its inputs during development. However, there is a short time window where the extent of neural plasticity will allow such gross abnormalities to develop - both studies used newborn animals therefore.

References
- Frost & Metin, Nature (1985); 317(6033): 162-4
- Sur et al. Science (1988); 242(4884): 1437-41

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  • $\begingroup$ I really like this answer, though I am also wondering if you know why brain regions outside of the sensory system are always associated with the same area of the brain? For example, why might executive functioning be primarily associated with the frontal lobes of the brain? $\endgroup$
    – Sydney Maples
    Aug 25 '15 at 17:47
  • $\begingroup$ @SydneyMaples - That's an intriguing question and your original question reflects the same thing. Quite frankly I don't know and therefore I chose to address sensory systems instead. I was already pretty baffled when reading about the 'rewired hamsters' :) Maybe someone else will take another shot. Thanks for accepting! $\endgroup$
    – AliceD ♦
    Aug 25 '15 at 23:33

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