The brain has plasticity in what areas perform specific tasks, like when a person has a brain injury in a visual processing region, another section can adapt to take over that function. So why are we born with specific functional regions that are in the same area of the brain for most people? I would expect that the organization would be randomly unique per person.
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Two main reasons. First, the structure and development of the brain isn’t random. The baseline structure of the brain is driven by genetics. Second, we are all human beings, and so share a lot genetically and experientially.
We don’t fully understand this, and we also aren’t 100% certain that all areas develop the same between people, since we don’t really understand what all the different brain areas do.
The areas we understand best are the ones that are closest to the inputs and outputs of the brain. Inputs being sensory information, and outputs being (basically) muscle commands. We understand those best because we can easily correlate the neural activity in these areas with things we can measure. For instance, we can see the visual cortex “light up” (become more active) when the person is looking at a high-contrast visual stimulus, and we can see different parts of the motor cortex light up when the person moves different parts of their body.
For these areas, it’s clear why they are (broadly) where they are: because that’s where the sensory and motor neurons go. The optic nerve, for instance, goes from the eyes, via the thalamus in the midbrain, to the occipital cortex at the back of the brain. Since that’s where visual information comes in, that’s where the visual cortex normally develops. If you’re born blind, then there is no visual information coming in, and that gives this part of the brain the freedom to develop into something else, like auditory cortex.
The visual cortex is made up of further specialized areas, though, and only the primary visual cortex receives direct input from the optic nerve. So the question remains: why do all the other visual areas develop (apparently) the same way in different people. For instance, there’s an area of the visual cortex that seems specialized in processing faces (the “fusiform face area”), and we always find it in roughly the same place (the fusiform gyrus). And we don’t fully understand why this is the case. There must be some genetic component to it, that pre-determines at least the broad connectivity structure between visual areas. However, there is likely also a component that is based on learning from experience, where the structure of the visual input itself helps determine “what goes where”.
And then there are regions of the brain like large swaths of the parietal and frontal cortex, which are involved with more abstract thought and decision-making, and we don’t really understand their functional organization into areas very well (if there is any). So there it’s harder to say for sure whether that organization is the same between individuals.
Finally, just to comment on this:
>like when a person has a brain injury in a visual processing region, another section can adapt to take over that function.
That isn’t always the case. The plasticity of the brain isn’t perfect and decreases with age. If you have a brain injury in your visual cortex when you’re very young, there’s a good chance other areas can take over. In particular, if it’s a local lesion that only affects part of your vision, then other parts of the visual cortex can especially easily take on the role of processing the visual input that otherwise had nowhere to go. But the larger the damage, the harder this is. Also, if the injury happens later in life, the brain is less plastic, and is less likely to be able to adapt.
Think of it like this: you’re training an orchestra where nobody knows any instrument yet. They’re all talented students, but they can only master one instrument. The person who happened to sit down in the violin section on the first day will end up playing the violin, the person who happened to sit in the oboe section will learn the oboe, and so forth. The better they learn how to play their instrument, the harder it is for them to switch to something else.
Now let’s say a bomb goes off somewhere in the orchestra. If this is during early training, and it takes out only the violins, then the some of the violas and cellists can step in and pick up the slack. But if it takes out the entire string section, you have a problem. Maybe some people from the adjoining sections can pick up a violin or a double bass, but you can’t fill the entire string section any more. Or, let’s say the bomb takes out the violinists when the orchestra is already far along in their training. The cellists are so specialized now that they can no longer learn to become violinists – at least not very well. So maybe you can partly replace some of the violinists, but they’ll never sound the same, and you’ll always have a gap in your orchestra.
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