Eli5: is the part of the brain responsible for seeing things through your eyes also responsible for seeing images in your mind?
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Edit: Ironically, my visual system failed to notice there wasn’t a “why” at the beginning of this question, so this is answering the “why is this the case?” version, but should still apply to the “is this the case?” version.
Because why waste energy having two visual processing centers when one visual processing center does the trick?
Of course, in reality evolution isn’t intentional. We evolved a really good system for interpreting electrical impulses coming from the eyes into a coherent picture. And it’s not just assembling an image, it’s also object recognition like how you instinctively understand that the phone you’re reading this on is a separate object to the hand you’re holding it with even though they’re connected, and even association between objects and activities, like how you don’t have to think about what hand shape to make to pick up that toilet roll you’re about to wipe your bum with. And it’s facial recognition and emotional processing, figuring out that you should be happy when you see the face associated with an entity you like. Honestly, our visual system is cool as shit.
So we’ve got this big chunk of brain whose job it is to process electrical signals it receives, which bear in mind carry no information besides which wire the signal is coming from, and “render” an image for whatever the “conscious mind” is to look at. When other signals come to this area then, it’s going to do what it evolved to do and try to figure out what image that signal describes. If those signals don’t come from the eyes, or have been garbled in transmission, it’s going to render an image that doesn’t match reality, which is what hallucinations sometimes are. Or, if the neurones within the visual cortex aren’t communicating like they normally do, the correct signals can be interpreted in a completely incorrect way, rendering a hallucination.
And all this incredible processing power is sitting there just being used to power the eyes. All it takes is a few mutations to the links between the visual cortex and areas of the brain related to things like memory, sound and language, and now signals from these regions can cause the visual cortex to create images the eyes aren’t sending, which is an incredibly useful thing that natural selection is going to love selecting for.
To evolve an entirely new visual cortex just for imagining things would take a lot of mutations each of which has to be either directly beneficial or fortunate enough to be associated with other events that are beneficial. This *could* have happened, but it’s much more likely that the existing visual cortex would end up expanded and repurposed, which is indeed what we see in humans.
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