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To clarify my earlier comment, if your goal is to prevent or delay dementia, then you would want to know whether dementia causes poor vision or poor vision causes dementia (via reduced visual stimulation or reduced engagement in cognitively engaging activities that require good vision like reading, some sports, etc.). This is because there may be effective interventions and strategies for dealing with vision impairments.

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Yes. What I meant is that in the absence of such perfect knowledge (whether dementia causes poor vision or poor vision causes dementia), which is usually the case[1], we can pursue these strategies anyway because the nervous system learns by association and by introducing a perturbation to its inputs, it could adapt toward our goal.

[1] Because the brain and body is a huge complex system and it's hard to untangle cause and consequence: There is usually no single cause and a lot of feedback cycles where the consequence could reinforce its cause.

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