I think its worth reiterating that while many have theories, I think Montoya is right to say that they cannot know for sure which is right, or which is the primary cause. Its the fable of the blind philosophers and the elephant all over again, except we are aware of just how blind we are.
The one thing I would like to make specific comment on is that Montoya confirmed that the fasciculi thickening is matched by connecting cortical thickening. This is itself confirmatory that something is going on. It tells us our brains are wired differently, and this is a structural change. For at least a subgroup this could prove very important.
For reference on what fasciculi are -
https://medical-dictionary.thefreedictionary.com/fasciculi