[moate]

Biologically, this doesn't make sense. One of the more accepted causes of graying-1 is a build of of H202 in the hair follicle resulting in the hair not holding as much melanin. It seems unlikely that this would come from the brain, through the skull since that's not how the circulatory system gets in/out of the brain (it mostly goes through the neck/spine).

1: https://www.nytimes.com/2009/03/10/health/10well.html

[truculation]

Indeed, but if control and resources (e.g. immune resources) are allocated via some kind of map in the brain then mightn't that make proximity relevant? (notwithstanding the bone barrier!) I admit it's a stretch...

[moate]

Again, not really how things work. You're assuming that the dermal layers closest to a section of the brain are somehow working with that section of the brain and that's not really how it works. The circulatory system (which is most likely what's responsible other than the skin/dermis itself) doesn't route from brain to skin that way (because of the bone).

There is a pretty large concentration of veins/arteries - 1 that come up around the temple, so that could be related to the frequency of graying starting there.

1- https://www.gettyimages.com/detail/news-photo/superficial-ar...