We know all the matter that the brain is made out of, and we know what interactions are possible with that matter, because we've done extensive experiments with particle accelerators. If there were something that affected baryonic matter besides the four fundamental forces, we would know about it.

In other words, we know with certainty that there is no physical mechanism by which some "external consciousness" could influence the brain. So the answer to that hypothesis is a definitive "nope, that's not how it works."

CalTech quantum physicist Sean Carrol explains: https://www.youtube.com/watch?v=x26a-ztpQs8&t=836