My own view is that mystical experiences correlate with neurophysiology, but that their correlation to the functioning of the nervous system does not warrant asserting that neurophysiology is their sole or ultimate cause. About that, I remain agnostic.
I was going to say something to this effect. There's always the exterior half of the interior experience. I have a problem with those who imagine everything we experience is caused by the brain. Does the brain decide for us what experiences it's going to give us this day? Are we just along for the ride where it is outside our own choices? Are our choices dictated by our biology? And so forth.
I am of the opinion that there is a sort of dance between our choices, our "will" and our bodies. The exterior and the interior dance together to the music of Life, to wax poetic here. Whatever the reality of the inner world is, we experience it in these bodies, and the way it manifests to us is the notes the body provides. Our experience of God (or what we experience as this nondual Absolute), does not define the Absolute as that experience itself. But our experience is the human experience of God. It is our Ultimate Reality as humans.
It's the same with how we experience love. The notes of the body, the chemicals, the activities, the hormones, the emotions, etc, are all responses of our bodies to the interior self that is 'feeling' its way through the undefinable Divine in the world. It's not easy to put my thoughts into words here. When it comes to mystical experience we can literally choose to have them. We can condition and train ourselves to "touch the face of God" at will, in a similar way to the way we can make ourselves experience love though simply a thought. However mystical experiences typically they seem to occur randomly, or "out of the blue". But these momentary "peak experiences" actually do have some internal things going on that precipitate the brain responding in that way where we have a "death" experience, which is a breaking down of all these dualistic tensions we feel trapped within.
Peak experiences are typically precipitated by a deep subconscious need for an existential release from internalized suffering. Even though on a conscious level we don't know what's happening "down there" in us, it's literally the subconscious breaking forth, and our
choosing to let it happen. We are choosing to experience "something" to have a break through this dark, nebulous tension, this "hood" covering us we need to be free of. And so, "Wham", like a lightening bolt, we experience release! The veil is pulled back and we see God. We passed through the door of death, letting go of the dualistic reality. And the light of the release is blasted into our experience through parts of the brain lighting up, producing an accepting of and release into death itself, as it does when our bodies are finished. The difference is, our bodies are still alive and not facing death, but we experience in our mystical experiences what it is to die, and yet live! Ever wonder about the common religious themes of death and resurrection? I believe this is it!
So when some "reductionist" bent on explaining away things claims that mystical experiences are the brain "misfiring", I think they are incredibly shortsighted and naive. If our brains are randomly misfiring neurologically, then we have a serious medical problem, and "God experiences" are probably the least concern! Yes, you may have instances of this happening in psychiatric disorders such as schizophrenia, but just because the experiences parallels what may be experienced in mystical states in otherwise normal and healthy people, it does not translate therefore into a brain dysfunction for those who have mystical experiences. The causes are not arbitrary misfirings, but the result of some deep subconscious need for a profound release, an "awakening" of the mind beyond it's own prison.
And then there is meditation practices where one learns how to let this "accident" happen pretty much at will. It makes you "accident prone", because you are deliberately putting yourself at the door of death, so to speak. Passing beyond this world into the next, which when you walk through "everything becomes clear".
"
Wanting nothing
With all your heart
Stop the stream.
When the world dissolves
Everything becomes clear.
Go beyond
This way or that way,
To the farther shore
Where the world dissolves
And everything becomes clear.
Beyond this shore
And the father shore,
Beyond the beyond,
Where there is no beginning,
No end.
Without fear, go."