Daniel M. Ingram (later post, same thread)
I realize that I am using somewhat scientific neuro-chemical theories to describe what is purely subjective experience, but that is my current best interpretation at this moment.
from original post:
this sense of a grayish veil over perception that I didn't even know was there felt like it ripped through the back of my head and vanished
I think the neuro-chemical theory is useful and think these reports that strongly note "back of the head..." are pointing to the vagus nerve branches (left and right) being de/stimulated - resulting in changes (temporary or permanent) in afferent and efferent nerve responses and their chemical triggers (e.g.,
norepinephrine releases into the amygdala, acetylcholine releasing slowing heart rate,etc).
Interestingly, there are similar reports of painful back of the head ripping sensations and keen/specific gut awareness in adverse vagal nerve stimulation (vasovagal syncopes like pressure or intense multi-day headaches creeping up the back of the neck upon bowel movements - these folks often report extreme pain in the LEFT side of the back neck). The nerve originating in the brain stem runs through many places (aortic arch, celiac branches, etc, the nerve is said to have the "widest distribution in the body") to the digestion system and persons experiencing adverse vagal response also sometimes report sharp sensitivity to intestine sensations, acids (
gastric nerves are branched from the vagus nerves.
Stephen Porges
Polyvagal Theory (and
here ) addresses primitive vagal portions (such as heart rate and emotion) and newer vagal portions and includes therapeutic vagal nerve stimulation. Elsewhere pain is reportedly reduced (aka:
nociceptive inhibition) with some forms of vagal stimulation... Adept meditators are often reported to have shifted in their experience of pain and beginning meditators often report the pain in the legs and back just vanishing or separating from emotion after a few days of concentration. Related to inhibition of reward pathway and eating vanilla-caramel ice-cream?
There is a lot of information to assimilate about the vagal nerves and related (it effects and is effected by hearing, emotion, tactility), chemical releases and adverse and beneficial stimulations. Personally, I see how a malady such as Parkinson's Disease and GERD can result from one aspect of de/stimulation, and how reports like yours (I'll call it a beneficial, welcome effects) are related to smoothing, beneficial, holistic stimulations (your practice indicates to me many physical and psychological efforts over many years).
So, there's a lot to digest for someone interested in the positive downstream results of de/stimulation of the vagus branches and their related where a person also is interested in the neuro-chemical changes occurring with meditation, mindfulness and their many various expressions (HAIETHMOBA, god-in-all-things, wonder, friendliness, anapanasati, anicca, non-separate, etc).
Thankfully, this nerve and meditative practices are being studied by medical scientists and practitioners, as in this example. of healing art.neurology.
As it's often said here on the DhO, a person tends to get what they optimize for. Why a person would not want to optimize for a calm, wonderful, useful mind, I do not know (what the heck was I doing for 37 years?). Perhaps the broad awareness of certain misery-stress causing ferocious behaviours worldwide today are driving people to understanding causality and the causality in one's own mind and body. This amazing internet is allowing us to see not just theses ferocious exploitations, but but also astounding personal and wholesome efforts of all sorts, including evidence of the human mind effecting its astounding altruistic/empathic/solutions/actual friendliness/curiosity of life-universe-here potential.
However, in a practice forum, if spare "time" is limited one can just do their practices (ethical discipline, joyous perseverance, anapanasati/kasina, sensate mindfulness,
metta, friendliness/receptivity, mindful attention to sense-cravings with action-restraint and sense-redirection, concentration, respect/friendliness for all, etc) and not spend time on "why the back of the head?" thing. Note to self

Thanks for sharing your experience, Daniel.
[edit: and
more hereon possibly why people report orgasmic sensations outside of of the reproductive zones during meditation or extensive mindfulness. Anywho...back to work]
felt like it ripped through the back of my head
(...)
This occurred after what I think of as my "peppermint phase", where this winter the sense of cold suddenly changed to the sense one gets when one puts peppermint oil on their skin and blows on it: much more defined and somehow clear and crisp.
(...)
It also followed a phase where it felt like I could perceive my stomach acid: may have just been GERD... ;)
It also occurred after a lot of hyper-relentless attention to the directness of visual experience in a wide yet inclusive and detailed way.
(...)
This has also altered the way I interact with others in a way that most shifts have not, and I find that I choose my words and gestures and facial expressions and where I put my eye attention and the like with much more automatic care and attention to each little bit of it, and this has actually made a real positive difference and I recommend it.
One more thing that is interesting relates to the way the mind imagines the inside of the head, neck, and trunk, what I will call the "inner space": there is simply much less imagined visual component to this than there was before, though it is not gone entirely, just substantially reduced.