The best neurological model we have at this point is mostly at the level of a working hypothesis--kundalini phenomena, like most upheavals resulting from meditation, result from disregulation of some inhibitory function of the nervous system.

I think that you can explain this with a combination of the predictive processing model and analogies to insight meditation. I think the resulting explanation also applies to the stages of insight meditation from Mastering the Core Teachings of the Buddha(MCT and stuff like Kundalini phenomena as well.

So the basic predictive processing model says that there’s a process of reconciling different predictions between top-down predictions with bottom-up sense data. At first the top-down model just overrides the bottom-up sense data. Each time the bottom-up data is surprising, this releases some small amount of a neurotransmitter. Eventually some cumulative threshold is reached and the top-down processing neurons starts noticing that they’re encountering errors and surprising information. They start to try to adjust their output to try to accurately predict the bottom-up data, emitting some slightly different neurotransmitters in the process. Eventually a combination of the right neurotransmitters and developing a better model of the world through trial and error allow both the top-down and bottom-up models to come to a mutually agreeable and consistent view of the world, and the basic phenomenon isn’t surprising anymore.

The basic premise of insight meditation is that most people’s top-down priors about their subjective experience of the world are incredibly wrong, in ways that are hard to understand until you’ve paid close enough attention to the details of your subjective experience. This includes stuff like believing that you’re a separate agent interacting with an objective reality, that objects continues to exist in your subjective experience even when you’re not thinking about them, and just generally that everything is way more solid and long-lasting than it actually is. Insight meditation or psychedelics are both methods for directly experiencing that that’s not true and changing your top-down and bottom-up models of the world.

Insight meditation follows the basic predictive processing model of trying to reconcile erroneous top-down perceptions to noisy/surprising bottom-up sense data. The difference is that insight meditation involves huge swathes of the brain, and that it involves having hugely surprising and improbable experiences. That adds up to megadoses of neurotransmitters being released, with the exact mix of neurotransmitters depending on where the brain is at in the process of reconciling top-down versus bottom-up data. The experiences of insight meditators can thus have a common similarities in how they’re experiencing the world, since the sequence of neurotransmitter release and surprise-reconciliation strategies changes in a predictable basis. This is the basis for the Progress of Insight stages that Daniel Ingram writes about in MCTB. When it comes to the states in concentration meditation, I like Leigh Brasington’s article on the neural correlates of the jhanas, which are a set of commonly-experienced states in concentration meditation. I haven’t read Surfing Uncertainty, but I’d bet that there’s a lot of overlap in the neurotransmitter sequence from the predictive processing model and in Leigh Brasington’s sequence of neurotransmitter evolution. And there’s definitely an overlap between the insight stages and the concentration states — Daniel Ingram discusses a mapping of this in his chapter on the vipassanna jhanas.

So what does all of this have to do with HPPD? Well, I beleive that predictive processing says that groups of neurons that consistently come up with surprising information get rewarded and listened. And insight meditation says that our priors about our subjective experience of the world are extremely wrong, and that the reality of what’s going on is extremely surprising. This difference between our current priors and reality means that any parts of our brain that know the right way to look have an abundance of things to point out as really surprising. And each time they point that out, they get rewarded with neurotransmitters and being listened to a little more carefully.

Under normal insight meditation conditions where someone continues practicing and is doing so deliberately, this happens mainly from the viewpoint of the person who is meditating, and they feel like it’s under their conscious control. But in the case of HPPD there’s no reason it couldn’t be a rogue group of visual neurons deciding to continually point out visual snow, or to point out that their perception of the wall changes many times a second based on new neural pulses. And in the case of Kundalini phenomena, there’s no reason that the neurons behind a basic physiological drive like lust or hunger can’t suddenly say “Wow, if I pay attention to the impermanence of sensations in this insight-meditation-like way, then I can constantly reap the rewards of lots of dopamine.” I like Kevin Simler’s discussion of this in Neurons Gone Wild, from his excellent blog post series.