Some people who take psychedelics continue hallucinating indefinitely

The parent commenter likely used fortified leaves given that they had such a distressful experience.

After about 3 months I had developed perma-tracers (I could draw shapes in the air with a lit cigarette), after about 6 months I saw after-images at night, usually while driving. (For example, a ghostly day-glow purple stop sign behind the actual stop sign). It wasn't enough to be actually disruptive to my life, besides the slight anxiety about it.

At about 9 months I developed a stutter which increased in frequency over the next few months, until it was an actual speech impediment. I stopped smoking pot and decreased my LSD use to a few times a year and it all went away within 6-9 months and never came back. It has never come back with LSD use, but it /did/ come back during a year in my 30s where I smoked a lot of pot and used no psycehdelics, so take that anecdata as you will.

These days I notice that if I do LSD more than about 4 times within 1 month, I'll develop some tracers and visual distortions that persist very slightly when sober, and more persistently when only stoned on pot. Tiled bathrooms breathe a little, lights sparkle and shimmer. But it always resolves within about a month of abstinence and has never caused me any trouble.

There are no (as far as I can tell) cognitive distortions or anything else while sober, and the visual distortions range from mildly amusing to slightly alarming. I've had tons of panic attacks (unrelated in my life) but none of them had the quality of any sort of "flashback", just plain old anxiety.

There has been a little bit (but not a ton) of research done on HPPD, and bluelight and other harm reduction forums had good discussions with those dealing with it for many years. For many (most) people it resolves on its own.

My completely out of my ass theory is that it basically works like water making a groove in dirt in that some neural pathways get overly "primed" to fire and fall into a habitual pathway.

/Most/ people I know that have either experimented heavily with LSD /or/ with intensive meditation have reported some sort of visual or auditory after effects that persist while sober and resolve on their own, but most people I've talked to haven't been too bothered by it.

That's not to discount the people that have experienced disruption to their life from HPPD, but this definitely seems to be the exception to the rule of either not having any persisting effects or not finding them that bothersome.

edit: typos

I'm going to quibble with you. When you say that individual humans vary widely, you're mostly assuming a baseline scale of typical human variation. Which is basically begging the question. Of course the fastest or smartest people look very extreme, when you're implicitly comparing them to typical people.

Usain Bolt is very fast for a human. But he's slower than the average housecat. On the flip side some people have really good memories. But even the dumbest humans manage to remember a huge number of symbols compared to what a chimp's capable of. Same for virtually everything else you listed. Food taste, aesthetic preferences, auditory capabilities, intoxicant consumption, etc.

Normal human variance is swamped by variance between different animal species. This is mostly invisible to us, because most of our day-to-day interactions are with other people. Not exotic organisms.

So I would guess just the opposite. I believe that sensory experiences across different people are very likely to be substantially similar. At least compared to an alien species, or AI, or even just an intelligence from a distant evolutionary branch (like an octopus).