> Third, it talks about HPPD-like visual hallucinations in people who have never taken psychedelic drugs. It says many people have them. [...] My only argument that this doesn’t disprove HPPD is how many people – including my survey respondents – describe these oddities starting (or getting much worse) right after they use psychedelics.
This is what I believe, and I think there's a simple reason why people often report the symptoms getting worse after taking psychedelics. They are more noticeable once you've seen the full-blown effects of the drug in comparison. Prior to that, you wouldn't have any reason to pick out those hallucinations and might unconsciously ignore them. But once you've seen the full effect then even mild instances are too obvious to ignore.