There's a disease called Hallucinogen Persisting Perception Disorder (HPPD) that some people get after heavy use of psychedelics. The theory I subscribe to is that they're actually overreacting (almost obsessively) to the defects in perception that we all have. For example, if you stare at a bright white screen, especially when tired, you'll eventually see "static" due to retinal fatigue happening at uneven rates. The brain filters out a lot, but when that filter is weakened or challenged, it can let "hallucinations" in. Most of us aren't bothered by "white screen static" and hardly notice it. For some people (such as those with HPPD, who may have strong emotional associations with "tripping") that can be very distressing. In extreme cases, they can't go out at night because they're bothered by (again) scotopic vision defects that all of us have, but that (due to their anxiety) they can't tune out.

What I've noticed with anxiety is that there are things that are hard to un-notice. For example, after reading that article, I itched like hell. I knew why, so I didn't need other explanations (I basically know that reading any article about health problems is going to be a bit masochistic) but there are some distressing thoughts that are just very hard to banish, especially in a state of fatigue (which is not uncommon in our overworked, bizarre society). None of us is fully in control of our focus, and sometimes it's possible to get into a feedback loop of anxiety producing weird symptoms (panic attacks, for example, can throw all kinds of shit... even phantom smells) that produce more intense focus and anxiety.