Ime, it was pretty random, at least on lsd that was the case. Even if you tried to focus on something else, blocked memories and emotions would spring forth anyway.
Other times, all I had were crazy, morphing/waving/web-like oev, and cev were whatever I was thinking about at the moment; fields, animals, different colors, etc.
At first, they increased (blocked memories). Later on, they occurred less often when I got "use" to lsd. For the record, I have no idea why.