[shadowgovt]

Concur. Also concur on external stimulus, and I'm pretty sure it works for exactly the reason you identified. I find the intrusive thoughts show up in the evening precisely because there's less stimulus (inside and out; my brain is quieting itself for sleep, giving the nastier, weirder parts of it their chance to say just the stupidest shit "out loud").

A major eye-opener for me was learning, awhile ago, that we differ from some other animals in that our limbic system is completely "wrapped" in malleable neurons. Some animals have (as far as we can tell) implastic wiring leading from some stimulus constructs straight into that system: they receive a stimulus, they respond immediately with physiological, emotional change. Ours is far more deeply tied to the plastic layers of the brain; we have a huge capacity to learn very complex stimulus-response ties between the world around us and how we should set our emotional state.

On the one hand, that's great! It's good to think that we can change our responses to stimuli in a way that, say, penguins can't.

On the other hand... "plastic" doesn't mean "you can will yourself to change them willy-nilly." It's more that we're capable of developing "mental allergies." We are unusual in that we can develop things like PTSD: you were shot at the same time you saw a Volkswagen Beetle drive by? Congratulations, you now experience a visceral survival-focused reaction every time you see one of the most classic examples of German engineering, and you can't will that feeling to stop because your brain's physiology changed under the dim hope that somehow avoiding vaguely-round cars avoids bullets, too.

Our brains are wonderful, transcendental things, but they're also machines and they're machines that can malfunction. It's good we have professionals who study this.