Imagine a sort of beat, but the sound isn't something sharp like a drumbeat. Maybe the sound of walking on gravel, or flicking the bristles on a toothbrush. This sound repeats regularly, roughly once a second. In my brain, it is as though these sounds follow a positive feedback loop (alternatively, like a resonance), rising in volume. Eventually (maybe after a few seconds, maybe a few tens of seconds) the imagined noise in my head is intolerably loud. That evening when I was eight or nine, I burst into tears at it. As I recall it, the news was on the TV, and Mum thought that I was reacting to the footage of the war zone (Yugoslavia?), trying to comfort me accordingly.
Over the years, this same sort of feedback-loop-thing repeated itself occasionally, ending more calmly than wild crying. My memory is that it was something I could almost command at will: imagining those regular, quiet sounds, and having them dominate the apparent noise inside of my brain. But it was long ago (how long ago? When did it stop? I don't know – in my teenage years maybe? Early adulthood??), and perhaps not as common or as controlled as I remember. I tried summoning those regular sounds just now, and I can only experience what feels like a faded ghost of what I remember: my brain stubbornly whirring away normally, saying only (in some metaphorical sense) "I know what you're trying to do, the sound used to get loud like this,", but this isn't actually overpoweringly loud.
Sometimes it wasn't sound-based. In what I associate more (though not exclusively) with dreaming was having the size of a ball (or a spherical rock) get larger very quickly. Perhaps the rock was on one end of a seesaw, and, without wanting to, I would imagine it rapidly getting many times larger than the seesaw. It would destroy any hope of imagining what I wanted to imagine about that ball or rock.
I was reminded of these old memories today. I'd been reading a discussion about photons and coherent states, and I pondered, as I occasionally do, how little I understand about quantum mechanics. What's an "observable" and why should it be a Hermitian operator in a Hilbert space? (Real eigenvalues, whatever, my main confusion is on representing something physically measured as a matrix. Or why non-commuting operators should exist. Turning Poisson brackets into commutator brackets just because. Totally weird stuff, though perhaps within the realms of "spend a few weeks looking at your old uni notes, in particular representing things in quantum by wavefunctions rather than kets, and you'll work it out".)
I went on to think of how, more generally, I don't understand things. Why does particle physics have Lie algebras in it? Why does anything exist? At the latter question, I imagined galaxies and the Big Bang and atoms and gravity and I had one of those weird positive-feedback-loop-things, my brain getting totally flipped out over the existence of anything at all, matter, energy, physics. It only lasted a second or so, but it was a powerful force in my head for that second, as though it was driving me fast towards a sort of existential madness*. Then it ended. The existence of the universe and physics is still really weird, but it's something that my brain can consider calmly and stably.
*Whichever meanings or connotations of 'existential' apply here, those are the ones that I mean.
I think this "getting briefly and excessively weirded out over the existence of anything" thing has happened to me before. Whether it belongs in the same category as the regular beats that made me cry when I was eight, I don't know, but it feels very similar.