One or more of the five senses getting too much input, too fast to handle. It could be just one, such as hearing. Too many people talking in a room so that you can't differentiate between one voice and another, so none of it makes any sense. Or touch. Tags or rough seams in clothing can be so irritating you can't concentrate on listening or reading, or anything for that matter.
If it becomes to overloading, a person might either explode in anger, shouting to make it stop, or withdraw into a proverbial shell, and no longer respond to anyone. The first is a meltdown, the second is a shutdown. Either one usually takes quite a while to come out of. It's quite a bit different from being startled and taking a few seconds or even minutes to recover from it.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...