ELI5:Why is it that when people sleep talk, they say random gibberish that is structurally correct, but syntactically wrong?

I'm an academic psychologist. It's out of my specialization, but I have a pet hypothesis that all of our thoughts always flow in the relatively random way that they do in our sleep. The difference is that, when we're awake, we have a preference to selectively attend to the ideas/words that make the most sense.

This is because, a la Pavlovian learning, many word associations we have don't make sense in most situations, but only some. For example, we've all heard people talk about roads and lines in the same sentence, because roads usually have lines: "They painted fresh lines on the road." Thus, our brain knows that sometimes roads and lines are associated.

Yet, oftentimes people will talk about roads without talking about lines: "She went down the road." In this case, our brain will start to activate the word, "lines", because they've been associated before, but will ultimately decide that "lines" isn't a relevant word/meaning in this situation.

For various reasons, sometimes teachers or therapists will ask people to write out their thoughts "as they flow." If you've tried this, you realize very quickly that, if you truly write out every thought you have, your writing won't make any sense. If you pay very close meta-cognitive attention, it seems that we quickly and easily decide which concepts not to include in our "flowing thought" writing, because they don't make narrative sense. Example: I'll start writing on the song I'm listening to:

"This song has nice drums. I always wanted to play drums. But the bass is nice. It makes me feel snarky. Snarky makes me think if sharks. Why are their teeth so large? Why do large teeth make me think of breakfast cereal? I don't even eat breakfast cereal often. It's usually better with marsh mellows. Marsh mellows aren't vegetarian. Those poor cows."

My writing might be biased by my pet theory, but it's just an example: I went from music to cows in no time, just by going with whatever thought came to mind. This doesn't make sense. If I was more willing to deny reject ideas/words that popped into my mind, the narrative would have been more coherent.

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