Degree Modifiers and Monotonicity

In a short squib, Zwicky (1970) wonders what could explain the assignment of certain adverbial functions.1 He observes that there exist pairs of expressions where the positive of the pair is a sentence adverbial, while the negative one is a degree modifier. For instance, unusually in (1) is a degree adverbial. That is, the example expresses that the children are noisy to a degree that is unusual. No such similar reading is available if we replace unusually with its positive counterpart usually. The example in (2) is instead interpreted as saying that it is usual for the children to be noisy.