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Peony is collecting seashells and she doesn't notice that it went from day to night until the sun was almost below the horizon and it was quite dark.
Why didn't Peony notice the changing light?
She didn't transduce the changing light.
She used top-down processing when she should have used bottom-up processing.
Habituation caused her to adapt before she processed the light.
The small changes in light were below her difference threshold.
The changing light was a 'hit' according to signal detection theory.