 Perceptual learning involves long-term changes in perception. This criterion rules out short term perceptual changes due to sensory adaptation (for more on sensory adaptation see Webster 2012). In the waterfall illusion, for instance, a person who looks at a waterfall for a minute, and then looks away at some rocks, sees the rocks as moving even though they are not. This is a short-term change in perception, lasting perhaps for fifteen to thirty seconds. Since it is not a long-term change in perception, however, it does not count as perceptual learning. In another short term adaptive change, a person who goes indoors after walking through a blizzard may have trouble as her eyes adjust to the new lighting. There is a change in her perception as a result of her experience in the blizzard. But it is not a long-term change, and so it does not count as perceptual learning.