The Perky effect, in psychology, refers to the process by which visual mental imagery is thought to interefere with visual perception. For example, if a person is instructed to project by imagination a tomato or a banana on a while screen, an a faint image of a tomato or banana is projected, the person may not be able to tell the difference between the real image and that which was imagined. Cheves W. Perky did the pioneering research on this back in 1910. This is the first meaning of the Perky effect.
There is another meaning: the Perky effect is what happens when you go braless on a cold day.
Thomasina loved cold days because of the perky effect.