the tendency for an imagined stimulus to interfere with seeing an actual target stimulus when the imagined form is close to that of the target. In one study, for example, a participant was positioned in front of a blank screen and asked to imagine a leaf, while simultaneously, without the participant’s knowledge, a blurry picture of a leaf was projected onto the screen, gradually becoming brighter; the intensity of the picture was well above the threshold for detection before the participant reported seeing it. [described in 1910 by Cheves West Perky (1874–1940), U.S. psychologist]