From John F. Kihlstrom:
Technically, the duck-rabbit figure is an ambiguous (or reversible, or bistable) figure, not an illusion (Peterson, Kihlstrom, Rose, & Glisky, 1992). The two classes of perceptual phenomena have quite different theoretical implications. From a constructivist point of view, many illusions illustrate the role of unconscious inferences in perception, while the ambiguous figures illustrate the role of expectations, world-knowledge, and the direction of attention (Long & Toppino, 2004). For example, children tested on Easter Sunday are more likely to see the figure as a rabbit; if tested on a Sunday in October, they tend to see it as a duck or similar bird (Brugger & Brugger, 1993).
But the more important point of this letter concerns attribution: the duck-rabbit was “originally noted” not by Wittgenstein, but rather by the American psychologist Joseph Jastrow in 1899 (Jastrow, 1899, 1900; see also Brugger, 1999), when the famous philosopher (b. 1889) was probably still in short pants. Along with such figures as the Necker cube and the Schroeder staircase, Jastrow used the duck-rabbit to make the point that perception is not just a product of the stimulus, but also of mental activity – that we see with the mind as well as the eye.