The spinning dancer illusion shows an ambiguous silhouette that appears to abruptly change direction.
What Do You See?
In this image, you see the silhouette of a woman spinning. Which direction is she turning? You may be surprised to learn that it is possible to see her spinning both clockwise and counter clockwise. How?
While it may be very difficult, you can probably get her to switch directions spontaneously. Try looking at the figure and then blink; she may appear to change directions immediately after you blink. Another strategy is to focus on a specific part of the figure.
How Does the Spinning Dancer Illusion Work?
After it was initially created by Nobuyuki Kayahara, the illusion was mistakenly referred to as a scientific personality test of right brain/left brain dominance by numerous websites and blogs. In reality, the spinning dancer illusion is related to bistable perception1 in which an ambiguous 2-dimensional figure can be seen in from two different perspectives.2 Because there is no third dimension, our brains try to construct space around the figure.
Reference The spinning dancer and the brain. http://greengabbro.net/2007/10/20/the-spinning-dancer-and-the-brain/