The illusion involves the apparent direction of motion of the figure. Some observers initially see the figure as spinning clockwise (viewed from above) and some anti-clockwise. Additionally, some may see the figure suddenly spin in the opposite direction.
If the foot touching the ground is perceived to be the left foot, the dancer appears to be spinning clockwise (if seen from above); if it is taken to be the right foot, then she appears to be spinning anti-clockwise. (Wikipedia)
The illusion involves the apparent direction of motion of the figure. Some observers initially see the figure as spinning clockwise and some anti-clockwise. Additionally, some may see the figure suddenly spin in the opposite direction.
Tipped down..or up…?
Bistable Black Blob -------- Bistable White Blob
Maximum Intensity Projection (MIP) of a whole body positron emission tomography (PET) acquisition of a 79 kg (170 lb) weighting female after intravenous injection of 371 MBq of 18F-FDG (one hour prior measurement). The investigation has been performed as part of a tumor diagnosis prior to applying a radiotherapy (tumor staging step). Besides normal accumulation of the tracer in the heart, bladder, kidneys and brain, liver metastases of a colorectal tumor are clearly visible within the abdominal region of the image. (Wikipedia)
This positron emission tomography scan of a woman has a similar effect when viewed spinning.
The Ferris wheel moves both clockwise and counter clockwise.
Spinning red stripes & wheel
Two directions of spinning motion of the red stripes & wheel. Some observers initially see the them as spinning clockwise and some counterclockwise.