New Scientist reported that the cause of the above optical illusion, where the spirals appear to flow, has finally been solved.
This illusion was made famous by a 1981 painting, Isia Leviant's Enigma
Researchers from the Barrow Neurological Institute in Arizona have demonstrated that the way our eyes constantly make tiny movements, called microsaccades, is responsible for the way concentric circles in the painting seem to flow before onlookers' eyes. For the experiment, the investigators had three participants view the painting while cameras recorded their eye movements 500 times every second. Next, the participants were asked to press a button when the speed of the optical 'trickle' of the illusion appeared to slow down or stop, and release it when the trickle seemed faster. The results showed that the illusion became more pronounced when microsaccades were happening at a faster rate. But, when the rate slowed to a stop, the illusion vanished. The authors said that their research may also explain other similar illusions