The Pinna-Brelstaff optical illusion contains concentric rings that appear to rotate. As you approach the rings, they shift clockwise. Move away, and they circle counterclockwise. Working with rhesus macaque monkeys, researchers monitored the activity of neurons in the brain that respond to the illusion. They found that the same set of neurons in the brain encode patterns for actual motion and the perceived movement of the static image. To discern whether the motion is real or not took the monkeys’ brains another 15 milliseconds, according to a report published yesterday (February 18) in the Journal of Neuroscience.
J. Luo et al., “Going with the flow: the neural mechanisms underlying illusions of complex-flow motion,” J Neurosci, doi:10.1523/JNEUROSCI.2112-18.2019, 2019.