If you take a look at this optical illusion, you might feel your brain straining for a flicker of a moment. It turns out, that’s not far off from what’s actually going on.

New research, published in the Journal of Neuroscience, has looked at the neural mechanisms underlying certain optical illusions, especially ones that evoke an illusory sense of movement. A good example of this is the Pinna-Brelstaff illusion, where concentric rings of shapes centered around a dot trick your mind into perceiving an unnerving jolty movement, despite the image being perfectly still.

They found that the brain experiences a 15-millisecond delay to process what’s going on in these types of movement-mimicking illusions. For those short few moments, the brain is effectively frozen.

To find this out, neuroscientists in China tracked the brain activity of macaque monkeys, which are a good model for understanding the human brain and visual system as they can also perceive illusory motion just like their human cousins. The researchers gathered nine human volunteers and two macaques and made them look at the illusion with their heads stabilized while they tracked the subtle moment of their eyes. All the participants, both monkey and human, displayed a rapid eye movement (called a saccade) that strongly suggests they were tracking movement.

Next up in their experiments, they recorded the brain activity of just the monkeys with electrodes in their brains. Even though the image is actually static, they noticed the illusion activates the same part of the brain as actual movement. Most curiously of all, there appeared to be a 15-millisecond delay between the activity of neurons that are associated with illusionary motion and those associated with actual motion. They concluded that humans most likely have a similar time delay. 

Study author Max Andolina, from the Chinese Academy of Sciences in Shanghai, told New Scientist that this 15-millisecond glitch is perhaps not a flaw. If anything, it displays the primate brain's ability to quickly adapt to new circumstances and unusual environments.

"The neural basis of the transformation from objective reality to illusory percepts of rotation, expansion and contraction remains unknown," the researchers conclude in their paper.

"Studying the mismatch between perception and reality helps us better understand the constructive nature of the visual brain."