The Pinna-Brelstaff Illusion is a tremendous fun: concentric rings of shapes, with inverse shading. When you move your head closer to or farther from the illusion, the rings appear to be rotate, expand, and contract (go ahead, we'll wait while you try this with the picture above).
We know that the shading effect plays a role in tricking our brains into perceiving motion, even though there is none at all; after all, it's well documented that our eyes are lying liars. But now a team of scientists has peered inside human and monkey brains in two separate studies to try to figure out what's really happening when you stare at those rings.
"The neural basis of transformation from objective reality to illusory percepts of rotation , expansion and contraction is still unknown, "researchers wrote in their paper."
"Studying the mismatch between perception and reality helps us better understand the constructive nature of the visual brain."
In the past year, researchers at the Chinese Academy of Sciences used a functional magnetic resonance imaging (fMRI) to study the brain of 42 humans observing the illusion under different conditions. However, fMRI is limited ̵
So the researchers turned to male rhesus macaques ( Macaca mulatta ), inserting electrodes into their brains to analyze the activity in more detail
First, they had to determine whether macaques can even perceive the illusions. They had nine human volunteers and two macaques to study the illusion (with their heads stably) to record rapid eye movements – called saccades – in response to motion sensation.
The humans quantified the effect of the illusion – for example, whether the rotation was clockwise or counter-clockwise, and whether it was expanding or contracting When the illusion moved closer or farther
Both the monkeys and the humans had similar saccade responses, meaning it is very likely that the monkeys really perceived the illusion similarly to how humans do.
Recording the brain activity was the next part. After recovering from the surgery to implant the electrodes, the monkeys were shown the illusion and animations. They were not told what was which; They were trained to indicate the direction of rotation and whether the figure was expanding or contracting.
The team found that illusions activate the same brain part as actual motion, indicating that the brain processes illusory and real motion with the same neuron
But it was a difference: the neurons took about 15 milliseconds longer to process illusory motion than real motion.
It is not exactly clear what causes this delay, but researchers believe that brain may be
In other words, it may look like motion, but the deep inner recesses of your brain may know something is fishy and take a fraction of a second to bubble it over We will need a little more work to confirm this, though.
We do know that humans and monkeys seem to perceive the Pinna-Brelstaff illusion in the same way; that the same region of the brain processes both real and illusory motion; and that monkeys have a 15-millisecond lag when processing illusions.
It seems to reason that human brain do the same, but it will require further research to confirm; "The question remains," the researchers wrote, "as to whether these higher brain areas in the primate's dorsal visual stream distinguish between real and illusory motions during active perception. "
The team's research has been published in Journal of Neuroscience .