The Pinna-Brelstaff illusion is a tremendous fun: concentric rings of shapes with inverse shading. When you move your head closer or closer to the illusion, the rings appear to rotate, expand, and contract
We know the shading effect plays a role in tricking our brains into perceiving motion, even though there's 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 is actually happening when you stare at those rings.
"The neural basis of transformation from objective reality to illusory perceptions of rotation
"Studying the mismatch between perception and reality helps us to better understand the constructive nature of the visual brain."
Last year, researchers at the Chinese Academy of Sciences used functional magnetic resonance imaging (fMRI) to study the brains 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 greater detail
First, they had to determine whether the macaques can even perceive the illusions. They had nine human volunteers and two macaques study the illusion (with their heads stabilized) to record rapid eye movements – called saccades – in response to the perception of motion
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 monkeys and humans had similar saccade responses, meaning that it is very likely that monkeys really perceived the illusion similar 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 illusion and animations. They were not told which was who; they were trained to indicate the direction of the rotation, and whether the figure was expanding or contracting
The team found that illusions activate the same part of the brain as actual motion, indicating that the brain processes illustrious and real motion with the same neurons
But there 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 the researchers believe that the brain might be
In other words – it might look like motion, but the deep inner recesses of your brain might know something is fishy and take a fraction of a second to mull it over.
We 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 stands to reason that human brains do the same, but it will require further research to confirm;
"The question remains," the authors write, "whether these higher brain areas in the primate dorsal visual stream distinguish between real and illusory motions during active perception. "
The team's research has been published in the Journal of Neuroscience .