You see circles turning, while you know that the figure is not moving. Chinese neuroscientists discovered how your brain deals with such an optical illusion: it processes the image with a delay.
Fix your gaze on the black dot and move your head towards and away from it. You see the rings rotating.
Keep your gaze fixed on the black dot in the center. Then move closer to the figure. You will see the circles rotating counterclockwise. If you move away from them, they turn clockwise. In reality, the different parts of the Pinna-Brelstaff illusion, as this figure is called, are standing still. So we see movement that is not there.
Researchers have been trying for some time to find out at the brain level why this figure seems to be spinning. A group of brain scientists from the Chinese Academy of Sciences in Shanghai took a big step in a study with macaque monkeys. They found that the figure activates a brain region responsible for detecting movement. The responsible brain cells perceive the illusory movement, but with some delay. The work appears this month in the journal JNeurosci.
Hierarchy in the visual brain
The information that enters your eye is relayed from one brain region to another. The first brain region to receive the information is the primary visual cortex, which lies at the back of our head, between the two hemispheres. This visual cortex transmits these signals to an area more in the middle, the MT (middle temporal area), which in turn transmits the whole thing to a nearby area called MSTd (medial superior temporal dorsal area). The ‘higher’ the brain region is in the visual hierarchy, the more refined the interpretation of the images.
The visual cortex (bottom right, purple) processes information that comes in through your eyes (bottom left, orange).
Wikimedia Commons , Wikimedia Commons
Additional processing time
First, the research group, led by neuroscientist Wei Wang, showed that macaques perceive the optical illusion in a similar way to humans. First, they trained the animals so they could indicate whether the figure was spinning counterclockwise or clockwise. This was followed by experiments with measurements on individual nerve cells, via electrodes that enter the brain. The Chinese showed monkeys the illusion. Meanwhile, they measured brain cell activity in two areas, the MT and the MSTd. These brain areas are known to be involved in seeing and processing moving images (see box).
The cells in both areas respond to both real and illusory movements, the measurements show. There is a difference, however: the MT processes the actual and illusionary movement at the same speed. However, the MSTd, which receives its information from the MT, needs a little more time. It takes an extra fifteen milliseconds to process the illusion, which is quite a lot for brain activity. This brain area therefore concludes with a fifteen millisecond delay that there is movement (even though it is not actually present).
That extra processing time shows how our brains sometimes struggle to distinguish between perception and reality. If there is no actual movement, but the input to the MSTd conforms to certain patterns – in this case the small blocks that make up the circles – then an illusion can arise. The MSTD then makes a global moving image of it.
Watch with your brain
“What we learn from this is that the visual brain is very complicated. The information that enters through the eye is processed by our brain and sometimes even distorted,” says Anne van Ham, who also conducts research into visual perception at the Netherlands Institute for Neuroscience.
The eyes and brain do not work like a camera, according to Van Ham. “It’s not like we’re taking a ‘picture’ by looking, and processing what we see on it and considering it to be the truth. Our brains can see and interpret things differently, as in the case of illusions.”
Van Ham thinks that Wang and his colleagues show particularly well that many brain areas are involved in seeing. They process the incoming visual information step by step, until we can finally perceive the image in its entirety. Whether it reflects reality or not.