This optical illusion breaks your brain for 15 milliseconds

https://www.newscientist.com/article/2194158-this-optical-illusion-breaks-your-brain-for-15-milliseconds/

This image is not moving, despite what your brain thinks

This image is not moving, despite what your brain thinks

JUNXIANG LUO

Move your head towards these rings of dashed lines and the circles will appear to turn clockwise. Pull your head away and the motion reverses. This is the Pinna-Brelstaff illusion – and it has just been explained. It seems to be due to a communication delay between the regions of your brain that process vision.

“It’s kind of like if you’re at a party where you’re listening to a voice amongst lots of noise,” says Ian Max Andolina at the Chinese Academy of Sciences in Shanghai. “The physical motion is like background noise and the illusion is the voice in the noise you have to pick out. It takes a little longer to do that.”

Andolina and his colleagues trained two macaques to indicate whether they saw any rotation in images that were actually in motion. Then they showed them the Pinna-Brelstaff illusion, and found that the macaques perceive illusory motion similarly to nine human observers.

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Macaques were used because they have a very similar vision processing system to humans. The macaques in this experiment had electrodes in their brains, allowing the researchers to see exactly how they processed the optical illusion.

The team found a 15-millisecond delay between the activity of neurons that perceive global motion – in this case the illusion that the entire set of lines is moving – and those that perceive local motion, in this case that there is actually no movement.

Our brains probably have the same delay, which may seem like a flaw, says Andolina, but they are just being efficient. When we see something, our brain tries to quickly guess what it is. Normally, that guess is pretty accurate because the physical rules of our environment are usually consistent. Here, your brain is using a shortcut, substituting apparent motion for actual motion.

“As you move your head in and out, the way these elements slide across your retina is going to have two motion directions: an outward expansion, but also the rotation induced by the fact that the edge is slanted,” says Paul Azzopardi at the University of Oxford. “That’s different from something that just rotates or just expands, so you might expect that it takes a little more work to resolve what the motion is telling you.”

Journal reference: Journal of Neuroscience, DOI: 10.1523/JNEUROSCI.2112-18.2019

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