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Do our brains see what our eyes see?

There are many optical illusions in our everyday lives, but the reason we see these illusions isn’t just a trick of the eye.

For the past month, I have been road-tripping across Canada. Along the way, I’ve seen vast grasslands, snow-capped mountains, and beautiful lakes. And mirages! A mirage is a type of optical illusion that happens as a result of light passing through air of different temperatures. The type of mirage that I have been seeing is commonly referred to as a “highway mirage”, where the sky is reflected on the heated pavement, causing our brain to interpret this reflection as water.

So why do we see illusions this way? Our bodies use what is called the visual system to process light in the environment around us. First, our eyes refract visual light to produce a small image on the retina. Next, the retina converts this information into electrical signals that are carried by the optic nerve to the optic chiasm. Here the nerves cross to organize the information coming from each eye to the correct side of the brain: the right field of view from each eye is combined and sent to the left side of the brain, and vice versa. Once reorganized, the nerves travel in what are called optic tracts to the lateral geniculate nucleus (LGN). The LGN receives visual input from the retina as described, but it receives the majority of its information from other parts of the brain in order to integrate information before sending it off. Neurons from the LGN will send impulses through the optic radiation to the visual cortex. Finally, once the information reaches the visual cortex, that’s where we make sense of the visual image.

As you can see, there are certainly more to illusions than meets the eye. Literally. Information is captured by our eyes but processed by our brains. A 2019 study sought to find exactly where perception lies in the hierarchy of visual processing. Researchers showed participants a “double-drift” illusion as well as a similar non-illusory animation. In both cases, participants were asked to focus on the dot on the left while hooked up to functional magnetic resonance imaging, or fMRI. The fMRI allowed scientists to analyse the participants’ perception of the two animations by evaluating changes in the brain’s blood flow. In the illusory animation, the dot to the right is moving up and down only but appears to be moving diagonally. In the non-illusory animation, the dot to the right is moving diagonally. The fMRI revealed that at the level of the visual cortex, the two animations triggered different activation patterns. However, participants thought these two animations were the same. That means the visual representation deviates from our final conscious perception at a higher level than the visual cortex itself. The eyes aren’t fooled, but the brain is!

But let’s go back to those highway mirages. What we see on the pavement is the colour blue reflected from the sky, but it appears the brain does not process it as such. Instead, it processes it to be a body of water since the brain knows that the sky cannot be on the ground.

Another example is camouflage. Many animals (chameleons, leopards, stick bugs, and arctic hares, to name a few) appear to disappear into their surroundings because of their colour or pattern. Our brains assume that if it looks like the background, it is the background. In every case, our brains make assumptions based on context. Our brains don’t see what our eyes see.


Cat Wang recently graduated from McGill University with a Bachelor of Science (BSc) degree in the anatomy and cell biology program.

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