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Raise your eyes from this text for a moment. What do you see in front of you? Hopefully a splendid tree or a park, a painting or just a bare wall. Are you sure that what you are seeing is really there? Of course you are, you can see it, can’t you? You can inspect the painting from different perspectives, or touch it. Or you can go outside and walk around in the park or stand under the tree. So they are really there just the way you see them, aren’t they? Maybe they are, maybe not. But you don’t see things as they stand. You look with your eyes, but you see with your brain. And the brain does not receive an image the way a camera does. The brain is not passive. What you see does not enter the brain as it is, but is constructed in the brain.
Your eyes are not cameras
How do you see with your brain? An image falls on your retinas in your eyes. That image is converted to electrical signals and transported to the brain via neurons behind your retina. The signals are then disassembled and processed in the brain. Say one green leaf is moving on the tree you are seeing. That image is not processed in its entirety, or like a film as it were, in the brain. After the disassembly the colour green, the shape of the leaf, the movement and the borders are transported to different parts of the brain and processed. Next, this processed information is sent through to other areas of the brain where form and movement are recognized. For this, the processed visual information is combined with existing knowledge about form and movement. This knowledge comes from your memory where it is stored. Finally the information is transferred to other brain areas where objects are recognized in their entirety, also in collaboration with your memory. And that is not all. What you recognise in your visual field also depends on the context in which you are looking. Information about relevancy in that context is combined in yet other brain areas with the objects you recognised. When you study trees you see leaves. But when you think a tiger is stalking you, it is not the leaves you see in that same scene, but -hopefully – a striped animal.
Does your brain fool you?
So when you see a beech leaf that moves in the breeze, that image is actually reconstructed from data that is compiled through your eyes and your memory. How do you know if there really is a beech leaf there, if the image in your brain is not a film but a reconstruction? Are your brains fooling you with that image? Usually not. The whole process of seeing has developed through evolution to perceive the outside world in such a way that we can react adequately. So what we see is usually a very good approximation of what really is out there. Imagine what would happen if people mistook a tiger for an antelope. Then our forefathers would have fallen prey to tigers and the human race would be extinct.
…well, not always….
How does it matter if what you see is exactly the reality, or a construct made by the brain that usually resembles the reality? In both cases you see a tiger if it is a tiger. That disassembling and reconstructing has a few advantages. If you were to see the exact unprocessed image that falls on the retina, then there would be a large blind spot in it. Behind the eye there is a nerve which has no light receptors in it. Colours would only be properly differentiated in the central area of the image, because there are less colour receptors near the edges. You blink on average once every second, but you do not register this in the image you see. The brain processes and constructs the image in such a way that these “ mistakes” are corrected.
….but sometimes it does
But sometimes mistakes can arise through this processing and constructing. So the brain does fool you sometimes. A well-known example is shown here below:
The two red lines are perfectly straight and run parallel to each other. Check for yourself with a ruler. Because of the blue lines in the background our brains “ read” them as curved lines.
And there are dozens of this sort of illusions known to us1. Diagrams like this one are mostly seen as amusing novelties: “ see how our brains can be fooled” or “ see how our brains fool us”. But it is a lot scarier than that. Our brains construct everything we perceive: our thoughts, our memory, our self-image, our consciousness, our subconscious, our free will and even our ego. There is nothing of which you can know for certain if it is the reality or an illusion. This is not something that should shake your confidence in your power of perception, because it has few consequences for daily life. But it is essential for our understanding of how the brain functions.
Clarke, A. and L. K. Tyler (2015). Understanding What We See: How We Derive Meaning From Vision. Trends in Cognitive Sciences 19(11): 677-687.
Dehaene, S. (2014). Consciousness and the brain; deciphering how the brain codes our thoughts. Viking, Penguin Group. ISBN 978-0-698-15140-6, p.60
Hoffman, D. D. (2011). The Construction of Visual Reality. Hallucination: Theory and Practice. J. D. Blom and I. E. C. Sommer, Springer Verlag. ISBN 978-1-4614-0959-5, Hfst. 2, p. 7-16
Eagleman, D. (2011). Incognito; the secret lives of the brain. ISBN 978-1-4614-0959-5. ISBN 978-1-4614-0959-5, Hfst.2
Frith, C. D. (2007). Making up the Mind; how the brain creates our mental world, Blackwell Publishing. Hfst. 1. ISBN 978-1-4051-6022-3