Translated by Rumia Bose
This post is a revised version of “What you see…” published on 11-05-2017
Raise your eyes from this text for a moment. What do you see in front of you? If you are lucky then you see a splendid tree or a park, a painting or otherwise just a bare wall. You probably experience that which you see in the way that you would a photo or a film. A good photo or film is uniformly well-defined and represents colours faithfully. But this does not hold true for what you see. You look with your eyes, but you see with your brain. And the brain does not receive an image the way a camera or videorecorder does. The brain is not passive, the light that falls on your eyes does not enter the brain as it is. It is processed in the eyes and in the brain.
Your eyes are not cameras
Light falls on the retina of your eyes. That light is converted to electrical signals and transported to the brain via neurons behind your retina. How this happens depends on all sorts of circumstances. In the shadow, for instance, you hardly see any colours. This is because the colour-sensitive receptors, or cones, need more light to be activated than the colour-insensitive receptors or rods ( see: What is colour ? How blue can look white…). And if it is practically dark then you see as good as nothing. Wait a while with your eyes open and you will see more and more. The receptors in the rods become progressively more sensitive as they continue to receive little light. If you still can’t see what is in front of you well enough, then you should look towards one side. Then you will see better. This is because there are more rods at the edges of the retina, but also because their sensitivity continues to adjust and synchronize with the other rods depending on the amount of light. The light that falls on your retina is therefore converted to electrical signals and processed before it is transported to the brain.
You see with your brain
The signals are then further processed in the brain. A simple illustration of this is the fact that you blink around six times per minute without noticing that you see nothing during the time your eyes are closed. This happens because your brain filters away the time in which you see nothing and replaces it with the image it constructed before the interruptions. In this way the film you think you are watching does not waver. But there is much more going on in your brain.
Say one green leaf is moving on the tree you are seeing. The signals for the colour green, the shape of the leaf, the movement, and the lines of transition from objects to background, are transported to different parts of the brain where they are processed. Next, this processed information is sent through to other areas of the brain where form and movement are recognized. For this, existing knowledge about form and movement is retrieved from your memory. The information is then transferred to other brain areas where objects are recognized in their entirety, also in collaboration with your memory (see also: Recognising faces).
And it does not end there. What you recognize also depends on what you expect to see in that context. When you look at trees you see leaves. But when you think a tiger is stalking you, it is not the leaves you notice in that same scene, but -hopefully – a striped animal. This prediction of what you expect to see enables you to distil the most relevant aspects out of all the excessive and often unclear information that you receive. Look at the next illustration before you read further.
What do you see? It is usually quite difficult to identify what this picture shows. I see a dog, a Dalmatian, because I am familiar with this picture, and therefore expect to see a Dalmatian. Once you have identified this, you will thereafter always see the Dalmatian. Though not everyone can see the Dalmatian even after this revelation.
Do you really see what there is?
What you see is therefore a complex processed form of the light that falls on your eyes. What’s more, what you see is also influenced by what you subconsciously expect to see. How do you know that what you are seeing is really a beech leaf? Is your brain playing tricks with your perception of 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. Imagine what would happen if people always saw a tiger for an antelope. Then our forefathers would all have fallen prey to tigers and the human race would be extinct. You might think this is a ridiculous supposition. See what Donald Hoffman has to say on the subject in this TED talk.
But sometimes mistakes can arise through your expectations and this processing and constructing, as shown 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 construct them as curved lines. And there are dozens of this sort of illusions in circulation1 More funny examples (see also Let the sunshine in). Diagrams like the Hering illusion are mostly seen as amusing novelties: “ see how our brains fool us”. But it is a lot stranger than that. It in fact illustrates the normal working of our brain. Fueled by our expectations, our brains construct everything we perceive: what we see, hear, smell, feel.
What is reality?
You never really know if something is exactly what it appears. Just as you never really know if what another person describes as what they see, hear, or smell is exactly the same as what you perceive ( see What is colour? How blue can look white…). Some scientists and philosophers say that everything we perceive is an illusion, and nothing exists in reality. That seems rather far-fetched. Trees, leaves, tigers, antelopes are for real. But you never know for sure if your perception is accurate and congruent with most other observers2see also “Can you smell it too?”.
Khan, A.G. and S.B.Hofer (2018). Contextual signals in visual cortex. Current Opinion in Neurobiology 52:131-138.
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, Ch. 2, p. 7-16
Eagleman, D. (2011). Incognito; the secret lives of the brain. ISBN 978-1-4614-0959-5, Ch.2
Frith, C. D. (2007). Making up the Mind; how the brain creates our mental world, Blackwell Publishing. ISBN 978-1-4051-6022-3 Ch. 1