Seeing isn’t believing

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Duck-Rabbit_illusion  Seeing is believing.  Or is it?  We feel that we see the world as it truly is, but in reality, what we see is merely an illusion created by our brains.

You find yourself in a dark room but feel the presence of some dangerous animal behind you.  It begins to chase you.  You break out into a run but it gets closer and closer – you can’t get away.  It jumps out to catch you and . . . you wake up.  We all dream, and we’re all familiar with this kind of experience.  And while we’re dreaming, that‘s exactly what it is – an experience.  To most of us, the world of our dreams feels as real as our waking life.  This suggests something interesting about our experience of the world around us – our brains do not passively view the world through the windows of our eyes, but instead must actively construct it.  Essentially, the world as you see it is an illusion.

But the world obviously isn’t pure invention.  Most of us agree that a physical world exists around us– although, given the power of our brains to fool us, as they do when we dream, we can never prove that we’re not currently in a dream.  This argument was initially proposed by Rene Descartes but it has stayed with us to the present day:

But let’s assume the world does exist.  How would your brain begin to go about the job of allowing you to see the world around you?  Rene Descartes also had an answer for this.  Descartes noted that the eye focuses a picture of the world on the back of the retina and suggested that the brain or the soul of the person simply had to look at this image in order to see.

However, there’s a major problem with this idea – it doesn’t answer the question of how the actual seeing gets done.  In this theory, the task of seeing is simply passed along to something inside your head that does the seeing for you. But if vision works by something inside you looking at the image on your retina, how does vision work for that something inside your head?  Perhaps there’s something inside it too that allows it to see – and so on to infinity.  The actual question of how vision works is never answered.

So how does vision work?  Focusing an image on the retina by itself is not enough – if it were, your camera would be able to see.  It turns out that, despite how effortless it feels, seeing is an extremely active process.  We now know that the brain extracts features of the visual world and combines them in order to rebuild the image inside your head, but in a code that the brain understands.  This liberates the brain from passively viewing whatever is currently in front of it and allows it to store images in memory, combine them with sounds and smells, and even to see when our are eyes closed during dreams.

It also allows our brains to constantly guess at what we’re seeing.  The world doesn’t always give us complete images, and we often have to use context to inform us of what it is that we are seeing.  For example, the image below could be a duck or a rabbit but without context, both are equally likely.  As a result your brain will reinterpret what you are seeing every few seconds, causing what you see to change even though the image in front of you is always the same.


The creativity of the brain results in it not being the perfect camera-like viewing machine that we might otherwise imagine that it is.  On top of this, our visual experience of the world is actually shaped by our own thought processes.  This can be illustrated through a phenomenon known as change blindness.

Sometimes we do not see what is right in front of us, but at other times we see things that aren’t really there.  Certain stimuli, such as faces, are very important to us.  In the wild, a face could belong to a predator or prey, and so it was very important for our ancestors to detect these faces so that they could respond appropriately.  As a result, our brains constantly detect faces in random patterns and everyday objects:

Similarly, if you’re in the woods at night, you might very easily mistake a bush for a person.  Our low threshold for detecting others is massively beneficial for our survival.  Mistaking a bush for a dangerous person only scares us temporarily but mistaking a dangerous person for a bush could lead to far worse consequences.  The creative power of the brain means that hallucinations are only a small step away from the normal way our brain works.

So, if you’ve ever witnessed anything “supernatural,” a ghost or a strange creature for example, you can rest assured that the laws of the universe have not changed– you’re brain is just doing what it evolved to do.  Or, to look at it another way: to the visual parts of your brain that ghost is just as “real” as the next person you see.

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James Cooke
James Cooke is a Neuroscience D.Phil. student, interested in cortical function and circuit organisation. He completed a B.A. in Experimental Psychology and an M.Sc. in Neuroscience at the University of Oxford where he is also based for his D.Phil. His research involves using electrophysiological and optogenetic techniques in order to investigate the biophysical basis of cortical computations. When he's not in the lab, he's usually either playing Jazz piano or running along a trail somewhere. You can find more of his work at @NeuralNoises

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