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How the Brain Fills in the Blanks

... and how it can be fooled to perceive things that do not exist.

Key points

  • Our brains construct parts of our experiences.
  • We do not see the world as it is; we see it as we are.
  • The brain can be fooled to perceive things that do not exist.

You probably don't remember the first time you heard music through a pair of stereo headphones. It's actually an amazing experience: The music sounds like it's right inside your head. You're so used to it that you don't realize that this experience is an illusion; there's no orchestra inside your head, of course.

Similar mechanisms create all forms of experience: Everything you see, hear, and feel is a magical performance created for you by your brain. This statement may seem surprising and scary. I will show that it is the best way to describe how conscious experiences are generated.

Try the following experiment: Close one eye and gently press the side of the eyeball of the other eye with your index finger. Even though you are sitting still, the world seems to be rocking. It is easy to get a feeling of dizziness.

This simple experiment shows that your experience of the world outside you is actually created by the vision process in your brain (and by other sensory processes). What you experience is not the world itself, to use Immanuel Kant's term, but your brain's construction of it: the world for you. This construction is extremely efficient: for example, the world seems to stand still even when we turn our heads, even though the image on the retina changes rapidly.

The visual image we experience is adapted to our needs. For example, when we look at an object, we perceive it as having contours. However, the contours are not present in the light that hits the retina but are an addition created at an early stage of the visual process. Another form of adaptation is that we see what we need: Our field of vision covers only 180° in width and 135° in height. Compare this to many birds and animals that need to be aware of predators and therefore have a field of vision of almost 360°.

A common thought is that the visual process works like a camera. It is easy to show, however, that this is wrong. For example, we feel that everything is sharp when we look at it. But in fact, only two angular degrees of the field of vision are sharp (about two fingers' width at arm's length); in the periphery, we see things as blurred we have more difficulty perceiving colors there. But every time we look at something, it becomes sharp and we therefore get the illusion that the entire field of vision is sharp. It's a bit like the light being on every time we look into the fridge.

Another illusion is that we do not experience any gaps in the field of vision. But there is an area of the retina that has no receptors; the blind spot means that there is a part of the visual field in which we see nothing. This could be a possible explanation for why we sometimes miss something in the environment when we think we have looked. "Looking without seeing" is said to be the third most common cause of road accidents. For example, a car hitting a train is much more common than a train hitting a car.

If my perception of the world is an illusion, what does the world really look like? It doesn't look like anything at all. There is nothing that can see, hear, smell, or feel unless there is a brain creating an experience. If an avalanche rages far from where people and animals are, there is no sound. There are, of course, a lot of sound waves, but no one hears them. The philosopher Alfred North Whitehead says that poets are wrong to wax lyrical about nature. Instead, they should congratulate themselves on the fact that their minds create all that they perceive as wonderful in the world around them. Nature itself is colorless, soundless, and scentless; it's just matter moving around meaninglessly. Our senses give it content.

The brain is therefore not a passive recipient of images and sounds from the world around us. It actively looks for patterns and interprets the world around it. Our experience of the body is also a construct built up through feedback from the senses. The brain not only improves, it also invents. You have probably tried crossing your middle finger over your index finger and then feeling the tip of your nose. It feels like you have two noses. The explanation is that the outer side of the middle finger does not usually touch the same object as the inner side of the index finger, which leads the brain to create a perception of two objects.

The following experiment provides an even more surprising illusion: Stand close behind another person put your right index finger on the other person's nose and close your eyes. Then ask a third person to slowly move her index finger up and down over your nose while holding your right index finger and moving it in sync over the other person's nose.

After a while you will experience that, like Pinocchio, you have a very long nose. Because the movements are simultaneous, your brain assumes that what your index finger feels is in the same place as what your nose feels, creating the illusion of a long nose. The result is that you do not perceive the body directly, but a virtual body created by your brain, just as you see a constructed world and not the "real" one.

A similar phenomenon can be induced in people who have so-called phantom limbs. A phantom limb means that after an amputation, you feel an arm or leg that is no longer there. The phantom limb itself is a result of the body model continuing to simulate the arm or leg as part of the body despite the lack of real feedback from the limb. Some patients experience the phantom limb as paralyzed, meaning they have no control over it.

Neuroscientist Vilayanur Ramachandran conducted an experiment with a patient who had a "paralyzed" phantom left arm. The patient was asked to put his "hands" into a box constructed so that he could see his real right hand, while a mirror created an image of the left "hands" (which was only a mirror image of the right). He was now told to move his "hands" in a symmetrical way.

The patient protested and said that he absolutely could not move his left hand. But through the construction of the box, two hands seemed to move in a similar way, and to his own great surprise, the patient felt that he was in control of his phantom limb. As soon as he closed his eyes, the feeling of control disappeared again.

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