Why is the brain wired contralaterally

Question to the brain

Answer by: Onur Güntürkün, Professor of Biopsychology at the Ruhr University in Bochum.

The brain is divided into two halves, called the hemispheres of the cerebral cortex. In humans and many other animals, the halves of the brain and body are cross-linked: The left hemisphere controls many functions of the right side of the body and vice versa. In people with damage to the left hemisphere of the brain, for example through a stroke, the effects are mainly evident in the right area of ​​the body. For example, you can no longer move your right arm.

But why does a lot cross between the brain and the body? The historically oldest explanation comes from the Spanish physician Santiago Ramón y Cajal. It refers to the junction of the optic nerve fibers. His argument: if the optic nerve fibers did not cross, the outside world in the brain could not be represented correctly as in a "head cinema". Cajal's explanation has now turned out to be wrong for many reasons, including because we don't represent the world like a movie in our brains.

The scientific explanation that is more popular today takes us back to a time long before ours. At that time, vertebrates lived in water, had worm-shaped bodies, and their brains consisted of little more than a backbone and a brain stem. Such an animal probably didn't even have the complex lens eyes that we have today. It had primitive eyes with which it probably couldn't see anything other than light and shadow.

When an enemy swam towards the animal, it meant for our animal that a shadow grew larger. In order to turn away from its enemy, it had to shorten its body on the opposite side of the stimulated "eyes". So if the visual stimulus came from the right side, it had to contract the muscles on the left opposite side as quickly as possible in order to initiate the escape movement. For this purpose there had to be an optic nerve junction. In this case from the right rudimentary eye to muscle groups on the left side. This is the shortest connection to activate the muscle groups on the opposite side of the body.

We cannot say with certainty whether this was actually the case. But there is a possible explanation.

Answer recorded by Christian Wolf