Maps in the Brain

Sensory Homunculus

The handsome man here on the left is called Homunculus (a beautifull name for a beautifull man) and his presence in this post will become clear soon. But first things first. In our previous post we tried to understand how human societies here on Earth work by looking at them from the Moon. We found out it was not easy at all (but nobody said it was, right?, hence the song). We also saw that trying to understand how the brain works is in some ways similar to the task of attempting to disentangle the human civilization from somewhere far away in the space. But there is a critical difference between the two enterprises (apart from the fact that one of them is completely imaginary/absurd): when studying the brain we can interact with it, which means that we can manipulate a specific parameter involved in its functioning (for instance the level of activity in a given region) and see what happens. From the Moon, the only thing you could do was to observe the Earth (and hope for the best).

Wilder Penfield was one of the first scientists (he was actually a medical doctor, but anyone with enough curiosity can be scientist!) who used systematically this approach to establish a direct correspondence between what we perceive and a specific region of the brain. Being a medical doctor, his original goal was to actually treat people. He wanted to cure patients that suffered from severe epilepsy. To do so, he developed the Montreal Procedure that consisted on applying a local anesthetic to the skull of the patient and opening a small cranial window on it so as to expose the brain and try to find the focus of the epileptic seizures. Once the focus was found, Penfield removed a small part of the brain containing it. And, believe or not, the technique worked pretty well. So well that a similar procedure is still used nowadays (of course with much more precise tools) in epileptic patients that cannot be treated pharmacologically.

Sensory Homunculus

Penfield was able to cure more than half of his patients using the Montreal Procedure. But he did not stop there. The procedure actually required him to electrically stimulate different brain areas of the patients and observe their responses before performing any irreversible surgery… And yes, this is exactly the kind of experiment I was talking about a couple of paragraphs ago: you change something in the brain and see what happens (or in this case you may ask the patient 'what's up?'). Wilder Penfield (once called “the greatest living Canadian”) did not miss this opportunity. By means of his new developed procedure, he discovered that by stimulating the temporal lobe (one of them) of his patients, he was sometimes able to elicit vivid and long forgotten memories in them. Moreover, every time he stimulated the exact same place, the exact same memory came up. Just think about it: this means that just by activating a small subset of neurons he could elicit the memory of that afternoon from the patient’s childhood when he was drinking hot milk in the kitchen, where his mother was preparing a turkey dish with smash potato that smelt incredibly good while singing the same song she always sang when she cooked. All this, his mother, the kitchen of the house where he grew up, the taste of the hot milk, the smell of the turkey, the song… all, just by applying a small amount of electricity to a very tiny area of the patient's brain. Pretty cool, uh??

Another crucial contribution made by Penfield was a detailed map of the tactile cortex, i.e. a look-up table that tells us which part of the brain encodes the (tactile) information about each part of the body. You can see the map above this paragraph. Maybe you notice that some parts of the body are actually over-represented in the brain. The hands, the lips, the tongue... An actually there is a whole region of the brain specifically dedicated to each finger of the hand but toes just deserve a common single region. This is because we do many more things with our hands than with our feet, and so we need many more neurons to encode what is going on with our hands than with our toes. Now, if we build a human body in which the size of each of its parts is proportional to the size of the region in the brain that is in charge of it we get a nice little man as the one you can see at the beginning of this post: the (Cortical) Humunculus.

The maps developed by Wilder Penfield are still used today. And he obtained them with a (conceptually) very simple methodology: altering the normal function of the brain and observing the consequences.

Manuel Molano

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