A homunculus is a representation of body parts in the motor and somatosensory cortex of the brain. Through these representations it is possible to distinguish between different parts of the brain, for example, so that you can control all your fingers separately, or so that you know your feet are on the ground.
The part of the cortex that is assigned to a particular body part depends on how much that part is used. For example, in the somatosensory areas, the fingertips are much larger than the arms because they are often used to sense small details. As you can imagine, these areas are even larger in people who read Braille. In the motor cortex, the same kind of distribution can be seen, but then there are overrepresentations of other areas. One can think of the representation of the legs, which we use to move around, both walking and cycling.
What is remarkable about the homunculus is that areas that are close together on the body are also close together in the homunculus. Thus, the representations of the fingers are adjacent to the representations of the hands. In addition, a homunculus is actually an inverted picture of the body, because the feet are represented in the dorsal part, and the face is represented in the ventral part. Furthermore, a homunculus contains representations of only half of the body, as we often see in the brain. Thus, a homunculus in the left hemisphere has only representations of right body parts.
Author: Myrthe Princen (translated by Melanie Smekal)