The Brain's Grotesque Little Man
This strange, distorted figure is the sensory homunculus, a neurological map of the human body. The word "homunculus" is Latin for "little man," a name Wilder Penfield, its creator, gave his creations, which he also called "grotesque creatures." The model is not a representation of the body's physical form, but a map of the brain's primary somatosensory cortex. The size of each body part is directly proportional to the amount of cortical tissue dedicated to processing its sensory information. This is why the hands, lips, and tongue appear enormous, while the torso and legs are small and shrunken. The areas with the highest density of sensory receptors command the most brain space.
This concept in neuroscience was developed right here in Montreal. In 1934, the American-Canadian neurosurgeon Wilder Penfield founded the Montreal Neurological Institute (MNI) at McGill University. It was at the MNI, through the 1930s and 40s, that Penfield mapped the brain. He developed a technique for treating severe epilepsy, which became known as the Montreal Procedure. During surgery, with the patient awake under only local anesthesia, Penfield would use an electrode to apply small electrical currents to the surface of the brain. By asking patients what they experienced—a touch on the finger, a sensation on the tongue—he could systematically chart the functions of the cerebral cortex. This process allowed him to remove the brain tissue causing seizures without damaging areas responsible for functions.
A Map That Can Change
The brain contains multiple homunculus. The sensory map in the parietal lobe has a counterpart: the motor homunculus, which governs movement from the primary motor cortex. The two maps are similar but distinct, both showing disproportionately large hands and facial features necessary for fine motor control and speech. The body is mapped in a curious, upside-down order on the cortex, with the toes at the top of the hemisphere and the face near the bottom. The left side of the body is mapped to the right side of the brain, and vice versa.
These brain maps are not static. The brain exhibits neuroplasticity; it can reorganize and form new neural pathways throughout life. The homunculus can change based on experience. For instance, the cortical map for the fingers of a violinist's left hand is larger than in a non-musician. If a person loses a hand, the area of the cortex that once received input from that hand does not go silent. Instead, it can be remapped, often becoming responsive to touch on the face or upper arm. This reorganization is a factor in the phenomenon of phantom limb sensations, where an amputee continues to feel the presence of a missing limb. The brain's body map is dynamic representation, updated by our experiences.