A WHAT Lives Inside My Brain?

Map of the motor cortex i

The neurons in your motor cortex are grouped by body part. Parts with a complex range of fine movement, like the hand, take up more space. Parts with a limited range of movement, like the hip, require less space. Together, the various parts form a full, representative map of your body, called a "homunculus." Posit Science hide caption

itoggle caption Posit Science
Map of the motor cortex

The neurons in your motor cortex are grouped by body part. Parts with a complex range of fine movement, like the hand, take up more space. Parts with a limited range of movement, like the hip, require less space on the cortical map. Together, the various parts form a full representative map of your body, called a "homunculus."

Posit Science

How Do You Amputate A Phantom Limb?

Many amputees say they can still feel the presence of a missing limb, and often what they feel is intense pain. But how does a doctor treat pain in an arm or a leg that no longer exists? Oddly enough, one researcher used a cardboard box and a $2 mirror.

A 'Homunculus Man' i

Put together all the disproportionate body parts from your cortical maps, and you get homunculus men — little guys who look more cartoon than human. In both of the "little men," the fingers appear enormous. In the somatosensory homunculus (seen above left), that's because your fingers receive so much sensory data as they touch the world around you. In the motor homunculus (seen above right), the fingers are even larger because of the complex range of motion involved in activities like typing or grasping an object. Wikimedia Commons hide caption

itoggle caption Wikimedia Commons
A 'Homunculus Man'

Put together all the disproportionate body parts from your cortical maps, and you get homunculus men— little guys that look more cartoon than human. In both of the "little men," the fingers appear enormous. In the somatosensory homunculus (left), that's because your fingers receive so much sensory data as they touch the world around you. In the motor homunculus (right), the fingers are even larger because of the complex range of motion involved in activities like typing or grasping an object.

Wikimedia Commons

Reach down and rub the rug or floor below you. Seems like a simple task. You read the directions, and your brain gives your fingers a set of instructions that say, "Rub." But how did your mind deliver the message to your fingers?

At the same time, your fingers are giving your brain a set of instructions about the floor as well. They send a series of impulses that your brain interprets as texture — rough or smooth. How does the message travel?

The 'Little Man' In Your Brain

The signals in both directions — brain to finger and finger to brain — run along particular pathways, says Michael Paradiso, a Brown University neuroscientist. In the brain, those lines converge in a region called the cortex. The motor nerves that helped you move your finger connect to the "motor cortex," while the sensory nerves that tell your brain about a feeling (like texture) connect to the "somatosensory cortex."

The neurons in the motor cortex and somatosensory cortex aren't just thrown together in a big jumble of cells. Instead, they're grouped by body part — all of the hand neurons together, then all of the face neurons, then the tongue neurons and so on, as you can see in the diagram of the motor cortex. Together, they form a full, representative map of your body called a "homunculus," which means "little man" in Latin.

Neurosurgeon Wilder Penfield first mapped the cortex in humans while operating on the brains of patients with epilepsy and other disorders. The patients were awake during the surgery, and Penfield would stimulate different parts of their brains and then ask them what they felt. He found that certain parts of the brain activated sensations on corresponding parts of the body (always on the opposite side). From these experiments, Penfield was able to construct representative maps of the cortex, which he published in 1950.

But there's something funny about the limbs on the maps. On the motor cortex map, the foot looks freakishly small compared to the monstrous hand, and the arm is just a tiny spindle. On the somatosensory map, the fingers look enormous compared to a tiny hip.

The body parts look out of proportion because they're represented in terms of range of motion and feeling, rather than physical size of the body part.

Think of all the intricate motions required in your tongue for language or all of the sensations your fingers can feel as you touch the world around you. The data sent from your tongue and fingers are much more detailed than those of your foot or arm, so they take up more space in your cortex accordingly.

Put all those disproportionate limbs together, and you get homunculus man — a distorted little guy who looks more cartoon than human.

Phantom Sensations

But what happens if one of your limbs is amputated? The pathways that run throughout your body are truncated but don't disappear entirely, and neither does the representation of the limb in your brain.

Up in the somatosensory cortex, the neurons of an amputated hand are still functional, but they're now deprived of sensory input. Without stimulation from the hand itself, the neurons sometimes start responding to neighboring signals in the cortex instead. In this case, the closest signals come from the face, which is right next to the hand on the map.

According to V.S. Ramachandran, a researcher at the University of California, San Diego, a gentle stroke of the face ordinarily just activates neurons in the face area of the cortex. But if the hand area of your brain has nothing else to respond to, sensory inputs to the face might actually start "invading" the space around it and activating the desperate neurons in the hand area of the cortex as well.

For some amputees, touching the face can actually create feeling in the phantom fingers. Trickling water down the face of an amputee can even feel like water trickling down the phantom hand.

This phenomenon is a great example of brain plasticity. You might be born with very specific cortical maps in place, but your brain has an impressive ability to adapt to the changes in your body.

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