The Homunculus

Scientists can map which parts of the brain control various parts of the body. The mapping is done by stimulating the sensory or motor cortex with a weak electric current. The stimulation often produces tingling or movement in part of the body.

Humans put great emphasis on speech and manipulation of objects by the hands, so humans have large amounts of cortex devoted to mouth, tongue, and hands. Different species have different patterns. Rats get a lot of information from their whiskers, so they have large amounts of sensory cortex devoted to their whiskers.

The following diagram represents a slice of cortex near the fissure of Rolando, running from the top of the head down toward the ear. The diagram indicates the location and amount of cortex devoted to each part of the body. At location #22, for example (just above the lateral fissure by the ear), stimulation produces a swallowing reflex. A location #3, at the top of the head, stimulation results in toe movement. Altogether, the map of brain connections to the body in this particular strip of cortex looks like this, with the amount of cortical tissue represented by the size of the body part in the diagram:

How is the homunculus mapped out?

The Homunculus, based on Penfield's classic diagram

The diagram looks a bit like a grotesque little man, so it is called the homunculus (ho-MUN-q-lus) which means "little man" in Latin. The first homunculus diagram was drawn by Wilder Penfield in the 1940s and looks similar to the one above. Notice that the hands, lips, and tongue are large, because of the large areas of cortex devoted to these areas of the body in humans.

What sort of humorous references to the homunculus are common?

The homunculus is a textbook diagram, certainly is not a self or center of consciousness in the brain. However, humorous references to the homunculus as a little person in the head are common among psychologists. One psychologist might say to another, "But how exactly is this mental activity carried out? Does the homunculus do it?" This is a way of saying, "You have not given us an adequate explanation!"

What is evidence that cortical mapping can change with experience?

Actually there are many homunculi in the brain, if the word refers to an area of cortex where body surfaces are mapped. Such maps can change with experience. People who read Braille (which is done with an index finger) develop large areas responsive to stimulation from the index finger. A homunculus mapped on the motor cortex of such a person would have a huge index finger.

This flexibility in the brain inspired some therapies for brain-damaged patients. In one study, people who suffered partial paralysis of an arm after a stroke were able to regain full use of the arm by having the other (good) arm immobilized (prevented from moving). This encouraged development of the cortex that controlled the "bad" arm, resulting in partial recovery of the patient's ability to move that arm (see "Behavioral Treatment of Pain").

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Copyright © 2007-2011 Russ Dewey