Mapping Your Brain
Maps of brain function are highly individual and exhibit wild exceptions.
Posted Jun 25, 2020
For 200 years, scientists have speculated that different parts of the brain were responsible for different types of activity and behaviors and have tried to build maps of the brain, carving it up into regions with different functions such as vision, speech, movement, and so on. The sensory nerves from the eyes, ears, tongue, and nose generally deliver their information about the external world to different parts of the brain, as do the nerves sensing touch in the body, so this is not an outrageous speculation.
But do different parts of the brain indeed direct different capabilities and behaviors? The earliest evidence came from cases of brain damage, but the results were always ambiguous. Out of 37 reported cases of frontal lobe damage, for instance, 21 showed damage to speech capability but the others did not.
Stimulating parts of the brain to understand function
Progress was made in 1870 when German neurologist Eduard Hitzig and physiologist Gustav Fristch took on a curious set of experiments. Forbidden to carry them out at the University of Berlin, where both held an appointment, they performed these experiments in Fritsch’s bedroom. Exposing a strip of the cortex (the folded outer brain layer) of a dog, they used a probe to apply electrical stimulation to different areas.
The dog, of course, couldn’t report what it experienced or if memories were evoked, but they discovered that they could map regions of the brain to muscle contractions arising in different parts of the body. This was one of the most solid pieces of evidence that stimulating one region of the brain, but not another, could drive a particular movement or motor behavior.
It was not until 1890 that such stimulation was carried out in humans. This was done on a 30-year-old woman by an American surgeon in Cincinnati called Roberts Bartholow. The woman had fallen into a fire and badly burned her scalp as an infant. The wig she wore to cover the burn had somehow eroded a part of her skull over the years to such a degree that a pulsing infection of the brain could be observed below it.
Despite this, she had no obvious loss in any movement or sensation. After removal of the pus, which appeared to cause no damage, he inserted needles into her brain to electrically stimulate it and see if he could elicit movement. His initial stimulation with very low current produced no pain whatsoever and motor movement in the arm on the opposite side of her body. However, when he increased the current, her arm lifted and went into clonic spasms. Her lips then went blue, her pupils dilated and she began frothing at the mouth. Five minutes later she went into a coma. In a few days, she died.
Despite this initial tragedy, stimulation in humans was not abandoned. And much of our understanding comes from work done by the Canadian neurosurgeon Wilder Penfield, who performed stimulation experiments on 163 patients on whom he operated in the 1920s.
Systematically stimulating different parts of the brain to identify where the patient exhibited movement or felt sensation, he created what is known as the cortical homunculus. The homunculus is a map of where different body parts are represented in the brain and forms a strip at the top of the brain on both sides of a groove called the central sulcus.
In this map, the body parts on the left side of the body are represented on the right side of the brain and vice versa, whilst the lips and hands occupy disproportionately larger parts of the cortex. However, even as he identified a "general" pattern, Penfield points out that the average map that he had drawn for any point will only be correct for one in six individuals and not for the others. Thus the map was only approximate but not individually precise. Our brains have their fair share of variability.
Penfield’s work also documented strange cases which he stated were “unexplained but must be accepted." It is in these atypical responses that our understanding is challenged. For instance, in some people, he could produce movement in response to stimulation of areas of the brain far away from the homunculus. Second, sensory and motor responses were typically on the opposite side of the body from the lobe of the brain that he stimulated. Yet three people reported sensations in both sides of the face and jaw. One woman even produced a repeatedly verified response on the same side of the face, which was opposite to everyone else.
Even more curious was that while repeated stimulation of the same point on the cortex during the same surgical session produced a consistent response, this was not so across surgeries or sessions. In five patients where a second surgery was carried out several months later, it was found that locations that produced quite active movement responses during the first surgery were mute to stimulation and vice versa. So altogether, the map of which parts of the brain control movement of different parts of the body can be wildly variable across people and even move around over time.
More interesting perhaps is that while it is possible to draw an approximate map of movement by stimulating different parts of the brain, it was not possible to make maps of emotion or sensation. In general, stimulation of the cortex at any location produced no gastro-intestinal or emotional response. However, Penfield found that he could reliably induce nausea in four patients and crying in three. As for sensation, while he could induce all kinds of sensations by stimulation such as numbness or tingling, cold or heat, or a feeling of thickness or swelling, this varied wildly across people and he could not predict what would happen in one person compared to another, let alone pinpoint a particular location.
The picture that emerges is that you can identify some common patterns of how function is localized in the cortex along some dimensions such as movement, but even this is highly individual, moves around, and is capable of wild exceptions. So next time you read a scientific study about how some part of the brain is associated with some kind of function or behavior, take it with a big pinch of salt. It may have been true for some people that day, in that particular situation, but maybe it won’t be true for you.