Lab Talk


Stimulation, Sensation and Localization in the Cortex

Some aspects of cortical localization are well established but it may be the atypical responses to cortical stimulation that hold the keys to the the nature of mind.

The quest to build maps of brain function is over 200 years old.  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.

see related post Deconstructing a Thought

The first cortical stimulation experiment

Then in 1870 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 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 motor behavior.

Cortical stimulation in humans

It was not until 1890 that such stimulation was carried out in humans.  This was done by an American surgeon in Cincinnati called Roberts Bartholow on a 30 year old woman who 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 there was no obvious sensorimotor loss leading him to believe it would be safe to insert needles to stimulate after removal of the pus.  His initial stimulation with very low current produced no pain whatsoever and motor movement in the contralateral arm.  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. Subsequent attempts produced similar results.

The homunculus and beyond

Still, stimulation in humans was not abandoned, and for our understanding of its effects we owe much to Canadian neurosurgeon Wilder Penfield who performed stimulation experiments on 163 patients on whom he operated in 1928.  The best known outcome of this work is the cortical homunculus – a map of the representation of the body in the pre and post central gyrus (see the image above) and largely laid out in his paper Somatic Motor And Sensory Representation In The Cerebral Cortex Of Man As Studied By Electrical Stimulation. written with Edwin Boldrey in 1936.  However, Penfield documented his work at a time when scientific papers were not simply graphs and charts with p-values that often buried or averaged away the most interesting phenomenon in the pursuit of a clean result.  Rather even as he looked for recurring themes of localization across his patients using very weak stimulation, his work documents the cases of the strange and unexplainable as well.  It is in these atypical responses that Penfield stated were “unexplained but must be accepted” that our understanding is challenged.  Here are some of those:

Individual Variability

The somatosensory responses mapped typically to the pre and post central gyrus along the central sulcus. However, Penfield points out that the average map that he has drawn for any point on the precentral gyrus will only be correct for one in in six individuals and not for the others. Thus the map is approximate but not individually precise.

see related post The Myth of the Average Brain

Displaced responses

Some cases produced movement in response to stimulation of atypical areas far removed from the pre and post central gyrus. One such instance consistently produced the same motor response from the temporal lobe.  Second, sensory and motor responses were typically contralateral to the stimulated lobe, yet three cases reported bilateral sensations in the eyes, face and jaw.  One woman even produced a repeatedly verified ipsilateral response in the face.

Instability of Localization

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 responses during the first surgery were mute to stimulation and vice versa. In two cases they even found that they could induce movement of a motor response from the posterior to the anterior lip of the precentral gyrus by rhythmic stimulation.

see related post The Curious Outcomes of Neurosurgery

Atypical responses

In general stimulation of the cortex at any location produced no gastro intestinal or emotional response.  However, in four patients nausea could be reliably induced by stimulation and in three patients crying could be elicited.

No localization for quality of sensation

Somatosensory response involved both movement as well as sensations of the body.  However, while eliciting some kind of sensation of different body parts mapped to typical locations, there was no localization of the reported quality of stimulation.  Some reported the sensation of numbness or tingling, others cold or heat, some a feeling of thickness or swelling and at times the sensation of movement even in its absence. A few reported pain, but not strong enough that the patient objected.

From Brain to Mind

The picture that emerges is that localization in the cortex has some common patterns but is yet highly individual, dynamic and capable of wild exceptions. Further in the framework of localization motor response is easier to understand but sensation is more baffling and raises questions about the nature of mind and brain.  I shall leave you with Penfield’s own thoughts on this from some of his last writings before his death (from The Electrode, The Brain and The Mind, 1972), still relevant today.

As scientists, we should reserve judgment as to the ultimate nature of things. Meantime we can only use the language of dualism, and speak thus of the mind and the brain. There is no other medium of analytical discussion. The ancient riddle of how brain and mind do interact is still unsolved. But, we begin to understand the brain, if not the mind. The mind receives messages. It seems to direct brain action in the focussing of attention and in voluntary activity.

There is a special mechanism for the mind. It can be activated from a distance by an electrode on the interpretive cortex. The mechanism is sometimes arrested by epileptic discharge in centrally placed gray matter. During this activation, consciousness is not lost. During this arrest, consciousness is lost and since other mechanisms continue without control from the mind’s mechanism, the individual becomes an automaton.

Fritsch and Hitzig had stumbled upon the truth. The brain does function, as they said, by “isolated mechanical means.” And we can now perceive the outline of a further truth: The mind is matched by a specific corresponding mechanism in the brain. Human behaviour is determined by interaction of brain and mind.


One thought on “Stimulation, Sensation and Localization in the Cortex

  1. Dear Tara,
    thank you for your post. Do you have a pdf of the article “The Electrode, The Brain and The Mind, Penfield 1972”. It is behind a paywall.

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