Wilder Penfield, Neural Cartographer


The patient lies on the operating table, with the right side of his body raised slightly. The anaesthetist sterilizes his scalp and injects it with Nupercaine to produce analgesia – the patient will remain fully conscious throughout the procedure. Behind the surgical drapes, three large incisions are made in his scalp. A large flap of bone is then cut from his skull, and turned downward to expose the surface of his brain. The ultraviolet lights which illuminate the operating theatre and keep the air sterile are positioned in such a way that they do not shine directly upon the cortex.

Using an atomizer, the surgeon sprays a small amount of Ringer’s solution onto the brain substance, to keep it moist. He then manoeuvres an electrode attached to a special holder which is clamped to the margin of the opening in the skull, so that it comes into direct contact with the brain. He adjusts a dial on the stimulator to 0.5 volts, and a current with a frequency of 60 cycles per second is applied to the patient’s cortex. After asking the patient if he feels anything, and getting a negative response, the surgeon reaches for the stimulator again. He turns the voltage dial up a notch so that it reads 1 volt, and applies another current. This time, the patient reports a tingling sensation in his face and, when asked to indicate exactly where, raises an arm and points to his left cheek and temple.

The surgeon dictates these results, via a microphone, to a secretary in the viewing stand. He then places a small numbered ticket on the part of the brain he has just stimulated, and manipulates the electrode again, bringing it to bear on another point several millimetres away from the last. Upon application of current, the patient reports a sensation on the inner surface of his left forearm, and there is a slight movement of his left thumb. He knows that his face, arm and thumb have not been touched; although aware that his thumb moved, he knows fully that he did not will the action, and realizes that he can reach across with his right hand to prevent the movements.

The patient is an 18-year-old male with a small tumour in his right frontal lobe. It is an unusual type of hemangioma that may have been present since early on in the patient’s life, located immediately adjacent to the primary motor cortex, one of the regions of the brain which controls voluntary movement. It therefore causes waves of abnormal electrical activity to sweep across the motor cortex, leading to epileptic seizures which begin with a sensation in the left side of the body and are followed by rapid, jerking movements of the left arm and left leg. The patient’s epilepsy is intractable – his symptoms were not alleviated by phenytoin, the most widely used anticonvulsant at the time – and so, as a last resort, it has been decided that his tumour should be surgically removed.

The surgeon, Wilder Penfield, faces a major challenge. Because the motor cortex controls the muscles in the throat and tongue which are needed for articulation, as well as those involved in limb movements, he must remove the tumour without damaging these parts of the cortex, in order to avoid paralysing the patient, or leaving him with a speech deficit. Using a technique he had developed more than 20 years earlier, Penfield identifies these crucial areas by electrically stimulating the tissue surrounding the tumour with his electrode. This elicits a movement in that part of the patient’s body controlled by that specific region of cortex. By carrying out multiple stimulations and meticulously recording the patient’s response to each one, Penfield can identify the abnormal tissue causing the patient’s seizures and delineate those areas surrounding the tumour which he must avoid damaging.

Penfield was one of the greatest neurosurgeons of the twentieth century, whose pioneering work revolutionized the discipline. His technique for treating intractable epilepsy, which was developed with his colleague Herbert Jasper, came to be known as the Montreal Procedure; it was ground-breaking because it applied of the principles of neurophysiology to the practice of neurosurgery. In using this technique to carry out presurgical evaluations of his patients, Penfield amassed a large body of data, which together constituted the first detailed large scale functional map of the human cerebral cortex. Penfield not only revolutionized neurosurgery, but also left a long-lasting influence on related fields such as neurology and neuropsychology. But less well known is that he was also a talented researcher whose experiments resolved a debate about the cellular structure of the brain.

The Montreal Procedure was based upon the earlier work of a number of great clinicians and investigators. One of these was John Hughlings Jackson (1835-1911), the father of English neurology. Prior to Jackson’s work on epilepsy, seizures were thought to originate in the medulla oblongata, the lower part of brain stem which is attached to the spinal cord. In the 1860s, while at the National Hospital for the Paralysed and Epileptic (now the National Hospital for Neurology and Neurosurgery) in London’s Queen Square, Jackson observed and described numerous patients with epilepsy. Some of these had sustained blunt head injuries, which, as Jackson noted, usually resulted in first seizures, and then paralysis, in the opposite side of the body. Based on these clinical observations, Jackson concluded that seizures were the result of abnormal electrical discharges in the cerebral cortex.

Jackson also observed that patients with an external injury lying over the posterior region of the frontal lobe usually experienced speech difficulties, either during or in between their seizures. These observations were consistent with those of the French physician Paul Broca (1824-1880), who had been working with stroke patients and examining their brains after they died. Aphasia, or an inability to speak, is a common symptom of stroke, and Broca noted that his patients consistently presented with damage in the same discrete region of the left frontal lobe. He therefore concluded that the faculty of speech is located in that part of the left hemisphere, which subsequently came to be known as Broca’s area.

In 1870 the German physiologists Gustav Theodor Fritz (1838-1927) and Julius Eduard Hitzig (1838-1907) performed the first direct electrical stimulations of the mammalian cerebral cortex. Using electrodes, they stimulated the front half of the brains of lightly anaesthetized dogs, and found that this produced movements of the leg on the opposite side of the body. Stimulation of the posterior half, however, did not elicit any movements. Several years later, this work was extended by the Scottish experimental neurologist David Ferrier, who obtained similar results in both dogs and monkeys. Ferrier also noticed that lesioning the frontal cortices of his animals led to a loss of the movements elicited by electrical stimulation.

These early studies, together with case studies of brain-damaged patients such as Phineas Gage, strongly suggested that the cortex of man and animals was divided into functionally distinct areas and, as Ferrier said at a meeting of the Royal Society, paved the way for “a scientific phrenology”. When Penfield began his career as a surgeon, the ground work had thus already been laid, but the human cerebral cortex was still largely uncharted territory. Using the technique he and Jasper developed, Penfield made the “scientific phrenology” to which Ferrier had alluded a reality. Today, he is best known for the functional mapping of the human cortex, but he also made a significant contribution to the then emerging field of neurocytology. Before drafting his map of the human cortex, Penfield surveyed the brain’s microgeography, and fully characterised, for the first time, a type of cell called the oligodendrocyte.

Although Penfield studied under Harvey Cushing, the father of modern neurosurgery, it was the eminent British physiologist Sir Charles Sherrington who inspired him to become a surgeon. In 1914, as an undergraduate medical student at Princeton, Penfield secured a Rhodes Scholarship, and enrolled in Sherrington’s mammalian physiology course at Merton College, Oxford University. This 3 year course consisted of a series of what Sherrington called “exercises”, involving various procedures, such as the dissection of the peripheral nerves and spinal cord of animals. Penfield thus learned how to handle living tissues with the greatest of care, and how to dissect them with fine surgical instruments while preserving the vital functions of the experimental animals. 


A young Wilder Penfield in Charles Sherrington’s laboratory for mammalian physiology in 1916. (Wilder Penfield Archive)

Upon completing this course, Penfield finished the clinical part of his medical training at Johns Hopkins University, and, after graduating in 1918, was interned at Peter Bent Brigham Hospital in Boston, where he did his apprenticeship with Cushing. He had been interested in epilepsy since his undergraduate years, during which he had read Jackson’s work. After finishing his internship, Penfield took a position at the New York Presbyterian Hospital, and it was here that he first operated on epileptic patients.

Penfield believed that a class of non-neural cells called microglia were involved in the formation of the scars which develop in damaged brain tissue. These scars form at the site of an injury and are part of the healing process, but they are also the origin of the abnormal electrical activity underlying epileptic seizures. In the early 1920s, he was therefore looking for a method to stain scarred brain tissue, in order to gain a better understanding of the origins of epilepsy. His work reached a dead end, however, because he could not find a way to stain the non-neuronal cells. At Oxford, Penfield had experienced some problems staining neurons, and he remembered that Sherrington had told him not to give up until he had tried the methods developed by the outstanding Spanish neuroanatomist Santiago Ramon y Cajal.

In the late nineteenth and early twentieth century, Cajal had used a modified version of a staining technique developed by Camillo Golgi to carry out a comprehensive and systematic study of the cellular architecture of the nervous system in a wide variety of species. It was largely because of his work that the Neuron Doctrine, which states that the nervous system consists of cells instead of a continuous network of tissue, came to be accepted in the 1890s. As well as describing the structure of neurons and astrocytes in great detail, Cajal also identified a mysterious and poorly understood “third element” of the nervous system, which apparently consisted of 2 types of non-neuronal cell that were different from astrocytes. Later, his student Pio Del Rio-Hortega developed a technique for staining one of these, the microglial cells, and published detailed drawings of their structures. However, his method did not stain the other type well, so they remained improperly characterized. They were, Penfield wrote, “no more than ghosts”.

The cells that Penfield was most interested in were amongst this third element, so he decided that he needed to travel to Spain to work with Cajal and Rio-Hortega, to “study the brain of man, and then move on to the effects of disease on the brain”.  In January 1924, Penfield approached Allen Whipple, chief of surgery at the Presbytarian Hospital, to try to obtain funds. Whipple was supportive, and contacted Mrs Percy Rockefeller, on whose daughter he had recently operated free of charge. He managed to secure a grant from her, and funds were also obtained from several other benefactors. Some time later, Penfield set sail to Spain with his wife and two children. He was so enthusiastic that he and his family embarked on their journey before receiving any confirmation – it was only when they were half-way across the Atlantic that Rio-Hortega replied, in a telegram containing a single word: “venga” (“come”).

Penfield spent 5 months working with Cajal and Rio-Hortega at the Laboratorio de Histopatologia in Madrid, and the collaboration proved to be a successful one. Penfield learned Rio-Hortega’s “ammoniacal silver carbonate” method for staining the non-neuronal cells, then modified and improved it, thus developing the first reliable stain for oligodendrocytes. Using his new technique on brain tissue from rabbits, Penfield was able to provide a detailed description of the cells. He published his results, complete with beautiful diagrams of the cells, in a seminal 1924 paper in the journal Brain:


Irregular stumps or short projections of cytoplasm have been frequently described in the third element…these projections are in reality the bases of expansions of considerable length and complexity. The heavier expansions pass across surrounding neuron fibres or more often are parallel to them…these fibres give rise to flattened branches that form an incomplete envelope about the adjacent myelin sheaths…the expansions of oligodendroglia are seen to be broad and short…[and] give rise to the longer and more slender branches which pass along with the nerve fibres and parallel to them…small expansions encircle the nerve fibres…but there is always a clear zone between the expansion and nerve fibres. This zone corresponds to unstained myelin sheaths. Inasmuch as oligodendroglia fibres form an irregular and incomplete network about the myelin tubes of the central nervous system, they may be said to form a discontinuous sheath for the myelin in a manner analogous to the sheath of Schwann about peripheral myelinated fibres (Penfield, 1924).

Before Penfield’s visit to Spain, whether oligodendrocytes constituted a distinct type of cell in the central nervous system was still a matter of debate. Unable to stain them properly himself, Cajal argued that they were not, and that the “third element” of the nervous system consisted solely of microglia. Penfield’s research showed that oligodendrocytes were indeed distinct from other types of glial cells, and that they could be distinguished from astrocytes because they lacked “sucker feet” (structures which today are called vascular endfeet). It also clearly showed that there was a difference in how astrocytes and the other types of glial cells are related to the blood vessels in the brain: whereas both oligodendrocytes and microglia contact blood vessels with their cell bodies, astrocytes do so only with their endfeet.

Penfield and Rio-Hortega also investigated how the non-neuronal cells in the brain are involved in pathological processes, and gained considerable insight into scar formation following injury. The reason that Penfield had travelled to Spain was to elucidate this process. The observations he and Rio-Hortega made of these processes were published 3 years later. Their description of how scar tissue forms in the brain is very similar to that of modern neuropathologists:

The formation of a simple cicatrix [contracting scar] in the brain presents the following stages: The first cellular change is observed in microglia cells which begin their phagocytic activity early and continue it for a long period. Later, the neuroglia astrocytes about the wound become swollen and those closest to the area of destruction or to obliterated vessels undergo clasmatodendrosis [loss of distal processes]. There follows rapid amitotic division of the other astrocytes and the cells then become fibrous and arrange themselves typically in a radial fashion about the wound. Most of their expansions…are arranged like the spokes of a wheel with the site of the former stab as the hub…A connective tissue core forms at the center, connective-tissue collagen fibrils are laid down and the wound contracts (Rio-Hortega & Penfield, 1927).

Upon his return to New York, Penfield resumed his work at the Presbytarian Hospital. Several years later, in 1927, he decided to compile “a textbook on the general principles of neuropathology without describing specific diseases”, with his colleague William Cone. Modestly, Penfield believed that others could write parts of the book better than he, and so wrote to a number of researchers asking them to contribute. By then, Penfield had made a very good reputation and was widely respected. All but one of his requests for contributions to the book were accepted. It was Cajal who refused the request to write a chapter, “saying he had advancing arteriosclerosis,” Penfield explains, “the histologist’s way of describing old age”. Even so, Cytology and Cellular Pathology of the Nervous System was eventually  published in 1932; it was dedicated to Cajal, and instantly became both successful and influential.

Uncertain about his future in New York, Penfield had moved to Montreal in 1928, where he hoped to establish a neurosurgical unit with William Cone. Later that year, Penfield’s sister Ruth developed a tumour, and he was called upon to surgically remove it. Ironically, her tumour was an oligodendroglioma, consisting of the very same cell type that Penfield had characterized while in Spain. On December 11th, 1928, Penfield performed a rather radical procedure to remove it, but he was unsuccessful – he did not remove it entirely, and it grew back some time later. In November of 1930, Cushing attempted a second operation, but she died the following year.


First functional map of the human cerebral cortex, outlining the areas of the brain which produce movements and auras. (Foerster & Penfield, 1930)

Shortly afterwards, Penfield visited the German neurologist and neurosurgeon Otfried Foerster, who showed him a method for excising scars from the brains of epileptics, involving the electrical stimulation of the cortex while the patient was under local anaesthesia. In 1930, the two of them published the first cortical map, albeit an incomplete one, based on the observations they made during over 100 operations. They also examined under the microscope the damaged tissues they removed from these patients, and so gained further understanding of the scar formation process:

There was always fibrous tissue, especially near the surface, and adhesion to the meninges. Thus connective tissue and an astonishingly rich plexus of vessels were invariably present in the scars, intermingled with fibrous astrocytes whose fibres were in general arranged in parallel and extended in the direction of the obvious traction, that is upward toward the cicatrix. Deeper down in the brain the astrocytes and blood vessels still continue to form the only framework capable of withstanding tension, the vaso-astral framework (Foerster & Penfield, 1930).

Subsequently, Penfield received a $1,232,000 donation from the Rockefeller Foundation, and used it to establish the Montreal Neurological Institute (MNI) at McGill University. The Institute opened in 1934, with Penfield as its first director; he remained in that position until 1960. Under his direction, the MNI quickly became a centre of excellence for both the practice and teaching of neurosurgery. Just like its founder, the Institute was a pioneering place: Clarence Greene, the first Afro-American board-certified neurosurgeon in North America, trained there under Penfield between the years of 1947-1949, at which time the United States was still racially segregated.

With his expertise in neurocytology and neurophysiology, Penfield now considered himself as “a neurologist in action”, and he set about working on a way to improve surgical treatments for patients with epilepsy. In his new position as director of the MNI, he was not constrained in his work, and so began to develop Foerster’s method in the hope of improving the outcome of his operations. Perhaps he was compelled to do so because of his sister’s death; more likely, given his long-held interest in epilepsy, he would have done this regardless.

Penfield used platinum electrodes with glass handles, sterilized by autoclaving before the operation, and attached by means of insulating wires to an instrument called a thyratron stimulator, with which the amplitude and frequency of the current were controlled. A local anaesthetic would be administered to the patient’s scalp, and a craniotomy performed to expose the surface of the brain. First, small electrical stimuli, insufficient to elicit a response, would be applied to the cortex. The current would then be gradually increased until the patient gave a positive response. Usually, the first thing Penfield did was to outline the central sulcus, a deep chasm which separates the undulating folds of the frontal and parietal lobes. This served as a prominent landmark which enabled him to get his bearings as he navigated his way around this most complex of organs. 

Sketch of a patient’s brain, annotated throughout the operation, to show the areas of motor and somatosensory cortices which elicit movements of, or sensations in, the mouth and lips. (Penfield & Boldrey, 1937).

The primary motor cortex, or precentral gyrus, is a strip of tissue lying immediately anterior to (in front of) the central fissure in the frontal lobe, and extending into its banks; it contains cells which send axons down through the spinal cord, where they form synaptic connections with the motor neurons which project out to the limb muscles. Immediately posterior to (behind) the central sulcus, in the parietal lobe, lies another strip of tissue called the primary somatosensory cortex, or postcentral gyrus, which receives tactile stimuli from the entire body. Flanked by these regions, the central sulcus could be found easily. A stimulus of sufficient amplitude, applied to the motor cortex, would elicit in the patient a movement of a specific muscle or muscles in the opposite side of the body. If applied on the other side of the central sulcus, the same stimulus would instead elicit the sensation of being touched, again in a specific part of the opposite side of the body.

In this way Penfield carried out, over the course of many years, a meticulous survey of the surface of the brain. For the patient lying on his operating table, the sensation elicited by electrical stimulation of a region of somatosensory cortex is indistinguishable from the one that occurs when the part of the body represented by that region is actually touched. In the latter situation, the pressure of a touch activates the nerve endings of the sensory nerves, which convey the information into the spinal cord and from there up to the brain. The sensation is only “felt” once the information reaches the cells in the somatosensory cortex. If those cells are instead activated by electrical stimulation, exactly the same sensation is felt. Because the patient is local anaesthesia, she would be fully conscious, and so able to report accurately where in her body she felt a sensation. Some of the verbal responses given by patients while Penfield pinpointed the central sulcus included: “I feel a numbness in my finger”, “My right thumb is tickling”, “I felt as though I could not speak” and “My tongue seemed to be paralyzed”.

By recording the responses of his many patients, Penfield refined the cortical map produced earlier with Foerster. Each operation made possible the delineation of a previously unrecognized landmark on the map. When trying to remove tissue from a part of the brain he had not operated on before, Penfield would first stimulate the surrounding tissues and so begin to understand,  from the patient’s responses, what their function might be. Every time a stimulus from the electrode elicited a response in the patient, he would place a small square piece of paper, numbered or lettered, on the exact point on the brain’s surface. He also gave a running description of the details of the operation, so that they could be recorded by a stenographer and, after each stimulation, used a sterile paper and pencil to record the exact location on a sketch of the patient’s brain (above).

One of the many significant discoveries Penfield was that the postcentral gyrus contains a somatotopic representation of the body. The body is faithfully mapped, in an orderly manner, onto the somatosensory cortex, so that inputs from adjacent parts of the body are encoded in adjacent parts of the strip. In almost every patient who presented with a tumour or scar tissue there, stimulation of a point located behind the ear invariably elicited a tingling sensation in the tongue. Stimulating the area slightly higher up evoked a tactile sensation in the lips. Application of a current further up always produced a sensation somewhere in the hand. Further up still, in positions successively closer to the brain’s midline, the electrode would elicit sensations in the wrist, elbows, shoulder, trunk, hip and knee. Stimulation of the tissue lying on the medial surface, within the longitudinal fissure which separates the two hemispheres, produced sensations in the ankle and toes.  

Here is the case of a boy suffering from focal epileptic seizures which are characterized by a sudden sensation on the right side of his body and movements of the right hand. The first 14 responses were obtained by stimulation of the postcentral gyrus from above down and the remainder (beginning with the point labelled G) from the precentral gyrus, also from above down:


Numbered tickets on the brain of  a patient indicating the responses obtained by electrical stimulation of each point (Penfield & Boldrey, 1937).

14. Tingling from the knee down to the right foot.

13. Numbness all down the right leg, excluding the foot.

12. Numbness over the wrist, lower border, right side.

11. Numbness in the right shoulder.

3. Numb feeling in hand and forearm up to just above the forearm.

10. Tingling feeling in the fifth or little finger.

9. Tingling in first three fingers.

4. Felt like a shock and numbness in all four fingers but not in the thumb.

8. Felt sensation of movement in the thumb.

7. Same as 8.

5. Numbness in the right side of the tongue.

6. Tingling feeling in the right side of the tongue, more at the lip.

15. Tingling in the tongue, associated with up and down vibratory movements.

16. Numbness, back of tongue, midline.

5. Numbness in the right side of the tongue.

(G) Flexion of knee.

18. Slight twitching of arm and hand like a shock, and felt as if he wanted to   move them.

2. Shrugged shoulders upwards; did not feel like an attack.

(H) Clonic movement of the right arm, shoulders, forearm, no movement in trunk.

(A) Extreme flexion of wrist, elbow and hand.

(D) Closure of hand and flexion of his wrist, like an attack.

17. Felt as if he were going to have an attack, flexion of arms and forearms, extension of wrist.

(E) . Slight closure of hand; stimulation followed local flushing of brain; this was repeated with the strength at 24. Flushing was followed by pallor for a few seconds.

(B) Patient states that he could not help closing his right eye but he actually closed both.

(C) Made a little noise; vocalization. This was repeated twice. Patient says he could not help it. It was associated with movement of the upper and lower lips, equal on the two sides.

Although the exact size of somatosensory cortical region devoted to each body part differed in size among patients, the sequence of sensations elicited by repeated stimulations that began behind the ear and moved successively closer to the midline was always exactly the same. The region of somatosensory cortex devoted to each part of the body was not found to be related to the size of that body part, but rather to its sensitivity. Thus the lips, tongue and hands are disproportionately represented in the somatosensory cortex, with the vast majority of the strip of tissue being devoted to them. Less sensitive regions of the body, such as the trunk, are represented by much smaller regions. 

Similarly, stimulations of the primary motor cortex elicited an invariant sequence of movements in Penfield’s patients. A large area to the side of the motor cortex produced either movements of the jaw, lips or tongue when stimulated; sometimes stimulation would elicit salivation. The movements were similar to those produced during chewing or swallowing and sometimes made it difficult for the patient to speak. Stimulation further up almost always led to flexion of one of the digits; yet further upwards, to movements of the whole hand, wrist, elbow, and so on. In the motor cortex, the lips, tongue and hands are again disproportionately represented, because these parts of the bodies contain a large number of muscles. Penfield illustrated all these findings in the sensory and motor homunculi (singular homunculus, or “little man”), the well known visual depictions which show the areas of motor and somatosensory cortex devoted to each part of the body, and their proportions relative to one another.


Three-dimensional clay model of the sensory homunculus (left), and an original diagram of the motor homunculus (right, from Penfield & Rasmussen, 1950). 

We now know that the somatosensory system reorganizes itself following injuries such as stroke, and are beginning to understand the cellular and molecular mechanisms by which it does so. This reorganization occurs because the brain remains “plastic” throughout life, and in some cases leads to considerable recovery of function. Something similar takes place following amputation, and is widely believed to underly phantom limb syndrome, in which amputees experience sensations, including painful ones, from their missing limb. At the time, such processes were thought to be impossible – that the adult human brain is not plastic was one of the central dogmas of brain research, which persisted until relatively recently. Penfield, however, may have unknowingly observed functional reorganization of the cortex more than 70 years ago; he seems to be alluding to it in this passage:

During any one exploration the responses of the sensorimotor cortex vary little, if at all. After stimulation has been carried up and down both sides of the fissure of Rolando [the central sulcus] and tickets placed upon each positive response, the whole process may be repeated with the same intensity of stimulation and the result will usually be identical. The same areas will give no response and the positive points will repeat themselves. But if the same hemisphere be explored at a later operation after a lapse of time, as we have done in five cases, the result may be quite different because areas quite active at the first operation may be mute at the second, and areas which gave no response may later be easily activated (Penfield & Boldrey, 1937).

During the course of his career, Penfield operated on approximately 400 patients, and eventually summarized his findings in a 1950 book called The Cerebral Cortex of Man. The book, which was written with his colleague Theodore Rasmussen, is a remarkable document of Penfield’s electrical explorations of the human brain. These explorations went far beyond the borders of the somatosensory and motor cortices – as Penfield charted the functions of parietal and temporal lobes, the tip of his electrode elicited in his patients dreams, smells, long-lost memories, auditory and visual hallucinations and even out-of-body experiences.

When Penfield operated on the visual cortex, some patients reported simple phenomena such as  “stars lower than the bridge of [the] nose and over to the right” or “stars [which] seemed to go from the midline [of the visual field] a little across to the right”. Upon temporal lobe stimulation, others reported more complex things, such as distortions of visual perception in which objects seemed either larger and nearer or farther and smaller than they actually were. Stimulation of the temporal also evoked auditory phenomena; some patients reported that sounds seemed louder than normal, while others reported hearing music.

In a number of patients undergoing temporal lobe stimulation, Penfield elicited an inextricable combination of dreams, memories and hallucinations. Because he could not separate them from one another, Penfield concluded that these three phenomena rely on the same neural mechanisms in the temporal lobes. Before him, other researchers had tried in vain to locate the elusive memory trace, or “engram”. Karl Lashley, for example, taught rodents to find their way through a maze, then performed lesioned their brains in many different locations. No matter where he made a lesion, Lashley did not succeed in erasing the animals’ memories of the route through the maze. In postulating the involvement of the temporal lobes in memory, Penfield was ahead of his time; as we shall see below, some of those who trained with him would go on to show that temporal lobe structures are indeed critical for memory formation. 

Penfield was one of a dynasty of  researchers who made enormous contributions to our understanding of the brain. He stood on the shoulders of the giants who came before him and subsequently trained the next generation of brilliant scientists. Among those who studied with him at the MNI were Donald Hebb and Brenda Milner, both of whom have since become highly influential, like Penfield before them. Hebb joined the Institute in 1937, aged 34, and began to study the effects of injury and neurosurgery on brain function. Five years later, working with Karl Lashley at the Yerkes National Primate Research Center, he wrote a ground-breaking book called The Organization of Behaviour,  in which he postulated, among other things, the  mechanism of long-term potentiation, which today is widely regarded to be the neural basis of learning and memory.  

Milner obtained her Ph.D. under Hebb’s supervision and joined Penfield at the MNI. In 1953, a patient with severe, intractable epilepsy, known as Henry M. or simply H.M., was referred to a neurosurgeon called William Scoville, who used Penfield’s method to evaluate him. On September 1st, 1953, Scoville surgically removed a structure called the hippocampus from both hemispheres of H.M.’s brain, in an effort to alleviate his symptoms. The procedure had severe consequences – H.M. was left was complete anterograde amnesia, an inability to form any new memories, apart from simple sequences of movements. Soon after the operation, Milner began to evaluate H. M.’s memory function. (She continues to do so, and has now known H.M. for more than 50 years, but he still does not recognize her.) Her early assessments of H.M. led to two seminal papers, published with Penfield and Scoville, which would lay the foundations of cognitive neuropsychology.

Even with modern techniques such as functional neuroimaging at their disposal, neurosurgeons of today still use Penfield’s method in their presurgical evaluations of patients with intractable epilepsy. But instead of using a single electrode as he did, they now use electrode arrays embedded in thin sheets of plastic. Penfield’s cytological research was also highly influential. His belief that non-neuronal cells are involved in various pathological processes pre-empted modern neuroscience by nearly 80 years: astrocytes are now known to be involved in numerous neurological conditions, including epilepsy.

“Brain surgery is a terrible profession,” Penfield wrote in a letter to his mother in 1921. “If I did not feel it would become very different in my lifetime, I should hate it”. At the time, it is unlikely that the self-effacing young surgeon could have imagined how radically his profession would change in the following decades, or that he himself would be the main instigator of that change.


Del Rio-Hortega, P. & Penfield W.G. (1927) Cerebral cicatrix: The reaction of neuroglia and microglia to brain wounds. Bull. Johns Hopkins Hosp. 41: 278-303.

Feindel, W. (2007). The physiologist and the neurosurgeon: the enduring influence of Charles Sherrington on the career of Wilder Penfield. Brain 130: 2758-2765.

Foerster, O. & Penfield, W. (1930). The structural basis of traumatic epilepsy and results of radical operation. Brain 53: 8-119.

Gill, A. et al (2007). Wilder Penfield, Pio Del Rio-Hortega, and the discovery of oligodendroglia. Neurosurgery 60: 940-948.

Penfield, W. (1924). Oligodendroglia and its relation to classical neuroglia. Brain 47:430-452.

Penfield, W. & Boldrey, E. (1937). Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60: 389-443.

Penfield, W. & Rasmussen, T. (1950). The Cerebral Cortex of Man. Macmillan, New York.


  1. #1 Luci
    August 27, 2008

    Lovely profile of Dr. Penfield, for whom pioneer seems much too earthbound and banal a descriptor. Decades ahead of his time, with the intuition and tenacity to carry cartography into the strangest world of all. One can only dream (the right temporal is covering this…) what he could have done with PET, SPECT, MRI, stereotactical surgical tools, and the whole toolbox of modern neurosurgery. And one can only dread where modern neurosurgery, that ‘terrible profession’ would be with Penfield’s trailblazing.

    Some superheroes wear surgical masks. When we have to trust someone to open up our own skulls, it’s more than a little comforting to know that they won’t get lost in there.

  2. #2 Jon
    August 27, 2008

    Today is an especially interesting prospect with much more extensive cartographic variation in brain modularisation and lateralisation. For instance, what induces right-hemispheric language function, which is decidedly rarer than left-hemispheric dominance (to put it simply)? The field of possibility is not limited to one general framework of travelling and sight-seeing, so to speak, though it cannot be denied that a basic conception or outline would be necessary for a refined idea to be proposed in the future. With history we should remark what trajectory it has given to our present and where others, pioneers themselves surely, will take us in understanding this peculiar quality of the human brain. This process toward the differentiation of a multiplicity of functions could very well be merely a preliminary stage to a fuller, more comprehensive understanding of the brain, particularly the neocortex, as a single uniform organ — a tool of behaviour, observation, reflection, and anticipation such that it can catch horrible fears and wondrous dreams in its various and complex web of changeable linkages and chemical analogies.

  3. #3 Vaughan
    August 28, 2008

    Excellent piece Mo. Enough said.

  4. #4 Michael Meadon
    August 28, 2008

    Excellent thanks!

    This might also be the single longest blog post I’ve ever seen. The word count is 5700+. I wish I had that much stamina! :-)

  5. #5 holyoke
    August 28, 2008

    A nice post. The MNI is a short walk from my house.

    And only just yesterday I was looking at Kandel et al (“Principles of Neural Science”) and Penfield’s “homunculus” diagram is one of the oldest illustrations in there (outside of “historical” diagrams from before the 20th century)

    And I wondered out loud if someone is planning on doing this work over again, with the improved equipment and techniques we have now.

  6. #6 Jeff
    August 28, 2008

    I agree with Vaughan. Brilliantly written.

  7. #7 Victor
    August 29, 2008

    Brilliant piece indeed, Mo – glad you pointed me to it! Kept me from going to sleep when I should be resting for tomorrow’s conference :)

  8. #8 Alice
    August 30, 2008


  9. #9 CopperKettle
    September 5, 2008

    Very interesting, thanx..

  10. #10 dan
    September 6, 2008

    amazing piece of work! Thanks for this gem!

  11. #11 Tero Pikkarainen
    October 16, 2008

    Fascinating! Inspiring! Wonderfully written! Thank You very much!

  12. #12 Labinot Marku, M.D., PhD
    January 27, 2009

    Thanks for the text, I really almost felt like I was in that operation room.

  13. #13 Bernard Z. Friedlander
    February 19, 2009

    Whoever wrote this Wilder Penfield piece deserves the highest praise for the excellence of the presentation–at many levels. Reading this refreshed my recollection of what a fine education I had on these topics more than 50 years ago when much of the Penfield and related work was still quite new. I have already modified something I’m writing now as a result of what I’ve learned from reading this article.

  14. #14 Evgeni Sergeev
    July 24, 2009

    This is very well written. Congratulations.

    I’m always wondering how did they know what to do in the first place. I mean, if a surgeom cuts open someone’s skull and knows what they are doing, there must have been preceeding a sequence of similar events when they did not.

  15. #15 Dan
    September 5, 2009

    very great post. it’s amazing that his work still has relevance in the OR today!

  16. #16 gregorylent
    October 9, 2009

    the new phrenology, cool

    when oh when is neuroscience going to stop looking at the effects, brain, and start having a look at the cause, consciousness …

    most backward science in the world

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    March 25, 2011

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  19. #19 David
    August 4, 2011

    I love this article. Dr. Penfield did wonders for modern medicine.