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The Discovery of Motor Cortex and its Background

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Journal of the History of the Neurosciences, 16:320–331, 2007Copyright © Taylor & Francis Group, LLCISSN: 0964-704X print / 1744-5213 onlineDOI: 10.1080/09647040600630160

NJHN0964-704X1744-5213Journal of the History of the Neurosciences, Vol. 15, No. 4, July 2006: pp. 1–24Journal of the History of the Neurosciences

The Discovery of Motor Cortex and its Background

The Discovery of Motor CortexCharles G. Gross CHARLES G. GROSS

Department of Psychology, Princeton University, Princeton NJ

In 1870 Gustav Fritsch and Edvard Hitzig showed that electrical stimulation of thecerebral cortex of a dog produced movements. This was a crucial event in the develop-ment of modern neuroscience because it was the first good experimental evidence fora) cerebral cortex involvement in motor function, b) the electrical excitability of thecortex, c) topographic representation in the brain, and d) localization of function indifferent regions of the cerebral cortex. This paper discusses their experiment andsome developments in the previous two centuries that led to it including the ideas ofThomas Willis and Emanuel Swedenborg, the widespread interest in electricity and thelocalizations of function of Franz Joseph Gall, John Hughlings Jackson, and PaulBroca. We also consider the subsequent study of the motor cortex by David Ferrierand others.

Keywords motor cortex, Fritsch and Hitzig, Ferrier, localization of function

Introduction

The modern neurophysiology of the cerebral cortex began in 1870 with the discovery byGustav Fritsch (1838–1927) and Edvard Hitzig (1838–1907) that electrical stimulation ofthe cerebral cortex produces movements. Their discovery was important for several rea-sons. First, it was the first clear experimental demonstration of a region of the cerebralcortex involved in motor function. Second, it was the first evidence that the cortex waselectrically excitable. Third, it was the first experimental evidence of a topographicallyorganized representation of the body in the brain. Finally, it was the first strong experi-mental evidence for localization of function in the cerebral cortex. As Fritsch and Hitzig(1870) put it:

… by the results of our own investigations, the premises for many conclusionsabout the basic properties of the brain are changed not a little. (Fritsch &Hitzig, 1870, translated by von Bonin, 1960, p. 79)

… some psychological functions, and perhaps all of them … need cir-cumscribed centers of the cerebral cortex. (Fritsch & Hitzig, 1870, translatedby von Bonin, 1960, p. 96)

In this paper, we review Fritsch and Hitzig’s classic experiment and its background in theeighteenth and early nineteenth centuries.

Address correspondence to Charles G. Gross, Department of Psychology, Princeton University,Princeton, NJ, 08540. Fax: 609 258-1113. E-mail: cggross@princeton.edu

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Fritsch and Hitzig’s Experiment

At the time of their 1870 experiment Fritsch and Hitzig were young physicians associatedwith the Berlin Physiological Institute. Fritsch, before and after this collaboration, wasinterested in studying hair and eye color in non-European societies with a view towardestablishing the superiority of the white race (Grundfest, 1963). Hitzig continued researchon motor cortex after his paper with Fritsch and, in addition, became a successful psychia-trist. He came from a distinguished Jewish family; he was later described by one of hisbiographers as “a stern and forbidding character of incorrigible conceit and vanity compli-cated by Prussianism” (Finger, 2000; Kuntz, 1953). After Fritsch and Hitzig were famousthe American anatomist, C. L. Herrick described them as “splendid specimens of physicaldevelopment and German culture at its best” (Herrick, 1892).

In their now classic experiment, Fritsch and Hitzig strapped their dogs down on FrauHitzig’s dressing table, as there were no animal facilities at the institute (Kuntz, 1953). Intheir early experiments they used no anesthesia or analgesic, although ether surgical anes-thesia had been introduced in 1846 and morphine analgesia in 1803 (Magner, 1992). Laterthey did use “morphine narcosis.” They began by removing the cranium and cutting thedura, the dog showing “vivid pain.” They stimulated the cortex with platinum wires with“galvanic stimulation”: brief pulses of monophasic direct current from a battery at theminimum current that evoked a sensation on their tongue.

The usual response to this stimulation was a muscle twitch or spasm (“Zuckung”).Their central findings were that a) the stimulation evoked contralateral movements, thecrossed laterality confirming observations dating back to Hippocrates in the 5th C. B.C.E.(Hippocrates, 1927), b) only stimulation of the anterior cortex elicited movements, c)stimulation of specific parts of the cortex consistently produced the activation of specificmuscles, and d) the excitable sites formed a repeatable, if rather sparse, map of move-ments of the body laid out on the cortical surface (Fig. 1). They went on to show that

Figure 1. Drawing after Fritsch and Hitzig’s (1870) figure of stimulation sites on the dog’s cortex. Δ,twitching of neck muscles; +, abduction of foreleg; †, flexion of foreleg; #, movement of foreleg; �,facial twitching (Brazier, 1988 after Fritsch and Hitzig, 1870). Note that the topography is not impressive.

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lesion of a particular site impaired the movements produced by stimulation of that site.The loss of function was not complete, suggesting to them that there were other motorcenters that had not been impaired by the lesion.

The roots of their discoveries lay in several developments beginning in the eighteenthcentury that are outlined in the next sections.

Electricity and the Nervous System

The eighteenth century was a period of great activity and interest over the new discoveriesabout electricity in both the salons and laboratories of the time: such intriguing gadgets aselectrostatic machines, the Leydon jar, and the gold leaf electroscope. At this time it wasrealized that man-made electricity and lightning were the same phenomenon as that foundin the electric fish, (an animal whose shocking properties had been known since classicaltimes). There were a number of attempts to use electricity for therapeutic purposes(including by the French revolutionary Marat and the American savant and revolutionaryBenjamin Franklin) and even studies of electrical stimulation of various brains from frogto dead human (Brazier, 1959, 1984; Finger, 2000).

The modern study of the electrical nature of nervous activity began with Luigi Gal-vani’s (1737–1798) demonstration that electrical stimulation of the sciatic nerve in a sev-ered frog’s leg resulted in contraction of the attached muscle. These findings sparked afierce debate between Galvani and Alessandro Volta (1745–1827) concerning the sourceof the electricity that caused the legs to become “reanimated” (Brazier, 1984; Finger,2000). Volta considered the cause to be the use of dissimilar metals, whereas Galvani wasconvinced that the electricity came from within (“animal electricity”) (Brazier, 1984).Eventually this led, on one hand to the development of the electric battery and, on theother hand, through the work of Emil du Bois-Reymond, Julius Bernstein, and others tothe discovery of the action potential. Galvani’s results soon prompted attempts to stimu-late other nervous structures, but the vast majority of experiments on the electrical stimu-lation of the cerebral cortex were negative, reinforcing the prevailing view that the cortexhad no significant functions (Brazier, 1959, 1984; Finger, 2000).

Cortex as “Rind”

In the eighteenth century, the cerebral cortex was usually dismissed as an insignificant“rind,” which indeed the Latin “cortex” means. The first to microscopically examine thecortex was Marcello Malpighi (1628–1694), professor in Bologna, the founder ofmicroscopic anatomy and discoverer of capillaries (Meyer, 1971; Clarke & O’Malley,1996). He saw it as made up of little glands with attached ducts (Fig. 2). Similar “glob-ules” were reported by many subsequent microscopists (Meyer, 1971; Clarke &O’Malley, 1996). Perhaps they were observing pyramidal cells. At least in Malpighi’scase, artifact is a more likely possibility, since modern studies have shown that his glob-ules are more prominent in boiled tissue (which is what he used) than in fresh tissue(Meyer, 1971; Clarke & Bearn, 1968). Malpighi’s view of the brain as a glandular organwas commonly held in the seventeenth and eighteenth centuries, perhaps because it fitwith the much earlier, but still persisting, Aristotelian view that the brain was a coolingorgan (Gross, 1995) and the Hippocratic theory that it was the source of phlegm (Meyer,1971).

Another common eighteenth century view of the cortex was that it was largelymade up of blood vessels. One of the earliest advocates of this was Frederik Ruysch

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(1628–1731), Professor of Anatomy in Amsterdam, who noted: “the cortical substanceof the cerebrum is not glandular, as many anatomists have described it, nay have posi-tively asserted, but wholly vascular” (Clarke & O’Malley, 1996). In this view the con-volutions were viewed as mechanisms for protecting the delicate blood vessels of thecortex.

Willis Gives the Cortex Cognitive Function

Prior to the nineteenth century there were only a very few figures who advocated signif-icant functions for the cerebral cortex. The first and most important of these wasThomas Willis (1621–1675), who held the Chair of Natural Philosophy at Oxford andwas one of the founders of the Royal Society (Dewhurst, 1982). His Cerebri Anatomie(Willis, 1664) was the first monograph on the brain and dealt with brain physiology,brain chemistry, and clinical neurology as well as brain anatomy. Many of its illustra-tions such as Fig. 3 were by the architect Sir Christopher Wren, then Professor ofAstronomy at Oxford.

Willis implicated the “cortical and grey part of the cerebrum” in the functions ofmemory and will. In his scheme, sensory signals came along the sensory pathways into thecorpus striatum where the common sense was located. They were then elaborated into per-ceptions and imagination in the overlying white matter (then called the corpus callosum orhard body since it was harder than the cortex) and then passed to the cerebral cortex wherethey were stored as memories. According to Willis, the cortex initiates voluntary move-ment whereas the cerebellum is involved only in involuntary movement.

Willis’s ideas on brain function came not only from his dissections but also from hisexperiments on animals and his correlation of symptoms and pathology in humans. Willisnoticed that whereas the cerebellum was similar in a variety of different mammals, the

Figure 2. Malpighi’s “cortical glands” from his De Cerebri Cortice (1666) with their attachedfibers. Although he may have seen brain cells, these drawings are likely to have been of artifacts(Meyer, 1971; Clarke & Bearn, 1968). Swedenborg used these supposed cortical elements to buildan elaborate theory of brain function that has close similarities with the neuron doctrine.

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complexity of the cerebral convolutions varied greatly among different animals and thisvariation was correlated with intellectual ability.

In spite of the relative importance of the cerebral cortex in Willis’s schema, there isno detailed drawing of the cortex in his works: he apparently never asked Wren or any-body else to produce one (See Fig. 3). In fact, for another 150 years the cortex continuedto be drawn as Erasistratus of Alexandria (200 B.C.E.) had first suggested: as coils of thesmall intestine (Schiller, 1965) (Fig. 4).

Although Willis was a major figure in his time, his ideas on the importance of thecerebral cortex soon disappeared and the views of the cortex as a glandular, vascular, orprotective rind returned to their original dominance. There were two figures that did chal-lenge this view. The first was Francois Pourfour du Petit (1644–1741), a French army

Figure 3. Ventral view of the brain from Willis, Cerebri Anatomie (1664), drawn by the architectSir Christopher Wren. Note the detailed drawing and labeling of the cranial nerves and basal brainstructures (including the circle of Willis) in contrast to the vague and partially obscured representa-tion of the cerebral cortex all of which has the single designation “A.”

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surgeon. He carried out a series of systematic experiments on the effects of cortical lesionsin dogs and related them to his clinico-pathological observations on wounded soldiers(Kruger, 1963). From these studies he realized that the cerebral cortex plays a critical rolein normal movement and that this influence is a contralateral one. However, his observa-tions were totally ignored until they were rediscovered much later. Perhaps this wasbecause he did not hold an academic post and published his account in a very limited edi-tion. Yet, his observation that the cortex was insensitive to touch was repeatedly cited tosupport the views of von Haller who, as discussed below, was the dominant physiologistof the day. Thus, du Petit’s work demonstrating motor functions of cortex was probablyignored largely because of the anticortex ideology of the time, not because it was pub-lished in a minor journal.

Swedenborg’s Lost Speculations

The second major eighteenth century figure advocating the importance of the cortex wasEmanuel Swedenborg (1688–1772), the founder and mystical prophet of the “New Jerusa-lem” or Swedenborgian Church (which is still active in United States and Great Britain).On the basis of reviewing the literature, Swedenborg arrived at an amazing set of prescientideas on the importance of the cerebral cortex in sensation, cognition, and movement(Gross, 1997).

He argued that the cortex was the highest sensory and motor structure of the brain. Inan anticipation of neuron theory, he called the cortical “glandules” described by Malpighi

Figure 4. The depiction of the cerebral convolutions by Guilo Casserio (1561–1617). The convolu-tions are not differentiated in any way and, following Erasistratus, look like intestines (Clarke andDewhurst, 1972).

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“little brains” (“cerebella”) and suggested that they were functionally independent andconnected by thin fibers. These fibers also ran through the white matter and medulla downto the spinal cord and then by way of nerves to parts of the body. The operations of thesecerebella were the basis of sensation, mentation, and movement. He seems to have had theidea of somatotopic organization of motor function in the cerebral cortex: He (correctly)localized control of the foot in the dorsal cortex (he called it the “highest lobe”), the trunkin an intermediate site, and the face and hand in the ventral cortex, his third lobe (Gross,1997).

Swedenborg’s philosophical and religious writing had a major impact on Europeanand American philosophers, writers, and artists. Indeed, he continues to be a subject ofreligious, philosophical, and fringe science tracts. However, he never had any impact onthe scientific study of the brain. There is no evidence that contemporary physiologists andanatomists even read his writings on the brain. He never held an academic post or had stu-dents, colleagues, or even scientific correspondents. He never carried any systematicempirical work on the brain and his speculations were in the form of baroque and grandi-ose pronouncements embedded in lengthy books on the human soul by one whose famewas soon to be that of a mystic or madman. Furthermore, some of his more advancedideas, such as on the organization of motor cortex or the functions of the pituitary gland,did not appear in print until after they were no longer new (Gross, 1997).

Haller Declares the Cortex Insensitive

In spite of Willis, du Petit, and Swedenborg, who all thought the cortex was a crucialbrain structure, the opposite view was very much the dominant one in the eighteenth cen-tury. Much more representative and influential was Albrecht von Haller (1708–1777),professor at Tuebingen and later Bern, who dominated physiology in the middle of theeighteenth century (Neuburger, 1981). In his monumental Elementa Physiologiae Cor-poris Humani and his Icones Anatomicae, he divided the organs of the body, as well asparts of the nervous system, into those “irritable” (such as muscle) and those “sensible”(such as the sense organs and nerves). Using animals, he tested the “sensibility” of vari-ous brain structures with mechanical stimuli such as picking with a scalpel, puncturingwith a needle, and pinching with forceps as well as with electrical and chemical stimuli,such as silver nitrate, sulfuric acid, and alcohol. With these methods he found the cortexcompletely insensitive. By contrast, he reported the white matter and subcortical struc-tures to be highly sensitive; their stimulation, he said, produced expressions of pain andmovement. He concluded that all parts of the cortex had the same function because theywere equally insensitive and all subcortical regions were also equivalent because theirstimulation had equally positive effects. Because of his prestige and many students andfollowers, Haller’s views of the insensitivity and equipotentiality of cortex suppressedcontrary observations and ideas on the importance of cortex and persisted well into thenext century (Neuburger, 1981).

Phrenology Calls Attention to the Cortex

The study of the localization of different psychological functions in different regions ofthe cerebral cortex begins with (or immediately after), Franz Joseph Gall (1758–1828) andhis collaborator J. C. Spurzheim (1776–1832), the founders of phrenology. The centralideas of their system was that the brain was an elaborately wired machine for producingbehavior, thought, and emotion and that the cerebral cortex was a set of organs with

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different functions (Gall & Spurzheim, 1835; Gross, 1987, 1998; Young, 1970). They pos-tulated about “35 affective and intellectual faculties” and assumed:

a) these were localized in specific organs of the cerebral cortex,b) the size of the cortical organ reflected the development or prominence of the corre-

sponding psychological faculty,c) the size of each cortical organ was indicated by the morphology of the overlying skull,

i.e. in cranial bumps.

All of the phrenological localizations were of “higher” intellectual and personality traits:the highest sensory functions were still in the thalamus and the highest motor functions inthe corpus striatum.

The primary method of data collection by Gall and Spurzheim was examining theskulls of a great variety of people from lunatics and criminals to the eminent and accom-plished. Neuropsychological and animal experimental data, even gathered by them, wereviewed as only minor and ancillary evidence (Gall & Spurzheim, 1835; Gross, 1987,1998; Young, 1970).

The absurdity of their dependence on cranial morphology was quickly recognized, atleast in the scientific community. Furthermore, Gall’s ideas about the cortex as a set ofpsychological organs stimulated investigation of the effects of cortical lesions in humansand animals, of structural variations across different cortical regions (“cytoarchtectonics”and “myeloarchtectonics”) and of the anatomical pathways leading to and from specificcortical areas. In these ways as well as by generating interest in the brain, phrenology hada lasting influence on the development of modern neuroscience (Gross, 1987, 1998;Young, 1970).

In 1861, Paul Broca (1824–1880) described several patients with long standing diffi-culties in speaking, which he attributed to damage to their left frontal lobes. This was thefirst generally accepted evidence for localization of a specific function in the cerebralcortex. At the time it was viewed as a general vindication of Gall’s ideas on localization offunction in the brain and a specific confirmation of Gall’s localization of language in thefrontal lobes (Broca, 1861; Schiller, 1979).

John Hughlings Jackson

The first intimations of a somatotopically cerebral motor mechanism (aside fromSwedenborg’s lost ideas) came from John Hughlings Jackson (1815–1911) who is oftencalled “the father of English neurology” because of his many neurological discoveries.Jackson reasoned that the cerebrum should have basic sensory-motor functions and gath-ered clinico-pathological evidence for this view. In studying epileptic seizures, includingthose of his wife, he noticed that there was a consistent systematic spread of convulsionsfrom one body part to the next. From this he inferred that different parts of the brain mustbe involved in the control of different muscle groups and that these parts must be arrangedin a way to mimic the organization of the body (Young, 1970; Finger 2000; Temkin,1971). Before 1870, he appeared unsure whether this somatotopy is found in the corpusstriatum, the traditional highest motor structure or in the cortex (Jackson, 1958; Temkin,1971; Young, 1970). But by 1870, Jackson placed the somatotopically organized struc-tures that are responsible for movement primarily in the convolutions (Jackson, 1870;Temkin, 1971).

Fritsch and Hitzig (1870) had not mentioned Hughlings Jackson in their first paper,an omission that upset David Ferrier (1843–1928), an admirer of Jackson who replicated

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Fritsch and Hitzig’s 1870 study as described below. (For the priority dispute betweenFritsch and Hitzig and Ferrier, see Taylor & Gross, 2002 and Young, 1970.) However, itshould be noted that Jackson did not clearly implicate cortex until 1870 and then only inthe Transactions of St. Andrews Medical Graduates’ Association, which the Germansmay well have not seen. Much later Hitzig (1900) commented that his experiments withFritsch “had only confirmed the conclusions reached by Jackson on clinical evidence.”Jackson himself later claimed he had been clearer about the role of cortex earlier than hisnumerous, difficult, and ambiguous writings in scattered medical journals demonstrate(Jackson, 1875).

Fritsch and Hitzig’s Motivations

What led Fritsch and Hitzig (1870) to electrically stimulate the cortex of a dog? Asdescribed above, this was a period of very active exploration of the nervous system withelectrical stimulation. While there were reports of effects of stimulation of the spinal cordand brain stem, all attempts at eliciting effects of stimulation of the cerebral cortex hadbeen universally ineffective, including by such skilled experimenters as Albrecht vonHaller, Francois Magendie, and Carlo Matteuci. As Fritsch and Hitzig put it in the intro-duction to their paper:

Even in other fields than in physiology, there can hardly be a question aboutan opinion which seems to be so unanimous, which seems to so completelysettled as that of the excitability of the cerebral hemisphere. (Fritsch & Hitzig,1870, trans. Von Bonin, 1960, p.75)

One impetus to their experiment was the paradox that some Central Nervous System(CNS) structures were excitable and yet the cortex didn’t seem to be. Another was theirown previous observation. Hitzig had tried electrical stimulation of the human head fortherapeutic purposes and had noticed it caused eye movements (Hitzig, 1870). He thentried rabbits and also elicited movements. Fritsch, while working as a battlefield surgeon,had apparently noticed that the contralateral limbs twitched while dressing an open headwound (Walker, 1998).

Ferrier’s Replication and Subsequent Developments

Soon after the Fritsch and Hitzig (1870) paper appeared, the young Scotch physicianDavid Ferrier set out to follow up the Germans’ work on motor cortex (Viets, 1938). Fer-rier had been heavily influenced by John Hughlings Jackson and realized that they hadconfirmed Jackson’s ideas. He was eager to study the relation of their findings to Jackson’sobservations about epilepsy as well as to determine the generality of their findings acrossa variety of species, especially monkeys. Ferrier replicated their basic findings that stimu-lation of the cortex can produce specific movements and that there is a topographic “motormap” in the cerebral cortex (Ferrier, 1873, 1874–1875, 1875; Taylor & Gross, 2003).

Although not noticed at the time, there were actually considerable differences in themethods, results and interpretation of Fritsch and Hitzig’s and Ferrier’s experiments onmotor cortex, as described in detail elsewhere (Taylor & Gross, 2003). The former usedbrief direct current pulses and obtained localized muscle twitches whereas Ferrier usedmuch longer duration biphasic stimulation that tended to produce complex integratedmovements rather than muscle twitches. Both Fritsch and Hitzig’s and Ferrier’s papers on

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motor cortex were initially greeted with considerable and equal skepticism. Their resultswent against the generally accepted views that the striatum was the highest motor centerand that the cortex was inexcitable. The critics usually interpreted the evidence for thelocalization of motor function in cortex as artifactual due to “spread of current” to the stri-atum. To overcome these criticisms, Horsley, Sherrington, and others began meticulous“punctate” mapping of cortex using the minimum current to elicit the smallest discernablemovement (e.g. Horsley & Schafer, 1888; Beevor & Horsley, 1887; Brown & Sher-rington, 1915). This resulted in both an acceptance of the reality of a motor cortex and astress on motor cortex controlling individual muscles, that is, in a Fritsch and Hitzig-likeview.

The extent to which motor cortex controls muscles, as opposed to complex move-ments, continues to remain an issue in the study of motor cortex with most contemporaryinvestigators leaning toward a Ferrier-like view of complex movements rather than aFritsch and Hitzig-type view of the punctate control of individual or small groups of mus-cles by a circumscribed region of cortex (e.g. Kakei et al., 1999; Georgopoulos et al.,1986; Graziano et al., 2002a, 2002b).

Concluding Summary

A major step in the development of modern neuroscience was the report in 1870 byFritsch and Hitzig that stimulation of portions of the cerebral cortex elicited movementsand that the stimulation sites were somatotopically arranged. This demonstration of theelectrical excitability of cortex and the topographic cortical localization of motor functionwas an outgrowth of several developments in the eighteenth and nineteenth centuries. Onewas the increasing knowledge and use of electrical phenomena in biology and medicine. Asecond was the growth of interest in the functions of the cerebral cortex, particularlyinspired by phrenology.

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