«by Roland Bauchot, honorary professor of biology University of Paris Denis Diderot In 1861, in the bulletin of the Paris Anthropology Society, a ...»
Paul Broca’s discovery of the area of the brain
governing articulated language
by Roland Bauchot, honorary professor of biology
University of Paris Denis Diderot
In 1861, in the bulletin of the Paris Anthropology Society, a short four-page
note penned by Paul Broca (1824–1880) was published that would revolutionise
neuroscience (Text no. 1). Paul Broca is not unknown. In Paris a hospital is
named after him; there is also a street bearing his name in the 13th
arrondissement, as well as in Bordeaux, Reims and Mantes-la-Jolie and no doubt elsewhere in France, and a square named after him in Sainte-Foy-la-Grande (Dordogne), where he was born.
THE BIRTH OF ANTHROPOLOGY
Anthropology The term anthropology appeared in 1655 in an anonymous English text. In 1735 the Swede Carl Linnaeus (1707–1778) introduced mankind into his zoological classification, including him alongside apes in the order of primates. In 1749 Georges Louis Leclerc, Count de Buffon, published his Natural History of Man. It was the Germans Johann Friedrich Blumenbach (1752–
1840) in 1795, then Emmanuel Kant (1724–1804) in 1798, who introduced the term “anthropology” as it is now understood. In 1827 Jean-Baptiste Bory de Saint-Vincent (1778–1846) published his Zoological Essay on the Human Species. In 1855 a Chair of Anthropology was established in Paris for Armand de Quatrefages (1810–1892).
The Anthropology Society founded by Broca in 1859 followed on from the Society of the Observers of Man (Société des Observateurs de l’Homme), created in 1800, and the Paris Ethnology Society, founded in 1839 by the Frenchman William Edwards (1777–1842). These societies focused essentially on the ethnological (racial) and ethological (behavioural) aspects of human populations, neglecting the physical anthropology dear to Paul Broca. Broca also founded the Laboratory of Anthropology at the École des Hautes Études in 1868, the Revue d’Anthropologie in 1872, and the School of Anthropology in 1875.
The Paris Anthropology Society intended to put humankind in its proper place in the scale of living organisms and to study humans using the classic scientific methods of zoology and comparative anatomy (which the Frenchman Georges Cuvier (1769–1832) had developed), thereby rejecting the moral, religious and philosophical considerations that prevailed at the time. For the first time in the history of anatomy, the members of the society would undertake quantitative and statistical studies of humans, including one related to the size of the encephalon, or encephalisation.
Encephalisation, or the study of the size of the encephalon
The two texts on aphasia are an epiphenomenon in the works of Paul Broca. Another of his activities, directly related to the creation of the Anthropology Society, shows his concern to apply scientific methods to the study of mankind. One crucial issue consisted in linking intelligence, whether animal or human, to encephalic volume. Georges Cuvier (1769–1832), in his Leçons d’anatomie comparée (Lessons in Comparative Anatomy, 1802), had studied this question and shown that the relationship between encephalic mass and body mass, which favoured small animals, was a poor method for measuring
1. For more information on encephalisation, see Roland Bauchot, « L’encéphalisation, aperçu historique», Journal de psychologie normale et pathologique, 1986, 81: 5–29.
THE LATERALISATION OF FUNCTIONS V. PERFECT SYMMETRYLet’s return to the note of 1861, which describes the circumstances that led Paul Broca, after the autopsy of the famous Tan-Tan, to hypothesise that the language centre was located in a convolution in the left frontal lobe.
M. Broca, in his statement, presents the brain of a man who died in his ward at the Bicêtre hospital, and who for the last twenty-one years of his life had lost his faculty of speech … he could now pronounce only a single syllable, which ordinarily he repeated twice; whatever the question asked, he always replied tan, tan, accompanied by very varied expressive gestures.
… but one need only glance at the organ to see that the principal source and primitive seat of this softening is the middle section of the frontal lobe of the left hemisphere; that is where one finds the oldest and most extensive and advanced lesions … Everything indicates, therefore, that in the present case, the lesion to the frontal lobe was the cause of the loss of speech.
It is remarkable that Broca arrived at such a conclusion given the extent to which understanding of the structure of the encephalon was still in limbo at this time. Indeed, it was not until 1875 that the Italian Camillo Golgi (1843–1926) developed a technique to stain cells using metal impregnation – which proved extremely useful for studying nerve cells and their projections – and it was only in 1899 that the Spaniard Santiago Ramon y Cajal (1852–1934), using Golgi’s method, published the first images of the cellular structure of nervous tissue.
This method, named black reaction by Golgi, and which uses silver nitrate, is based on the precipitation of silver when it comes into contact with cellular cytoplasm.
For reasons that are not fully understood, only a small number of neurons are impregnated by silver salt, which has two advantages. First, when a neuron is impregnated, its entire cytoplasm is affected: the silver stains not only the pericaryal area (which surrounds the nerve nucleus) but also the neuron’s various dendritic or axonic projections. Second, as only a very small number of neurons are stained, it is possible to observe the path of the various dendrites and the axon against the colourless background of cerebral grey matter. The silver can be replaced by gold or osmium.
In 1861, when Broca put forward his hypothesis, little more was known about the structure of the encephalon than the various afferent and efferent tracts (white matter), those bundles of fibres that conduct nerve impulses. The rest of the encephalon (grey matter) was considered an amorphous mass of tissue acting, in each function it performed, as an undifferentiated whole (this opinion was upheld by Marie Jean Pierre Flourens, 1794–1867, the father of anaesthetics). It is true that, following Albert the Great (Albert von Bollstädt, c.1200–1280), scientists had sought to identify the various cerebral functions, but their work remained purely speculative. The phrenology of Franz Josef Gall (1758–1828) and Johann Gaspar Spurzheim (1776–1832) – which suggested haphazard localisations on the basis of bumps (bosses) on the skull (an idea the very same Flourens opposed) – was quickly abandoned, but not before spawning
2. Afferent neural pathways carry impulses towards the spinal cord or the encephalon (our reference here);
efferent neural pathways carry impulses from the spinal cord or the encephalon to the organs.
Bertillonage. Phrenology left us with classic French expressions such as “Nicolas Bourbaki a la bosse des maths” (“Nicolas Bourbaki is a born mathematician”).
In short, Broca was the first to localise one of the cerebral functions, language, on the basis of sound experimental evidence, and Broca was the true father of cerebral localisations.
@@@@@@@ But his achievements did not stop there. Indeed, he localised the language centre in the left frontal lobe, for the autopsy indicated that Tan-Tan’s right frontal lobe was normal. Similarly, in 1874, the German Carl Wernicke (1848–
1905) localised a nerve centre in the left temporal gyrus, close to the auditory cortex and connected by a nerve bundle to Broca’s area; a lesion to this nerve centre produces another form of aphasia, known as Wernicke’s aphasia. The afflicted patient is not unable to articulate words, but speaks in a kind of unintelligible jargon.
Figure 2: The superior temporal lobe, crucial to the comprehension of language, notably comprises the primary auditory cortex (CAud), which receives information from the ear, and Wernicke’s area in its posterior portion. In adults, the lower frontal region (Broca’s area) is involved in verbal production. (Image Direction des sciences de la vie – CEA) In 1860, however, the encephalon was thought to be perfectly symmetrical.
We know that each of the brain’s hemispheres receives sensory information from the opposite side of the body and that it then issues the latter its motor orders.
The localisation of the language centre in a single hemisphere is therefore rather
3. Alphonse Bertillon (1853–1914) claimed to be able to identify criminals by measuring the contours, size and shape of their skull. He is also known for testifying for the prosecution as a handwriting expert during the Dreyfus Affair.
4. [Translator’s note] Literally, “Nicolas Bourbaki has a mathematician’s skull”.
surprising. What, then, is the role of the symmetrical region in the right frontal lobe, which does not have this language function – for it appeared quite normal during Tan-Tan’s autopsy – and did not compensate for the destruction of the contralateral area. Broca was perfectly aware of such an incongruity. Indeed, he would return to the unilateral localisation of the language centre on several occasions, publishing eleven notes on the subject between 1861 and 1866. We reproduce here the more structured of the articles, produced in 1863 (Text no. 2), in which he reports on other cases of aphasia observed by himself or by his colleagues and which are due to the destruction of a convolution in the left frontal lobe. The document gives an insight into the discussion that pitted doctors against anthropologists at this time. Some, including Louis Pierre Gratiolet (1815–1865) – who, like Broca, was born in Sainte-Foy-la-Grande – did not accept that the absence of speech could coexist with an otherwise intact intelligence, as is generally observed among aphasiacs, who understand what is said to them perfectly but do not have the means to reply in speech.
M. LINAS. M. Broca would therefore distinguish the ability to articulate words and the faculty of language.
M. BROCA. This distinction is obvious. One of the patients whom I mention had retained the ability to pronounce five words. Most aphemics have a limited vocabulary, which they use, however, to prove that they are still able to articulate words, though their faculty of language is extinguished.
M. GRATIOLET. These observations raise a great philosophical difficulty.
How can the retention of intelligence coinciding with the loss of language be understood?
“Articulated language”, in this interesting debate, should be understood as intelligible language. The aphasic patient, though he may appear to have lost all possibility of making himself heard, has lost neither the vocabulary, grammar nor syntax necessary to correctly communicate with others. The only defect lies in the motor neurons in Broca’s area, which operate various muscles (pharynx, tongue, lips, vocal chords) involved in the articulation of spoken language.
Figure 3: Stylised (Penfield) representation of the human cortical homunculus, in the ascending frontal convolution in the cerebral cortex. The electrical stimulation of a point on this surface will produce a movement in the corresponding body part. Note the relatively large size of the area corresponding to the buccopharyngeal muscles (which are involved in the articulation of language).
The Dax affair
In 1863 Gustave Dax submitted two papers to the Academy of Medicine, one apparently written by his father in 1836 and which he had recently discovered in a drawer, and one penned by himself, which affirms that the “cerebral organ of speech has been found”. For Dax, this organ was located on the left side of the brain. Marc Dax’s (1770–1837) argument was based on the fact that the aphasics he had met were all more or less paralysed on their right side, which indicated that the language centre was located in the left hemisphere. According to his son, this paper had been delivered at a medical congress in Montpellier in 1836, though there is no trace of such a speech. Most curious of all is that Gustave, his son, does not once cite Broca in his paper, and bases his argument on only a very large sample of cases of aphasia-related motor paralysis, without having performed a single autopsy. Paul Broca never called into question the work of Marc Dax, of which he was most probably unaware. The fact remains, however, that he was the first to provide tangible evidence of this localisation.