The Brain as a Double Organ: Localization without Lateralization
At the time, lateralization, or cerebral specialization, was not under debate. Xavier Bichat [8], seeing that all paired structures in the brain “resemble each other on every side,” declared what came to be known as the law of symmetry: “two parts essentially alike in their structure cannot be different in their mode of acting” (pp 8, 14). Gall, likewise, envisioned the organs in each pair as anatomically and functionally identical, and overlooked the soldier’s injury being on the left.
Not everyone saw the hemispheres that way. Vicq d’Azyr [9] and Malgaigne [10] were among those who remarked on their asymmetrical appearance. For Malgaigne, it meant that “if the phrenological organs are situated in these convolutions, the necessary conclusion will be that these organs are not the same in the 2 hemispheres” (p 371). But these were minority views, and the law of symmetry remained a core physiological principle.
Left-Hemisphere Specialization
It was in the 1860s when Paul Broca came on the scene (Fig. 2). By then, despite the critics, some physicians, like Bouillaud [11], remained steadfast in support of the principle, at least about speech and the anterior lobes. Broca [12] himself, while doubting the feasibility of “constructing a detailed system of localization” like Gall’s, credited Gall with the “incontestable merit of proclaiming the great principle of cerebral localization” (p 191). That he also accepted the law of symmetry becomes clear in his reports on his soon-to-be-famous patient, Louis Victor Leborgne (whose full name we finally know, thanks to Cezary Domanski [13]).
Fig. 2. Paul Broca (1824–1880). Wellcome Library. Public Domain.
Speech and the Frontal Lobe
Leborgne was a 51-year-old patient at the Bicêtre, a hospital on the outskirts of Paris, where Broca was on staff. He had been epileptic since his youth, and in 1850, following a series of strokes, became paralyzed on the right side, losing his ability to speak. In the hospital, he became known as “Tan” because, with one exception to be noted later, it was the only articulate sound he could make. Broca called the condition aphemia; the term we have come to use, aphasia, was Armand Trousseau’s [14] (see Ryalls [">15]). In 1861, when gangrene set in, Leborgne was treated by Broca but died soon after. Upon examining his brain, Broca found softening throughout the left frontal lobe apparently centered around F3, the posterior part of the third, or inferior, frontal convolution.
Broca’s three reports [16-18] on Leborgne and a second speechless patient, Lezare Lelong, focused on the frontal location, but as evidence mounted, so did his appreciation for the “remarkable fact” ([19], p 202) that the damage was always on the left. By 1863, he had ten such cases, and, finally, in 1865 [20], with still more, he famously declared, “we speak with the left hemisphere” (“nous parlons avec l’hémisphère gauche”), with F3 at the center (p 384).
More evidence soon came in support, either of speech loss after left-anterior injury or no loss after injury on the right. But as Broca [21] knew, “in pathology and especially in cerebral pathology, there is hardly any rule without exception” (p 385), and here the most challenging were reports of persons who became aphasic after injury to F3 on the right. Broca [20] acknowledged “the existence of a small number [petit nombre]” who “speak with the right hemisphere” (pp 385–386) and analogized them to another exceptional group, left-handers. Many assumed he meant it was left-handers who “speak with the right,” but Broca rejected any necessary “coincidence” between the groups (p 387). Among other reasons, the numbers did not match, and Broca cited reports estimating the incidence of left-handedness at 1 in 10 but only 1 in 20 for right-hemisphere speech. The evidence for lateralization converted most but not all believers in the law of symmetry. Bastian [22], for one, said it was “only fair” to suppose that the hemispheres “should be endowed with like functions,” since they, like the sense organs, “may be said to have a bilateral symmetry.” He therefore was “strongly inclined still to believe in (their) similarity of function, … notwithstanding all that has been said of late in opposition to this doctrine” (p 455). In years past, the doctrine may have dissuaded some, like Bouillaud [7], from even looking for differences. There were 29 patients in his report linking aphasia to anterior-lobe lesions. From the list, Benton [23] determined that 25 had unilateral lesions, 11 left, 14 right, and that 8 (73%) of the former were aphasic, only 4 (25%) of the latter.
If Bouillaud fell victim to conventional wisdom, it was only in the sense that he failed to compare his left- and right-side cases or chose not to mention the difference. Marc Dax, in a paper written in 1836, actually proposed that the left hemisphere was the speech side after observing that aphasic patients more often have right than left hemiplegia. Dax prepared his paper for a medical society meeting but, evidently, never delivered it, and it remained unknown and unheralded until his son, Gustave, published it in 1865, along with his own report on the same subject ([24, 25]; for reviews, see Finger and Roe [26]; Joynt and Benton [27]).
What Was the Underlying Deficit?
Leborgne could not speak, but what was the underlying deficit? To understand Broca’s answer, we must consider the state of theory and evidence on another issue – the motor excitability of the cerebral cortex. In the 1860s, this was still unknown. No one, that is, had distinguished between an anterior “motor” region and a posterior “somatosensory” region, approximately encompassing, respectively, the pre- and post-central gyri. That revelation came later, most importantly in 1870 from Fritsch and Hitzig’s [28] experiments on dogs, and, in 1873, from Ferrier’s [29] experiments on monkeys. Until then, the cortex was seen as purely for mental acts, with the “motor”