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Broca: the cortical area for language; Fritsch and Hitzig: the motor cortex

Although the first experiments indicating a specialized area of the cortex for motor control were reported by Fritsch and Hitzig in 1870, it could be claimed that the first evidence for cortical specialization was that reported by Paul Broca (1824–1880) in 1861 for speech. In that year Broca reported the results of an autopsy on the cortex of one of his patients, a Monsieur Leborgne, who had suffered from loss of speech (aphasia). Broca found a lesion in the left anterior (frontal) lobe, which, he suggested, was the language area of the cortex. Subsequently it became known as Broca’ s area.⁷⁷

There was little progress in the understanding of the function of the cortex for nearly 200 years, between the death of Thomas Willis in 1675 and, as we shall see, the experiments of Fritsch and Hitzig around 1870. For example, the leading French physiologist of the day, Marie-Jean-Pierre Flourens (1794–1867), claimed, as a consequence of his researches on pigeons (c. 1824), that the cortex was concerned only with perception, intellectual abilities and the will, not with motor action.⁷⁸ He showed in 1858 that the motor action involved in respiration could be delimited to the medulla, and did not involve the brain. Furthermore, according to Flourens, these functions of the cortex could not be ascribed to different areas of the cortex, for this acted as a whole. All sensations, all perception and all volition, he argued, occupy concurrently the same seat in these organs. The faculty of sensation, perception and volition, he therefore concluded, is essentially one faculty.

Discovery of the motor cortex: Fritsch and Hitzig

It was not until the second half of the nineteenth century that progress was made on the motor control functions of the cortex. In 1870 Gustav Fritsch (1838–1891) and Edouard Hitzig (1838–1907) published their monumental work ‘Über die elektrische Erregbarkeit des Grosshirns’ (1870), in which they described the results of their experiments on stimulating the brains of dogs with galvanic currents, which led them to the idea of a ‘motor cortex’. In these experiments, the exposed cortex of dogs was excited at different sites with levels of electrical stimulation just detectable when applied to the human tongue. They found areas on the surface of the cortex that gave rise to muscular contractions involving the face and neck on the opposite side of the dog to the hemisphere being stimulated, as well as forepaw extension and flexion. On unilateral ablation of the forepaw area of the cortex, they observed that sensation was unaffected, but that the dog possessed impaired motor activity and posture. On this they commented:

A part of the convexity of the hemisphere of the brain is motor, another part is not motor. The motor part, in general, is more in front, the non-motor part more behind. By electrical stimulation of the motor part, one obtains combined muscular contractions of the opposite side of the body. These muscle contractions can be localized on certain very narrowly delimited groups by using very weak currents. The possibility to stimulate narrowly delimited groups of muscle is restricted to very small foci which we shall call centers.⁷⁹

This led them not only to the hypothesis that a discrete area of the cortex possessed a motor function, but also to the generalization that other functions might also be localized in specific areas of the cortex. This conception of cortical localization was the first major advance in our understanding of cortical function since the time of Willis.

Somatotopic organization of motorcortex: Jackson and Ferrier

Following this work of Fritsch and Hitzig on dogs, John Hughlings Jackson (1835–1911) reached similar conclusions concerning the existence of a motor cortex in humans, based on his observations on patients with epilepsy reported in 1863: ‘In very many cases of epilepsy, and especially in syphilitic epilepsy, the convulsions are limited to one side of the body; and, as autopsies of patients who have died after syphilitic epilepsy appear to show, the cause is obvious organic disease on the side of the brain, opposite to the side of the body convulsed, frequently on the surface of the hemisphere.’⁸⁰ Of particular interest was the temporal pattern of contraction across muscle groups during seizures in spreading epilepsy. This led Hughlings Jackson to speculate that the motor cortex must be organized along somatotopic lines, so that the hands, face and feet, which possess the greatest capacity for specialized movement, are allocated the largest area in the motor cortex. These brilliant suggestions of Hughlings Jackson were confirmed by the work on primates by David Ferrier (1843–1928) in 1874. Using alternating current stimulation of discrete sites on the cortex, he was able to delineate clearly the area of the cortex that produces the twitching of muscles, as well as movements that in some cases resembled attempts at walking. On introducing small lesions into the motor area of the cortex which he had mapped, Ferrier showed that, in some cases, these resulted in a paralysis of the opposite hand and forearm, and in another case, to the paralysis of the biceps muscle. By contrast, these animals showed normal sensitivity to touch and noxious stimuli. Such observations clearly pointed to a somatotopic organization of the motor cortex.⁸¹ This work on primates was subsequently confirmed and extended by Victor Horsley (1857–1916), who, in 1887, showed that the precentral gyrus was predominantly motor, and the postcentral sensory, so that the motor cortex was to be found exclusively anterior to the Rolandic fissure.⁸²

Caton’ s and Beck’ s discovery of electrical phenomena in the cortex support the idea of a motor cortex

In 1875 Richard Caton (1842–1926) discovered that electrical oscillations could be recorded through two electrodes placed on the surface of the cortex of a monkey, and that these oscillations were altered by sensory stimulation, anoxia and anaesthesia. Caton comments that:

In every brain hitherto examined, the galvanometer has indicated the existence of electrical currents on the areas shown by Dr Ferrier to be related to rotation of the head and to mastication, negative variation of the current was observed to occur whenever those two acts respectively were performed. Impressions through the senses were found to influence the currents of certain areas; e.g. the currents of that part of the rabbit’ s brain which Dr Ferrier has shown to be related to movements of the eyelids, were found to be markedly influenced by stimulation of the opposite retina by light.⁸³

The electrical changes due to stimulation of the retina with light were later confirmed by Adolf Beck (1863–1942), adding credence to Caton’ s observations on the localization of electrical activity during motor acts of the kind predicted by Ferrier’ s work.⁸⁴