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
1.6 The Integrative Action of the Nervous System: Sherrington
It is to Charles Sherrington (1857–1952) at the end of the nineteenth and the beginning of the twentieth century that one must turn to find an experimental plan for elucidating the mechanisms of the ‘true spinal marrow’. The thoroughness and methodical nature of Sherrington’ s researches on the subject are at a new level. These were not dependent on technical advances at the time so much as on the brilliance and clarity of his thinking, coupled with a formidable and indefatigable capacity for experiment. Sherrington first elucidated the spinal origin of the efferent nerves innervating a particular muscle.85 In 1905 his experiments indicated that stimulation of the afferent nerves of a particular muscle could produce contraction of that muscle independent of contraction of opposing muscles of the joint.86 In 1910 he published his great paper, nearly 100 pages long, on ‘Flexion-reflex of the limb, crossed extension-reflex, and reflex stepping and standing’.87 In this he first describes the flexion-reflex and identifies the extension reflex as well as the crossed-extension reflex. This work, together with his earlier papers of 1897 and 1907, laid down the conceptual scheme for the analysis of the role of the spinal cord in stepping and standing. In doing this, Sherrington completed the research programme initiated eighty years earlier by Marshall Hall, with the consequence that the notion of a ‘spinal soul’ was finally eliminated from further consideration.
Although Ferrier in 1886 had first located the motor cortex in primates as a distinct area, it was Grunbaum and Sherrington in 1902 who first gave a detailed description of the spatial extent of this area on the cortex of primates.88 They noted that the ‘motor’ area does not at any point extend behind the sulcus centralis. In this Sherrington and Grunbaum clearly distinguished for the first time the motor area from the area behind the sulcus centralis that we now know as somatosensory.89 Their method of unipolar Faradization (alternating current) stimulation of the cortex allowed for much finer localization than had been possible with the double-point electrodes used up to this time.90 Their classic paper, published at the beginning of the twentieth century, established without equivocation the conception of a motor cortex and therefore that different parts of the cortex are specialized for different functions.
The notion of a ‘spinal soul’ had been put to rest, largely through Sherrington’ s detailed elucidation of spinal reflexes. However, the relationship between the soul and the cortex, or between the mind and the brain, still bedevilled Sherrington, as it had neuroscientists and philosophers for more than two millennia. Sherrington engendered similar concern for it amongst his protégés. We shall explore this in the next chapter.
1.6.1 The dependence of psychological capacities on thefunctioning of cortex: localization determined non-invasivelyby Ogawa and Sokolof
Neural activity in the cortex is reflected in vascular activity
Sherrington began his revolutionary research on the integrative action of the nervous system towards the end of the nineteenth century with an observation that was to have great impact on neuroscience research towards the end of the twentieth century. This was his discovery, with Charles Smart Roy (1854–1897), that there is an intrinsic ‘mechanism for controlling the cerebral circulation … by which the blood supply of various parts of the brain can be varied locally in accordance with local requirements’.91 They go on to say that ‘We conclude then, that the chemical products of cerebral metabolism contained in the lymph which baths the walls of the arteriole of the brain can cause variations of the calibre of the cerebral vessels: that in this reaction the brain possesses an intrinsic mechanism by which its vascular supply can be varied locally in correspondence with local variations of functional activity.’92 This vascular supply provides glucose and oxygen to the brain, in which glucose acts as a source of carbon for oxidization, necessary for the energy required to sustain neuronal electrical activity. Roy and Sherrington had thus discovered that a principal determinant of this energy supply was the activity of the neurons themselves.
Vascular activity can be measured using BOLD brain imaging
Exactly 100 years after this discovery Seiji Ogawa (1934– ) and his colleagues showed that brain nuclear magnetic resonance (NMR) spectroscopy
