CORTICAL LOCALISATION

The physiology of the brain has, since the year 1870, been engrossed by the one question of localisation. Is there a localisation of different functions in different parts of the brain, are its different parts concerned with different offices, is there a division of cerebral labour? or does every part subserve every office, can any office be administered by any part? To these questions it may be answered at once that the balance of evidence is in favour of localisation, but that the counter-evidence has shown that localisation is not sharply defined, but blurred. The state of doctrine may be made plain by an analogy. The various activities making up the business of the brain do not all take place all over its surface, as in a country without towns or villages, where all kinds of industry go on in every hut or tent; nor are the different activities absolutely restricted to certain spots, as if in walled towns. The brain cortex is not comparable with either of these extreme cases; its territory must be recognised as possessing towns with special industries, but towns with straggling and overlapping suburbs, and industries that are, indeed, predominant each in a given centre, but not exclusive of all other industries in that centre, nor excluded from other centres in which other industries predominate. In this qualified sense localisation of function in different parts of the brain must be considered to be established.

The chief facts by which the doctrine of localisation has been formed have been :—

(1) The proof given by Fritsch and Hitzig in 1870 that the cortex of the brain responds to experimental stimulation, and that the excitation of certain different spots provokes certain different muscular movements; (2) the consequent detailed study-by Ferrier, Munk, Schäfer, Horsley, Exner, Nothnagel, Beevor, Goltz, Franc, Heidenhain, Luciani, Tamburini, Seppilli, Löb, and many others of the relation between cortical areæ, destroyed or stimulated, and resulting muscular paralysis or spasm; (3) the scientific clinical study of epilepsy by Hughlings Jackson, completed by post-mortem recognition of the seats of lesion; (4) the association found to exist between aphasia and lesion of the left third frontal convolution.

The cortex of the brain is experimentally excitable. This simple proposition was established by Fritsch and Hitzig in con

FIG. 281.-HUMAN BRAIN; LATERAL ASPECT OF LEFT HEMISPHERE. (After Ecker.)

FIG. 282.-HUMAN BRAIN; MESIAL ASPECT OF RIGHT HEMISPHERE. (After Ecker.)

FIG. 283.-HUMAN BRAIN; LATERAL ASPECT OF LEFT HEMISPHERE. TO ILLUSTRATE LOCALISATION OF FUNCTION.

FIG. 284.-HUMAN BRAIN: MESIAL ASPECT OF RIGHT HEMISPHERE. TO ILLUSTRATE CORTICAL LOCALISATION OF FUNCTION.

tradiction of the heretofore universally accepted statement by Flourens, Magendie, Longet, and the older physiologists, to the effect that the surface of the brain may be lacerated, pricked, burnt, mechanically, chemically or electrically excited, without provoking muscular contraction in any part of the body. Fritsch and Hitzig used dogs, exposed the surface of an entire hemisphere, and tested it by interruptions of the constant current and by induced currents; they found the lateral and posterior parts of the cortex to be non-excitable, but in the frontal region they found certain spots, stimulation of which produced combined movements of the anterior or of the posterior extremities, of the neck, and of the face on the opposite side of the body; they found that on closing or reversing the constant current, the anode was more effectual than the kathode, and they observed epileptic attacks following repeated stimulation, and originating in spasms of the previously-excited muscles.

Objections. The electrical excitability of the cortex, the keystone of the doctrine of localisation, was not at once admitted as proved. It was objected that the diffusion of currents in the brain substance is such that stimuli cannot be limited to given areæ, and that the effects are due to the excitation of the subjacent white matter or of the basal ganglia. To this objection the answers are (1) No doubt the corona radiata is excitable; a localisation of effects is in fact demonstrable by excitation of different bundles from the cortex, as well as of different parts of the cortex itself; cortical stimulation remains without effect if the cortex is separated by an incision parallel to the surface, and replaced in situ; electrical diffusion to deeper parts is unaltered, but the physiological connection between cortex and base is interrupted. (2) Stimulation of the island of Reil, which is much closer to the basal ganglia than the Rolandic area, remains without effect. (3) Localised reactions are obtainable by mechanical stimulation of the Rolandic area. (4) Comparison of the 'lost times' in the two cases, viz. excitation of cortex and of subjacent white matter, reveals the fact that time is occupied in the physiological transmission of stimuli through the cortex; Franc and Pitres found, for example, on the dog, that the contraction of the front paw commenced 065 sec. after a cortical stimulus, 045 sec. after a stimulus to the subjacent white matter; i.e. they obtained 02 sec. as the time-value of cortical delay. These results were confirmed by Bubnoff and Heidenhain, and supple

mented by a still more convincing proof of cortical excitability. They showed that the muscular contraction obtained by cortical excitation of a morphinised dog follows after a much longer interval, and is much more prolonged than that obtained immediately afterwards by sub-cortical excitation. (5) Confirmatory proof of the excitability of grey matter is afforded by the effects of anæsthetics or of interrupted blood-supply; by either of these means cortical excitability is abolished, and that far too rapidly for it to be possible to admit that the subjacent white matter has been thus influenced.

The investigation opened up by Fritsch and Hitzig was soon afterwards pursued and extended by Ferrier (1874-78), who employed monkeys, and tested the cortical excitability by weak induced currents. He distinguished between direct and reflex excitability by differences in the character of motor reactions, regard

FIG. 285.-MUSCULAR CONTRACTIONS PROVOKED BY CORTICAL AND BY
SUB-CORTICAL EXCITATION. (Bubnoff and Heidenhain.)

ing the reactions obtained by stimulation of the Rolandic area as direct motor reactions, and those obtained by stimulating the temporal and the occipito-angular area as reflex reactions, such as might be expected in consequence of auditory or visual sensations. Excitation of the left occipital lobe causes conjugate movement of the two eyes towards the right-i.e. the eyes turn, as if an object on the right, casting its image upon the left halves of the retina, had created an impression in the left occipital lobe.' In harmony with this view, Schäfer finds that the latent time of the ocular movements is greater to stimulation in the occipital (or visual) area than to stimulation in the frontal (or oculo-motor) area. Ferrier found that direct excitability is limited to the convolutions in the vicinity of the fissure of Rolando, viz. the ascending frontal and ascending parietal. This region he therefore designated as the motor area' of the cortex; and it may be remarked in this place, that Ferrier's experimental results on monkeys are in general agreement with the deductions that