LOCALISATION OF SPEECH CENTRES IN THE BRAIN
In 1863 Broca showed the importance in all right-handed people (that is in about ninety-five per cent of all human beings) of the third left frontal convolution for speech (vide figs. [16] and [17]); when this is destroyed by disease, although the patient can understand what is said and can understand written and printed language, the power of articulate speech is lost. Motor Aphasia. This portion of the brain is concerned with the revival of the motor images, and has been termed by Dr. Bastian "the glosso-kinæsthetic centre," or the cortical grey matter, in which the images of the sense of movement of the lips and tongue are formed (vide [fig. 17]). A destruction of a similar portion of the cortex in a right-handed person produces [!-- pagenumber --]no loss of speech; but if the person is left-handed there is aphasia, because he, being left-handed, uses the third right inferior frontal convolution for speech. These facts have for long been accepted by most neurologists, but recently doubts have been cast upon this fundamental principle of cerebral localisation by a most distinguished French neurologist, M. Marie; he has pointed out that a destructive lesion of the cortex may be accompanied by subcortical damage, which interrupts fibres coming from other parts of the brain connected with speech.
In the study of speech defects it is useful to employ a diagram; a certain part of the brain corresponds to the Speech Zone there indicated, and lesions injuring any part of this area in the left hemisphere cause speech defects (vide [fig. 17]). All neurologists, M. Marie included, admit this, and the whole question therefore is: Is a destruction of certain limited regions of the superficial grey matter the cause of different forms of speech defects, or are they not due more to the destruction of subcortical systems of fibres, which lie beneath this cortical speech zone?
There is a certain portion of the speech zone which is assumed to be connected with the revival of written or printed [!-- pagenumber --]language, and is called the visual word-centre. There is another region connected with the memory of spoken words—the auditory word-centre; you will observe that it is situated in the posterior third of the first temporal convolution, but this does not [!-- pagenumber --]comprise nearly the whole of it, for there is an extensive surface of grey matter lying unseen within the fissure, called the transverse convolutions, or gyri. Lesions of either of these regions give rise to Sensory Aphasia, which means a loss of speech due to inability to revive in memory the articulate sounds which serve as verbal symbols, or the graphic signs which serve as visual symbols for language.
Fig. 17
FIG. 17.—Diagram to illustrate the Speech Zone of the left hemisphere (Bastian). This scheme is used to explain the mechanism of speech, but probably the centres are not precisely limited, as shown in the diagram; it serves, however, to explain disorders of speech. Destruction of the brain substance in front of the central fissure gives rise to what is termed Motor Aphasia and Motor Agraphia, because the patient no longer recalls the images of the movements necessary for expressing himself in articulate speech or by writing. Destructive lesions behind the central fissure may damage the portion of the brain connected with the mental perception of the sounds of articulate language, or the portion of the brain connected with the mental perception of language in the form of printed or written words—Sensory Aphasia; the former entails inability to speak, the latter inability to read.
This speech zone acts as a whole, and many disorders of speech may arise from destructive lesions within its limits. It has a special arterial supply, viz. the middle cerebral, which divides into two main branches—an anterior, which supplies the motor portion, and a posterior, which supplies the posterior sensory portion. The anterior divides into two branches and the posterior into three branches, consequently various limited portions of the speech zone may be deprived of blood supply by blocking of one of these branches. The speech zone of the left hemisphere directly controls the centres in the medulla oblongata that preside over articulation and phonation; innervation currents are represented by the arrows coming from the higher to the lower centres.
These several cortical regions are connected by systems of subcortical fibres to two regions in front of the ascending frontal convolution (vide [fig. 17]), called respectively the "glosso-kinæsthetic" (sense of movement of tongue) and the "cheiro-kinæsthetic" (sense of movement of hand) centres. Now a person may become hemiplegic and lose his speech owing either to the blood clotting in a diseased vessel, or to detachment of a small clot from the heart, which, swept into the circulation, may plug one of the arteries of the brain. The arteries branch and supply different regions, consequently a limited portion of the great brain may undergo destruction, giving rise to certain localising symptoms, according to the situation of the area which has been deprived of its blood supply. Upon the death of the patient, a correlation [!-- pagenumber --]of the symptoms observed during life and the loss of brain substance found at the post-mortem examination has enabled neurologists to associate certain parts of the brain surface with certain functions; but M. Marie very rightly says: None of the older observations by Broca and others can be accepted because they were not examined by methods which would reveal the extent of the damage; the only cases which should be considered as scientifically reliable are those in which a careful examination by sections and microscopic investigation have determined how far subcortical structures and systems of fibres uniting various parts of the cortex in the speech zone have been damaged. Marie maintains that the speech zone cannot be separated into these several centres, and that destruction of Broca's convolution does not cause loss of speech (vide figs. [16], [17]). There are at present two camps—those who maintain the older views of precise cortical centres, and those who follow Marie and insist upon a revision.
Herbert Spencer says that "our intellectual operations are indeed mostly confined to the auditory feelings as integrated into words and the visual feelings as integrated into ideas of objects, their relations and their motions."
Stricker by introspection and concentration of attention upon his own speech-production came to the conclusion that the primary revival of words was by the feeling of movements of the muscles of articulation; but there is a fallacy here, for the more the attention is concentrated upon any mental process the more is the expressive side brought into prominence in consciousness. This can be explained by the fact that there is in consequence of attention an increased outflow of innervation currents to special lower executive centres, thence to the muscles, but every change of tension in the speech muscles is followed by reciprocal incoming impressions appertaining to the sense and feeling of the movement. The more intense the sense of movement, the greater will be the effect upon consciousness. In fact, a person who reads and thinks by articulating the words, does so because experience has taught him that he can concentrate his attention more perfectly; therefore his memory or understanding of the subject read or thought of will be increased. Very many people think and commit to memory by this method of concentrating attention; they probably do not belong to the quick, perceptive, imaginative class, but rather [!-- pagenumber --]to those who have power of application and who have educated their minds by close voluntary attention. Galton found a large proportion of the Fellows of the Royal Society were of this motor type. But the fact that certain individuals make use of this faculty more than others does not destroy the arguments in favour of the primary revival of words in the great majority of persons by a subconscious process in the auditory centre, which is followed immediately by correlated revival of sensori-motor images. Although the sensori-motor images of speech can be revived, it is almost impossible without moving the hand to revive kinæsthetic impressions concerned in writing a word. Both Ballet and Stricker admit this fact, and it tends to prove that the sense of hearing is the primary incitation to speech.
Charcot in reference to the interpretation of speech defects divided persons into four classes—auditives, visuals, motors, and indifferents. There are really no separate classes, but only different kinds of word-memory in different degrees of excellence as regards the first three; and as regards the fourth there is no one kind of memory developed to a preponderating degree. Bastian doubts the second class, but does [!-- pagenumber --]not deny that the visual type may exist; for Galton has undoubtedly shown that visual memory and power of recall of visual word images varies immensely in different individuals, and it is unquestionable that certain individuals possess the visualising faculty to an extraordinary degree; some few, moreover, can see mentally every word that is uttered; they give their attention to the visual symbolic equivalent and not to the auditory. Such persons may, as Ribot supposes, habitually think and represent objects by visual typographic images. Lord Macaulay and Sir James Paget were notable possessors of this visualising faculty. The former is said to have been able to read a column of "The Times" and repeat it verbatim; the latter could deliver his lectures verbatim as he had written them. Both saw mentally the print or MS. in front of them.
Nevertheless it is a question of degree how much motor images enter into silent thought and into the primary revival of words in different individuals. Mach in "Analysis of Sensations" says: "It is true that in my own case words (of which I think) reverberate loudly in my ear. Moreover, I have no doubt that thoughts may be directly excited by the ringing of [!-- pagenumber --]a house-bell, by the whistle of a locomotive, etc., that small children and even dogs understand words which they cannot repeat. Nevertheless I have been convinced by Stricker that the ordinary and most familiar, though not the only possible way, by which speech is comprehended is really motor and that we should be badly off if we were without it. I can cite corroborations of this view from my own experience. I frequently see strangers who are endeavouring to follow my remarks slightly moving their lips."