§ 3. Immediate Correlation of Conscious and Nervous Process.—In the cortex at least we have a direct relation between nervous process and conscious process, and are so brought face to face with the question of the ultimate nature of their connexion. To some extent this is a question of fact, and can be settled by special evidence. But there is also a wide field for speculation, in which we cannot be at all certain of our conclusions.
The facts show that there is not only a general correlation between conscious process as a whole and cortical process as a whole, but also that special parts of the cortex are connected with special constituents of the mental life. A generation ago the prevailing doctrine among physiologists was opposed to this "localisation of function," as it is called. It was held that the cortex discharged each of its separate functions as a whole, so that injury done to it, or removal of part of its substance, did not involve the loss or impairment of any special mental process rather than any other. The only result was a general impairment of mental power. Just as a man still breathes as before when he has only one lung, except that he does so less efficiently, so it was supposed that a part of the brain might in the same manner be substituted for the whole. On the other side, the phrenologists maintained a very definite theory of localisation. But their doctrine was a mass of psychological and physiological crudities. They mapped out the brain into organs corresponding to complex faculties, such as acquisitiveness, combativeness, ideality, orderliness, constructiveness, and the like. Such a scheme is a psychological absurdity. Each of these faculties involves the cooperation of a vast number of fundamental processes, and the same processes enter in varying combination into the constitution of the different faculties. Thus the procedure of phrenology is like that of a man who should assume a different board and a different set of pieces for every game of chess, or a separate alphabet for every word. Besides this there is a very obvious anatomical objection against the supposed evidence adduced by the phrenologist. This consisted in the reading of character by the feeling of bumps; but as a matter of fact, the external conformation of the skull is far from accurately corresponding to the development of the brain.
But the most crushing refutation of phrenology is supplied by what has been ascertained in recent years about the modes in which cerebral functions actually are localised. So far as the cortex has been mapped out on good evidence, it is found that the division of function among different parts corresponds, not to complex faculties, but to the bodily organs of sensation and movement. One portion of the cortex, anatomically connected with the eye, is specially correlated with visual consciousness, in the way of sensation or mental imagery. Another, anatomically connected with the ear, has a similar relation to auditory experience. Another is specially connected with touchsensations, and with movements of the limbs. The evidence on which these conclusions are based is partly gathered from experiments on animals, and partly from pathological data. The pathological evidence is most important and unambiguous. Diseases affecting communication of ideas by means of language have been especially useful. Under the general name aphasia are embraced many defects of varying kinds. The patient may be simply unable to articulate words, although he can understand them when he hears them. This is motor aphasia, and it has been definitely connected with lesion of a special part of the brain called the third frontal convolution.* Again, a man may be able to articulate, and yet lack the power to distinguish words as such when he hears them. He hears them indeed as a confused stream of sound, but they are not for him words. This is sensory, or, more accurately, perceptual aphasia, and it is connected with lesion of a special portion of the auditory area of the cortex. Similarly, inability to recognise written words for what they are is connected with lesion of a special portion of the visual area. These indications may serve to show what is meant by localisation+ of cerebral functions, and the methods by which it is determined. But it must be remembered that our ignorance is still incomparably greater than our knowledge. The student must also be warned against supposing that localisation is definite and precise in its nature. "The various activities making up the business of the brain do not take place all over its surface, as in a country without towns and 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."*
• Of the left hemisphere in righthanded persons, and of the right hemisphere in lefthanded persons.
+In localisation what is locally marked off: is a certain portion of the brain and the material processes which take place in it. The corresponding conscious process is not, strictly speaking, localised. The nature of its connexion with the localised brainprocess remains to be discussed.
Data of the kind described afford us no efficient help when we come to consider precisely how cortical process is related to the corresponding conscious process. There are, in the main, three alternative possibilities,—interaction, onesided action, and simple concomitance. On the interaction hypothesis, a cerebral process may produce a state of consciousness, just as a nervous process may produce a muscular contraction, or as change in one part of the cortex may produce change in another part; and, inversely, a conscious process, such as a volition, may act on the cortex, just as the cortex acts on subcortical centres, and these on the muscles.
The main objection to this view is that the kind of interaction presupposed is utterly incongruous with the conception of causation on which the whole system of our knowledge both of physical and psychical process is based. It is the function of science to explain how events take place, or, in other words, to make their occurrence intelligible; but this is only possible in so far as we can discover such a connexion between cause and effect as will enable us to understand how the effect follows from the cause; or, in other words, we must exhibit cause and effect as parts of one and the same continuous process. To explain is to exhibit a fact as the resultant of its factors. This is the ideal of science, and it is never completely attained. But in so far as it is unattained, our knowledge is felt to be incomplete.
* Waller's Physiology, pp. 534-535.
Now when we come to the direct connexion between a nervous process and a correlated conscious process, we find a complete solution of continuity. The two processes have no common factor. Their connexion lies entirely outside of our total knowledge of physical nature on the one hand, and of conscious process on the other. The laws which govern the change of position of bodies and of their component atoms and molecules in space, evidently have nothing to do with the relation between a material occurrence and a conscious occurrence. No reason in the world can be assigned why the change produced in the grey pulpy substance of the cortex by light of a certain wavelength should be accompanied by the sensation red, and why that produced by light of a different wavelength should be accompanied by the sensation green. It is equally unintelligible that a state of volition should be followed by a change in the substance of the cortex and so mediately by the contraction of a muscle.