Popular Science Monthly/Volume 27/May 1885/The Nervous System and Consciousness II

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THE NERVOUS SYSTEM AND CONSCIOUSNESS.
By W. R. BENEDICT.,

PROFESSOR OF PSYCHOLOGY AND LOGIC IN THE UNIVERSITY OF CINCINNATI.

II.

MY former paper gave an outline account of the structure of the cerebro-spinal nervous system. The functions of this system were examined as far as to the cerebral hemispheres. It was said that we lacked evidence for the appearance of consciousness in connection with the activities of the spinal cord, the medulla oblongata, the pons Varolii, and the cerebellum. It was also affirmed that, if consciousness be associated with the activities of any organs below the cerebrum, this consciousness is of a general and vague kind, not the intelligence of clear perception.

The present paper is to state the functions of the cerebral hemispheres, as far as these functions are thought to be established by recent experiment and pathology.

We shall need to refresh our minds by a general view of the cerebrum. Looking at this organ from the side, we readily distinguish its so-called lobes or divisions. These are made by the fissures or furrows which dip down from the surface, penetrating, more or less deeply, the entire mass.

The prominent fissures are the fissure of Sylvius (S, Fig. 1) and the fissure of Rolando (R, Fig. 1). The fissure of Sylvius separates, in part, the temporo-sphenoidal lobe from the lobes above, and has two branches, a longer, horizontal branch (s), and a shorter, perpendicular branch (s'). If we push apart the brain-mass at the horizontal branch, we will see the nerve-matter called the Island of Reil. This is simply an additional fold of cell and fiber substance lying over the corpus striatum. The fissure of Rolando separates the frontal lobe, F, from the rest of the brain. It begins at the great longitudinal division between the hemispheres, and pursues an uninterrupted course to within a short distance of the horizontal branch of the Sylvian fissure. Back of the fissure of Rolando is the external perpendicular fissure (E); it

PSM V27 D079 Fissures and convolutions of the human brain.jpg

Fig. 1. Fissures and Convolutions of the Human Brain. (Wundt.) Left side. S, Sylvian fissure—s, perpendicular, s'. horizontal, branches of this fissure; R, fissure of Rolando; E, external perpendicular fissure; F3, third frontal convolution or convolution of Broca; AF, ascending frontal convolution; AP, ascending parietal convolution; AG, angular gyrus or pli courbe; F, frontal lobe; P, parietal lobe; T.S, temporo-sphenoidal lobe; O, occipital lobe.

appears as a simple notch on the upper edge of the hemisphere. It is a prolongation, on the convex or lateral surface of the brain, of the deep fissure of the internal zone. This fissure marks the rear limit of the parietal lobe (P), which therefore lies between the fissure of Rolando and this furrow. Back of the parietal lobe is the occipital lobe (O). This region is less exactly defined; an ideal prolongation of the external perpendicular fissure would determine its anterior and inferior limits. The temporo-sphenoidal lobe (T) has already been noticed as lying below the fissure of Sylvius. Among the various convolutions formed by these fissures there are three or four which must be named, because it is with them that the experiments in brain-functions are chiefly concerned. In the frontal lobe there are two of these convolutions (F 3), the third frontal convolution, or the convolution of Broca, and (A F) the fourth frontal convolution or ascending frontal fold. Broca's convolution has somewhat the shape of a horseshoe, and is formed around the ascending branch of the Sylvian fissure. The ascending frontal fold lies directly to the left of the fissure of Rolando, which it follows throughout. In the parietal lobe we notice (A P) the ascending parietal convolution immediately to the right of the fissure of Rolando, and (A G) the angular gyrus or pli courbe. This latter convolution is very complex in man.

It was believed for a long time that the cerebral hemispheres were insensible and inexcitable to direct stimulation. The Germans Fritsch and Hitzig discovered, however, that parts of the cerebrum would respond to a very gentle current of electricity. This beginning has been carefully followed up by Ferrier, Munk, Goltz, and many others, until we now have, amid much disagreement and uncertainty, some results that are interesting, to say the least.

All experiments on the cerebrum are of two kinds (stimulation of the surface and destruction of the surface), and are necessarily made on the lower animals. Dupuy offered an objection to experiment by electrical stimulation, which, if well founded, would destroy the entire value of the undertaking. He claimed that the effects produced by electricity at the surface of the hemispheres were due wholly to conduction of the current through the mass to the corpora striata below and so to the muscles. Dupuy proved that conduction did take place through the cell-mass of the hemispheres. He placed the leg of a frog in contact with the rear of a brain, and by application of electricity to the front of this brain produced strong movements in the limb. Ferrier's answers to Dupuy are a sufficient refutation of the objection.

If the effects observed under electrical stimulation are due to conduction, we could not have (as is the case) strikingly different results from application of the electrodes to very closely adjacent areas. Further, when the striata themselves are stimulated, there is always a general contraction of muscles on the entire opposite side of the body. There is no limitation of the movements to special groups of muscles, as always happens when particular centers on the brain-surface are stimulated. Again, there are many portions of the brain which give no response to electrical stimulus. How can this be so if such movements as are produced result from conduction, especially since many of these silent regions of the brain are no more remote from the striata than the responsive ones?

Experiment and pathology, despite all the contradictions, seem to point to the existence of a motor zone on the surface of the hemispheres. This means that certain parts of the brain are directly concerned with the movements of particular muscles and groups of muscles; also, that these parts can not be shown to be connected with sensations. The natural, primary occasions of their activity may be the states of consciousness which we call volitions; they are not, so far as evidence goes, the states of consciousness we call sensations.

It is of interest to observe that these motor regions are situated in the anterior portions of the hemispheres, and occupy here a relatively small space. They lie above the Sylvian fissure, and are mostly on the fourth frontal and ascending parietal convolutions.

The experiments have been performed on a great variety of animals, and repeated a large number of times. The monkey is, of course, the most interesting of these animals to us, from the striking resemblance between his brain and the human brain.

Hitzig's investigations, published in Berlin in 1874, give all the results gained up to that time by the stimulation experiments. Ferrier's book, "The Functions of the Brain," London, 1876, better known PSM V27 D081 Lateral aspect of monkey brain.jpgFig. 2.—Lateral Aspect of Monkey's Brain, showing the relative positions of the so-called "Motor Centers" in the left Cerebral Hemisphere. Ferrier. to English readers, has special merit in two respects. It displays a very intelligent comprehension of the consequences of electrical stimulation, and seems to give a juster account of the motor regions in the monkey's brain than was furnished by Hitzig.

Figs. 2 and 3 will show the character and results of these experiments in sufficient detail.

When center No. 1 is stimulated, the hind-limb on the opposite side of the body advances as in the act of walking; when No. 5 is stimulated, the opposite arm and hand reach forward as if to touch something. These movements go together and are essentially the PSM V27 D081 Upper aspect of monkey brain.jpgFig. 3.—Upper Aspect of Monkey's Brain, showing the relative positions of some of the so-called "Motor Centers" in the left Cerebral Hemisphere, (Ferrier.) same. Centers 2 and 3 work together; when 2 is stimulated, there are combined movements of the opposite thigh, leg, and foot, and the foot is brought to the middle line of the body as in scratching that part, or in seizing something with the foot; 3 gives movements of the tail. An interesting fact should be noted at this point. There is no center No. 2 in the brain of cat, dog, or jackal, while No. 3 is present in each. These animals do not grasp with the foot, and the monkey alone uses the rear foot for seizing. That No. 2 should be present, and of great size, in the monkey's brain, while absent elsewhere, is confirmatory of the accuracy of the experiments. The centers marked a, b, c, d, are on the ascending parietal convolution. When stimulus is applied there, the fingers and wrist move with separate and combined movements that end in closing the fist; in connection with these centers we may note Nos. 4 and 5; they produce movements of the opposite arm and hand. It is plain that 4 and 5, and a, b, c, d, are closely related to one another.

According to the theory of localization of functions, we should expect the centers, a, b, c, d, to be extensive in the monkey's brain, and to be wanting in the brains of lower animals. As matter of fact, they are absent in cat, dog, and jackal, except that a is found in the brain of the cat. This animal uses the front-paw for seizing and holding. Upon stimulating center No. 6, the fore-arm bends, and the hand lifts to the mouth. This movement is constant with the monkey. There is no corresponding center in the brain of the dog or cat. The centers marked 7, 8, 9, 10, 11, are all concerned with movements of the mouth—such as elevating the angle of the mouth, depressing the lower lip, thrusting out and withdrawing the tongue. No. 12 lies quite to the front of the brain; when it is stimulated, the eyes open widely, the pupils dilate, head and eyes turn toward the opposite side.

These are the centers in the brain which, by some authorities, are thought to have a purely motor significance. The centers marked 13, 13', 14, and 15, give movements—the former of the eyes, the latter of the nostrils but they are believed to be primarily connected with sensations.

Aside from the centers enumerated, no other parts of the brain respond to stimulation.

I have purposely stated the results of Ferrier's earlier experiments on the so-called motor zone. These experiments have been, in general, confirmed by other investigators. That is to say, the movements above described have been found by many to follow stimulation. It is, however, a part of the present confusion and contradiction which prevails respecting cerebral localization that the interpretation of these movements is disputed.

Munk appears ("Transactions of the Physiological Society of Berlin," 1876-1878) with a series of experiments which, as he thinks, prove that the motor zone is primarily a zone of feeling. He therefore divides this portion of the brain into spheres of feeling—one for the forward limbs, one for the head, one for the eyes, one for the ears, etc. Munk believes that the animal's movements are affected by destruction of these centers, because four distinct kinds of feeling are destroyed. For example, loss of the center concerned with movements of the fore-limb would, according to Munk, cause a loss—1. Of the consciousness of pressure on the limb; 2. Of the consciousness of the position of the limb; 3. Of the consciousness of the motions belonging to the limb; and, 4. Of the consciousness of touch in the limb. Whereas Ferrier and others find sensibility, both general and special, intact after destruction of these motor regions, Munk finds a loss of sensibility so well defined and persistent as to justify the fourfold division above stated.

We have now to inquire as to the testimony of pathology respecting these motor areas in the brain. Charcot and Pitres, in "Revue Mensuelle," November, 1878, and February, 1879, cite fifty-six cases of brain-dieases bearing on this subject. Twenty-one of these cases show lesions in the brain outside the motor zone, and unaccompanied by motor trouble. Charcot's deductions from these cases are that "there exist in the cortex of the cerebrum tracts which are independent of voluntary motion, and when lesions occur in these tracts there are no permanent affections of the motor functions." The remaining cases cited by Charcot show lesions in the motor zone, and are accompanied by varied degrees of paralysis in keeping with the situation and extent of the lesion. Dr. Bechstrew, in the "Medicinische Wochenschrift," St. Petersburg, details a number of cases which confirm the recent views on the motor functions of the areas about the middle convolutions. Other confirmatory cases have been cited by Burdon and Maragliano, by Dr. Henry Obersteiner, and many more. It is well known, however, that a number of opposing instances are on record—that is, of lesions in the so-called motor zone without paralysis, and of paralysis unaccompanied by lesions in these portions of the brain. There is a fundamental objection to this kind of evidence: it is selected evidence, chosen to make for or against a theory. What we really need is a collection of all cases of injuries to parts of the hemispheres, and a full statement of consequences without regard to the bearing of the example.

This is the proper place to mention a brain disorder more or less commonly known under the name of aphasia. Aphasia is a disturbance of the power of speech. It appears in two distinct forms, viz., amnesic and ataxic aphasia. The person suffering from amnesic aphasia forgets substantives and names, other parts of speech being properly used; or he forgets a language which he once knew, or he misapplies terms, "using pamphlet for camphor, horse for man," etc. In ataxic aphasia the power of articulation is completely lost. The person understands fully the word to be used, and makes vigorous effort to use it, but is unable to do so. Sometimes articulation is half destroyed, so that the first part of the word can be spoken, but not the other. Sometimes automatic phrases can be uttered, such as yes and no, while it is perfectly clear that these exclamations do not satisfy the person. Another form of this general trouble is agraphia, or the inability to express ideas in writing; this is frequently complete, and all attempts at writing end in a scrawl. It is noticeable that aphasia is sometimes, though seldom, unaccompanied by insanity. As early as 1861 Broca, in Paris, expressed the opinion that aphasia was connected with disease in the third frontal convolution. While a large number of cases have been cited for and against this conclusion, many pathologists are disposed to regard it as substantially correct. It would seem just, then, to connect these central functions which are concerned in speech with the peculiarly developed region of the human brain that lies on the anterior and lower limit of the Sylvian fissure; Wundt adds that perhaps the Island of Reil should be joined to this territory.

We are now brought to consider directly the relation of portions of the brain to specific states of consciousness. I shall state the location of the senses as formerly made by Ferrier and by Munk, and will give a specimen experiment from each investigator. Sight is located by Ferrier in the angular gyrus (A, Fig. 1), by Munk in the occipital lobe (O, Fig. 1); hearing, by both, in the temporo-sphenoidal lobe (H, Fig. 1). Ferrier places smell and taste in the lower and inner aspect of the temporo-sphenoidal lobe (IT, Fig. 4). These centers are not distinguished by Munk. Ferrier names also a tactile center (H, Fig. 4). This he locates in what is known as the hippocampal region. If we separate the hemispheres from one another by cutting through the corpus callosum, we shall obtain a view of the median aspect of the hemispheres (see Fig. 4).

Attention has been called to the fact that Munk disagrees with all

PSM V27 D084 Right hemisphere of the human brain.jpg

Fig 4.—Median View of Right Hemisphere of Human Brain. (Ecker.) c c, Corpus callosum, connecting band between the hemispheres, longitudinally divided; w, lower and inner portion of temporo-sphenoidal lobe, center of smell, according to Ferrier; H, hippocampal fold, touch.

the authorities, except Schiff, in maintaining that a destruction of the motor centers destroys sensibility. Munk, therefore, does not indicate a special tactile center, but finds centers of feeling for head, neck, and back.

Ferrier's experiment with regard to vision was as follows: He chloroformed the animal, a monkey, and destroyed the angular gyrus on the left hemisphere. He bandaged the left eye, and allowed the animal to recover from the chloroform. "Upon recovery it began to grope about a little in loco, perfectly alert, but would not move from its position; hearing and the other senses were not affected, for there was always a prompt reply to stimulation of these senses." The animal remained in this position for an hour. The bandage was then removed from the left eye. "It instantly looked around, ran quickly to the cage and joined its companions. When brought to the light, as before, it flinched and turned away its head." Ferrier describes the change in the animal's manner, after removal of the bandage, as most complete and remarkable. On the following day the left eye was again bandaged, but "the animal gave plain signs of vision, it ran swiftly and accurately to the bars of the cage, thrust its head between them, and began to drink from a cup of water." In his next experiment Ferrier destroyed the angular gyrus on both hemispheres. He found great difficulty in forming a right test for vision, one which should discriminate between sight as a state of consciousness and simple reflex reaction to visual stimulation.

The animal sat perfectly still and would not move from its position. "The pupils contracted to light, and light flashed in the eyes caused the animal to wince." It was utterly unwilling to move from its place; nothing else showed lack of vision. Ferrier's test was a cup of tea, which the animal liked very much. Ferrier placed the tea to the monkey's lips; it began at once to drink eagerly. The cup was then removed, but barely removed, from contact with the lips. "The monkey seemed intensely anxious to drink, but could not find the tea, though both eyes were looking straight at it." As soon as contact was established, the monkey buried his head in the cup and followed it around the room, as the cup was slowly lowered.

Munk's experiments on vision led him to different results. He removed the entire angular gyrus from the left hemisphere; he then raised the lids of the left eye with his fingers and touched parts of the eye softly; immediately there were blinking and vigorous movement of the head and muscles of the eye. The animal made every effort to draw back its head, and almost always accompanied these efforts by striking with the left front-limb. With the right eye, however, the case was entirely different. This eye could be pressed and pinched constantly, and the animal remained perfectly quiet.

If the finger or hand was brought suddenly up close to the left eye there was blinking; if to the right eye, no blinking at all resulted, unless the lids were actually touched. Munk removed the center, marked O, Fig. 1, from the occipital lobe in both hemispheres. He says: "In from three to five days after the operation there was nothing abnormal in the hearing, smell, taste, movements, or sensations of the animal, only in the territory of sight was there any peculiar disturbance. The animal moved about the room or garden with perfect freedom, he did not strike against any object, and if things were put in his way he uniformly avoided them. There was, however, a striking difference. He regarded very coldly those men whom he used to greet most affectionately. He was indifferent to the dogs he always played with before. However hungry and thirsty he was, he did not go to the corner of the room where his food was, as formerly; and, if food and water were placed directly in his path, he would go round and round them without noticing them at all. The sight of the whip, which used to drive him into the corner, did not now produce the slightest effect. He used to raise his paw when your hand was moved before his eyes; now he will not lift it, however much the hand is moved."

From these and similar facts, Munk draws a conclusion which, to say the least, seems a trifle broader than the premises. He says: "There can be no doubt about the meaning of these observations. By the extirpation of this portion of the brain, the dog has become soul-blind. He has lost the sight-perceptions which he once possessed; his recollection-images of things seen before are gone, so that he can not recognize what he sees—still he sees; sensations of sight come to his consciousness, so that he receives a knowledge of the existence, form, and position of external objects, but he does not know what these mean—this knowledge must be learned anew. The dog has been set back to his earliest years, to the time when he first opened his eyes; he must learn to see."

As removal of this part of the occipital lobe causes soul-blindness, so a removal of a portion of the temporo-sphenoidal lobe causes soul-deafness.

Until lately the defenders of localization seemed to be justified in believing that something had been established as to a motor area of the brain; they might well feel, also, that a beginning had been made toward connecting certain parts of the cortex with specific sensations and might hope that further experiment would remove, in considerable degree, the present disagreements. The doctrine of localization, both as a whole and in detail, has, however, received a severe blow at the hands of Professor Goltz, of Strasburg. In 1876 this distinguished experimenter began the publication of a series of papers in "Pflüger's Archiv fur die gesammte Physiologic" In September, 1881, this series was finished and published by itself. Professor Goltz fearlessly declares that he has overthrown all the conclusions about division of the brain into motor and sensory areas, and brought back our knowledge of brain-function to the old view of Flourens, viz., that the cerebrum is one organ, having one function throughout. Professor Goltz's experiments were confined entirely to dogs, and their chief significance is due to the fact that he was able to keep the animal alive after removal of larger masses of the cerebrum than any other experimenter. These experiments seem to have been abundant and thorough. As a result of them, Professor Goltz concludes that the degree of the disturbance of function from destruction of brain-substance depends upon the quantity removed, not upon the location of the lesion. He says, most positively, that "no extirpation of the motor centers, or of any other portion of gray matter, could cause permanent paralysis to any muscle in the body." His emphasis is upon the word permanent. Very many of the effects insisted on by advocates of localization did follow these brain-lesions, but the effects were not lasting, and they did not depend upon removal of specific portions of the substance. Blindness follows destruction of the angular gyrus, but it is temporary; the animal will see again in time. Professor Goltz admits a compensation of brain-functions, so that remaining portions of the organ may take up the work of a part destroyed; but this is not at all the compensation talked of by the supporters of localization. Their compensation requires that the additional work shall be done by the corresponding part in the other hemisphere. Professor Goltz destroys the angular gyrus on—sides and still his dog sees. Professor Goltz believes, however, that there are some permanent disturbances resulting from brain-lesions, such as "a certain dullness in the sensation of touch, a diminished power of vision, everything appearing cloudy to the eye, and some awkwardness in the movements." It will disturb the opponents of vivisection to know that Professor Goltz sacrificed fifty-one dogs in attempting to determine the effects of lesion in both hemispheres. He found that what happened only to one side of the body, and that the opposite, if one hemisphere was dealt with, happened on both sides of the body if both cerebral masses were affected. In all these cases mental weakness increased with the increasing quantity of matter removed. When considerable portions were taken away on both sides, the dog presented a demented appearance, very plain to be recognized. He could walk, run, see, hear, smell, and taste, but he was imbecile in all these activities.

It was not to be supposed that so fierce an attack upon localization would go unchallenged. Professor Goltz certainly did not shrink from the demand to make good his assertions. He took up basket and dog and journeyed from Strasburg to London. Here, in 1881, he came before the physiological section of the International Medical Congress, opened his basket, and, taking out the dog, placed him over against the almost equally celebrated monkey of Professor Ferrier. The dog walked, ran, saw, heard, tasted, and smelt; this was as his master desired, yet he should not have behaved so, for he had lost almost all the centers for these respective functions. Large territories in both hemispheres were gone. He was clearly weak-minded, but, on the whole, he was not the kind of dog believed in by the advocates of localization. Professor Yeo even went so far as to say before the section, "I candidly admit that, should the entire of the so-called motor centers prove to be destroyed in this case, Professor Goltz has succeeded in completely changing my views on cerebral localization."

After the dog there was the monkey. Professor Ferrier introduced him. He had lost the motor zone in the left hemisphere seven months previously. Of him Professor Ferrier said: "As to any independent volitional action of the right arm and leg we have not seen a single indication since the operation was made. The animal is, in every other respect, perfectly well, and as to its tactile sensibility there is not the slightest sign of impairment." It is pleasing to know that, as the dog had been faithful to his master, so the monkey was true to his friend; he displayed the proper amount of paralysis on the opposite side of the body. In this connection Dr. Ireland's words are suggested. He says, "It is to be hoped, in the interest of the martyrs of cerebral physiology, that definite results will be attained as quickly and with as little suffering as possible."

Whatever may or may not be accomplished in finding definite centers of the brain for special movements and sensations, one thing stands fast—the cerebral hemispheres are the sole organs of the higher intellectual manifestations. From the time of Flourens, experiment has again and again shown that complete removal of the hemispheres is followed by stupor. All that resembles intellect disappears—spontaneous volition is gone. The animal remains buried in the profoundest repose. He originates no action. A low form of sensation and a low form of volition may remain. The animal when pinched gives evidence of pain; when set in motion, continues the motion till stopped by external hindrances. A frog deprived of the cerebrum and thrown into the water will swim until land is reached; a pigeon thrown into the air will fly until stopped by an obstacle or by exhaustion. It is to be particularly observed that the motions of these animals are strictly normal, i. e., pure motions; they are no longer connected with the higher power that once controlled them. They continue because they must continue.

A writer in the "Journal of Anatomy," of Paris, 1870-'71, gives a clear account of this matter. He says: "As a summary, alike in the inferior and superior animals, the removal of the hemispheres does not cause to disappear any of the movements that previously existed, but these movements assume certain peculiar characters. They are regular, for no psychical influence intervenes to modify them. They take place inevitably after excitation. The physiologist can, at will, in an animal deprived of the brain, determine such and such an act, limit it, arrest it. He can predict all the movements that will take place as certainly as a chemist knows in advance the reaction he will obtain from mixing certain bodies.

Pathology confirms our conclusion respecting these higher functions of the cerebrum. Loss of cerebral substance, in man, is followed by a weakening of the intellectual powers. They make a very childish mistake who attempt to deal with the physiological materialism of our day by citing the American crow-bar case, or any number of cases of brain-loss unaccompanied by marked intellectual enfeeblement. It is equally puerile to cite instances of small brains with great intellectual power. In the first place, these small brains may be of superior quality, as small muscles often are; or, in the second place, the boasted greatness of mind may be anything and everything but greatness of mind. Learning of a very extensive kind may coexist with small mental caliber. A monkey is shrewd and quick, and cunning and smart a parrot is learned, up in a variety of languages, speaking, as many human parrots do, some French, some Italian, some Spanish: there is no great-mindedness here.

Proof from size of the brain is, on the whole, reliable. There is, in general, a remarkable decrease in weight corresponding to the intellectual enfeeblement. Many idiots between the ages of sixteen, forty, and fifty years, have shown brains weighing 193/4, 253/4, and 221/2 ounces. There is on record the case of a deaf-mute idiot, forty-three years old, who showed an idiocy of the lowest kind, yet his brain weighed over forty-eight ounces. Such cases are not to overthrow an induction based upon a large majority of opposing instances.

It remains for the succeeding paper to consider the question propounded by the physiology of to-day, respecting the kind of relation which holds between the brain and consciousness. If we were to accept the judgment of the younger leading physicians, we should believe that modern Physiology had answered her own question. A distinguished physician of my city says, in his published "Lectures on Physiology": "The so-called voluntary movements are only the final responses to impressions made upon the special senses at the time or in the past. The highest expressions of the intellect of man may be resolved into the more perfect transmutations of outside forces, by machinery made more perfect by original constitution or by labor."

Without believing that such a correlation between brain and consciousness as is here asserted can be rationally accepted, there are, as I think, two general conclusions which may be drawn with certainty:

A constant relation obtains between nerve-matter and those manifestations which are usually said to belong to the soul. This relation is so important, so constant, as to determine in large measure the intellectual and moral well-being of every individual.

The origination of our states of consciousness, their character and conduct, are conditioned by physical processes antecedently occurring in the brain.