Spring 2022 Mid-Term Exam Solution On Biology – Human Brain: California Institute Of Technology
Explain in detail how the cortical regions and networks of the human brain implements distinct cognitions such as face recognition, social cognition, language, and music.
The human brain is neither the largest brain in the animal kingdom nor is it the largest by comparison with body size. An elephant's brain is about four times heavier than a man's, and many monkeys have a body-brain ratio of 20:1 whereas the human ratio is nearer 50:1. Nevertheless, even though mankind has been judge and jury in dubbing itself sapiens, in reference to itself as the pinnacle of evolution, in distinguishing between man and the animals, there is nothing quite like the human brain. (somewhat equivalent to its bulk representing 2% of the human body, but it receives much fuller treatment in The Mind, a companion volume to The Body.)
As ever there are innumerable conundrums. Brain bulk is undoubtedly related to brain ability. The animals and mankind's evolutionary ancestors bear out this generalization; but, within the species man, brain bulk seems strangely unimportant. One brain maybe twice the size of another without showing any apparent difference in ability. The largest human brains, twice average size, are those of idiots. Machines can be made to operate far faster than any brain and mathematical problems of mind-boggling proportions are mechanically calculated before a brain can absorb them, let alone provide an answer; but no machine has a memory with a capacity in the same class as the human brain. Many humans say that they never forget a face, one more configuration of the same old theme of two eyes, a nose and a mouth, and yet the memory of one new telephone number spoken by that face is instantly elusive. Surely a face is more complex than a small finite series of figures? Louis Pasteur suffered a cerebral haemorrhage which caused moderate one-sided paralysis but did not prevent him from doing some of his best work. At post-mortem, years later, the injury was found to be so extensive that he was said to have been living on half a brain. The brain is undoubtedly sensitive to wrong cuts by the surgeon's knife, and gloved fists cause at least a dozen deaths a year in the boxing ring, but Phineas Gage achieved fame when he had 131 lbs. (6 kgs.) tamping iron blasted into his skull and his brain. He lost consciousness briefly, walked to the surgeon's office and recovered physically. Admittedly he became a drunkard and lost jobs thereafter, but he still had the wit to sell his skeleton, cash in advance, to several different medical schools, and the pushing of an iron bar with the diameter of an old English penny through his the delicate and sensitive brain, demonstrated remarkable cerebral resilience. (In fact, he was to die only twelve years after the accident when aged thirty-seven.)
Wars and surgery have, since Mr Gage's accident a hundred years ago, amply demonstrated man's ability to lose substantial portions of the brain without undue suffering. In 1935 a London conference was informed of the stabilising effect upon a chimpanzee caused by the surgical severing of much of its frontal lobes. In 1936 a Paris conference was yet more fascinated to hear of a similar operation upon a human being. Since then thousands of the mentally unbalanced have suffered the deliberate severance of the foremost part of each cerebral hemisphere, and many have benefited psychologically from this deliberate destruction.
Finally, although the brain is anatomically symmetrical, and although many of its functions are entirely bilateral, some others are quite one-sided. One half is called dominant and its loss is the greater loss. Had Pasteur suffered his haemorrhage on the other side there would have been no question of good work; either death or a pathetic existence would have followed. For those parts of the brain which are bilateral and unaffected by dominance, extra complexity is caused by the fact that each half tends to organise the opposite half of the body. Neither dominance nor this crossing over simplifies comprehension of man's fantastic neuro-logical anatomy.
Anatomy The human brain is a soft lump of some 14 thousand million cells, it weighs slightly more than 3 lbs. (1-4 kgs.) and is so full of water that it tends to slump like a blancmange if placed without support on a firm surface. Predominantly its structure is the huge pair of cerebral hemispheres sit on top of 10-15% of the remaining tissue. This smaller fraction is much more varied, being composed of several entirely different portions of brain tissue, and fits into such space as exists in the cranium. between the top end of the spinal cord and the huge convoluted growth of the two cerebral hemispheres. The complexity has been accentuated by man's upright stance. From being a tetrapod creature whose spine and head were both horizontal, to becoming a bipedal creature whose spine was vertical and whose head continued to look horizontally, the central nervous system of the spinal cord plus brain has had to suffer this 90' flexure. Small wonder there is confusion at the point where the spinal column joins the brain.
In fish, everything is simpler. In even less advanced creatures the central nervous system is merely a tube running the length of the body which swells modestly at the front end where sense organs are predominantly situated, and where sensory impressions are predominantly received. By the time of the fishes, the brain is in three parts, three separate swellings each linked to a sense organ. The front swelling receives impulses from the olfactory organs, the middle one from the eyes, and the back one from the organ of balance, later to be the ear as well. These three, the fore-brain, mid-brain and hind-brain, persist with their three roles in more advanced forms, but the expanding fore-brain is to take over all co-ordination from the other two.
It may be an over-simplification to say so, but if one part of the brain is to be expanded it had better be the front part. To enlarge elsewhere is plainly a greater problem. Simplification or not, this is what happened. The paired lump of the fore-brain became increasingly important as the centre of sensory correlation. To react suitably is not to react according to stimuli from one sense organ, but to sift through all available information from the exterior in one central control. By the time of mammals the fore-brain was dominant in this regard, and the importance of the mammalian sense of smell was intimately bound up with the swelling olfactory lobes. When the primates were evolved smell was less important, but the fore-brain was by then firmly established as the centre for interpretation and correlation. Therefore, again to simplify, but with good reason, if greater correlation and association are both required and advantageous, the fore-brain needs to be expanded still further. Besides, unlike the mid-brain and hind-brain, its expansion continues as before to present less of a space problem anatomically.
During evolution the brain tissue doing all this correlating moved increasingly towards the outer surface of the fore-brain. Called the pallium, or cloak, or cortex, this correlating tissue made up the most advanced portion of the brain. Its area expanded when fissures and grooves were formed, as in a walnut, and a brain should be judged for its level of advancement more on the area of its cortex than on the body of brain tissue. In human beings, the cloak of the cortex is only about one-eighth of an inch (3 mm.) thick or less, but its area, enlarged by all the folds, convolutions, gyri and sulci, is 400 sq. ins. (2,580 sq. cms.). Beneath that cortex lies the maze of fibres and pathways which lead to and from the thin crucial skin of cortical tissue.
The human brain is the most exaggerated form of this development. The original fore-brain, now so heavily grooved and with four paired and major lobes of its own, is over five-sixths of the human brain. Known as the cerebrum, with its two halves being the cerebral hemispheres, it performs all the higher and highest roles of human activity, the correlation of sensory impulses, of memory, of thought. The remainder of the human brain is tucked underneath the huge mushroom of the cerebrum, and it also consists of four parts. These are the:
Midbrain. Short and narrow. Still linked with vision in that it receives impulses from the retina, and still acts as a centre for visual reflexes, but it mainly consists of fibres going from elsewhere to elsewhere.
Pons- Just below the midbrain. Dominantly tracts of fibres.
Medulla oblongata - The continuation of the spinal cord where it first begins to broaden. Mainly tracts. Contains much of the crossing-over of fibres causing the left cerebral hemisphere's association with the body's right-hand side, and vice versa.
Cerebellum - Lies behind the midbrain, pons and medulla, and below rear regions of cerebral hemispheres. It is also in the form of two hemispheres and, like the archaic hind brain, is associated with the inner ear. The main cerebellar activity is to assist muscular co-ordination. Victims of a poor cerebellum move jerkily and without accuracy.
The cerebrum Forgetting these four other parts, and returning again to the cerebral hemispheres, a further division into four is necessary because the four lobes of each hemisphere have distinct roles, far more distinct than the lobes themselves.
Frontal lobe The 'motor lobe'. Situated at the front of the head. The precise motor area, which controls muscular movement, is only a narrow band of cortex at the back of this lobe. The band moves from the side of each lobe up to the top and down into the central fissure between each hemisphere. Each region of the body is linked with a part of this band. Stimulation of any portion of the band will always cause a muscle, even a particular muscle, to twitch in response in the appropriate bodily region. Progressing along the band from each side of the brain to the centre, the bodily areas involved are the mouth, including speech (but more about speech later under left-sided dominance), the face, eye, neck, thumb, fingers, hand, wrist, elbow, shoulder, trunk, hip, knee, ankle and toes in that order. The amount of band involved is certainly not proportional to the size of the body controlled by it: the hands and fingers use as much of the band as the remainder of the arms plus the legs and trunk. Digital skill is correspondingly more advanced.
In front of this motor band lies the pre-motor area. Its stimulation does not ever lead to the twitching of a single muscle, but to a more co-ordinated movement of a whole area. Presumably, this area affords general control whereas the motor band affords greater precision. The remainder of the frontal cortex is not only far larger but far more mysterious. Called the silent zone. great quantities of it have sometimes been cut away or severed, as in tumour operations, to leave the patient remarkably unimpaired. A small nick in the motor cortex and there is immediate partial paralysis: a huge carving up of the silent cortex and, although personality is involved, notably mood, criticism, drive and concentration, the functional ability is scarcely modified.
Parietal lobe - Separated by a deep fissure from the frontal lobe, but possessing a band at its front border adjacent to the frontal motor band, this second band is sensory. It receives sensations of warmth, cold, touch and general movement from the body. The order along the band is virtually the same as the order for the motor band: the mouth, including taste, is low down on each side and the toes are in the central fissure.
Temporal lobe - The lobe nearest to the ears. Reasonably it is linked with hearing, and the lobe on each side receives impulses from both ears. The area actually linked with hearing, as with most lobe areas positively linked with any function, is quite small. There is believed to be an association between memory and this lobe.
Occipital lobe- The lobe nearest to the back of the head. Linked with vision. A small wound at the back of the head has often caused no harm other than total blindness. If only the visual cortex of the left lobe is destroyed, vision will still remain good for the right half of each eye's retina, and vice versa.