by Carl Sagan
The ability to connect auditory with visual stimuli is also localized in the temporal lobe. Lesions in the temporal lobe can result in a form of aphasia, the inability to recognize spoken words. It is remarkable and significant that brain-damaged patients can be completely competent in spoken language and entirely incompetent in written language, or vice versa. They may be able to write but unable to read; able to read numbers but not letters; able to name objects but not colors. There is in the neocortex a striking separation of function, which is contrary to such common-sense notions as that reading and writing, or recognizing words and numbers, are very similar activities. There are also as yet unconfirmed reports of brain damage that results only in the inability to understand the passive voice or prepositional phrases or possessive constructions. (Perhaps the locale of the subjunctive mood will one day be found. Will Latins turn out to be extravagantly endowed and English-speaking peoples significantly short-changed in this minor piece of brain anatomy?) Various abstractions, including the “parts of speech” in grammar, seem, astonishingly, to be wired into specific regions of the brain.
Face described by a patient as an apple. (Otherwise: apple described by a physician as a face.) After Teuber.
In one case, a temporal-lobe lesion resulted in a surprising impairment in the patient’s perception of faces, even the faces of his immediate family. Presented with the face on this page, he described it as “possibly” being an apple. Asked to justify this interpretation, he identified the mouth as a cut in the apple, the nose as the stem of the apple folded back on its surface, and the eyes as two worm holes. The same patient was perfectly able to recognize sketches of houses and other inanimate objects. A wide range of experiments shows that lesions in the right temporal lobe produce amnesia for certain types of nonverbal material, while lesions in the left temporal lobe produce a characteristic loss of memory for language.
Our ability to read and make maps, to orient ourselves spatially in three dimensions and to use the appropriate symbols—all of which are probably involved in the development if not the use of language—are severely impaired by lesions in the parietal lobes, near the top of the head. One soldier who suffered a massive wartime penetration of the parietal lobe was for a full year unable to orient his feet into his slippers, much less find his bed in the hospital ward. He nevertheless eventually experienced an almost complete recovery.
A lesion of the angular gyrus of the neocortex, in the parietal lobe, results in alexia, the inability to recognize the printed word. The parietal lobe appears to be involved in all human symbolic language and, of all the brain lesions, a lesion in the parietal lobe causes the greatest decline in intelligence as measured by activities in everyday life.
Chief among the neocortical abstractions are the human symbolic languages, particularly reading and writing and mathematics. These seem to require cooperative activities of the temporal, parietal and frontal lobes, and perhaps the occipital as well. Not all symbolic languages are neocortical however; bees—without a hint of a neocortex—have an elaborate dance language, first elucidated by the Austrian entomologist Karl von Frisch, by which they communicate information on the distance and direction of available food. It is an exaggerated gestural language, imitative of the motions bees in fact perform when finding food—as if we were to make a few steps towards the refrigerator, point and rub our bellies, with our tongues lolling out all the while. But the vocabularies of such languages are extremely limited, perhaps only a few dozen words. The kind of learning that human youngsters experience during their long childhood seems almost exclusively a neocortical function.
While most olfactory processing is in the limbic system, some occurs in the neocortex. The same division of function seems to apply to memory. A principal part of the limbic system, other than the olfactory cortex, is, as we have mentioned, the hippocampal cortex. When the olfactory cortex is excised, animals can still smell, although with a much lower efficiency. This is another demonstration of the redundancy of brain function. There is some evidence that, in contemporary humans, the short-term memory of smell resides in the hippocampus. The original function of the hippocampus may have been exclusively the short-term memory of smell, useful in, for example, tracking prey or finding the opposite sex. But a bilateral hippocampal lesion in humans results, as in the case of H. M., in a profound impairment of all varieties of short-term memory. Patients with such lesions literally cannot remember from one moment to the next. Clearly, both hippocampus and frontal lobes are involved in human short-term memory.
One of the many interesting implications of this is that short-term and long-term memory reside mostly in different parts of the brain. Classical conditioning—the ability of Pavlov’s dogs to salivate when the bells rang—seems to be located in the limbic system. This is long-term memory, but of a very restricted kind. The sophisticated sort of human long-term memory is situated in the neocortex, which is consistent with the human ability to think ahead. As we grow old, we sometimes forget what has just been said to us while retaining vivid and accurate recollections of events in our childhood. In such cases, little seems to be wrong with either our short-term or our long-term memories; the problem is the connection between the two—we have great difficulty in accessing new material into the long-term memory. Penfield believed that this lost accessing ability arises from an inadequate blood supply to the hippocampus in old age—because of arteriosclerosis or other physical disabilities. Thus elderly people—and ones not so elderly—may have serious impairments in accessing short-term memory while being otherwise perfectly alert and intellectually keen.* This phenomenon also shows a clear-cut distinction between short-term and long-term memory, consistent with their localization in different parts of the brain. Waitresses in short-order restaurants can remember an impressive amount of information, which they accurately transmit to the kitchen. But an hour later, the information has vanished utterly. It was put into the short-term memory only, and no effort was made to access it into the long-term memory.
The mechanics of recall can be complex. A common experience is that we know something is in our long-term memory—a word, a name, a face, an experience—but find ourselves unable to call it up. No matter how hard we try, the memory resists retrieval. But if we think sideways at it, recalling some slightly related or peripheral item, it often follows unbidden. (Human vision is also a little like this. When we look directly at a faint object—a star, say—we are using the fovea, the part of the retina with the greatest acuity and the greatest density of cells called cones. But when we avert our vision slightly—when, in a manner of speaking, we look sideways at the object—we bring into play the cells called rods, which are sensitive to feeble illumination and so able to perceive the faint star.) It would be interesting to know why thinking sideways improves memory retrieval; it may be merely associating to the memory trace by a different neural pathway. But it does not suggest particularly efficient brain engineering.
We have all had the experience of awakening with a particularly vivid, chilling, insightful or otherwise memorable dream clearly in mind; saying to ourselves, “I’ll certainly remember this dream in the morning”; and the next day having not the foggiest notion about the content of the dream or, at best, a vague trace of an emotion tone. On the other hand, if I am sufficiently exercised about the dream to awaken my wife in the middle of the night and tell her about it, I have no difficulty remembering its contents unaided in the morning. Likewise, if I take the trouble of writing the dream down, when I awaken the next morning I can remember the dream perfectly well without referring to my notes. The same thing is true of, for example, remembering a telephone number. If I am told a number and merely think about it, I am likely to forget it or transpose some of the digits. If I repeat the numbers out loud or write them down, I can remember them quite well. This surely means that there is a part of our brain which remembers sounds and images, but not thoughts. I wonder if that sort of memory arose before we had very many thoughts—when it w
as important to remember the hiss of an attacking reptile or the shadow of a plummeting hawk, but not our own occasional philosophical reflections.
ON HUMAN NATURE
Despite the intriguing localization of function in the triune brain model, it is, I stress again, an oversimplification to insist upon perfect separation of function. Human ritual and emotional behavior are certainly influenced strongly by neocortical abstract reasoning; analytical demonstrations of the validity of purely religious beliefs have been proffered, and there are philosophical justifications for hierarchical behavior, such as Thomas Hobbes’ “demonstration” of the divine right of kings. Likewise, animals that are not human—and in fact even some animals that are not primates—seem to show glimmerings of analytical abilities. I certainly have such an impression about dolphins, as I described in my book The Cosmic Connection.
Mosaic II by M. C. Escher.
Nevertheless, while bearing these caveats in mind, it seems a useful first approximation to consider the ritualistic and hierarchical aspects of our lives to be influenced strongly by the R-complex and shared with our reptilian forebears; the altruistic, emotional and religious aspects of our lives to be localized to a significant extent in the limbic system and shared with our nonprimate mammalian forebears (and perhaps the birds); and reason to be a function of the neocortex, shared to some extent with the higher primates and such cetaceans as dolphins and whales. While ritual, emotion and reasoning are all significant aspects of human nature, the most nearly unique human characteristic is the ability to associate abstractly and to reason. Curiosity and the urge to solve problems are the emotional hallmarks of our species; and the most characteristically human activities are mathematics, science, technology, music and the arts—a somewhat broader range of subjects than is usually included under the “humanities.” Indeed, in its common usage this very word seems to reflect a peculiar narrowness of vision about what is human. Mathematics is as much a “humanity” as poetry. Whales and elephants may be as “humane” as humans.
The triune-brain model derives from studies of comparative neuroanatomy and behavior. But honest introspection is not unknown in the human species, and if the triune-brain model is correct, we would expect some hint of it in the history of human self-knowledge. The most widely known hypothesis that is at least reminiscent of the triune brain is Sigmund Freud’s division of the human psyche into id, ego and superego. The aggressive and sexual aspects of the R-complex correspond satisfyingly to the Freudian description of the id (Latin for “it”—i.e., the beast-like aspect of our natures); but, so far as I know, Freud did not in his description of the id lay great stress on the ritual or social-hierarchy aspects of the R-complex. He did describe emotions as an ego function—in particular the “oceanic experience,” which is the Freudian equivalent of the religious epiphany. However, the superego is not depicted primarily as the site of abstract reasoning but rather as the internalizer of societal and parental strictures, which in the triune brain we might suspect to be more a function of the R-complex. Thus I would have to describe the psychoanalytic tripartite mind as only weakly in accord with the triune-brain model.
Perhaps a better metaphor is Freud’s division of the mind into the conscious; the preconscious, which is latent but capable of being tapped; and the unconscious, which is repressed or otherwise unavailable. It was the tension that exists among the components of the psyche that Freud had in mind when he said of man that “his capacity for neurosis would merely be the obverse of his capacity for cultural development.” He called the unconscious functions “primary processes.”
A superior agreement is found in the metaphor for the human psyche in the Platonic dialogue Phaedrus. Socrates likens the human soul to a chariot drawn by two horses—one black, one white—pulling in different directions and weakly controlled by a charioteer. The metaphor of the chariot itself is remarkably similar to MacLean’s neural chassis; the two horses, to the R-complex and the limbic cortex; and the charioteer barely in control of the careening chariot and horses, to the neocortex. In yet another metaphor, Freud described the ego as the rider of an unruly horse. Both the Freudian and the Platonic metaphors emphasize the considerable independence of and tension among the constituent parts of the psyche, a point that characterizes the human condition and to which we will return. Because of the neuroanatomical connections between the three components, the triune brain must itself, like the Phaedrus chariot, be a metaphor; but it may prove to be a metaphor of great utility and depth.
* This rule on the relative parental concern of mammals and reptiles is, however, by no means without exceptions. Nile crocodile mothers carefully put their fresh hatchlings in their mouths and carry them to the comparative safety of the river waters; while Serengeti male lions will, upon newly dominating a pride, destroy all the resident cubs. But on the whole, mammals show a strikingly greater degree of parental care than do reptiles. The distinction may have been even more striking one hundred million years ago.
* The heads and bodies of anthropods can briefly function without each other very nicely. A female praying mantis will often respond to earnest courting by decapitating her suitor. While this would be viewed as unsociable among humans, it is not so among insects: extirpation of the brain removes sexual inhibitions and encourages what is left of the male to mate. Afterwards, the female completes her celebratory repast, dining, of course, alone. Perhaps this represents a cautionary lesson against excessive sexual repression.
* Indeed, there is a range of medical evidence on the connection between blood supply and intellectual abilities. It has long been known that patients deprived of oxygen for some minutes can experience permanent and serious mental impairment. Operations to remove material from clogged carotid arteries in an effort to prevent stroke yield unexpected benefits. According to one study, six weeks after such operations, the patients showed an average increase in IQ of eighteen points, a substantial improvement. And there has been some speculation that immersion in hyperbaric oxygen—that is, oxygen under high pressure—can raise the intelligence of infants.
4
EDEN
AS A METAPHOR:
THE EVOLUTION
OF MAN
Then wilt thou not be loth
To leave this Paradise, but shalt possess
A Paradise within thee, happier far …
They hand in hand with wandering steps and slow
Through Eden took their solitary way.
JOHN MILTON
Paradise Lost
Why didst thou leave the trodden paths of men
Too soon, and with weak hands though mighty heart
Dare the unpastured dragon in his den?
Defenseless as thou wert, oh, where was then
Wisdom, the mirrored shield …?
PERCY BYSSHE SHELLEY
Adonais
OR THEIR surface area, insects weigh very little. A beetle, falling from a high altitude, quickly achieves terminal velocity: air resistance prevents it from falling very fast, and, after alighting on the ground, it will walk away, apparently none the worse for the experience. The same is true of small mammals—squirrels, say. A mouse can be dropped down a thousand-foot mine shaft and, if the ground is soft, will arrive dazed but essentially unhurt. In contrast, human beings are characteristically maimed or killed by any fall of more than a few dozen feet: because of our size, we weigh too much for our surface area. Therefore our arboreal ancestors had to pay attention. Any error in brachiating from branch to branch could be fatal. Every leap was an opportunity for evolution. Powerful selective forces were at work to evolve organisms with grace and agility, accurate binocular vision, versatile manipulative abilities, superb eye-hand coordination, and an intuitive grasp of Newtonian gravitation. But each of these skills required significant advances in the evolution of the brains and particularly the neocortices of our ancestors. Human intelligence is fundamentally indebted to the millions of years our ancestors spent aloft in the trees.
And after we returned to the savannahs and abandoned the trees, did we long for those great graceful leaps and ecstatic moments of weightlessness in the shafts of sunlight of the forest roof? Is the startle reflex of human infants today to prevent falling from the treetops? Are our nighttime dreams of flying and our daytime passion for flight, as exemplified in the lives of Leonardo da Vinci or Konstantin Tsiolkovskii, nostalgic reminiscences of those days gone by in the branches of the high forest?*
Other mammals, even other nonprimate and non-cetacean mammals, have neocortices. But in the evolutionary line leading to man, when was the first large-scale development of the neocortex? While none of our simian ancestors are still around, this question can nevertheless be answered or at least approached: we can examine fossil skulls. In humans, in apes and monkeys, and in other mammals, the brain volume almost fills the skull. This is not true, for example, in fish. Thus by taking a cast of a skull, we can determine what is called the endocranial volume of our immediate ancestors and collateral relatives and can make some rough estimates of their brain volumes.
The question of who was and who was not an ancestor of man is still being hotly debated by the paleontologists, and hardly a year goes by without the discovery of some fossil of remarkably human aspect much older than anyone had previously thought possible. What seems certain is that about five million years ago, there was an abundance of apelike animals, the gracile Australopithecines, who walked on two feet and had brain volumes of about 500 cubic centimeters, some 100 cc more than the brain of a modern chimpanzee. With this evidence, paleontologists have deduced that “bipedalism preceded encephalization,” by which they mean that our ancestors walked on two legs before they evolved big brains.
