The Limbic System

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The limbic system appears to generate strong or particularly vivid emotions. This immediately suggests an additional perspective on the reptilian mind: it is not characterized by powerful passions and wrenching contradictions but rather by a dutiful and stolid acquiescence to whatever behavior its genes and brains dictate.

Electrical discharges in the limbic system sometimes result in symptoms similar to those of psychoses or those produced by psychedelic or hallucinogenic drugs. In fact, the sites of action of many psychotropic drugs are in the limbic system. Perhaps it controls exhilaration and awe and a variety of subtle emotions that we sometimes think of as uniquely human.

The "master gland," the pituitary, which influences other glands and dominates the human endocrine system, is an intimate part of the limbic region. The mood-altering qualities of endocrine imbalances give us an important hint about the connection of the limbic system with states of mind. There is a small almond-shaped inclusion in the limbic system called the amygdala which is deeply involved in both aggression and fear. Electrical stimulation of the amygdala in placid domestic animals can rouse them to almost unbelievable states of fear or frenzy. In one case, a house cat cowered in terror when presented with a small white mouse. On the other hand, naturally ferocious animals, such as the lynx, become docile and tolerate being petted and handled when their amygdalas are extirpated. Malfunctions in the limbic system can produce rage, fear or sentimentality that have no apparent cause. Natural hyperstimulation may produce the same results: those suffering from such a malady find their feelings inexplicable and inappropriate; they may be considered mad.

At least some of the emotion-determining role of such limbic endocrine systems as the pituitary amygdala, and hypothalamus is provided by small hormonal proteins which they exude, and which affect other areas of the brain. Perhaps the best-known is the pituitary protein,. ACTH (adrenocorticotropic hormone), which can affect such diverse mental functions as visual retention, anxiety and attention span. Some small hypothalamic proteins have been identified tentatively in the third ventricle of the brain, which connects the hypothalamus with the thalamus, a region also within the limbic system. The stunning pictures on page 65, taken with an electron microscope, show two close-ups of action in the third ventricle. The diagram on page 73 may help clarify some of the brain anatomy just described.

There are reasons to think that the beginnings of altruistic behavior are in the limbic system. Indeed, with rare exceptions (chiefly the social insects), mammals and birds are the only organisms to devote substantial attention to the care of their young-an evolutionary development that, through the long period of plasticity which it permits, takes advantage of the large information-processing capability of the mammalian and primate brains. Love seems to be an invention of the mammals.*

* 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.

Much in animal behavior substantiates the notion that strong emotions evolved chiefly in mammals and to a lesser extent in birds. The attachment of domestic animals to humans is, I think, beyond question. The apparently sorrowful behavior of many mammalian mothers when their young are removed is well-known. One wonders just how far such emotions go. Do horses on occasion have glimmerings of patriotic fervor? Do dogs feel for humans something akin to religious ecstasy? What other strong or subtle emotions are felt by animals that do not communicate with us?

The oldest part of the limbic system is the olfactory cortex, which is related to smell, the haunting emotional quality of which is familiar to most humans. A major component of our ability to remember and recall is localized in the hippocampus, a structure within the limbic system. The connection is clearly shown by the profound memory impairment that results from lesions of the hippocampus. In one famous case, H. M., a patient with a long history of seizures and convulsions, was subjected to a bilateral extirpation of the entire region about the hippocampus in a successful attempt to reduce their frequency and severity. He immediately became amnesic. He retained good perceptual skills, was able to learn new motor skills and experienced some perceptual learning but essentially forgot everything more than a few hours old. His own comment was "Every day is alone in itself-whatever enjoyment I've had and whatever sorrow I've had." He described his life as a continuous extension of the feeling of disorientation many of us have upon awakening from a dream, when we have great difficulty remembering what has just happened. Remarkably enough, despite this severe impairment, his IQ improved after his hippocampectomy. He apparently could detect smells but had difficulty identifying by name the source of the smell. He also exhibited an apparent total disinterest in sexual activity.

In another case, a young American airman was injured in a mock duel with another serviceman, when a miniature fencing foil was plunged into his right nostril, puncturing a small part of the limbic system immediately above. This resulted in a severe impairment of memory, similar to but not so severe as H. M.'s; a wide range of his perceptual and intellectual abilities was unaffected. His memory impairment was particularly noticeable with verbal material. In addition, the accident seems to have rendered him both impotent and unresponsive to pain. He once walked barefoot on the sun-heated metal deck of a cruise ship, without realizing that his feet were being badly burned until his fellow passengers complained of the uncomfortable odor of charring flesh. On his own, he was aware of neither the pain nor the smell.

From such cases, it seems apparent that so complex a mammalian activity as sex is controlled simultaneously by all three components of the triune brain-the R-complex, the limbic system and the neocortex. (We have already mentioned the involvement of the R-complex and the limbic system in sexual activity. Evidence for involvement of the neocortex can be easily obtained by introspection.)

One segment of the old limbic system is devoted to oral and gustatory functions; another, to sexual functions. The connection of sex with smell is very ancient, and is highly developed in insects-a circumstance that offers insight into both the importance and the disadvantages of reliance on smell in our remote ancestors.

I once witnessed an experiment in which the head of a green bottle fly was connected by a very thin wire to an oscilloscope that displayed, in a kind of graph, any electrical impulses produced by the fly's olfactory system. (The fly's head had only recently been severed from its body-in order to gain access to the olfactory apparatus-and was still in many respects functional.*) The experimenters wafted a wide variety of odors in front of it, including obnoxious and irritating gases such as ammonia, with no discernible effects. The line traced out on the oscilloscope screen was absolutely flat and horizontal. Then a tiny quantity of the sex attractant released by the female of the species was waved before the severed head, and an enormous vertical spike obligingly appeared on the oscilloscope screen. The bottle fly could smell almost nothing except the female sex attractant. But that molecule he could smell exceedingly well.

* 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.

Such olfactory specialization is quite common in insects. The male silkworm moth is able to detect the female's sex attractant molecule if only about forty molecules per second reach its feathery antennae. A single female silkworm moth need release only a hundredth of a microgram of sex attractant per second to attract every male silkworm in a volume of about a cubic mile. That is why there are silkworms.

Perhaps the most curious exploitation of the reliance on smell to find a mate and continue the species is found in a South African beetle, which burrows into the ground during the winter. In the spring, as the ground thaws, the beetles emerge, but the male beetles groggily disinter themselves a few weeks before the females do. In this same region of South Africa, a species of orchid has evolved which gives off an aroma identical to the sex attractant of the female beetle. In fact, orchid and beetle evolution have produced essentially the same molecule. The male beetles turn out to be exceedingly nearsighted; and the orchids have evolved a configuration of their petals that, to a myopic beetle, resembles the female in a receptive sexual posture. The male beetles enjoy several weeks of orgiastic ecstasy among the orchids, and when eventually the females emerge from the ground, we can imagine a great deal of wounded pride and righteous indignation. Meanwhile the orchids have been successfully cross-pollinated by the amorous male beetles, who, now properly abashed, do their best to continue the beetle species; and both organisms survive. (Incidentally, it is in the interest of the orchids not to be too consummately attractive; if the beetles fail to reproduce themselves, the orchids are in trouble.) We thus discover one limitation to purely olfactory sexual stimuli. Another is that since every female beetle produces the same sex attractant, it is not easy for a male beetle to fall in love with the lady insect of his heart's desire. While male insects may display themselves to attract a female, or-as with stag beetles-engage in mandible-to-mandible combat with the female as the prize, the central role of the female sex attractant in mating seems to reduce the extent of sexual selection among the insects.

Other methods of finding a mate have been developed in reptiles, birds and mammals. But the connection of sex with smell is still apparent neuro-anatomically in higher animals as well as anecdotally in human experience. I sometimes wonder if deodorants, particularly "feminine" deodorants, are an attempt to disguise sexual stimuli and keep our minds on something else.

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