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FRONTIERS OF BIOLOGY also are clearly under the control of two independent half-brains, without fusion of individual inputs. Accordingly, in man, each hemisphere is capable of independent conscious awareness. Surgery leaves "split-brain" people with two separate minds, indicating that the higher functions are not the exclusive domain of a major hemisphere. Such studies offer great promise of revealing the highest orders of organization of the brain. BEHAVIOR Scientists of diverse backgrounds investigate the ways animals behave toward each other and toward their environment. These "behavioral biologists" include zoologists, physiologists, psychologists, psychiatrists, and anthropologists, among others. Some observe whole animals; others, small parts of their nervous systems. Some study the physiological mechanisms of behavior in adults; others, the development of behavior in embryos. Some, concerned with specific problems, use any particularly appropriate animal as a tool; others are fascinated by particular kinds of animals. Some approach animal investigation primarily as a means of understanding human behavior; others are not primarily motivated by the potential social utility of their work. In sum, however, their interrelated work illuminates in diverse ways the behavior of organisms. This field of endeavor is in its infancy. Nevertheless, starting from dif- ferent points of view, studying quite different animals, formulating different problems, a coherent picture is emerging of the way in which behavior has historically evolved and the way it develops in individuals, and of the physiological mechanisms that make behavior possible. Clearly, this en- deavor shades into the totality of experience in observational and experi- mental psychology. So large is the field and so disparate the parts that no adequate summary is possible here. We can, however, offer a small sam- pling of the kinds of studies that, in recent times, have contributed to growing understanding of the physiological bases for behavior. Evolution, Inheritance, and the Development of Behavior The biologist assumes that behavior is not an accidental feature of the existence of a species- that it has, itself, been subject to selection in the evolutionary process. In some instances, unique behavioral patterns clearly relate to the ecological niche in which a species has lodged, as, for example, various aspects of the behavior of the kittiwake, a cliff-nesting gull, which 109
110 THE LIFE SCIENCES are shared with the swallow-tailed gull of the Galapagos, thousands of miles distant. Both of these birds, however, engage specifically in cliff- nesting. Not all aspects of behavior can be so identified, and indeed some may be irrelevant, but surely the remarkable formalized courtship and mating patterns of many species have served to preserve their genetic dis- tinctiveness and to assure their continuity. There can be little doubt that behavioral qualities can be treated as genetic traits and followed in longi- tudinal breeding experiments. Few are simple Mendelian traits governed by single genes. Nevertheless, maze performance in rats, aggressiveness in many species, geotropic responses in flies, can all be manipulated by breed- ing, although the chemical functions of the genes involved have not been identified. At the same time, the genetic contribution to behavior can be modified by individual experience; e.g., producing mild behavior in geneti- cally aggressive animals by frequent fondling during their early lives. However, assorted allegations of causality with respect to behavior asso- ciated with genes known to affect specific biochemical functions have, in the main, proved instead to be associated only by chromosomal linkage with genes of unknown biochemical function. A clear correlate of general behavior with genetic endowment is illustrated by individuals afflicted with Down's syndrome, the result of trisomy (the relatively rare occurrence of three rather than two chromosomes) and by recent suggestions of excessive violence in some proportion of human males whose sex chromosomes are XYY. In the main, the nervous system is "prewired" in the sense that most of its connections are the consequence of genetic endowment. Few connec- tions, if any, are formed simply on the basis of reaction to the environment. In this sense the newborn child is not simply a "clean slate," but has built-in attributes that are subject to modification with experience. Examples of such environmental influences include the modified behavior of the geneti- cally aggressive rats cited above; the impaired visual-motor coordination of an adult monkey if, during the early months of life, it is prevented from visually perceiving the consequences of voluntary movements; the strik- ingly abnormal social and sexual behavior of monkeys that are reared without normal social association with other monkeys of their own age; and the well-known "imprinting," as the maternal object, of the first moving object to come within the ken of a newborn duckling. Physiological Analysis of Behavior A concerted and many-sided attack on the ways in which the nervous and endocrine systems influence behavior is now under way. One challenging
FRONTIERS OF BIOLOGY problem is the "clockwork" responsible for a large group of biological rhythms with a range of frequencies from a few seconds to several years, occurring at all levels of organization. Some occur in single cells or small cell aggregations, such as the pacemaker of the heart or rhythmic impulses from the brain stem; some involve almost the totality of the endocrine system, as in the ovarian cycle; some are the basis of diurnal or annual rhythmic behavior, as in the breeding and migration patterns of many species; some are responsible for cycles in the size of entire populations. Clearly, some of these have been imposed by the environment- by the length of daylight or seasons of the year. But most circadian rhythms' such as sleeping patterns, are basically of internal origin in the individual and become entrained in some aspect of the environment. In no instance to date has the intrinsic clockwork actually been established. Simple motivational behavior lends itself readily to experimental ap- proach, permitting analysis of questions concerning such behavioral attri- butes as drive and satiation, reward and punishment, pleasure and pain, and, particularly, physiological regulations of hunger, thirst, sexual and maternal behavior, and temperature regulation. In each of these the activity of the hypothalamus at the base of the brain has proved to be central, exhibiting excitatory and inhibitory mechanisms that accelerate or brake be- havior. For example, experimental destruction of the excitatory mechanism can result in fatal refusal to eat or drink in the presence of an abundance of food and water. Conversely, destruction of the inhibitory mechanism elsewhere in the hypothalamus leads to gross hyperphagia and inordinately obese animals. These regions of the brain can be manipulated by electrical or chemical stimuli, causing animals to eat or to starve themselves on signal. Evidently, the normal hypothalamus is responsive to internal sig- nals such as the blood concentrations of sugar, sodium ions, estrogens, and androgens. Normal man regulates these behaviors with remarkable pre- cision; thus, for example, one can maintain relatively constant weight for 30 or 40 years of adult life. At the next level of understanding, it should be possible to obtain sufficient information to permit clinical management of the obese, the anorexic, the depressed, and persons with damaged brains in whom these normal controls have failed. ORIENTATION AND HOMING Spatial orientation to the surroundings is one of the most characteristic behavioral attainments of animals, including man. Many animals, however, orient themselves by sensing mechanisms or behavioral discriminations that pose perplexing mysteries because they differ considerably from those that affect human behavior. Echolocation enables a bat with a brain weighing 111
112 THE LIFE SCIENCES less than a gram to locate and intercept minute flying insects within a frac- tion of a second, but only very limited echolocation has been achieved by blind humans. Similar abilities are obviously exercised by marine mammals in murky waters. Migratory animals pose elementary questions for which, despite great effort, there are as yet no adequate answers. The aspects of their environ- ment that provide directional information and the sensory channels that convey that information to the central nervous system remain obscure. A catalog of the achievements of migratory animals never fails to astonish. Birds such as Arctic terns and some of the shearwaters migrate thousands of miles. On land, some mammals such as caribou migrate great distances under conditions so difficult as to pose extremely complex questions con- cerning how they manage their orientation and sustain their motivation. Sea turtles swim across hundreds of miles of ocean to locate relatively small islands where they lay their eggs on a few beaches. Many fish and eels are known to migrate similarly over thousands of miles of ocean. Even some marine invertebrates go through life cycles that include extensive migratory behavior. The migrations of whole populations of insects such as locusts have been notorious since biblical times because of their destruction of human crops. Tagging programs have revealed that monarch butterflies migrate for more than a thousand miles and in directions that cannot be accounted for by passive transport by the wind. Not only do the sensory mechanisms involved in their orientation require analysis, but also attention must be given to the equally significant questions of motivation, genetic programming in relation to individual learning, and the ecological advan- tages of migration, which suffice to offset the exertion and hazards of such extensive travel. The homing behavior evident in numerous species is a related phenom- enon. It is usually studied by artificial displacement of animals by consider- able distances to unfamiliar surroundings. Success in their performance is even more remarkable than that of long-distance migrations. Conceivably, the latter are guided by sun or star compass orientation, which also requires compensation for the apparent motion of celestial bodies due to the earth's rotation. But homing after arbitrary displacement requires that the animal select the homeward direction as well as keep moving in that direction. We know how some animals obtain the equivalent of a compass, but we know almost nothing about how, in homing, they obtain the equivalent of a map. The heart of this problem is the search for sensory cues not apparent to man. Bees orient themselves by the polarization of light, and some fish detect distortion of the electric fields generated by their own electrical organs. Conceivably, other attributes of the environment are as important as the star map appears to be to migrating birds.
FRONTIERS OF BIOLOGY The effect of the endocrine system on behavior has been extensively explored for some time, largely because it lends itself readily to such ex- perimentation. Well known are the profound consequences on male and female sexual behavior that result from castration and restoration of normal behavior by administration of appropriate sex hormones. More recently, it has been shown that if radioactive hormones are administered, they bind to a relatively small group of cells in the brain stem, and it appears that subsequent behavior is the consequence of such binding. As we have already seen, a converse set of relationships is mediated by the hypothalamus and pituitary glands. Some such relations require more complex interactions. For example, stimulation of the mother's nipples by the suckling young is _ recognized In tne Drain and, in turn, through the hypothalamus, initiates . . ~. . · . . . · . ~ L the secretion of pituitary lactogenlc hormone, which, in its turn, stimulates the mammary glands to manufacture milk. In a mother rat even the sound or smell of hungry young sets off this train of events. The nervous system of a newborn infant is already so constructed as to exhibit either male or female character. Injection of the opposite sex hormone into adolescent or adult animals does not elicit a striking change in sexual behavior. However, if the hormone is administered during embryonic life, the nervous system takes on the character associated with the sex of the administered hormone, at least as measured by the ability of the adult animal to respond to the sex hormone when it is administered later. As yet, there is no clue as to the nature of the "wiring diagram" or any other attribute of the nervous system that gives it its male or female character. Such studies have deep relevance for that unfortunate group of individuals whose genetically intrinsic sex is mistaken during early life. Learning and Memory Learning is the process whereby behavior is modified by experience; it is a measurable and lasting change in behavior produced by a specifiable set of environmental circumstances. All learning can be reduced to specific parameters operating under the following principles: (1) There must be continuity in time and space of the items learned (stimuli to be associated with other stimuli or response). (2) There must be repetition of the asso- ciated items, although occasionally there is successful one-try learning. (3 ~ There must be reinforcement, whether reward or punishment, for with reinforcement the learned response grows in force and, in its absence, decays (forgetting). (4) An interference or intermission process is required to produce forgetting, presumably by covering over memory rather than destroying it, because there is evidence that the basic process of learning 113
114 THE LIFE SCIENCES is essentially permanent. But having made these general statements, one must admit that the nature of the learning process, in its essence, remains obscure. Primitive organisms can engage in the simplest learning, if only as avoidance responses. Ascending the phylogenetic ladder, there is a con- tinual increase in the complexity that the organism can handle in learning. A wide variety of experimental approaches have been brought to bear on this process, but without genuine success. It is apparent that no one area of the brain constitutes its memory. Memory is diffused over much of the total cortex. No one lesion in the brain, even a sizable one, produces permanent amnesia or inability to learn, although such lesions can produce amnesia for recently learned material. A variety of investigative techniques have yielded similar findings. Lesions placed in various areas of the brain, violent electric shocks, epileptic seizures, and drugs that inhibit protein synthesis, specifically puromycin, can all abolish a very recent learning experience, but none removes well- established memory. Investigators engaged in such studies have contem- plated that memory may take one of several forms. Among these are electrical circuits in the brain that are self-sustaining for long periods; con- nections between nerve cells, viz., synapses, which may either be brought into being or broken; and new outgrowths or sproutings of such connecting terminals, which might be generated. Finally, there may be specific chem- ical changes, conceivably in the transmitter molecules, but, more likely, experience could be encoded within macromolecules of the nerve cells. The favorite candidates for the latter, if this possibility is valid, have been RNA, proteins, or even the complex polysaccharides. Each affords sufficient variability to give rise to speculation that learning is an experience that can be coded within its structure. The antibiotic puromycin, which inhibits protein synthesis, can totally eliminate memory of a recent learning experience in mice or fish. In view of the wholesale protein-synthesis characteristic of cells of the nervous system, some relationship between protein synthesis and learning may not be entirely unexpected. But the matter is not simple, because cycloheximide, which also inhibits protein synthesis, has no effect on memory. An important clue may reside in the fact that puromycin blocks protein synthesis by interrupting growth of polypeptide chains, whereas cycloheximide somehow prevents the normal breakdown of polysomes so that the mRNA remains in the cell, inaccessible to the enzymes that normally degrade it after about 30 minutes. These are the most recent observations, and the fact that the experiments are at all productive gives hope of the possibility that the intrinsic nature of the learning process may yet come to light. A vast literature deals with sensation and perception of the environment and the behavioral consequences of these processes. Since these are thor