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HISTORY OF BIOLOGICAL PSYCHOLOGY
The great questions of philosophy, the mind–body problem and the nature of knowledge, were also the questions that drove early developments in the pathways to modern psychology. This is especially true of biological or physiological psychology. Wilhelm Wundt, who founded experimental psychology, titled his major work Foundations of Physiological Psychology (1874/1908). William James, the other major figure in the development of modern psychology, devoted a third of his influential text Principles of Psychology (1890) to the brain and nervous system. Both Wundt and James studied medicine and philosophy, and both considered themselves physiologists. Their goal was not to reduce psychology to physiology but rather to apply the scientific methods of physiology to the study of the mind. The other driving force in early biological psychology was the study of the brain and nervous system.
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The major topics in modern biological psychology are sensoryprocesses,learning and memory,motivation and emotion, and most recently cognition—in short, behavioral and cognitive neuroscience. A number of other areas began as part of physiological psychology and have spun off to become fields in their own right. We treat the major topics in biological psychology separately in the text that follows. But first we sketch very briefly the recent philosophical and physiological roots.
THE MIND
The history of such issues as the mind–body problem and epistemology is properly the domain of philosophy, treated extensively in many volumes and well beyond the scope of this research paper. Our focus in this brief section is on the history of the scientific study of the mind, which really began in the nineteenth century.
Perhaps the first experimental attacks on the nature of the mind were the observations of Weber as generalized by Gustav Fechner. Ernst Weber, a physiologist, was attempting in 1834 to determine whether the nerves that respond to the state of the muscles also contribute to judgments about weights. He found that the just noticeable difference (jnd) in weight that could be reliably detected by the observer was not some absolute amount but rather a constant ratio of the weight being lifted. The same applied to the pitch of tones and the length of lines.
Fechner realized that Weber had discovered a way of measuring the properties of the mind. Indeed, in his Elements of Psychophysics (1860/1966) he felt he had solved the problem of mind and body. He generalized Weber’s observations to state that as the psychological measurement in jnd’s increased arithmetically, the intensity of the physical stimulus increased geometrically—the relationship is logarithmic. Fechner, trained as a physicist, developed the classical psychophysical methods and the concepts of absolute and differential thresholds. According to Edwin Boring (1942), he had a nervous breakdown and resigned his chair at Leipzig in 1839. During the last 35 years of his life, he devoted himself to panpsychism, the view that mind and matter are one and thus that mind is all. He viewed the psychophysical law as the paradigm for the transformation of the material into the spiritual. In any event, the methods Fechner developed were of great help to such early experimental psychologists as Wundt and his student Tichener in their attempts to measure the attributes of sensation.
Tichener identified the elements of conscious experience as quality, intensity, extensity, protensity (duration), and attensity (clearness) (see Tichener, 1898). But for all their attempts at scientific observation, the basic approach of Wundt and Tichener was introspection, but other observers (e.g., Külpe at Bonn) had different introspections. Boring studied with Tichener and was for many years chair of the psychology department at Harvard. He attempted to recast Tichener’s views in more modern terms (The Physical Dimensions of Consciousness, 1933) by emphasizing that the dimensions listed earlier related to discrimination of physical stimuli. His student S. S. Stevens showed that trained observers could reliably form judgments of sounds in terms of pitch, loudness, “volume,” and “density” (see also Boring, 1950).
At Harvard, Stevens later introduced an important new method of psychophysics termed direct magnitude estimation. The subject simply assigned a number to a stimulus, a higher one to a more intense stimulus and a lower number to one that was less intense. Some what surprisingly this method gave very reliable results. Using this method, Stevens found that the proper relationship between stimulus intensity and sensationis not logarithmic, as Fechner had argued, but rather a power function: The sensation, that is, sensory magnitude, equaled the stimulus intensity raised to some power, the exponent ranging from less than to greater than one.This formulation proved very useful in both psychophysical and physiological studies of sensory processes (see Stevens, 1975).
The key point of all this work on psychophysics is that it is not necessary to be concerned at all about subjective experience or introspection. The observer simply pushes a button or states a word or number to describe his or her judgment of the stimulus. The more the observer practices, the more reliable the judgments become and the more different observers generate the same results. Psychophysics had become purely behavioral.
As Hilgard (1987) notes, Fechner was troubled by the question of where the transformation between stimulus and judgment occurs. Fechner distinguished between “inner” and “outer” psychophysics, outer referring to the relation between the mind and external stimuli and inner to the relation between the mind and excitation of the sensory apparatus. Fechner opted for a direct correspondence between excitation and sensation, a surprisingly modern view. Indeed, Stevens(1961) argued with evidence that the psychophysical transformation occurs at the receptor–first-orderneurons, at least for intensity.
We take an example from the elegant studies of Mountcastle, Poggio, and Werner (1963). Here they recorded the action potentials of a neuron in the somatosensory thalamus of a monkey driven by extension of the contralateral knee. The relation between degrees of joint angle () and frequency of neuron discharge (F) is F 13.90.429 24, where 13.9 and 24 are constants determined by conditions. So the power exponent is 0.429, within the general range of exponents for psychophysical judgments of the relation between joint angle and sensation of movement. In other words, the relationship is established by ascending sensory neuron activity before the level of the cerebral cortex, presumably at the receptor– first-order neuron.
The modern era of psychophysics can perhaps be dated to a seminal paper by John Swets in 1961: Is there a sensory threshold? His answer was no. He and David Green developed the theory and methodology of signal detection theory (Green & Swets, 1966). There is always noise present with signals. When one attempts to detect a signal in noise, the criteria used will determine the outcome. This approach has proved immensely useful in fields ranging from the telephone to psychophysical studies in animals to detection of structural failures in aircraft wings to detection of breast cancer. But where is the mind in decision theory? It has disappeared. The initial hope that psychophysics could measure the mind has been reduced to considerations of observer bias. A similar conclusion led to the downfall of introspection.
BIOLOGICAL PSYCHOLOGY RESEARCH PAPER EXAMPLES:
THE BRAIN
Until the nineteenth century, the only method available to study brain function was the lesion, either in unfortunate humans with brain damage or brain lesions done in infrahuman animals. The key intellectual issue throughout the history of the brain sciences was localization. To state the question in simplistic terms: Are psychological traits and functions localized to particular regions of the brain or are they widely distributed in the brain?
The history of ideas about localization of brain function can be divided roughly into three eras. During the first era, which spans from antiquity to about the second century A.D., debate focused on the location of cognitive function, although the discussion revolved around the issue of the soul, that is, what part of the body housed the essence of being and the source of all mental life (for reviews, see Finger, 1994; Gross, 1987; Star, 1989). In an early and particularly prophetic Greek version of localization of function, the soul was thought to be housed in several body parts, including the head, heart, and liver, but the portion of the soul associated with intellect was located in the head (McHenry, 1969). The individual whom many historians have viewed as having the greatest influence during this era was Galen, an anatomist of Greek origin. Using animals, he performed experiments that provided evidence that the brain was the center of the nervous system and responsible for sensation, motion, and thinking (Finger, 1994; Gross, 1987).
In the second era (spanning the second to the eighteenth centuries), the debate focused on whether cognitive functions were localized in the ventricular system of the brain or in the brain matter itself. The influence of the church during this era cannot be overstated; for example, ethereal spirits (and ideas) were believed to flow through the empty spaces of the brain’s ventricles. Nevertheless, by the fifteenth and sixteenth centuries, individuals such as da Vinci and Vesalius were questioning the validity of ventricular localization. Finally, during the seventeenth century, partly as a result of the strongly held views and prolific writings of Thomas Willis, and during the eighteenth century, with the publication of clinical descriptions of cognitively impaired patients accompanied by crude descriptions of brain damage (e.g., Baader), the view that intellectual function was localized in brain matter and not in the ventricles became solidified (Clenending, 1942).
The nineteenth century to the present makes up the third era, and here debate has focused on how mental activities (or cognitive processes) are organized in the brain. An early idea, which became known as the localizationist view, proposed that specific mental functions were carried out by specific parts of the brain. An alternative idea, which became known as the equipotential view, held that large parts of the brain were equally involved in all mental activity and that there was no specificity of function within a particular brain area (Clark & Jacyna, 1987).
Perhaps the most influential idea about localization of brain function derived from Franz Joseph Gall during the early nineteenth century. Gall had been influenced somewhat by the earlier ideas of Albrecht von Haller (Clarke & Jacyna, 1987). In the mid-eighteenth century, Haller had developed a doctrine of brain equipotentiality, or a type of action commune. He believed that the parts of a distinguishable anatomical component of the brain—the white matter, for instance— performed as a whole, each area of white matter having equivalent functional significance (Clarke & Jacyna, 1987). Indeed, one might characterize Gall’s ideas as a reaction against the equipotential view of Haller. Gall’s insight was that, despite its similarity in appearance, brain tissue was not equipotential but instead was actually made up of many discrete areas that had different and separate functions. Eventually, Gall was able to characterize 27 different regions, or organs, of the brain in a scheme that he called organology. Later, the term phrenology came to be associated with Gall’s work. However, this term was coined by Gall’s colleague, Spurzheim, with whom he had a falling out, and Gall himself never used the term (Zola-Morgan, 1995).
Gall’s ideas about the localization of cognitive functions began to tear at the religious and social fabric of the nineteenth century. In particular, various governmental and religious authorities saw his notion that various mental faculties were represented in different places in the brain as in conflict with moral and religious views of the unity of the soul and mind. Gall’s organology, and later versions of phrenology, faced similar critiques from philosophy and science. Clerics and metaphysicians were concerned with the larger theological implications of the phrenological system. For example, in Flourens’s critique of phrenology in 1846 (dedicated to Decartes), Gall and his followers were declared guilty of undermining the unity of the soul, human immortality, free will, and the very existence of God (Harrington, 1991). Rolando, the famous Italian neuroanatomist, recognized the elegance of Gall’s dissection techniques and his tracing of fiber tracts from the spinal cord to the cerebrum. However, he found no logical connection between the tracings of the fibers and the distinct organs in the convolutions of the brain proposed to house particular mental faculties.
Another scientific criticism had to do with the questionable way in which Gall had determined the locus and extent of each of the 27 organs. For example, Gall had localized the carnivorous instinct and the tendency to murder (organ 5) above the ear for three reasons: (a) This was the widest part of the skull in carnivores; (b) a prominence was found there in a student who was fond of torturing animals; and (c) this region was well developed in an apothecary who later became an executioner (Barker, 1897).
Another scientific issue critics raised during the nineteenth century was that Gall never specified the precise extent or the anatomical borders of any of the organs. This lack of rigor, it was argued, made it impossible to correlate a specific faculty with the size of an organ or cranial capacity (Sewall, 1839). Related criticisms involved Gall’s seeming failure to acknowledge that there were variations in the thickness of the skull, that is, variations from one individual specimen to another and from one locus to another within the same skull (Sewall, 1839).
An oft-cited example of a specific contribution Gall made to our understanding of brain function is the idea that he anticipated the discovery by Broca in 1861 of a specific speech area of the brain (Ackernecht & Vallois, 1956; Bouillaud, 1848). However, we believe that a careful reading of the facts surrounding this discovery tells a somewhat different story. In fact, Broca never mentioned Gall’s name in his 1861 report. Moreover, he referred to Gall’s doctrine in a rather negative way. Nevertheless, Broca’s work stands as a clear example of a modern idea of localization of function built on the foundation and fundamental idea, established by Gall a half century earlier, that specific parts of the brain mediate specific behaviors.
Both Gall and Bouillaud seemed to be vindicated in 1861 with the publication of the proceedings from a meeting of the Société d’Anthropologie de Paris. Broca, assisted by Alexandre Ernest Aubertin, Bouillaud’s son-in-law and a strong believer in localization and in Bouillaud’s hypothesis, presented the neuropathological findings from the brain of his patient, Monsieur Leborgne. [This patient subsequently was referred to by the name “Tan,” the only utterance Broca ever heard Monsieur Leborgne make (Broca, 1861).]
Broca’s finding from his patient Tan has been regarded by some historians as the most important clinical discovery in the history of cortical localization. Moreover, within the decade, what some historians regard as the most important laboratory discovery pertaining to cortical localization was reported when Gustav Fritsch and Eduard Hitzig (1870) discovered the cortical motor area in the dog and proved that cortical localization was not restricted to a single function (Finger, 1994). The discoveries of the speech area by Broca and the motor area by Fritsch and Hitzig were seen as vindication for Gall’s ideas and reestablished him as the father of localization.
Following the pioneering study by Fritsch and Hitzig on the localization and organization of the motor area of the cerebral cortex, localization of function quickly won the day, at least for sensory and motor systems. In the last three decades of the nineteenth century, the general locations of the visual and auditory areas of the cortex were identified. The field of physiology, in particular neurophysiology—for example, in the work of Sir Charles Sherrington—together with clinical neurology and neuroanatomy, were exciting new fields at the beginning of the twentieth century.
At this time, the only experimental tools for studying brain organization and functions were ablation and electrical stimulation. Neuroanatomy was in its descriptive phase; thanks in part to the Golgi method, the monumental work of Ramon y Cajal was completed over a period of several decades beginning near the end of the nineteenth century. Neurochemistry was in its descriptive phase, characterizing chemical substances in the brain.
The first recording of a nerve action potential with a cathode-ray tube was done by Gasser and Erlanger in 1922, but the method was not much used until the 1930s.The human EEG was rediscovered in 1929 by H. Berger, and the method was applied to animal research and human clinical neurology, particularly epilepsy, in the 1930s by, for example,Alexander Forbes, Hallowell Davis, and Donald Lindsley.
The pioneering studies of Adrian in England (1940) and of Wade Marshall, Clinton Woolsey, and Philip Bard (1941) at Johns Hopkins were the first to record electrical evoked potentials from the somatic sensory cortex in response to tactile stimulation. Woolsey and his associates developed the detailed methodology for evoked potential mapping of the cerebral cortex. In an extraordinary series of studies, they determined the localization and organization of the somatic sensory areas, the visual areas and the auditory areas of the cerebral cortex, in a comparative series of mammals. They initially defined two projection areas (I and II) for each sensory field; that is, they found two complete functional maps of the receptor surface for each sensory region of the cerebral cortex, for example, two complete representations of the skin surface in the somatic-sensory cortex.
In the 1940s and 1950s, the evoked potential method was used to analyze the organization of sensory systems at all levels from the first-order neurons to the cerebral cortex. The principle that emerged was strikingly clear and simple—in every sensory system the nervous system maintained receptotopic maps or projections at all levels from receptors—skin surface, retina, basilar membrane—to cerebral cortex. The receptor maps in the brain were not point-to-point; rather, they reflected the functional organization of each system— fingers, lips, and tongue areas were much enlarged in the primate somatic cortex, half the primary visual cortex represented the forea, and so on.
The evoked potential method was very well suited to analysis of the overall organization of sensory systems in the brain. However, it could reveal nothing about what the individual neurons were doing.This had to await development of the microelectrode (a very small electrode that records the activity of a single cell). Indeed, the microelectrode has been the key to analysis of the fine-grained organization and “feature detector” properties (most neurons respond only to certain aspects, or features, of a stimulus) of sensory neurons. The first intracellular glass pipette microelectrode was actually invented by G. Ling and R. W. Gerard in 1949; they developed it to record intracellularly from frog muscle. Several investigators had been using small wire electrodes to record from nerve fibers, for example, Robert Galambos at Harvard in 1939 (auditory nerve; see Galambos & Davis, 1943) and Birdsey Renshaw at the University of Oregon Medical School in the 1940s (dorsal and ventral spinal roots). Metal electrodes were generally found to be preferable for extracellular single-unit recording (i.e., recording the spike discharges of a single neuron where the tip of the microelectrode is outside the cell but close enough to record its activity clearly). Metal microelectrodes were improved in the early 1950s; R. W. Davies at Hopkins developed the platinum-iridium glass-coated microelectrode, D. Hubel andT.Wiesel at Harvard developed the tungsten microelectrode, and the search for putative stimulus coding properties of neurons was on. The pioneering studies were those of Mountcastle and associates at Hopkins on the organizationofthesomatic-sensorysystem(Mountcastle,Davies,& Berman, 1957), those of Hubel and Wiesel (1959) at Harvard on the visual system (and Maturana and Lettvin’s work at MIT on the optic nerve fibers of frogs, see Maturana, Lettrin, McCulloch, & Pitts, 1960), and those of Rose, Hind,Woolsey, and associates at Wisconsin on the auditory system (see Hind et al., 1960).
It was not until many years later that imaging methods were developed to study the organization and functions of the normal human brain (see following text). Heroic studies had been done on human brain functioning much earlier in neurosurgical procedures (heroic both for the surgeon and the patient, e.g., Penfield & Rasmussen, 1950). However, these patients typically suffered from severe epilepsy. The development of PET, fMRI, and other modern techniques is largely responsible for the explosion of information in the aspect of biological psychology termed cognitive neuroscience.
SENSORY PROCESSES
We select two examples of sensory processes, color vision and pitch detection, that illustrate very well the historical development of the study of sensory systems. They are both extraordinary success stories in the field of biological psychology.
COLOR VISION
Color vision provides an illustrative case history of the development of a field in biological psychology with feet in both physics and physiology. Isaac Newton was perhaps the first scientist to appreciate the nature of color. The fact that a prism could break up white light into a rainbow of colors meant that the light was a mixture that could produce spectral colors. But Newton recognized that the light rays themselves had no color; rather, different rays acted on the eye to yield sensations of colors (1704/1931). Oddly, the great German literary figure Goethe asserted it was impossible to conceive of white light as a mixture of colors (1810/1970).
In physics there was an ongoing debate whether light was particle or wave (we know now it is both). Interestingly, Newton favored the particle theory. Thomas Young, an English physicist working a century later, supported the wave theory. Newton had developed the first color circle showing that complementary pairs of colors opposite to one another on the circle would mix to yield white light. Young showed that it was possible to match any color by selecting three appropriate colors, red, green, and blue, and suggested there were three such color receptors in the eye. Helmholtz elaborated and quantified Young’s idea into the Young-Helmholtz trichromatic theory. Helmholtz, incidentally, studied with Müller and Du Bois-Reymond. He received his MD in 1842 and published two extraordinary works, the three-volume Treatis on Physiological Optics (1856–1866/1924) and On the Sensations of Tone (1863/1954). He was one of the leading scientists in the nineteenth century and had a profound impact on early developments in psychology, particularly biological psychology.
The basic idea in the trichomatic theory is that the three receptors accounted for sensations of red, green, and blue. Yellow was said to derive from stimulation of both red and green receptors, and white was derived from yellow and the blue receptor. But there were problems. The most common form of color blindness is red-green. But if yellow is derived from red and green, how is it that a person with red-green color blindness can see yellow? In the twentieth century, it was found that there are four types of receptors in the human retina: red, green, blue (cones), and light-dark (rods). But what about yellow?
Hering (1878) developed an alternative view termed the “opponent-process” theory. He actually studied with Weber and with Fechner and received his MD just two years after Wundt in Heidelberg. Interestingly, Hering disagreed with Fechner about the psychophysical law, arguing that the relationship should be a power function, thus anticipating Stevens. Hering proposed that red-green and blue-yellow acted as opposites, along with white-black. In modern times, Dorothea Jameson and Leo Hurvich (1955) provided an elegant mathematical formulation of Herring’s theory that accounted very well for the phenomena of color vision.
Russell De Valois, now in the psychology department at the University of California, Berkeley, provided the physiological evidence to verify the Herring-Jameson-Hurvich theory, using the monkey (see De Valois, 1960). Ganglion neurons in the retina that respond to color show “opponent” processes. One cell might respond to red and be inhibited by green, another will respond to green and be inhibited by red, yet another will respond to blue and be inhibited by yellow, and the last type will respond to yellow and be inhibited by blue. The same is true for neurons in the visual thalamus. De Valois’s work provided an elegant physiological basis for the opponent-process theory of color vision. But Young and Helmholtz were also correct in proposing that there are three color receptors in the retina. It is the neural interactions in the retina that convert actions of the three color receptors into the opponent processes in the ganglion cells. It is remarkable that nineteenth-century scientists, working only with the facts of human color vision, could deduce the physiological processes in the eye and brain.
An interesting chapter in the development of color-vision theory is the work of Christine Ladd-Franklin (Hilgard, 1987). She completed her PhD in mathematics at Johns Hopkins in 1882 but was not awarded the degree because she was a woman. Later she spent a year in Müller’s laboratory in Göttingen, where he gave her private lectures because, as a woman, she was not allowed to attend his regular lectures. She developed a most interesting evolutionary theory of color vision based on the color zones in the retina. The center of the fovia has all colors and the most detailed vision. The next outer zone has red and green sensitivity (as well as blue and yellow), the next outer zone to this has only blue and yellow sensitivity (and black-white), and the most peripheral regions have only black-white (achromatic) sensitivity.
She argued that in evolution, the achromatic sensitivity (rods) developed first, followed by evolution of blue and yellow receptors and finally red and green receptors. The fact that red-green color blindness is most common is consistent with the idea that it is the most recent to evolve and hence the most “fragile.”
Modern molecular biology and genetics actually provide support for Ladd-Franklin’s evolutionary hypothesis. The Old World monkey retina appears to be identical to the human retina: Both macaques and humans have rods and three types of cones. It is now thought that the genes for the cone pigments and rhodopsin evolved from a common ancestral gene. Analysis of the amino acid sequences in the different opsins suggest that the first color pigment molecule was sensitive to blue. It then gave rise to another pigment that in turn diverged to form red and green pigments. Unlike Old World monkeys, New World monkeys have only two cone pigments, a blue and a longer wavelength pigment thought to be ancestral to the red and green pigments of humans and other Old World primates. The evolution of the red and green pigments must have occurred after the continents separated, about 130 million years ago. The New World monkey retina, with only two color pigments, provides a perfect model for human red-green color blindness. Genetic analysis of the various forms of human color blindness, incidentally, suggests that some humans may someday, millions of years from now, have four cone pigments rather than three and see the world in very different colors than we do now.
The modern field of vision, encompassing psychophysics, physiology, anatomy, chemistry, and genetics, is one of the great success stories of neuroscience and biological psychology. We now know that there are more than 30 different visual areas in the cerebral cortex of monkeys and humans, showing degrees of selectivity of response to the various attributes of visual experience, for example, a “color” area, a “movement” area, and so on. We now have a very good understanding of phenomena of visual sensation and perception. The field concerned with vision has become an entirely separate field of human endeavor, with its own journals, societies, specialized technologies, and NIH institute.
PITCH DETECTION
As we noted, Helmholtz published a most influential work on hearing in 1863 (On the Sensation of Tone). The fundamental issue was how the nervous system codes sound frequency into our sensation of pitch. By this time, much was known about the cochlea, the auditory receptor apparatus. Helmholtz suggested that the basilar membrane in the cochlea functioned like a piano, resonating to frequencies according to the length of the fibers. The place on the membrane so activated determined the pitch detected; this view was called the place theory of pitch. The major alternative view was the frequency theory (Rutherford, 1886), in which the basilar membrane was thought to vibrate as a whole due to the frequency of the tone activating it. Boring (1926) presented a comprehensive theoretical analysis of these possibilities.
One of Boring’s students, E. G. Wever, together with C. W. Bray, recorded from the region of the auditory nerve at the cochlea and found that the recorded electrical signal followed the frequency of the tone up to very high frequencies, many thousands of Hertz (Wever & Bray, 1930). So the frequency theory was vindicated. But there were problems. A single nerve fiber cannot fire at much greater than 1,000 Hertz. The attempted answer was the volley theory: Groups of fibers alternated in firing to code higher frequencies.
Wever and Bray’s discovery is an interesting example of a perfectly good experiment fooled by biology. As it happens, there is a process in the cochlea much like the pizoelectric effect—a tone generates electrical activity at the same frequency as the tone, now termed the cochlear microphonic. It is thought to be an epiphenomenon, unrelated to the coding functions of the auditory system.
The solution to the question how the cochlea coded tone frequency was provided by Georg von Békésy. Born in Budapest, he received his PhD in physics in 1923 and was a professor at the University of Budapest from 1932 to 1946. In 1947, he accepted a research appointment in the psychology department at Harvard, where he worked until 1964. During his time at Harvard, he was offered a tenured professorship but did not accept it because he disliked formal teaching.
During his years of full-time research at Harvard, he solved the problem of the cochlea, for which he received the Nobel Prize in 1961. In 1964, he accepted a professorship at the University of Hawaii, where he remained until his death.
By careful microscopic study of the cochlea, Békésy determined the actual movements of the basilar membrane in response to tones (see Békésy, 1947). When William James Hall was built at Harvard to house the psychology department, a special floating room was constructed in the basement for Békésy’s experiments. The entire room floated on an air cushion generated by a large air compressor. Furthermore, the experimental table floated within the floating room on its own compressor. For Békésy’s experiments it was necessary to avoid all external building vibrations. (One of the authors, R.F.T., had the opportunity to use this facility when at Harvard.)
Békésy discovered that the traveling waves of the basilar membrane induced by a given tone establish a standing wave pattern that maximally displaces a given region for a given tone and different regions for different tones. The pattern of displacement is more complicated than the Helmholtz theory but nonetheless provided a triumph for the place theory.
Actually, another kind of physiological evidence provided strong support for the place theory in the 1940s. Woolsey and Walzl (1942) published an extraordinary study in which they electrically stimulated different regions of the auditory nerve fibers in the cochlea (the fibers are laid out along the basilar membrane) in an anesthetized cat and recorded evoked potentials in the auditory cortex. The place stimulated on the cochlea determined the region of the auditory cortex activated. An important practical outcome of all this work is the cochlear prosthesis developed for deaf individuals.
More recent studies recording the activity of single neurons in the auditory cortex have verified and extended these observations (e.g., Hind et al., 1960). When the ear is stimulated with low-intensity pure tones (anesthetized cat), neurons—in particular, narrow dorsal-ventral bands in the primary auditory cortex—respond selectively to tones of different frequency. The regions of the cochlea activated by pure tones are represented in an anterior-posterior series of narrow dorsal-ventral bands along the primary auditory cortex, a cochlea-topic representation.
Like the visual sciences, the modern field of the hearing sciences has become an entirely separate field with its own societies, journals, and NIH institute focusing on psychophysics and the neurobiology of the auditory system. We know a great deal less about the organization of auditory fields in the cerebral cortex in primates and humans, incidentally, than we do about the visual system. The human auditory areas must be very complex, given our extraordinary speciesspecific behavior of speech.
LEARNING AND MEMORY
Karl Lashley is the most important figure in the development of physiological psychology and the biology of memory in America. He obtained his PhD at Johns Hopkins University where he studied with John Watson and was heavily influenced by Watson’s developing notions of behaviorism. While there he also worked with Sheherd Franz at a government hospitalin Washington; they published a paper together in 1917 on the effects of cortical lesions on learning and retention in the rat. Lashley then held teaching and research positions at the University of Minnesota (1917–1926), the University of Chicago (1929–1935), and at Harvard from 1935 until his death in 1958. During the Harvard years, he spent much of his time at the Yerkes Primate Laboratory in Orange Park, Florida.
Lashley devoted many years to an analysis of brain mechanisms of learning, using the lesion-behavior method, which he developed and elaborated from his work with Franz. During this period, Lashley’s theoretical view of learning was heavily influenced by two congruent ideas—localization of function in neurology and behaviorism in psychology.
Lashley describes the origins of his interest in brain substrates of memory and Watson’s developing views of behaviorism in the following letter he wrote to Ernest Hilgard in 1935:
In the 1914, I think, Watson called attention of his seminar to the French edition of Bechterev, and that winter the seminar was devoted to translation and discussion of the book. In the spring I served as a sort of unpaid assistant and we constructed apparatus and planned experiments together. We simply attempted to repeat Bechterev’s experiments. We worked with withdrawal reflexes, knee jerk, pupil. Watson took the initiative in all this, but he was also trying to photograph the vocal cord, so I did much of the actual experimental work. I devised drainage tubes for the parotid and submaxiallary ducts and planned the salivary work which I published. As we worked with the method, I think our enthusiasm for it was somewhat dampened. Watson tried to establish conditioned auditory reflexes in the rat and failed. Our whole program was then disrupted by the move to the lab in Meyer’s clinic. There were no adequate animal quarters there. Watson started work with the infants as the next best material available. I tagged along for awhile, but disliked the babies and found me a rat lab in another building. We accumulated a considerable amount of experimental material on the conditioned reflex which has never been published. Watson saw it as a basis for a systematic psychology and was not greatly concerned with the nature of the reaction itself. I got interested in the physiology of the reaction and the attempt to trace conditioned reflex paths through the nervous system started my program of cerebral work. (Letter of May 14, 1935, K. S. Lashley to E. R. Hilgard, reproduced with the kind permission of E. R. Hilgard)
It was in the previous year, 1913, that Watson published his initial salvo in an article entitled “Psychology as the Behaviorist Views It.” He was elected president of the American Psychological Association in 1914.
As we noted earlier, localization of function in the cerebrum was the dominant view of brain organization at the beginning of the twentieth century. In Watson’s behaviorism, the learning of a particular response was held to be the formation of a particular set of connections, a series set. Consequently, Lashley argued, it should be possible to localize the place in the cerebral cortex where that learned change in brain organization was stored—the engram. (It was believed at the time that learning occurred in the cerebral cortex.) Thus, behaviorism and localization of function were beautifully consistent—they supported the notion of an elaborate and complex switchboard where specific and localized changes occurred when specific habits were learned.
Lashley set about systematically to find these learning locations—the engrams—in a series of studies culminating in his 1929 monograph, Brain Mechanisms of Intelligence. In this study, he used mazes differing in difficulty and made lesions of varying sizes in all different regions of the cerebral cortex of the rat. The results of this study profoundly altered Lashley’s view of brain organization and had an extraordinary impact on the young field of physiological psychology. The locus of the lesions is unimportant; the size is critically important, particularly for the more difficult mazes. These findings led to Lashley’s two theoretical notions of equipotentiality and mass action: that is, all areas of the cerebral cortex are equally important (at least in maze learning), and what is critical is the amount of brain tissue removed.
Lashley’s interpretations stirred vigorous debate in the field. Walter Hunter, an important figure in physiologicalexperimental psychology at Brown University who developed the delayed response task in 1913, argued that in fact the rat was using a variety of sensory cues; as more of the sensory regions of the cortex were destroyed, fewer and fewer cues became available. Lashley and his associates countered by showing that removing the eyes has much less effect on maze learning than removing the visual area of the cortex. Others argued that Lashley removed more than the visual cortex. Out of this came a long series of lesion-behavior studies analyzing behavioral “functions” of the cerebral cortex. Beginning in the 1940s, several laboratories, including Lashley’s and those of Harry Harlow at the University of Wisconsin and Karl Pribram at Yale, took up the search for the more complex functions of association cortex using monkeys and humans.
Perhaps the most important single discovery in this field came from Brenda Milner’s studies with patient H. M. who, following bilateral temporal lobectomy (removing the hippocampus and other structures), lives forever in the present. Work on higher brain functions in monkeys and humans is one of the key roots of modern cognitive neuroscience, to be treated later. Since Milner’s work with H. M., the hippocampus has been of particular interest in biological psychology. Another facet of hippocampal study in the context of the biological psychology of memory is long-term potentiation (LTP), discovered by Bliss and Lomo (1973). Brief tetanic stimulation of monosynaptic inputs to the hippocampus causes a profound increase in synaptic excitability that can persist for hours or days. Many view it as a leading candidate for a mechanism of memory storage, although direct evidence is still lacking.
Yet another impetus to study of the hippocampus in the remarkable discovery of “place cells” by John O’Keefe (1979). When recording from single neurons in the hippocampus of the behaving rat, a give neuron may respond only when the animal is in a particular place in the environment (i.e., in a box or maze), reliably and repeatedly. There is great interest now in the possibility that LTP may be the mechanism forming place cells. A number of laboratories are making use of genetically altered mice to test this possibility.
Lashley’s influence is felt strongly through the many eminent physiological psychologists who worked or had contact with him. We select two examples here—Austin Riesen and Donald O. Hebb. We discuss Roger W. Sperry’s work next in the context of cognitive neuroscience. The basic problem of the development of perception fascinated Lashley and his students. How is it that we come to perceive the world as we do? Do we learn from experience or is it told to us by the brain? Riesen did pioneering studies in which he raised monkeys for periods of time in the dark and then tested their visual perception. They were clearly deficient.
This important work served as one of the stimuli for Hebb to develop a new theory of brain organization and function, which he outlined in The Organization of Behavior (1949). This book had an immediate and profound impact on the field. Hebb effectively challenged many traditional notions of brain organization and attempted to pull together several discordant themes—mass action and equipotentiality, effects of dark rearing on perception, the preorganization of sensory cortex, the lack of serious intellectual effects of removal of an entire hemisphere of the brain in a human child—into a coherent theory. Important influences of Gestalt notions can be seen in Hebb’s theory. He is a connectionist but in a modern sense: Connections must underlie brain organization but there is no need for them to be in series.
One concept in Hebb’s book has come to loom large (too large perhaps) in modern cognitive-computational neuroscience—the Hebb synapse:
When an axon of Cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased. (1949, p. 62)
Lashley’s pessimistic conclusions in his 1929 monograph put a real but temporary damper on the field concerned with brain substrates of memory. But other major traditions were developing. Perhaps the most important of these was the influence of Pavlov. His writings were not readily available to Western scientists, particularly Americans, until the publication of the English translation of his monumental work Conditioned Reflexes in 1927. It is probably fair to say this is the most important single book ever published in the field of behavioral neuroscience. Pavlov developed a vast and coherent body of empirical results characterizing the phenomena of conditioned responses, what he termed “psychic reflexes.” He argued that the mind could be fully understood by analysis of the higher order learned reflexes and their brain substrates. As an example of his influence, Clark Hull, in his Principles of Behavior (1943), wrote as though he were a student of Pavlov.
- Horsley Gantt, an American physician, worked with Pavlov for several years and then established a Pavlovian laboratory at Johns Hopkins. He trained several young psychologists, including Roger Loucks and Wulf Brogden, who became very influential in the field. Perhaps the most important modern behavioral analyses of Pavlovian conditioning are the works of Robert Rescorla and Allan Wagner (1972).
Although Pavlov worked with salivary secretion, most studies of classical conditioning in the West tended to utilize skeletal muscle response, à la Bechterev. Particularly productive have been Pavlovian conditioning of discrete skeletal reflexes (e.g., the eyeblink response), characterized behaviorally by Isadore Gormezano and Allan Wagner and analyzed neuronally by Richard Thompson and his many students, showing localization of the basic memory trace to the cerebellum (Thompson, 1986). Masao Ito and associates in Tokyo had discovered the phenomenon of long-term depression (LTD) in the cerebellar cortex (see Ito, 1984). Repeated conjunctive stimulation of the two major inputs to the cerebellum, mossy-parallel fibers and climbing fibers, yields a long-lasting decrease in the excitability of parallel fibers— Purkinje neuron synapses. Ito developed considerable evidence that this cerebellar process underlies plasticity of the vestibular-ocular reflex. Thompson and associates developed evidence, particularly using genetically altered mice, that cerebellar cortical LTD is one of the mechanisms underlying classical conditioning of eyeblink and other discrete responses.
Fear conditioning was characterized behaviorally by Neal Miller and analyzed neuronally by several groups, particularly Michael Davis (1992), Joseph LeDoux (2000), and Michael Fanselow (1994), and their many students. They showed that at least for classical conditioning of fear, the essential structure is the amygdala, which may contain the basic memory trace for this form of learning (but see just below). The process of LTP may serve to code the amygdalar fear memory.
Duncan’s discovery in 1949 of the effects of electroconvulsive shock on retention of simple habits in the rat began the modern field of memory consolidation. Hebb and Gerard were quick to point out the implication of two memory processes, one transient and fragile and the other more permanent and impervious. James McGaugh and his associates (1989) have done the classic work on the psychobiology of memory consolidation. He and his colleagues demonstrated memory facilitation with drugs and showed that these effects were direct and not due to possible reinforcement effects of the drugs (and similarly for ECS impairment).
The amygdala is critical for instrumental learning of fear. McGaugh and his associates demonstrated that for both passive and active avoidance learning (animals must either not respond, or respond quickly, to avoid shock) amygdala lesions made immediately after training abolished the learned fear. Surprisingly, if these same lesions were made a week after training, learned fear was not abolished, consistent with a process of consolidation (see McGaugh, 2000). The apparent difference in the role of the amygdala in classical and instrumental learning of fear is a major area of research today. Chemical approaches to learning and memory are recent. The possibility that protein molecules and RNA might serve to code memory was suggested some years ago by pioneers such as Gerard and Halstead. The RNA hypothesis was taken up by Hyden and associates in Sweden and by several groups in America. An unfortunate by-product of this approach was the “transfer of memory” by RNA. These experiments, done by investigators who shall remain nameless, in the end could not be replicated.
At the same time, several very productive lines of investigation of neurochemical and neuroanatomical substrates of learning were developing. In 1953, Krech and Rosenzweig began a collaborative study of relationships between brain chemistry and behavior. Krech did classic early work in animal learning (under his earlier name, Kreshevsky) and was a colleague of and collaborator with Tolman. Mark Rosenzweig received his PhD in physiological psychology at Harvard in 1949 and joined the psychology department at the University of California, Berkeley, in 1951. Soon after they began their joint work in 1953 they were joined by E. L. Bennett and later by M. C. Diamond. Their initial studies concerned brain levels of AChE in relation to the hypothesis behavior and included analysis of strain differences (see Krech, Rosenzweig, & Bennett, 1960). More recently, they discovered the striking differences in the brains of rats raised in “rich” versus “poor” environments.William Greenough (1984), at the Universityof Illinois, replicated and extended this work to demonstrate dramatic morphological changes in the structures of synapses and neurons as a result of experience.
The use of model biological systems has been an important tradition in the study of neural mechanisms of learning. This approach has been particularly successful in the analysis of habituation, itself a very simple form or model of learning. Sherrington did important work on flexion reflex “fatigue” in the spinal animal at the turn of the century. In 1936, Prosser and Hunter completed a pioneering study comparing habituation of startle response in intact rats and habituation of hindlimb flexion reflex in spinal rats. They established, for habituation, the basic approach of Sherrington, namely that spinal reflexes can serve as models of neural-behavioral processes in intact animals. Sharpless and Jasper (1956) established habituation as an important process in EEG activity. Modern Russian influences have been important in this field—the key studies of Evgeny Sokolov (1963), first on habituation of the orienting response in humans and more recently on mechanisms of habituation of responses in the simplified nervous system of the snail.
The defining properties of habituation were clearly established by Thompson and Spencer in 1966, and the analysis of mechanisms began. Several laboratories using different preparations—Aplysia withdrawal reflex; Kandel and his many associates (see Kandel, 1976); vertebrate spinal reflexes; Thompson, Spencer, Farel; crayfish tail flip escape; Krasne (1969), Kennedy—all arrived at the same underlying synaptic mechanism—a decrease in the probability of transmitter release from presynaptic terminals of the habituating pathway. Habituation is thus a very satisfying field; agreement ranges from defining behavioral properties to synaptic mechanisms. In a sense, the problem has been solved. Habituation also provides a most successful example of the use of the model biological systems approach to analysis of neural mechanisms of behavioral plasticity (see Groves & Thompson, 1970).
Special mention must be made of the elegant and detailed studies by Eric Kandel and his many associates on longlasting neuronal plasticity in the Aplysia gill-withdrawal circuit (Kandel, 1976; Hawkins, Kandel, & Siegelbaum, 1993). This simplified model system (together with work on the hippocampus) made it possible to elucidate putative processes that result in long-lasting synaptic plasticity, for example, biochemical models of memory formation and storage. Eric was awarded the Nobel Prize for Physiology and Medicine in 2000 in part for this work.
MOTIVATION AND EMOTION
Physiological and neural mechanisms of motivation and emotion have been a particular province of biological psychology and physiology in the twentieth century. In more recent years, the fields of motivation and emotion have tended to go separate ways (see Brown, 1961, 1979). However motivation and emotion have common historical origins. In the seventeenth and eighteenth centuries, instinct doctrine served as the explanation for why organisms were driven to behave (at least infrahuman organisms without souls). Darwin’s emphasis on the role of adaptive behavior in evolutionary survival resulted in the extension of instinct doctrine to human behavior. Major sources of impetus for this were Freud’s and McDougall’s notions of instinctive human motivation. Watson rebelled violently against the notion of instinct and rejected it out of hand, together with all biological mechanisms of motivation. As Lashley (1938) put it, he “threw out the baby with the bath.”
EMOTION
The dominant theory of emotion in the first two decades of the century was that of James and Lange—“We feel afraid because we run” (see James, 1884). Actually, James focused more on the subjective experience of emotion, and Lange, a Danish anatomist, focused on the physiological phenomena, believing that subjective experience is not a proper topic for science. But between them they developed a comprehensive theory of emotion. The basic idea is that we first perceive an emotionally arousing situation or stimulus (“a bear in the woods” is a favorite example), which leads to bodily (physiological) changes and activities, which result in the experienced emotion.
This general view was challenged by the American physiologist Walter B. Cannon in the 1920s and 1930s. He actually agreed with James and Lange that the initial event had to be perception of an emotion-arousing situation but argued that the development of autonomic (sympathetic) responses— release of epinephrine and other bodily changes—occurred concomitantly with the subjective feelings (see Cannon, 1927). However, his primary interest was in the physiology, particularly the peripheral physiology. Cannon’s view was championed by the distinguished Johns Hopkins physiologist Philip Bard, who stressed the key role of the brain, particularly the thalamus and hypothalamus, in both emotional behavior and experience (see Bard, 1934). Cannon, incidentally, also contributed the notion of homeostasis, which he developed from Bernard’s concept of the milieu interieur.
A key issue in these theories was the role of sympathetic arousal or activation in the experience of emotion. This issue was tested in a classic study by Stanley Schachter and Jerome Singer at Columbia University in 1962. They injected human subjects with either effective doses of epinephrine or a placebo. The epinephrine activated the sympathetic signs of emotions (pounding heart, dry mouth, etc.). Both groups of subjects were told they were receiving a shot of a new vitamin. Stooges acted out euphoria or anger in front of the subjects. The subjects were either informed of what the injection might do, for example, the autonomic side effects, or not informed. Results were dramatic. Uninformed epinephrine subjects reported emotional experiences like those of stooges but informed epinephrine subjects did not report any emotion at all. Emotion is more than sympathetic arousal—cognitive factors are also important.
Experimental work on brain substrates of emotion may be said to have begun with the studies of Karplus and Kreidl in 1910 on the effects of stimulating the hypothalamus. In 1928, Bard showed that the hypothalamus was responsible for “sham rage.” In the 1930s, S. W. Ranson and his associates at Northwestern, particularly H. W. Magoun, published a classic series of papers in the hypothalamus and its role in emotional behavior (Ranson & Magoun, 1939). In the same period, W. R. Hess (1957) and his collaborators in Switzerland were studying the effects of stimulating the hypothalamus in freely moving cats. A most important paper by H. Klüver and P. Bucy reported on “psychic blindness and other symptoms following bilateral temporal lobectomy in rhesus monkeys” in 1937. This came to be known as the KlüverBucy syndrome. The animals exhibited marked changes in motivation and aggressive behavior.
Pribram (Bucy’s first resident in neurosurgery) developed the surgical methods necessary to analyze the Klüver-Bucy syndrome.This analysis led to his discovery of the functions of the inferotemporal cortex in vision and to the exploration of the suggestions of J. W. Papez (1937) and P. D. MacLean (1949) that the structures of the limbic system (the “Papez” circuit) are concerned with motivation and emotion. However, modern neuroanatomy deconstructed the Papez circuit. The emphasis is now on the hypothalamus-pituitary axis, on descending neural systems, and on the amygdala.
MOTIVATION
Today most workers in the field prefer the term motivated behaviors to emphasize the specific features of behaviors relating to hunger, thirst, sex, temperature, and so forth. Karl Lashley was again a prime mover. His 1938 paper, “Experimental Analysis of Instinctive Behavior,” was the key. He argued that motivated behavior varies and is not simply a chain of instinctive or reflex acts, is not dependent on any one stimulus, and involves central state. His conclusions, that “physiologically, all drives are no more than expression of the activity of specific mechanisms” and that hormones “activate some central mechanism which maintains excitability and activity,” have a very modern ring.
Several key figures in the modern development of the psychobiology of motivation are Clifford Morgan, Eliot Stellar, Kurt Richter, Frank Beach, Neal Miller, Philip Teitelbaum, and James Olds. Morgan went to graduate school at Rochester, where his professors included E. A. K. Culler and K. U. Smith and his fellow graduate students included D. Neff, J. C. R. Licklider, and P. Fitts. He then became an instructor at Harvard, where he first worked in Lashley’s laboratory in 1939. He later moved to Johns Hopkins, where he remained until 1959. As a graduate student and later at Harvard, Morgan came to doubt Cannon’s then current notion that hunger was the result of stomach contractions. Morgan did a series of studies showing this could not be a complete or even satisfactory account of hunger and feeding behavior. Eliot Stellar and Robert McCleary, then undergraduates at Harvard, worked with Morgan. They focused on hoarding behavior and completed a classic analysis of the internal and environmental factors controlling the behavior.
Lashley’s general notion of a central mechanism that maintains activity was developed by Beach in an important series of papers in the 1940s and by Morgan in the first edition of his important text, Physiological Psychology (1943), into a central excitatory mechanism and ultimately a central theory of drive. This view was given a solid physiological basis by Donald B. Lindsley from the work he and H. W. Magoun, G. Moruzzi, and associates were doing on the ascending reticular activating system. Lindsley sketched his activation theory of emotion in his important chapter in the Stevens Handbook (1951). Hebb (1955) and Stellar (1954) pulled all these threads together into a general central theory of motivation.
Eliot Stellar worked with Clifford Morgan as an undergraduate at Harvard. After obtaining his doctorate in 1947 at Brown University, he spent several years at Johns Hopkins and joined the psychology department at the University of Pennsylvania in 1954. Stellar did extensive work on brain mechanism of motivation. He coauthored the revision of Morgan’s text in 1950 and published his influential central theory of drive in 1954.
Philip Teitelbaum (1955) did the classic work on characterization of, and recovery from, the lateral hypothalamic “aphagia” syndrome. He discovered the striking parallel with the ontogenetic development of feeding behavior. In addition, he discovered more general aspects of the syndrome, for example, “sensory neglect.”
Frank Beach received his doctorate from the University of Chicago under Lashley in 1940 and then joined the American Museum of Natural History in New York. He moved to Yale in 1946, and then to the University of California, Berkeley, in 1958. From the beginning, he focused on brain mechanisms of sexual behavior (see Beach, 1951). As the study of sexual behavior developed, hormonal factors came to the fore and the modern field of hormones and behavior developed. Beach played a critical role in the development of this field, as did the biologist W. C. Young of the University of Kansas. They and their students shaped the field as it exists today.
Even within the field of hormones and behavior, several fields have developed. Sexual behavior has become a field unto itself. Another important field is the general area of stress. The endocrinologist Hans Selye was an important intellectual influence. Kurt Richter, a pioneering figure in this field, took his BS at Harvard in 1917 and his doctorate at Johns Hopkins in 1921 and was a dominant influence at Hopkins. His early work was on motivation and feeding (see Richter, 1927). His pioneering “cafeteria studies” in rats are still a model (if given a wide choice of foods, they select a relatively balanced diet). Richter then focused on the adrenal gland, its role in diet and in stress. He also did pioneering work on circadian rhythms in mammals. The modern field of stress focuses on hormonal-behavioral interactions, particularly adrenal hormones, as in the work of Seymore Levine (1971).
Neal Miller represents a uniquely important tradition in biological psychology. From the beginning of his career, Miller was interested in physiological mechanisms of both motivation and learning. He took his doctorate at Yale in 1935 and stayed on at Yale for many years, with a year out in 1936 at the Vienna Psychoanalytic Institute. Throughout his career he has exemplified superb experimentation and an unusual ability to synthesize. He was a pioneer in early studies of punishing and rewarding brain stimulation and their roles in learning and in the study of conditioned fear (see Miller, 1948, 1961). In later years, his work focused on mechanisms of instrumental conditioning of autonomic responses—biofeedback techniques—and brain mechanisms of learning. The impact of his work is much wider than biological psychology, influencing learning theory, psychiatry, and clinical medicine as well.
James Olds, whose untimely death in 1976 cut short an extraordinary career, made the most important discovery yet in the field of motivation—rewarding electrical self-stimulation of the brain. He got his doctorate at Harvard and worked with Richard Solomon. Solomon, although primarily a behavioral student of learning, had considerable impact on biological psychology through his theoretical-experimental analysis of hypothetical central factors in learning. As a graduate student Olds read and was much influenced by Hebb’s Organization of Behavior and obtained a postdoctoral fellowship with Hebb at McGill in 1953. He began work there with Peter Milner. In his own words:
Just before we began our own work (using Hess’s technique for probing the brain), H. R. Delgado, W. W. Roberts, and N. E. Miller at Yale University had undertaken a similar study. They had located an area in the lower part of the mid-line system where stimulation caused the animal to avoid the behavior that provoked the electrical stimulus. We wished to investigate positive as well as negative effects (that is, to learn whether stimulation of some areas might be sought rather than avoided by the animal).
We were not at first concerned to hit very specific points in the brain, and, in fact, in our early tests the electrodes did not always go to the particular areas in the mid-line system at which they were aimed. Our lack of aim turned out to be a fortunate happening for us. In one animal the electrode missed its target and landed not in the mid-brain reticular system but in a nerve pathway from the rhinecephalon. This led to an unexpected discovery.
In the test experiment we were using, the animal was placed in a large box with corners labeled A, B, C, and D. Whenever the animal went to corner A, its brain was given a mild electric shock by the experimenter. When the test was performed on the animal with the electrode in the rhinencephalic nerve, it kept returning to corner A. After several such returns on the first day, it finally went to a different place and fell asleep. The next day, however, it seemed even more interested in corner A.
At this point we assumed that the stimulus must provoke curiosity; we did not yet think of it as a reward. Further experimentation on the same animal soon indicated, to our surprise, that its response to the stimulus was more than curiosity. On the second day, after the animal had acquired the habit of returning to corner A to be stimulated, we began trying to draw it away to corner B, giving it an electric shock whenever it took a step in that direction. Within a matter of five minutes the animal was in corner B. After this the animal could be directed to almost any spot in the box at the will of the experimenter. Every step in the right direction was paid with a small shock; on arrival at the appointed place the animal received a longer series of shocks.
After confirming this powerful effect of stimulation of brain areas by experiments with a series of animals, we set out to map the places in the brain where such an effect could be obtained. We wanted to measure the strength of the effect in each place. Here Skinner’s technique provided the means. By putting the animal in the “do-it-yourself” situation (i.e., pressing a lever to stimulate its own brain) we could translate the animal’s strength of “desire” into response frequency, which can be seen and measured.
The first animal in the Skinner box ended all doubts in our minds that electric stimulation applied to some parts of the brain could indeed provide a reward for behavior. The test displayed the phenomenon in bold relief where anyone who wanted to look could see it. Left to itself in the apparatus, the animal (after about two to five minutes of learning) stimulated its own brain regularly about once very five seconds, taking a stimulus of a second or so every time. (1956, pp. 107–108)
We think now that this brain reward circuit Olds discovered underlies addictive behaviors. It includes the medial forebrain bundle (MRB) containing the ascending dopamine (and other neurotransmitters) projection system to the nucleus accumbens and prefrontal cortex. Activation of this system appears to be a common element in what keeps drug users taking drugs. This activity is not unique to any one drug; all addictive drugs affect this circuit.
Another direction of research in motivation and emotion relating to brain stimulation concerns elicited behaviors, particularly from stimulation in the region of the hypothalamus. This work is in some ways a continuation of the early work by Hess. Thus, Hess described directed attack, from hypothalamic stimulation in cats, as opposed to the “sham” rage of decerebrate animals and certain other brain stimulation studies (“sham” because the animal exhibited peripheral signs of rage without integrated behavior) (see Hess, 1957). John Flynn, in a most important series of studies, was able to elicit two quite different forms of attack behavior in cats—one a quiet predation that resembled normal hunting and the other a rage attack (Flynn, Vonegas, Foote, & Edwards, 1970). Elliot Valenstein analyzed a variety of elicited consumatory-like behaviors—eating, drinking, gnawing, and so forth—from hypothalamic stimulation and their possible relations to the rewarding properties of such stimulation (Valenstein, Cox, & Kakolweski, 1970).
Current focus in the study of motivated behaviors is on detailed physiological processes, particularly involving mechanisms of gene expression of various peptide hormones in the hypothalamus and their actions on the pituitary gland, and on descending neural systems from the hypothalamus that act on lower brain systems to generate motivated behaviors (see e.g., Swanson, 1991). But we still do not understand the neural circuitries underlying the fact that seeing the bear in the woods makes us afraid.
COGNITIVE NEUROSCIENCE
The term cognitive neuroscience is very recent, dating perhaps from the 1980s. The Journal of Cognitive Neuroscience was first published in 1989. Indeed, Posner and Shulman’s comprehensive chapter on the history of cognitive science (1979) does not even mention cognitive neuroscience (human imaging techniques were not yet much in use then). Here we note briefly the biological roots of cognitive neuroscience (see Gazzaniga, 1995).
Karl Lashley was again a key figure. One of the most important aspects of cognitive neuroscience dates from the early days at the Orange Park laboratory, where young scientists like Chow and Pribram began studies of the roles of the association areas of the monkey cerebral cortex in learning, memory, and cognition.
The 1950s was an especially rich time of discovery regarding how cognitive function was organized in the brain. Pribram, Mortimer Mishkin, and Hal Rosvold at NIMH, using lesion studies in monkeys, discovered that the temporal lobe was critical for aspects of visual perception and memory. Work with neurologic patients also played a critical role in uncovering the neural substrates of cognition. One particular discovery became a landmark in the history of memory research. “In 1954 Scoville described a grave loss of recent memory which he had observed as a sequel to bilateral medial temporal resection in one psychotic patient and one patient with intractable seizures. In both cases . . . removals extended posteriorly along the medial surface of the temporal lobes . . . and probably destroyed the anterior two-thirds of the hippocampus and hippocampal gyrus bilaterally, as well as the uncus and amygdala. The unexpected and persistent memory deficit which resulted seemed to us to merit further investigation.”
That passage comes from the first paragraph of Scoville and Milner’s 1957 report, “Loss of Recent Memory after Bilateral Hippocampal Lesions.” This publication became a landmark in the history of memory research for two reasons. First, the severe memory impairment (or amnesia) could be linked directly to the brain tissue that had been removed, suggesting that the medial aspect of the temporal lobe was an important region for a particular aspect of cognition, that is, memory function. Second, comprehensive testing of one of the patients (H. M.) indicated that memory impairment could occur on a background of otherwise normal cognition. This observation showed that memory is an isolatable function, separable from perception and other cognitive and intellectual functions.
The findings from patient H. M. (Scoville & Milner, 1957) identified a region of the brain important for human memory, that is, the medial portion of the temporal lobe. The damage was originally reported to have included the amygdala, the periamygdaloid cortex (referred to as the uncus in Scoville & Milner, 1957), the hippocampal region (referred to as the hippocampus), and the perirhinal, entorhinal, and parahippocampal cortices (referred to as the hippocampal gyrus). Recently, magnetic resonance imaging of patient H. M. has shown that his medial temporal lobe damage does not extend as far posteriorly as originally believed and that damage to the parahippocampal cortex is minimal (the lesion extends caudally from the temporal pole approximately 5 cm, instead of 8 cm, as originally reported; Corkin, Amaral, Gonzalez, Johnson, & Hyman, 1997).
While these observations identified the medial temporal lobe as important for memory, the medial temporal lobe is a large region including many different structures. To determine which structures are important required that studies be undertaken in which the effects of damage to medial temporal lobe structures could be evaluated systematically. Accordingly, soon after the findings from H. M. were reported, efforts were made to develop an animal model of medial temporal lobe amnesia. During the next 20 years, however, findings from experimental animals with intended hippocampal lesions or larger lesions of the medial temporal lobe were inconsistent and difficult to interpret.
In 1978, Mishkin introduced a method for testing memory in monkeys that captured an important feature of tests sensitive to human memory impairment (Mishkin, 1978). This method allowed for the testing of memory for single events at some delay after the event occurred. The task itself is known as the trial-unique delayed-nonmatching-to-sample task, and it measures object recognition memory. In Mishkin’s study, three monkeys sustained large medial temporal lobe lesions that were intended to reproduce the damage in patient H. M. The operated monkeys and three unoperated monkeys were given the delayed-nonmatching-to-sample task in order to assess their ability to remember, after delays ranging from eight seconds to two minutes, which one of two objects they had recently seen. The monkeys with medial temporal lobe lesions were severely impaired on the nonmatching task, consistent with the severe impairment observed in patient H.M. on delay tasks. Thus, lesions that included the hippocampal region, the amygdala, as well as adjacent perirhinal, entorhinal, and parahippocampal cortices caused severe memory impairment. This work, together with work carried out in the succeeding few years, established a model of human amnesia in nonhumanprimates (Mishkin,Spiegler,&Saunders,1982;Squire& Zola-Morgan, 1983). Although other tasks have been useful for measuring memory in monkeys (object discrimination learning, the visual paired-comparison task; see below), much of the information about the effects of damage to medial temporal lobe structures has come, until recently, from the delayed-nonmatching-to-sample task.
Once the animal model was established, systematicandcumulative work eventually identified the structures in the medial temporal lobe that are important for memory. The important structures are the hippocampal region and the adjacent perirhinal, entorhinal, and parahippocampal cortices (for reviews, see Mishkin & Murray, 1994; Zola-Morgan & Squire, 1993). The amygdala proved not to be a component of this memory system, although it can exert a modulatory action on the kind of memory that depends on the medial temporal lobe system (Cahill & McGaugh, 1998).
The medial temporal lobe is necessary for establishing one kind of memory, what is termed long-term declarative or explicit memory. Declarative memory refers to the capacity for conscious recollection of facts and events (Squire, 1992). It is specialized for rapid, even one-trial learning, and for forming conjunctions between arbitrarily different stimuli. It is typically assessed in humans by tests of recall, recognition, or cued recall, and it is typically assessed in monkeys by tests of recognition (e.g., the delayed-nonmatching-to-sample task). The medial temporal lobe memory system appears to perform a critical function beginning at the time of learning in order that representations can be established in longterm memory in an enduring and usable form (see also Eichenbaum, Otto, & Cohen, 1994).
Another important discovery that paralleled in time the work on the medial temporal lobe system involved the understanding that there is more than one kind of memory. Specifically, work with amnesic patients and with experimental animals who sustained lesions to specific brain regions showed that other kinds of abilities (including skills, habit learning, simple forms of conditioning, and the phenomenon of priming, which are collectively referred to as nondeclarative memory) lie outside the province of the medial temporal lobe memory system. Nondeclarative forms of memory are intact in amnesic patients and intact in monkeys with medial temporal lobe lesions. For example, classical delay conditioning of skeletal musculature depends on the cerebellum (Thompson & Krupa, 1994), conditioning of emotional responses depends on the amygdala (Davis, 1992; LeDoux, 2000), and habit learning (win-stay, lose-shift responding) depends on the neostriatum (Packard, Hirsh, & White, 1989; Salmon & Butters, 1995). Nondeclarative memory thus refers to a variety of ways in which experience can lead to altered dispositions, preferences, and judgments without providing any conscious memory content.
Further work with monkeys has demonstrated that the severity of memory impairment depends on the locus and extent of damage within the medial temporal lobe memory system. Damage limited to the hippocampal region causes significant memory impairment, but damage to the adjacent cortex increases the severity of memory impairment. It is important to note that the discovery that larger medial temporal lobe lesions produce more severe amnesia than smaller lesions is compatible with the idea that structures within the medial temporal lobe might make qualitatively different contributions to memory function. This is because anatomical projections carrying information from different parts of the neocortex enter the medial temporal lobe memory system at different points (Suzuki & Amaral, 1994).
Another important brain area for memory is the diencephalon. However, the critical regions in the diencephalon that when damaged produce amnesia have not at the time of writing been identified with certainty. The important structures appear to include the mediodorsal thalamic nucleus, the anterior nucleus, the internal medullary lamina, the mammillo-thalamic tract, and the mammillary nuclei. Because diencephalic amnesia resembles medial temporal lobe amnesia in many ways, these two regions together probably form an anatomically linked, functional system.
These findings in monkeys are fully consistent with the findings from human amnesia. Damage limited to the hippocampal region is associated with moderately severe amnesia (Rempel-Clower, Zola, & Squire, 1996; Zola-Morgan, Squire, Rempel, Clower, & Amarel, 1992), and more extensive damage that includes the hippocampal region as well as adjacent cortical regions is associated with more severe memory impairment (Corkin, 1984; Mishkin, 1978; RempelClower et al., 1996; Scoville & Milner, 1957).
The same principle, that more extensive damage produces more severe impairment, has also been established for the hippocampus proper in the case of the rat (E. Moser, Moser, & Andersen, 1993; M. Moser, Moser, & Forrest, 1995). The dorsal hippocampus of the rat is essential for spatial learning in the water maze, and progressively larger lesions of this region produce a correspondingly larger impairment. Thus, in all three species it has turned out that the brain is organized such that memory is a distinct and separate cognitive function, which can be studied in isolation from perception and other intellectual abilities. Information is still accumulating about how memory is organized, what structures and connections are involved, and what functions they support. The disciplines of both psychology and neuroscience continue to contribute to this enterprise.
Roger Sperry was another key player in the origins of cognitive neuroscience. He received his doctorate in zoology at the University of Chicago and then joined Lashley for a year at Harvard and moved with Lashley to the Yerkes Primate Laboratory at Orange Park, where he stayed for some years. Sperry did his pioneering studies on the selective growth of brain connections during this time (see Sperry, 1951). Lashley was fascinated by the mind–brain issue—the brain substrates of consciousness (although he never wrote about it)—and often discussed this problem with his younger colleagues at Orange Park (Sperry, personal communication). In more recent years, Sperry and his associates at the California Institute of Technology tackled the issue with a series of commissurotomy patients—the human “split-brain” studies. This work proved to be extraordinary, perhaps the most important advance in the study of consciousness since the word itself was developed many thousands of years ago (Sperry, 1968).
Another key origin of the modern field of cognitive neuroscience is the study of humans with brain damage, as in Milner’s work on H. M. noted earlier. Other influential scientists in the development of this field were Hans-Lukas Teuber and Brenda Milner. Karl Pribram also played a critical role. Teuber received his early training at the University of Basel, obtained his doctorate at Harvard, and studied with Karl Lashley. He became chairman of the psychology department at MIT in 1961. In the 1940s, he published an important series of papers in collaboration with Bender and others on perceptual deficits following penetrating gunshot wounds of the brain. Later he also investigated the effects of frontal lesions on complex performance in humans.
Brenda Milner received her undergraduate training at Cambridge; then after the war she came to Canada and studied for her PhD at McGill University under Hebb’s supervision. Hebb arranged for her to work with Wilder Penfield’s neurosurgical patients at the Montreal Neurological Institute. Her work on temporal lobe removal in humans, including H. M., really began modern study of the memorial functions of the hippocampus (see earlier). She also collaborated on studies with Roger Sperry and with Karl Pribram.
Another very important influence in modern cognitive neuroscience comes from the Soviet scientist Alexander Luria, who died in 1977. Luria approached detection and evaluation of damage to higher regions of the human brain both as a clinician with extraordinary expertise in neurology and as a scientist interested in higher functions of the nervous system (e.g., his book Language and Cognition, 1981).
Yet another origin of cognitive neuroscience is recording the activity of the human brain, initially using the EEG. Donald Lindsley was a pioneer in this work. Lindsley did his graduate work at Iowa and worked with L. E. Travis, himself an important figure in psychophysiological recording. Lindsley then took a three-year postdoctoral at Harvard Medical School (1933–1935). The neurophysiologist Alexander Forbes was at Harvard doing pioneering studies on brain-evoked potentials and EEG in animals. The first human EEG recording laboratory was set up at Harvard, and Lindsley and other pioneering figures such as Hallowell Davis did the first EEG recording in America (Lindsley, 1936).
More recently, the method of averaging evoked potentials recorded from the human scalp made it possible to detect brain signals relevant to behavioral phenomena that could not be detected with individual trial recording. Donald Lindsley was a pioneer in this field as well, doing early studies on evoked potential correlates of attention. E. Roy John and others developed complex, comprehensive methods of quantitative analysis of EEG and evoked potential recordings.
But the techniques that have revolutionized the study of normal human brain organization and functions are of course the methods of imaging. The first such method was X-raycomputed tomography, developed in the early 1970s. The major innovation beyond simple X rays was complex mathematical and computer techniques to reconstruct the images.
Somewhat later, positron emission tomography (PET) was developed. It is actually based on a long used method in animal neuroanatomy—autoradiography. In this technique, a radioactive substance that binds to a particular type of molecule or brain region is infused and brain sections are prepared and exposed to X-ray film. For humans PET involves injecting radioactive substances, for example, radiolabeled oxygen (15O), in water. Increased neuronal activity in particular regions of the brain causes a rapid increase in blood flow to the regions, as shown years earlier in work by Seymore Kety and others. Consequently, the radioactive water in the blood becomes more concentrated in active brain areas and is detectable by radioactivity detectors.
The most widely used method at present is magnetic resonance imaging (MRI). This is based on the fact that changes in blood flow cause changes in the blood’s magnetic properties, which can be detected as changes in a strong imposed magnetic field. This method was first used in 1990 (Ogawa, Lee, Kay, & Tank). The current procedure is termed functional MRI (fMRI), involving very fast acquisition of images. A landmark publication in human brain imaging is the elegant book by two pioneers in the field, Michael Posner and Marcus Raichle, Images of Mind (1994). The fMRI procedures have several advantages, such as the fact that they are noninvasive—no radioactive substance is injected—and provide better spatial resolution than does PET imaging. Functional magnetic resonance imaging exploits variations in magnetic susceptibility that arise from molecular binding of oxygen to hemoglobin, which can be used to detect blood flow changes associated with neuronal activity. At the present time, these neuronal activity-related signals can be derived from areas of the brain with a spatial resolution of 1 to 2 mm. Moreover, the temporal resolution of this functional imaging technique is compatible with the time course needed to carry out most perceptual and cognitive operations. An important and promising strategy for the use of fMRI is its use in conjunction with other kinds of neurobiological techniques, including neurophysiology and anatomical and behavioral analyses. Thus, fMRI provides an extraordinary new window through which one can probe the neural machinery of cognition (Albright, 2000).
CONCLUSION
Physiological psychology, the field concerned with biological substrates of behavior and experience (mind), has to be the most important discipline in psychology and the life sciences. The two great questions in science are the nature of the universe and the nature of the mind. Over the past century, the field of physiological psychology has spun off a number of areas that are now separate fields in their own right: vision, audition, psychophysiology, behavioral genetics, behavioral neuroscience, and cognitive neuroscience. It seems that in this sense physiological psychology is destined to selfdestruct. But to participate in the process is surely among the most exciting intellectual endeavors of our time.
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