The Story of Psychology

by Morton Hunt

  Some of his own students, however, later in the century followed the lead of his honest confession of uncertainty and showed that all nerve transmissions possess the same characteristics and that it is indeed the end-location in the brain that determines the kind of experience created by the transmissions.20

  Nevertheless, Müller’s physiology began to answer one of the great questions that had puzzled philosophers and protopsychologists: How do the realities of the world around us become perceptions in our minds? A detailed picture of how perception works was beginning to emerge. The process starts with the optical properties of the eyeball or the auditory machinery of the ear (both of which Müller investigated in detail), continues with the nerves that convey the stimulation coming from the sensory organs, and concludes with the brain areas that receive and interpret those nerve impulses. As opposed to the ancients’ supposition that a tiny replica of whatever is perceived passes through the air and nerves to the brain, Müller showed that what is transmitted to the brain are nerve impulses; our perceptions are not replicas of, but analogues or isomorphs of, the objects around us. As he put it:

  The immediate objects of the perception of our senses are merely particular states induced in the nerves and felt as sensations either by the nerves themselves or by the parts of the brain concerned with sensation. The nerves make known to the brain, by virtue of the changes produced in them by external causes, the changes of condition of external bodies.21

  But how do we know that what our brains make of the incoming excitations corresponds to reality? This issue, which had so plagued earlier philosophers and psychologists, seemed to him to be readily answerable. The state of our nerves corresponds to that of objects in suitable and regular ways; the image on the retina, for instance, is a reasonably faithful portrayal of what is outside, and that is the stimulus the optic nerves carry to the brain. So, too, with the responses of the other sense organs and the messages they transmit.22 Müller thus answered the epistemological conundrum posed by Berkeley and Hume and transformed the untestable Kantian categories into testable and observable realities. Wrong in its details, his doctrine of specific energies was right in its most profound implications.

  Just Noticeable Differences: Weber

  At the University of Leipzig, in the early 1830s, a bearded young professor of physiology was conducting perception research totally unlike Müller’s. No scalpel and no laid-open frogs’ legs or rabbit skulls for Ernst Heinrich Weber; he chose to work with healthy, intact human volunteers—students, townspeople, friends—and to use such prosaic instruments as little apothecary’s weights, lamps, pen and paper, and thick knitting needles.

  Knitting needles?

  Let us look in on Weber on a typical day. He blackens the tip of a needle with carbon powder and gently lowers it perpendicularly onto the shirtless back of a young man lying prone on a table.23 It leaves a tiny black dot on the young man’s back. Now Weber asks him to try to touch that place with a similarly blackened little pointer. The young man, trying, touches a place a couple of inches away, and Weber carefully measures the distance between the two dots and records it in a workbook.24He does this again and again on different parts of the man’s back, then his chest, arms, and face.

  A year or so later, carrying on this line of inquiry, he opens a drafts-man’s compass and touches both ends to different places on the body of a blindfolded man. When the legs of the compass are far apart, the volunteer knows he is being touched by two points, but as Weber brings the legs closer together, the subject finds it ever harder to say whether there were two points or one until, at a critical distance, he perceives the two as one. The critical distance, Weber discovers, varies according to the part of the body. On the tip of the tongue, it is less than a twentieth of an inch; on the cheeks, half an inch; and along the backbone, anywhere up to two and a half inches—a more than fifty-fold range of sensitivities and a dramatic indication of the relative number of nerve endings in each area.

  All of Weber’s many experiments on the sensitivity of the sensory systems were similarly simple—and important in the history of psychology. At a time when most other mechanists were working only with reflexes and nerve transmission, Weber was looking at the entire sensory system: not just organs and the consequent nerve responses but the mind’s interpretation of them. Moreover, his were among psychology’s first true experiments; that is, he altered one variable at a time—in the two-point threshold test, the area of the body being tested—and observed how much change that caused in a second variable—the critical distance between the two compass points.

  To recognize how remarkable it was of Weber to conduct such experiments in the early 1830s, consider the period. James Mill, without budging from his desk, was espousing simplistic associationism; Johann Friedrich Herbart, occupying Kant’s chair at Göttingen, was maintaining, as Kant had, that psychology could never be an experimental science; Johann Christoph Spurzheim, at the peak of his popularity, was assuring crowds of enthusiasts that phrenologists could read a person’s character from the shape of his skull.

  Weber (1795–1878), born in Wittenberg in Saxony, was one of three brothers, all of whom became scientists of distinction and, at times, worked together. Wilhelm, a physicist, aided Weber in his research on touch; Eduard, a physiologist, discovered along with him the paradoxical effect of the vagus nerve, which, when stimulated, slows the heartbeat.25

  Like many another psychological mechanist, Weber had had medical training and specialized in physiological and anatomical research. Early in his career, he became interested in determining the minimum tactile stimulation necessary to produce a sensation of touch in different parts of the body, but soon moved on to a more complicated and interesting question about perceptual sensitivity. Many years earlier, the Swiss mathematician Daniel Bernoulli had made a psychologically shrewd observation: a poor man who gains a franc feels far more enriched by it than does a wealthy man; the perception of gain produced by any given sum of money depends on one’s economic status. This led Weber to formulate an analogous hypothesis: The smallest difference we can perceive between two stimuli—two weights, for instance—is not an objective, fixed amount but is subjective and varies with the weights of the objects.

  To test the hypothesis, Weber asked volunteers to heft first one small weight and then a second, and say which was heavier. Using a graduated series of weights, he was able to ascertain the smallest difference—the “just noticeable difference” (j.n.d.)—that his subjects could perceive. As he had correctly surmised, the j.n.d. was not a specific unvarying weight. The heavier the first weight, the greater the difference had to be before his subjects could perceive it, and the lighter the first weight, the greater their perceptual sensitivity. “The smallest perceptible difference,” he later reported, was “that between two weights standing approximately in the relation of 39 to 40: that is, one of which is about a fortieth heavier than the other.”26 If the first weight was an ounce, the j.n.d. of a second weight was a fortieth of an ounce; if ten ounces, a quarter of an ounce.

  Weber went on to conduct similar experiments on other sensory systems, determining the j.n.d. between, among other things, the length of two lines, the temperatures of two objects, the brightness of two lights, the pitch of two tones. In every case he found that the magnitude of the j.n.d. varied with the magnitude of the standard stimulus (the one with which a second was being compared) and that the ratio between the two stimuli was constant. Interestingly, the ratio of the j.n.d. to the standard varied widely among the different sensory systems. Vision was the most sensitive, detecting differences as small as a sixtieth in the intensity of light. In the case of pain, the minimum perceivable difference was a thirtieth; of pitch perception, a tenth; of smell, a quarter; and of taste, a third.27 Weber summed up the rule in a simple formula:

  which says that the ratio between the just noticeable stimulus, δ (R), and the magnitude of the standard stimulus, R, is a constant, k, for any sensory system. Known as Weber�
��s Law, it is the first statement of its kind—a quantitatively precise relationship between the physical and psychological worlds. It was the prototype of the kind of generalization that experimental psychologists would be looking for from then on.

  Neural Physiology: von Helmholtz

  In 1845, a handful of young physiologists, most of them former students of Müller’s, formed a little club, the Berliner Physikalische Gesellschaft (Berlin Physical Society), to promote their view that all phenomena, including neural and mental processes, could be accounted for in terms of physical principles. It was one of the group, Du Bois-Reymond, who had earlier stated the mechanist doctrine mentioned above, “No forces other than the common physical-chemical ones are active within the organism.”

  Du Bois-Reymond brought to the club a friend, Hermann Helmholtz (1821–1894), who was surgeon of a regiment stationed in Potsdam.28He was a shy, serious young man with a broad forehead and large intense eyes; neither by personality nor position did he seem likely to become the front-runner for the society’s radical theory. But within a few years he was just that. His research on nerve transmission, color vision, hearing, and space perception clearly showed that the neurological processes underlying mental functions are material and can be experimentally investigated.

  Helmholtz never thought of himself as a psychologist; his major interest was physics. Although the first twenty years of his career were devoted largely to physiology, his goal during that period was to explain perception in terms of the physics of the sense organs and nervous system; in so doing, he exerted a major influence on experimental psychology. Ironically, in his own time Helmholtz’s best-known scientific achievement was one that took him only eight days and that he himself considered minor—the invention of the ophthalmoscope, with which doctors could for the first time view the living retina.

  Although Helmholtz became one of the leading scientists of his century—his achievements earned him elevation to the nobility (hence the “von”)—he was totally unlike the scientist he most admired, the ferociously competitive, dour, reclusive Isaac Newton. Toward fellow scientists he was courteous and generous, if rather formal, and in private life he was a remarkably normal middle-class Herr Professor; his biography offers no frissons. He got a good grounding in the classics and philosophy from his father, a poorly paid teacher of philosophy and literature at the Potsdam gymnasium; went through medical training, wrote his dissertation under Müller, and served five years as a regimental surgeon; married when he received his first academic appointment and had two children; was widowed, married again, and had three more children. His career consisted of ever-better posts at ever-better universities, constant research and writing, and growing status and acclaim. He engaged in no priority fights and only one scientific controversy, and his only recorded indulgences were classical music and mountaineering.

  Helmholtz began his research career during his obligatory service in the military. Since it was peacetime, he had plenty of leisure, and he built a small laboratory in his barracks and conducted experiments on frogs with the aim of supporting a mechanistic view of behavior. He measured the energy and heat produced by the frog’s body and was able to account for it entirely in terms of the oxidation of the food the frog ingested. Today this sounds hardly revolutionary, but in 1845 many physiologists were “vitalists,” who believed that the processes of life were in part powered by an immaterial and imperceptible “vital force,” a sort of latter-day version of soul (though said to exist in all living things).

  Helmholtz, firmly opposed to this quasi-mystical view, wrote a paper titled “The Conservation of Force,” based on his frog data and his knowledge of physics, and presented it before the Berlin Physical Society in 1847. His thesis was that all machines obey the law of conservation of energy; therefore, perpetual motion is impossible. He then argued that this is true of organic processes, too, and that vital force, having no source of energy, would violate that law and hence did not exist. In short, he put physiology on a firmly Newtonian footing. The paper won him such respect that the Prussian government excused him from further military service, made him a lecturer on anatomy at the Berlin Academy of Arts, and a year later appointed him professor of physiology at the University of Königsberg.

  For the next two decades, Helmholtz devoted himself largely to studies of the physiology of sensation and perception. (From then on, he concerned himself chiefly with physics, at the University of Berlin.)

  His historic first research achievement was to measure the speed with which the nerve impulse travels along the nerve fiber. His mentor, Müller, like most other physiologists of the time, had taken Galvani’s discovery of the electrical nature of the nerve impulse to mean that the nervous system was somewhat like a set of continuous wires through which the current flowed at extremely high speed—roughly the speed of light, according to one reckoning. But Helmholtz’s friend Du Bois-Reymond had chemically analyzed nerve fibers and suggested that the impulse might be not purely electrical but electrochemical; if so, it would be relatively slow.

  In his laboratory at Königsberg, Helmholtz undertook to measure the speed of the impulse in a frog’s motor nerve. Since the high-speed chronoscope was not yet generally available—the first one was then in development—he ingeniously rigged a galvanometer to a frog’s leg (with the motor nerve attached) in such a way that a needle drawing a line on a revolving drum would show the time elapsed between the instant a current was applied to the upper end of the nerve and the subsequent kick of the foot. Knowing the distance between stimulus and the foot muscle, Helmholtz could then calculate the speed of the nerve impulse; it proved to be remarkably slow, about ninety feet per second.

  He also measured the speed of the nerve impulse in human subjects, asking volunteers to signal with a hand as soon as they felt a tiny current he applied either to toe or thigh. These experiments yielded figures ranging from 165 to 330 feet per second, but Helmholtz considered them less reliable than those based on the frog’s leg; something about the testing of humans made for wide variability.

  At first his results, published in 1850, were not widely accepted; they were too hard to believe. Physiologists were still wedded to the notion that either immaterial animal spirits or electricity flowed through the nervous system, and Helmholtz’s data supported a different theory, namely, that the nerve impulse consisted of the complex movements of particles. Moreover, his findings contradicted common experience. We seem to feel a touch on finger or toe the instant the contact is made; we seem to move a finger or toe the instant we mean to.

  Yet his evidence could not be gainsaid, and after initial resistance, his theory won general acceptance. Had he done nothing else, this alone would have made him one of the immortals of psychology, since it prepared the way, says Edwin Boring, “for all the later work of experimental psychology on the chronometry of mental acts and reaction times…It brought the soul to time, as it were, measured what had been ineffable, actually captured the essential agent of mind in the toils of natural science.”29

  Here we make a brief detour, looking ahead eighteen years to view a significant offshoot of Helmholtz’s study: the first attempt to measure the speed of higher mental processes.

  A Dutch ophthalmologist named Franciscus Cornelius Donders (1818–1889) with no background in psychology was intrigued by Helmholtz’s research on the speed of the neural impulse and speculated that because nerve impulses take time, higher mental processes probably do so, too.30 The lag between stimulus and voluntary response, he hypothesized, was due in part to nerve transmission and in part to the time taken by thought processes.

  In 1868, Donders devised and conducted an imaginative experiment to test his hypothesis and measure the mental processes at work. He asked subjects to respond to a nonsense sound, like ki, by repeating it as quickly as possible. A pointer making a track on a revolving drum would jiggle in response to the vibration of both ki s, and the distance between jiggles would be a measure of the time lag.

&
nbsp; In the simplest case, the subject knew what the sound would be and what the right response would be; the lag between stimulus and response was therefore simple reaction time. But what if subjects had to do mental work of some kind? What if the experimenter uttered any one of several sounds, such as ki, ko, or ku, and subjects had to imitate the sound as quickly as possible? If this took longer than simple reaction, Donders reasoned, the difference must be a measure of two mental processes: discrimination (among the sounds heard) and choice (of the correct response).

  Donders also thought of a way to disentangle these two mental processes and obtain a measure for each. If he told subjects that they might hear ki, ko, or ku but were to imitate only ki and remain silent in response to the others, they would, by not repeating ko or ku, be discriminating among the sounds but not choosing a response. By subtracting the discrimination time from the discrimination-plus-choice time, Donders would get a measure of choice time.

  The results were striking. On the average, discrimination took thirty-nine milliseconds more than simple reaction time, and discrimination-plus-choice seventy-five milliseconds longer than simple reaction time. Choice thus apparently accounted for thirty-six milliseconds.

  Donders optimistically created a number of more complicated procedures in the belief that the time each mental process took would add to the time the other processes had taken, and that each could be measured by the subtraction. But it did not work out well; the differences in times proved to be unreliable and only sometimes additive. Later psychologists would greatly modify Donders’s methods.