The Story of Psychology (22 page)

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.
²⁸
He 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.

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.

Still, he had shown beyond doubt that some of the time taken by responses involving cognitive activity was spent by that activity. Far more important, he had used elapsed time as a way to investigate unseen psychological processes; according to one recent appraisal of his work, “With Donders’s discovery of a means of apparently measuring the higher mental processes, a new era had begun.”
³¹

We retrace our steps to 1852 and to Helmholtz. Soon after establishing the speed of the nerve impulse and inventing the ophthalmoscope, he became interested in the problem of color vision. Ever since Newton’s discovery in 1672 that the white light of the sun was a mixture of light of all visible colors, physiologists and psychologists had tried to figure out how the eye and mind perceive colors. What was most puzzling was that we see white when light of all colors is mixed, but also when two complementary
colors, such as a particular shade of red and one of blue-green, are mixed; similarly, we see orange when exposed to pure orange light, but also when red and yellow light are mixed.

As a physicist, Helmholtz knew that three specific colors—particular hues of red, blue-violet, and green—could, mixed in the proper proportions, reproduce any other color; these are the
primary
colors.
*
He reasoned that this meant human vision can detect those three colors and hypothesized that the retina must have three different kinds of receptor cells, each furnished with a chemical sensitive to one of the primary colors. Relying on Müller’s doctrine of specific nerve energies, he suggested that the nerves leading from each receptor to the brain conveyed not just visual messages but specific color messages.

An English scientist, Thomas Young, had advanced somewhat the same theory in 1802, but without experimental evidence; it had been generally ignored. Helmholtz, however, amassed a variety of supportive evidence, including that of the colors we experience when lights of different hues are mixed, the afterimage of a complementary color that we see after staring at a strong color for a while, the kinds of color blindness that exist in some people and animals, the influence of particular brain lesions on color vision, and so on. He generously acknowledged Young’s priority, and his account of color vision has been known ever since as the Young-Helmholtz theory (or the trichromatic theory).

The color theory, a testable mechanistic explanation of how the mind perceives colors, was a stunning achievement. Link by link, from the outside world to the receptive area of the brain, Helmholtz had forged a chain of causal events that replaced the guesses and fantasies of philosophers and physiologists. It is still the reigning theory of color vision, though in more complex form and stripped of the notion that the nerves from each kind of receptor carried different kinds of energy.

As for the profoundly troubling question about perception asked by Democritus, Berkeley, Hume, and others—whether what we see is a true representation of what is out there—Helmholtz, far more mechanistic than Müller, dismissed it as being without meaning or value:

Thus the mechanist physiologist was, after all, a philosopher of psychology, and one to reckon with.

Helmholtz’s color vision research was only one facet of a comprehensive inquiry into visual perception that he carried on for a number of years. The fruits of this labor, his
Handbook of Physiological Optics
(1856–1867), ran to half a million words and covered all previous research in the field as well as his own; for several generations it remained the standard authority on the optical and neural properties of the eye. He also performed a similar service for hearing in another, not quite so massive, work.