by Morton Hunt
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:
In my opinion, there can be no possible sense in speaking of any other truth of our ideas except a practical truth. Our ideas of things cannot be anything but symbols, natural signs for things that we learn how to use in order to regulate our movements and actions. Having learned how to read those symbols correctly, we are able by their help to adjust our actions so as to bring about the desired result; that is, so that the expected new sensations will arise… Hence there is no sense in asking whether vermilion [mercuric sulfide], as we see it, is really red or whether this is simply an illusion of the senses. The sensation of red is the normal reaction of normally formed eyes to light reflected from vermilion… The statement that the waves of light reflected from vermilion have a certain length is something different; that is true entirely without reference to the special nature of our eye.32
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.
In Optics Helmholtz dealt chiefly with the physics and physiology of vision and made some keen observations about the psychological processes by which the mind interprets messages from the optic nerves. He drew an invaluable distinction that had eluded earlier psychologists between sensation (the excitation of the retina’s rods by light of whatever color, and the resultant impulses of the optic nerves) and perception (the meaningful interpretations the mind makes of the arriving impulses). He made the same differentiation about the input of other sensory systems.
The distinction was central to Helmholtz’s epistemology. He agreed with Kant that sensations are interpreted and given meaning by the mind, but disagreed that the mind innately possesses “categories” and “intuitions” that supply those meanings. Rather, he said, the mind learns to interpret sensations by means of trial and error—by learning which reactions to a visual sensation produce an expected result and which do not.
Space perception is a case in point. Kant said that the mind innately intuits spatial relationships; Helmholtz argued that we learn about space by means of unconscious inference. As infants, we learn little by little that such visual clues as size, direction, and intensity of hue are related to whether objects are closer or farther, to one side or the other of us, above or below us; through experience we gradually come to make correct judgments about spatial relations. (Every parent who has watched a three-month-old trying to grasp an object knows the process intimately.)
The British empiricist-associationists had said much the same thing but lacked experimental evidence to back it up; Helmholtz, an experimentalist through and through, supported his theory with research findings.
It occurred to him that if he could distort the spatial sensations reaching a subject’s brain—and if his theory was correct—the subject should adapt to the distorted vision and learn to interpret it correctly. He therefore constructed eyeglasses with prismatic lenses that shifted the apparent position of objects to the right of where they actually were. When subjects wearing the glasses tried to touch objects in front of them, they missed—they reached toward the apparent rather than the real position of the objects.
Next, for some minutes he had them reach for and handle the objects while wearing the lenses; at first they had to consciously reach to the left of where they saw the object, but soon they began to reach for objects where they actually were without having to think about it. They had made a perceptual adaptation; their minds had reinterpreted the messages arriving from the optic nerves and they now saw the objects in the context of reality.
Finally, when they took off the spectacles and reached for the objects, they missed again, this time erring to the left of the real position; it took a little while for their normal space orientation to reassert itself.
Helmholtz did agree with Kant about one innate capacity, the ability to interpret cause-and-effect relationships. For the rest, he maintained that virtually all knowledge and ideas are the result of the mind’s interpretation of sensory experience, and that these interpretations, particularly those having to do with spatial perception, are largely the product of unconscious inference.
This view was strongly opposed by psychologists who held that the mind is innately equipped to interpret its perceptions. A key function they explained in innate terms was the combining of the two images coming from the eyes to form a single three-dimensional image. Some said that each point on the retina receives exactly the same bit of information as the corresponding point on the other retina and that the two optic nerves thus combine their images into one. One opponent of Helmholtz’s ideas said that each retina is endowed with innate “signs” that distinguis
h height, right-left orientation, and depth and that enable the nervous system to fuse the images before they reach the brain.
Helmholtz brusquely dismissed these notions. Nativist theory, he wrote, was “an unnecessary hypothesis”; it relied on unprovable assumptions and added nothing to the demonstrable facts of empiricist theory.33 His strongest evidence that experience is what enables us to perceive paired images as a single one came from the stereoscope. Through this instrument, invented by Charles Wheatstone in 1833, the viewer sees not two identical images but two slightly different ones taken from slightly different angles. The images cast on the retinas therefore do not match point for point, yet after a novice viewer looks through the stereoscope for a little while, he or she suddenly sees a single image—in three dimensions. The fusion of two nonidentical images yields a result different from either one; the result comes from experience and takes place in the brain.
In the end, Helmholtz did not completely vanquish his opponents; nativism survived in one guise or another, including Gestalt psychology and, more recently, genetic psychology, studies of temperament, and, still more recently, evolutionary psychology. But the mainstream of psychology from Helmholtz’s time on has been largely empiricist and experimental. He, who did not consider himself a psychologist, would have been surprised to learn that he had a more profound and lasting influence on psychology than on physics or physiology.
Psychophysics: Fechner
While sensible, normal young Helmholtz was beginning to amass evidence for his mechanistic view of neural and psychic events, a visionary, neurotic middle-aged professor at the University of Leipzig was seeking to demonstrate that every person, animal, and plant in the universe is composed of both matter and soul. Gustav Theodor Fechner (1801–1887) failed in that aim, but in gathering data to show the mathematical relationship between stimuli (the world of matter) and the resulting sensations (the world of mind or soul)—which, he thought, confirmed his panpsychic philosophy—he developed research methods that have been used ever since by experimental psychologists to advance the materialist psychology he meant to invalidate.34
Fechner, born in a village in southeastern Germany, was the son of the local pastor. The father combined religious faith with a hard-headed belief in science, as would his son. He preached the word of the Lord but shocked the villagers by installing a lightning rod on the church, a precaution that in those days was seen as a lack of faith in God’s care of His own.
Fechner studied medicine at the University of Leipzig, but in 1822, after receiving his degree, switched his attention to physics and mathematics. For several years he supported himself by translating into German a number of French manuals on physics and chemistry—nine thousand pages’ worth of them in a few years—and from 1824 lectured on physics at the university, conducted a heavy research program on electrical currents, and wrote numerous technical articles. The hectic pace made his reputation in physics, but at a cost: he began suffering from headaches and spells of inability to control his thoughts, which would obsessively go around and around on matters of no importance.
Although only in his early thirties and prospering—he was able to marry by 1833 and was made a full professor in 1834—his condition continued to deteriorate. “I could not sleep and suffered from attacks of total exhaustion which robbed me of the ability to think and caused me to lose all interest in life,” he later said of this period.35 He sought relief in spas, but to no avail. He then distracted himself by studying afterimages—his first foray into experimental psychology—in the course of which he stared at the sun through tinted glasses for long periods. His research on afterimages was well received—Helmholtz, as we know, made use of the data—but as a result of it Fechner suffered severe photophobia and total emotional collapse.
Virtually blind, he immured himself in a darkened room, where he was tormented by pain, emotional distress, intolerable boredom, and severe digestive problems. (He resigned from the university but was granted a pension, although he had been teaching only half a dozen years.) At the nadir of three years of invalidism, he had his room painted black, remained in it day and night, and saw no one. Not laxatives, steam treatments, mesmerism, nor two kinds of shock treatments did any good. He continued to be troubled by repetitive thinking about minor matters; in addition he was torn between an exalted sense that he was close to discovering the secret of the world and a troubling feeling that he would have to demonstrate the truth of that secret by scientific methods.
At last he began spontaneously to improve and after a while could see without pain and talk to people. When he walked in the garden for the first time in many months, the flowers looked brighter, more intensely colored, and more beautiful than ever; he perceived an inner light in them, the significance of which he instantly grasped:
I had no doubt that I had discovered the soul of the flower, and thought in my strangely enchanted mood: this is the garden that lies behind the boards of this world. The whole earth and its very body is merely a fence around this garden for those who still wait on the outside.36
He soon wrote a book about the mental life of plants and for the rest of his many years sought to promote his panpsychist theory that consciousness coexists with matter throughout the world.
It was this mystical belief that led Fechner to his historic work in experimental psychology. Lying in bed on the morning of October 22, 1850, pondering how to prove to the mechanists that mind and body were two aspects of a fundamental unity, he had a flash of insight: If he could show a consistent mathematical relationship between the force of stimuli and the intensity of the sensations they produced, he would have shown the identity of body and mind.37
Or so it seemed to him; the logic of the reasoning may escape the nonmystic. But he had asked a valid and important question about the accuracy with which the mind perceives the outer world: Is there a consistent mathematical relationship between the magnitude of a stimulus and the magnitude of the sensation it creates? Intuitively, it might seem so: the brighter a light, the brighter it looks to us. But if you double the light, do you double the intensity of sensation? Or does some other, less verisimilar relationship prevail?
Fechner, trained in both physics and mathematics, sensed that as the intensity of a stimulus increased, it would require ever larger differences (in absolute terms) to produce increases of constant size in sensation. In mathematical terminology: Geometrical increases in the strength of the stimulus would produce arithmetical increases in the strength of the sensation. A contemporary illustration: In terms of energy delivered to the ear, an average clap of thunder is many times as powerful as ordinary conversation; in terms of decibels—a decibel is the smallest difference in loudness the human ear can recognize—it is only twice as loud.
To confirm his intuition experimentally, Fechner would have to solve a seemingly insoluble problem: He could easily measure stimulus intensity, but sensations are subjective and incapable of being measured. He reasoned, however, that though he could not observe and measure sensation directly, he could do so indirectly by using sensitivity as a guide. He could determine the smallest increase in stimulus strength at any level that would be just barely noticeable to the perceiver. Since “just barely noticeable” meant the same thing at any level, that would be a unit of measurement of sensation he could compare with the increase in stimulus necessary to produce that awareness.
Fechner later said that he did not get this idea from Weber, his former teacher, whose work on j.n.d.’s had been published a few years earlier. But he soon realized that he would be using and extending Weber’s Law. Weber had found that the ratio between two just noticeably different stimuli remains the same, whatever the magnitude of those stimuli; Fechner was saying that although the absolute difference between two stimuli increases as the magnitude of the stimuli does, the perceiver’s sensation of a just noticeable difference remains the same.
Imagine (Fechner later wrote) that you look at the sky through a tinted glass and pick out a cl
oud that is just noticeably different from the sky background. Now you use a much darker glass; the cloud does not vanish but is still just barely visible—because although the absolute levels of intensity are much lower through the darker glass, the ratio of intensities between cloud and sky has not changed.38
To express the relationship between stimulus intensity and sensation intensity, Fechner mathematically transformed Weber’s Law, integrating it and making it:
which means, in English, that stepwise increases in sensation intensity are the result of doublings of stimulus intensity (multiplied by some ratio or factor). Bending over backward to give credit to his former teacher, Fechner called this Weber’s Law—it was he who gave the name to Weber’s formula and his own—but later psychologists, giving credit where credit is due, have called the reformulation Fechner’s Law.
Fechner spent the next nine years in plodding experimentation, collecting data to confirm the law. Despite the mystical and poetic aspects of his personality, in the laboratory he was the very model of a compulsive and rigorous researcher. He tirelessly had subjects lift weights, look at lights, listen to noises and tones, look at color samples, and so on, and pronounce them either different or the same. Over those years he experimented with a wide range of intensities of each kind of stimulus, using three methods of measuring such judgments. With just one of those methods he tabulated and computed no fewer than 24,576 judgments.39He considered this first systematic exploration of the quantitative relationship between the physical and psychological realms a new scientific specialty and named it “psychophysics.”
