Story of Psychology - Morton Hunt [73]
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.