The Experimental Analysis of Behavior
The 1957 American Scientist article, reproduced in full
In speaking about colors projected on the key or the fact that a key is on the right or left, we are, of course, talking about stimuli. Moreover, they are stimuli which act prior to the appearance of a response and thus occur in the temporal order characteristic of the reflex. But they are not eliciting stimuli; they merely modify the probability that a response will occur, and they do this over a very wide range. The general rule seems to be that the stimuli present at the moment of reinforcement produce a maximal probability that the response will be repeated. Any change in the stimulating situation reduces the probability. This relationship is beautifully illustrated in some experiments by Norman Guttman  and his colleagues at Duke University on the so called stimulus generalization gradient. Guttman makes use of the fact that, after a brief exposure to a variable-interval schedule, a large number of responses will be emitted by the organism without further reinforcement (the usual extinction curve) and that, while these are being emitted, it is possible to manipulate the stimuli present and to determine their relative control over the response without confusing the issue by further reinforcement. In a typical experiment, for example, a monochromatic light with a wave length of 550 millimicrons was projected on the key during variable-interval reinforcement. During extinction, monochromatic lights from other parts of the visible spectrum were projected on the key for short periods of time, each wavelength appearing many times and each being present for the same total time. Simply by counting the number of responses made in the presence of each wavelength, Guttman and his colleagues have obtained stimulus generalization gradients similar to those shown in Figure 8. The two curves represent separate experiments. Each is an average of measurements made on six pigeons. It will be seen that during extinction responding was most rapid at the original wavelength of 550 millimicrons. A color differing by only 10 millimicrons controls a considerably lower rate of responding. The curves are not symmetrical. Colors toward the red end of the spectrum control higher rates than those equally distant the violet end. With this technique Guttman and his colleagues have studied gradients resulting from reinforcement at two points in the spectrum, gradients surviving after a discrimination has been set up by reinforcing one wavelength and extinguishing another, and so on.
The control of behavior achieved with methods based upon rate of responding has given rise to a new psychophysics of lower organisms. It appears to be possible to learn as much about the sensory processes of the pigeon as from the older “introspective” methods with human subjects. An important new technique of this sort is due to D. S. Blough . His ingenious procedure utilizes the apparatus shown in Figure 9. A pigeon, behaving most of the time in total darkness, thrusts its head through an opening in a partition at a, which provides useful tactual orientation. Through the small opening b, the pigeon can sometimes see a faint patch of light indicated by the word Stimulus. (How this appears to the pigeon is shown at the right.) The pigeon can reach and peck two keys just below the opening b, and it is sometimes reinforced by a food magazine which rises within reach at c. Through suitable reinforcing contingencies Blough conditions the pigeon to peck Key B whenever it can see the light and Key A whenever it cannot. The pigeon is occasionally reinforced for pecking Key A by the presentation of food (in darkness). Blough guarantees that the pigeon cannot see the spot of light at the time this response is made because no light at all is then on the key. By a well established principle of “chaining,” the pigeon is reinforced for pecking Key B by the disappearance of the spot of light. This suffices to keep responses to both keys in strength.
A further fact about the apparatus is that Key B automatically reduces the intensity of the spot of light, while Key A increases it. Suppose, now, that a pigeon is placed in a brightly lighted space for a given interval of time and then put immediately into the apparatus. The spot of light is at an intensity in the neighborhood of the bright adapted threshold. If the pigeon can see the spot, it pecks Key B until it disappears. If it cannot see the spot, it pecks Key A until it appears. In each case it then shifts to the other key. During an experimental session of one hour or more, it holds the spot of light very close to its threshold value, occasionally being reinforced with food. The intensity of the light is recorded automatically. The result is the “dark-adaptation curve” for the pigeon’s eye. Typical curves show a break as the dark adaptation process shifts from the cone elements in the retina to the rods.
By repeating the experiment with a series of monochromatic lights, Blough has been able to construct spectral sensitivity curves for the pigeon which are as precise as those obtained with the best human observers. An example is shown in Figure 10, where data for the pigeon are compared with data for an aphakic human —one who has had a crystalline lens removed for medical reasons. Such a person sees violet light more sensitively than normal subjects because the light is not absorbed by the lens. Even with this advantage the human observer is no more sensitive to light at the violet end of the spectrum than the pigeon. The discontinuities in the photopic curves (the lower set of open circles) of the pigeon appear to be real. The surprising correspondence in the scotopic curves (after dark adaptation, and presumably mediated by the rods) is remarkable when we recall that the avian and mammalian eye parted company in the evolutionary scale of things many millions of years ago.