Edwin R. Guthrie (essay date 1934)
SOURCE: "Pavlov's Theory of Conditioning," in the Psychological Review, Vol. 41, No. 2, March, 1934, pp. 199-206.
[In the following essay, Guthrie offers a critique of Pavlov's theory of the conditioned reflex.]
Pavlov's recent article, 'The Reply of a Physiologist to Psychologists,'1 deals with two items printed some time ago in the Psychological Review, one by Lashley on 'Basic neural mechanisms in behavior,' and one by the writer, 'Conditioning as a principle of learning.' The issues raised by Pavlov deserve some further discussion because they are fundamental. My own article would have been justified if its only effect had been to persuade Pavlov to additional writing on the conditioned reflex, since by his laboratory experience he is undoubtedly its most competent exponent. However, on a number of points I remain quite unconvinced after reading his 'Reply.'
The first difference of opinion that he mentions concerns a very general issue. He says,
The psychologist takes conditioning as a principle of learning, and accepting the principle as not subject to further analysis, not requiring ultimate investigation, he endeavors to apply it to everything and to explain all the individual features of learning as one and the same process. . . . The physiologist proceeds in quite the opposite way. At every phase of his investigation he endeavors to analyze the phenomena individually and in connection with facts, determining as much as is possible of the conditions for their existence, not trusting to mere deduction or to a single hypothesis.
This characterization is substantially correct. Pavlov has been reporting many detailed experiments with resulting generalizations as numerous as the varieties of experimental procedure. It was the writer's belief that the time had arrived when an hypothesis could be set up in order to direct experimental work. The hypothesis suggested was an old and familiar one, that the phenomena of learning, when described in terms of altered movement or secretion, may be described in terms of one principle, which was called the principle of conditioning. Its statement was this: Stimuli acting at the time of a response tend on their recurrence to evoke that response. In other words, it was suggested that the outstanding characteristics of learning, which have been expressed in terms of frequency, intensity, irradiation, temporary extinction, conditioned inhibition, forgetting, forward and backward conditioning, and so on, are all derivable from a more general law, the law of simultaneous conditioning or association by contiguity in time. To this end an analysis of these various phenomena was undertaken, an analysis that was, to the writer at least, very plausible.
Pavlov's second objection concerns this analysis in so far as it applies to backward conditioning. According to him the question is: What elementary properties of the brain-mass form the basis of conditioning? Backward conditioning, practice with the conditioned stimulus following the unconditioned, has a double effect, " . . . at first, temporarily, it assists in the formation of the conditioned reflex, and then destroys it,"—becoming eventually an inhibiting stimulus.
Pavlov's explanation of this is that
.. . the cell excited by the conditioned stimulus is inhibited or comes to an inhibited state with repeated concentration on the part of the unconditioned stimulus—and the conditioned stimulus in this way meets in its cell a permanent state of inhibition.
To this there is an objection. An understanding of the phenomenon of backward conditioning can be had only by finding the conditions under which it occurs. No properties of the brain-mass have been observed; no technique for observing states of inhibition in cells has yet been suggested. An explanation in these terms is and will remain unverifiable and entirely useless for prediction. In place of this unverifiable and useless hypothesis the writer had suggested that experimental search might disclose overlapping stimuli whose presence or absence would mark the presence or absence of backward conditioning. This would be to explain backward conditioning in terms of simultaneous conditioning. No act is instantaneous, and in backward conditioning the belated cue may accompany the later part of a sustained mascular contraction. This may be the explanation for the lessening effect of backward conditioning as the interval between cue and original stimulus is increased. In the writer's experiments with backward association2 with human subjects the cue could be practised before or after the original stimulus with like associative strength, and the writer is convinced that backward conditioning occurs only when, and in some measure to the degree that, there are overlapping stimuli.
Concerning remote forward conditioning, or "delayed and 'trace' reflexes," the writer had evidently not made himself clear. Pavlov says,
.. . if we grant with the author that not the bell but the centripetal flow of impulses from the motor act of listening is the true stimulus for the conditioned effect, why does that effect, in the case of delayed reflexes, nevertheless come out, not at once, but after an interval—and (furthermore) in accordance with the length of the interval between the beginning of the stimulus and the beginning of the unconditioned reflex?
For this Pavlov's explanation is two-fold.3
Many cyclic phenomena take place inside the animal's body. . . . The alimentary canal is periodically filled or emptied; and, in fact, changes in practically all the component tissues and organism are capable of influencing the cerebral hemispheres. This continuous cycle of direct and indirect influences upon the nervous activity constitutes the physiological basis for the estimation of the duration of time.
This is his first suggestion. The second is as follows:
Although prolonged for a significant length of time, the conditioned stimulus remains one and the same; but for the central nervous system (and it is especially necessary to think of the cerebral hemispheres) it is distinctly different in different periods of its course. This comes out particularly clearly with olfactory stimuli, which we sense at first very keenly, and then quickly as weaker and weaker, even if they remain objectively constant. Obviously the state of the stimulated cortical cell under the influence of an external stimulus undergoes successive changes and in the case of delayed reflexes only the state of the cell near the time of the addition of the unconditioned reflex acts as a signal for the conditioned stimulus.
The writer had made his suggestion expressly to account for the fact that the delayed reflex was...
(The entire section is 2909 words.)