Main points Pavlov devoted considerable time to the study of sleep and hypnosis; he considered both states as forms of progressive "cortical inhibition" of the nervous system. Representing them as two points along a continuum, Pavlov portrayed sleep as complete, diffuse internal inhibition of the cortex and hypnosis as a "partial sleep" state.
According to Pavlov, certain conditioned reflexes, such as the salivation response, remained in his animal subjects during hypnosis, while other reflexes related to movement disappeared. Pavlov concluded that the reflexes that remained did so either because they were governed by the subcortex rather than the cortex, or because the state of hypnosis was light and did not significantly inhibit the cortex.
Explanation In Pavlov's lecture "Conditioned Reflexes: Pathological Disturbances of the Cortex," he discussed experiments in which a dog was hypnotized by applying a physical restraint or by placing the animal on its back. According to Pavlov,
[t]he inhibitory influence of very strong stimuli can be regarded as a reflex of 'passive self-defense,' as, for instance, in the case of hypnosis. The immobility of the animal makes it less noticeable to the enemy, and thus abolishes or diminishes the aggressive reaction of the enemy.
Pavlov also described the use of "strong and unexpected stimuli" to induce hypnosis in cases of "hysteria" in man.
Strong stimuli were not the only triggers of a hypnotic response. Pavlov also described other "external stimuli which directly lead to inhibition of the cortical elements. These are of three kinds—monoto-nously reoccurring weak stimuli, very strong stimuli, and unusual stimuli." Repetitive, recurring conditioned stimuli gradually lulled dogs to sleep in several of Pavlov's experiments.
Pavlov believed hypnotic conditioning of humans was very similar to that of animals: According to Pavlov, "The classical method consisted in the performance of so-called 'passes'—weak, monotonously repeated tactile and visual stimuli, just as in our experiments upon animals." At present the more usual method consists in the repetition of some form of words, describing sleep, articulated in a flat and monotonous tone of voice. Such words are, of course, conditioned stimuli that have become associated with the state of sleep. In this manner any stimulus that has coincided several times with the development of sleep can now by itself initiate sleep or a hypnotic state. The mechanism is analogous to the inhibitory chain reflexes.
Pavlov also addressed the concept of hypnotic suggestion in terms of conditioned response. He theorized that because language is a excitatory stimulus,
[t]he command of the hypnotist, in correspondence with the general law, concentrates the excitation in the cortex of the subject (which is in a condition of partial inhibition) in some definite narrow region, at the same time intensifying (by negative induction) the inhibition in the rest of the cortex and so abolishing all competing effects of contemporary stimuli and of traces left by previously received ones. This accounts for the large and practically insurmountable influence of suggestion as a stimulus during hypnosis as well as shortly after it.
In other words, suggestion works because a) it is a novel stimulus and b) language, as Pavlov's "second signal," dominates and overrides all other competing stimuli.
Examples The concept of hypnotic inhibition as a reaction to a new and overwhelming stimulus was described in Pavlov's description of an experimental dog that was brought to a large lecture hall filled with people for a demonstration of the animal's conditioned reflexes. Because of the new location and the large audience, the dog became almost catatonic, and while it exhibited a digestive reflex when a conditioned stimulus was presented, it refused to take the food that was presented. A short time later, it fell asleep in its stand. Pavlov explained that the conditioned reflex remained because the dog was in a hypnotic state due to the unfamiliar stimulus of the lecture hall. After a time, the animal's diffuse inhibition had spread throughout the cortex and subcortex, triggering sleep.
Pavlov also related the story of a dog that was left in its stand (or experimental harness) for hours at a time between experiments. Eventually, the dog would shift into a hypnotic or pre-sleep stage immediately upon entering the experimentation room, and it would fall asleep within ten minutes if the experiment was not begun. It had been conditioned to associate the monotony of the room with the inhibited sleep state.
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