Hilgard begins his essay by denying the unity of consciousness. Partial consciousness involves selecting some features to attend to, while withdrawing attention from others. Hilgard's work on this problem has involved the study of hypnosis for almost forty years. In the 1960's, he found the concepts of dissociation and partial dissociation useful to explain divided attention and the effects of hypnotic suggestion on attention. In the 1970's, however, an unplanned demonstration involving a particularly hypnotizable subject led him to posit "the hidden observer" and to formulate the neodissociation hierarchical model.
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The case in question is briefly as follows. Hypnotic deafness was induced in the subject. No startle reaction to previously startle-inducing noises took place. Next, class members said things to the subject trying to provoke a response, without success. One of Hilgard's students then asked "whether some part of him [the subject] might know what was going on, for there was nothing wrong with his ears." Hilgard addressed the subject on the matter. In a quiet voice, while the subject was still hypnotically deaf, Hilgard said, "Although you are hypnotically deaf, perhaps there is some part of your mind that is hearing my voice and processing the information. If there is, I should like the index finger of your right hand to rise…" The subject's index finger rose, and he said, "Please restore my hearing so that you can tell me what you did. I felt my finger rise in a way that was not a spontaneous twitch, so you must have done something to make it rise." Hilgard used the signal for restoring hearing (touching the subject's right shoulder), and asked the subject what he had experienced. He reported hearing that at the count of three he would be deaf, and being told of the tactile signal indicating that hearing would be restored; that thereafter he was quiet for a while, then his finger rose. Hilgard next suggested that at another signal his finger would rise; then, that at yet another signal, Hilgard could be in touch with that part of the subject's mind which had heard Hilgard before, while the subject had still been hypnotically deaf. This signal was given. The subject was now asked to engage in "automatic talking" (free association) regarding what had happened while he had been hypnotically deaf. The subject said, "After you counted to make me deaf, you made some noises … behind my head. Members of the class asked me questions to which I did not respond. Then one of them asked if I might really be hearing, and you told me to raise my finger if I did. This part of me responded by raising my finger, so it's all clear now."
From this subject Hilgard concluded that a hypnotized subject, unaware of a sensory message, is nonetheless registering and processing sensory information; further, that the "hidden observer's" knowledge can under certain circumstances be recovered.
Hilgard's neodissociation hierarchical model includes an executive ego or central control structure which has monitoring (i.e., hidden observer) and executive functions. The central control structure is constrained by many factors (severely, under hypnosis) from autonomously "actuating" subordinate systems. These subsystems are themselves organized in a parallel and hierarchical fashion, and are considered latent in their available but not yet actuated state. "Many subsystems of habits, attitudes, prejudices, interests, and specialized abilities are available, although at any one time they may be latent; these are usually actuated according to the demands of the situation and the plans of the central system… A hierarchy of subsystems is implied, although it is a shifting hierarchy under the management of the control mechanisms." Hilgard goes on to describe the subsystems as continuing automatically, once activated. Concomitant with the automaticity is a decrease in consciousrepresentation of the control system itself. The automaticity also "allows such dual actions as carrying on a conversation while engaged in habitual activity."
The executive and monitoring functions can operate harmoniously and smoothly. "If one course of action does not work, another may be tried. Whether the second course works better is determined by monitoring; the executive acts on this information." However,
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in hypnosis and in some cases outside (Hilgard cites obsessive-compulsive behavior), "an alert monitoring function will be helpless in modifying executive action through feedback." Once activated in any way, many of the subsystems are self-sustaining, some with their own monitoring and control systems. Thus for the hypnotized subject carrying out a suggestion as well as for a person who decides to engage in an activity such as reading, once one is so engaged, awareness of the control function recedes as absorption in the task increases.
Hilgard closes by hoping that his brief review of some aspects of divided consciousness and dissociation will fit in with developments in the study of consciousness within a scientific psychology.