Posthypnotic amnesia is a condition that occurs when, with or withoutexplicit or implicit suggestions to do so, a subject is unable to remember some or all of the events that occurred in the hypnotic state when he is subsequently awakened. Typically these unavailable memories can be restored suddenly and without any intervening opportunity for relearning by means of a prear- ranged release signal. These memories are also freely retrievable in a sub- sequent hypnotic session. It is this property of reversibility or retrievability that differentiates true posthypnotic amnesia from some types of pseudo-amnesia, which may be caused by simple forgetting or by the failure to attend to or learn material while in the hypnotic state. The material lost as a result of this kind of pseudo-amnesia is not recoverable posthypnotically; the loss is per- manent. The phenomenon of reversibility also demonstrates that posthyp- notic amnesia is not caused by a failure to record material in the hypnotic state but by an interference with the normal retrieval or playback mechanism for gaining access to material in memory (Kihistrom, 1977; Kihisrrom and Evans, 1976; Nace, Orne, and Hammer, 1974; Orne, 1966b; Spanos and Bodorik, 1977). This conflicts with Hilgard's hypothesis that posthypnotic amnesia occurs because subjects under hypnosis suffer from a reduced ability to retain memories just as sleeping subjects do. This is particularly so in view of the findings of Nace, Orne, and Hammer (1974) that there were no sig- nificant differences between high- and low-susceptibility subjects in total recall of events experienced under hypnosis. Furthermore, Orne (1966b) dem- onstrated that the suggestion made to subjects in stage 1 sleep that their noses would itch when a cue word was spoken elicited scratching behavior in sub- sequent stage 1 sleep. This suggestion was also effective on the following night, even though the subjects were amnesic for the suggestion during the waking interval between the two laboratory sessions. This suggests that even sleeping subjects may have more capacity to retain memories than is gen- erally indicated (by studies showing that nocturnal dreams are usually for- gotten if a subject is not awakened within 10 minutes of the REM period during which the dream occurred). Perhaps it was the active response of the subject to the suggestion that enabled the memory trace to be recorded. While spontaneous posthypnotic amnesia is commonly regarded as a sign of somnambulism and is thought by some to be one of the signs of a deep hypnotic state, the experimental literature is in agreement that this phenom- enon rarely occurs in the laboratory (Barber and Calverley, 1966c; Kihistrom, 1977; Kihistrom and Evans, 1977).
Kihistrom and Twersky (1978) found that not only is posthypnotic am- nesia not caused by poor waking memory but subjects displaying marked posthypnotic amnesia actually had superior long-term retention of intention- ally learned material in the waking state.
Young and Cooper (1972) demonstrated the effect of implicit suggestion on the development of posthypnotic amnesia in subjects whose expectancies concerning the development of amnesia following hypnosis were manipu- lated. Half of their subjects were exposed to a prehypnotic lecture on hyp- nosis stating that posthypnotic amnesia invariably follows hypnosis, and the other half were told that it never occurs spontaneously. A significantly greater number of subjects expecting to develop posthypnotic amnesia developed it spontaneously.
In a study involving suggested rather that spontaneous posthypnotic am- nesia, Ashford and Hammer (1978) found a nonsignificant relationship be- tween inferred subject expectancies of posthypnotic amnesia and its subse- quent development following its suggestion on the HGSHS'.A. Simon and Salzberg (1985) also found that manipulating subjects' expectations had no effect on the occurrence of posthypnotic amnesia on the SHSS form A but hypnotic suggestion did. Hypnotic subjects given no specific suggestion for amnesia had less memory than nonhypnotized control groups, which sug- gests the possibility of self-suggestion. Perhaps the reason for the apparent conflict between this study and the findings of Young and Cooper was that in the present study subjects' expectancies were manipulated by having some of them read a paragraph denying the spontaneous occurrence of posthyp- notic amnesia. None was cued to expect this phenomenon, and since the initial expectancy of posthypnotic amnesia in these subjects seemed to have been low to begin with, this "manipulation" may not have produced two groups differing in expectancies. Orne (1966b), on the other hand, cites the cross-cultural occurrence of spontaneously developed posthypnotic amnesia, particularly in hypnotic-like religious and mystic experiences. He believes that this phenomenon deserves more attention than a glib dismissal of it as being due to implicit suggestion. Orne further notes that emotionally charged ma- terial relived by patients during hypnosis is usually forgotten spontaneously on awakening. This material is often related in language appropriate to an earlier stage of life, and he suggests that part of the difficulty in memory may involve the need to translate this material into adult patterns of thought. He reports that patients have difficulty in integrating this type of material into present consciousness even after they have the opportunity to listen to a tape recording of their hypnotic session while awake. Kline (1966) also notes that amnesia is more common following hypnotherapy than other types of hyp- nosis, and its extent seems to be related more to the material brought up under hypnosis than to the depth of the trance.
As in many other areas of controversy in hypnosis, perhaps both sides in this conflict are right. Although the development of spontaneous amnesia is rare in the laboratory, typical hypnotic research does not deal with affect- laden events, and there is no dynamic need for subjects to display an un- suggested amnesia. In clinical practice, however, where affect-laden material is routinely dealt with under hypnosis, spontaneous amnesia may be more common. Indeed, under these circumstances, the amnesia may be caused by the same dynamic factors that produced the original repression rather than by any special properties of the hypnotic state. Thus, as Orne suggests, there may be two different mechanisms involved in the production of posthypnotic amnesia: one based on suggestions in experimental work and one based on repression in clinical phenomena. His idea that dissociation may result from essential differences between the hypnotic and waking thought processes is more difficult to square with the apparent lack of spontaneous amnesia in experimental work, unless it is realized that clinical investigations typically deal with personal memories as opposed to material learned under hypnosis. Suggested posthypnotic amnesia has many subclassifications. Generally it is not an all-or-none phenomenon and can vary in degree from complete to slight. This is indeed fortunate, for the occurrence of partial posthypnotic amnesia makes it possible to study the effects of hypnotic suggestions on the mechanisms of memory retrieval. This would not be possible if amnesia were complete (Evans and Kilhstrom, 1973).
Suggested posthypnotic amnesia can be general—all memories of the hyp- notic experience are interfered with—or specific—only certain memories (either acquired under hypnosis or previous to it) are inhibited. In the former case, the subject may develop pseudo-memories and fill in the gaps with confabulations, as sometimes b . with patients having organic memory defects (Orne, 1966b). If a specific amnesia is suggested for a familiar name or a number, there will be marked differences in both the subjective experi- ence and objective behaviors of subjects responding to such a suggestion. Some subjects will report totally forgetting the name or number, while others will report remembering it but be unable to pronounce it when challenged to do so. It is quite common for such a suggestion made to a group of subjects to be interpreted differently by individual subjects. Hence these differences in responses are not due merely to the wording of the suggestions but also to the individual interpretations of these words made by each subject (and possibly to individual differences in hypnotic depth and the resulting literal- ness of understanding).
There was a time when it was widely believed that in order for a post- hypnotic suggestion to be effective it was necessary at the time of making the suggestion also to suggest a specific posthypnotic amnesia for it. Although this is no longer regarded as essential, Orne (1966b) believes that posthyp- notic suggestions made with suggestions of amnesia tend to last longer. In any event, subjects carrying out posthypnotic suggestions without awareness of the source of their behavior tend to justify their seemingly odd conduct with rationalizations. Subjects aware of the cause of their behavior tend to experience a compulsion to carry out the suggested actions (Estabrooks, 1957; Orne,1966b).
Posthypnotic amnesia may be divided into source amnesia or content am- nesia. Source amnesia is commonly produced when a hypnotized subject is given some obscure bit of information that he would have been unlikely to be aware of prior to hypnosis. Following a suggestion for a general posthyp- notic amnesia, it is found that he is immediately aware of this information on waking but is unaware of its source. This reaction, like most other hypnotic alterations of memory, is similar to the normal waking characteristics of mem- ory. Most people retain factual information of the type learned in school in isolation from the context in which it was learned. Thus the average adult will be unable to tell the circumstances under whic** ''" '"arned the date of the discovery of America or the Pythagorean theorem, source amnesia can be a source of torment for an author who remembers an appropriate quo- tation but cannot remember who said it. Memory that includes the contextual situation surrounding the information recalled is referred to as redintegration. It usually is related to personal experiences rather than factual or theoretical data. Unlike content amnesia, source amnesia is not often suggested explicitly under hypnosis and usually occurs spontaneously (Kilhstrom, 1977; Nace Orne, and Hammer, 1974; Orne, 1966b; Thorne, 1969).
Evans (1979) found that source amnesia occurred in 31% and 33% 29 and 12 deeply hypnotized subjects, respectively, who displayed a total recall amnesia for all other events under hypnosis, but it did not occur in 15 simulating subjects. Hence he concluded that it resulted from a dissociative phenomenon rather than the demand characteristics of the hypnotic situation or subtle cues given concerning the expectations of the experimenter (who was blind as to the hypnotic or simulating status of the subject).
Like all other posthypnotic phenomena, a posthypnotic amnesia can last for a variable period of time following termination of hypnosis. In some sub- jects, this period can be quite lengthy. A posthypnotic suggestion that a sub- ject will not develop a posthypnotic amnesia or that one developed will ter- minate is usually effective in preventing any spontaneous amnesia. Besides being terminated suddenly by a posthypnotic release cue or the reinduction of hypnosis with suggestions that the subject will now be able to regain all memories from the previous hypnotic experience, hypnotic amnesia can be permitted to dissipate with the passage of time.
A 1949 ftim, Unconscious Motiuation, was designed to demonstrate the effect of ui ous ideation on behavior. A male and a female college stu- dent were given the suggestion under hypnosis that as children they had failed to return a pocketbook they had found containing two coins and had used the coins to buy candy. The subjects were given a suggestion of post- hypnotic amnesia for this fantasy, and it was found to produce an unpleasant affective state in them, although they were unable to assign a reason for their feelings. In spite of their lack of conscious awareness of this ideation, it af- fected their responses on TAT-like and Rorschach-like tests, as well as word association responses. The amnesia was broken down without a prearranged release signal by the kinds of associations used in psychotherapy. Often in- complete memories obtainable under conditions of posthypnotic amnesia can be used as a starting point for associations to break the amnesia, and some- times total recall can be obtained soon after the first breakthrough is attained.
Orne (1966b) believes that memories retained during a suggested post- hypnotic amnesia relate to events during relatively light periods of the trance. Thus, he believes that the effectiveness of a suggestion for posthypnotic am- nesia is determined not by the overall depth of the trance but by its depth immediately preceding the suggestion of amnesia. A subject's failure to re- spond to suggestions early in the trance may not interfere with the devel- opment of the suggested amnesia, provided that he is given suggestions that he can respond to just prior to the suggestion for amnesia. The converse is also true; failed suggestions just prior to suggesting amnesia may interfere with its being developed in spite of previous successful tests of trance depth. This was demonstrated by giving the Harvard Group Scale of Hypnotic Sus- ceptibility (HGSHS) to two groups of subjects. Test items were given to one group in ascending order of difficulty and to the other in descending order of difficulty.