Post-hypnotic amnesia is the inability in hypnotic subjects to recall events that took place while under hypnosis. This can be achieved by giving individuals a suggestion during hypnosis to forget certain material that they have learned, either before or during hypnosis.Barnier, A., Bryant, R. A., & Briscoe, S. (2001). Posthypnotic amnesia for material learned before or during hypnosis: Explicit and implicit memory effects. International Journal of Clinical and Experimental Hypnosis, 49(4), 286–304. Individuals who are experiencing post-hypnotic amnesia cannot have their memories recovered once put back under hypnosis; it is therefore not state-dependent. Nevertheless, memories may return when presented with a pre-arranged cue. This makes post-hypnotic amnesia similar to dissociative amnesia, as it disrupts the retrieval process of memory.Kihlstrom, J.F. (1997). Hypnosis, memory and amnesia. Philos Trans R Soc Lond B Biol Sci. 352(1362), 1727–1732. It has been suggested that inconsistencies in methodologies used to study post-hypnotic amnesia cause varying results.Dorfman, J. & Kihlstrom, J. F. (1994). Semantic priming in post-hypnotic amnesia. Paper presented at the annual meeting of the Psychonomic Society, St. Louis, November 1994.
History Post-hypnotic amnesia was first discovered by Marquis de Puységur in 1784. When working with his subject Victor, Puységur noticed that when Victor would come out of hypnosis, he would have amnesia for everything that had happened during the session. Recognizing the importance of this power, Puységur soon began treating those who were ill with induced amnesia. When French physician Ambroise-Auguste Liébeault published a book on hypnotism in 1866, he proposed that post-hypnotic amnesia was a "symptom" and a varying degree of hypnotism.Hull, C.L. (1961). Hypnosis and suggestibility: An experimental approach. New York: Appleton-Century-Crofts. Similarly, 19th century French neurologist Jean Martin Charcot focused solely on post-hypnotic amnesia. Charcot introduced three states of hypnosis: fatigue, catalepsy, and somnambulism (sleepwalking); it was within this last state that Charcot believed individuals could be communicated to and could respond to suggestions. Charcot showed that if an individual (through post-hypnotic suggestion) self-suggested that they had a psychological trauma, those who were neurologically susceptible would display symptoms of psychological trauma. It was hypothesized that this was due to the dissociation of those ideas from the rest of the individual's consciousness. However, dissociation theory was put aside for Freud's psychoanalytic theory and the rise of behaviorism until Ernest Hilgard renewed its study in the 1970s.Lynn, S. J., & Kirsch, I.,. (2006). In Steven Jay Lynn and Irving Kirsch. (Ed.), Essentials of clinical hypnosis an evidence-based approach / (1st ed. ed.) Washington, D.C. : American Psychological Association, c2006.
Some of the earliest experimental studies on post-hypnotic amnesia were done by Clark Hull (1933). Hull's work showed that there was dissociation between explicit memory and implicit memory through studies on proactive interference and retroactive interference, pair associations and complex mental addition.
In the mid-1960s, Evan and Thorn produced studies on source amnesia. In one study, hypnotized individuals were taught answers to obscure facts and when brought out of their hypnotized states, one third of the individuals were able to produce the correct answers. Nevertheless, these same individuals had no conscious memory of where they learned this material.Evans, F.J. & Thorn, W.A.F. (1963) Source amnesia after hypnosis. American Psychologist, 18, 373.
Categories Spontaneous and suggested post-hypnotic amnesia can occur or be induced in an individual.
Spontaneous For most of the 19th century, scientific knowledge regarding post-hypnotic amnesia was minimal, and investigators reported that it only occurred spontaneously. Spontaneous post-hypnotic amnesia represents a slight memory impairment that results as a consequence of being put under hypnosis or being tested. This form of amnesia can also be experienced across susceptibility groups, but to a much lesser extent and magnitude than suggested post-hypnotic amnesia.Hilgard, E. R. & Cooper, L. M. (1965). Spontaneous and suggested posthypnotic amnesia. International Journal of Clinical and Experimental Hypnosis, 13(4), 261–273.
Spontaneous amnesia has also been difficult to determine as research bias has been found to influence results in many cases. In one study, participants were put into two groups; half to receive amnesic instructions and half not. The next day, the groups were reversed. Results showed that there was little spontaneous amnesia across all participants, leading to doubts about the actual occurrence of amnesia. It was later found that those more susceptible to hypnosis were more susceptible to suggested post-hypnotic amnesia and not spontaneous amnesia. These results suggest that spontaneous amnesia is less common than suggested amnesia and that when high results of spontaneous amnesia are recorded, some cases may be false.Hilgard, E. R. (1966). Posthypnotic amnesia: Experiments and theory. International Journal of Clinical and Experimental Hypnosis, 14(2), 104–111.
Suggested Suggested post-hypnotic amnesia involves the suggestion to hypnotized persons that, following hypnosis, they will be unable to accurately recall specific material (e.g. stimuli or events learned while under hypnosis) until they receive a reversibility cue.Barnier, A. J. (2002). Posthypnotic amnesia for autobiographical episodes: A laboratory model of functional amnesia? Psychological Science, 13(3), 232–237. This type of post-hypnotic amnesia is the most common in research surrounding post-hypnotic amnesia due to its controlled nature.
Suggested amnesia has been found to result in a more significant memory loss than spontaneous amnesia, regardless of the order of induction. On average, more individuals experience suggested amnesia and there appears to be a moderate effect across individuals of all levels of hypnotic susceptibility. Suggested post-hypnotic amnesia also involves a "temporary, retrieval-based dissociation between episodic and semantic memory". However, it is more common for highly hypnotizable individuals to remember less information than low hypnotizable individuals or controls while under suggested post-hypnotic amnesia.
Suggested post-hypnotic amnesia is reversible, a characteristic that distinguishes it from other forms of amnesia that arise primarily from traumatic brain injury. Whereas the retrieval of memories under retrograde amnesia is a slow and labour-intensive process, the reversal of hypnotically induced amnesia can occur with a simple suggestion or reversal cue (e.g., "when I clap my hands, you will remember everything").Perry, C. Key concepts in hypnosis. False Memory Syndrome Foundation. Retrieved 12 March 2012 from http://www.fmsfonline.org/hypnosis.html#hdham2
Types
Recall amnesia Post-hypnotic recall amnesia refers to an individual's inability to recall, when in a normal conscious state, the events that occurred during hypnosis. Evidence for this type of post-hypnotic amnesia is seen in a typical research model testing where nonsense syllables, that were paired during hypnosis, are unable to be recalled post hypnotically when a suggestion for amnesia was given during hypnosis. Recall amnesia for word associations tend to be very high when done by post-hypnotic individuals, with some studies showing one hundred percent total recall amnesia.Evans, F.J., & Thorn, W.A.F. (1966). Two types of posthypnotic amnesia: Recall amnesia and source amnesia. International Journal of Clinical and Experimental Hypnosis, 14(2), 162–179. This amnesia can also be measured by asking individuals, after their hypnosis has been terminated, to describe what they have been doing since they first laid down on the couch for their hypnosis session. When using this method for experimental testing, the hypnosis session will typically involve several tests or activities that the subject will engage in. Recall amnesia can then be measured by the amount of accurate tasks and activities the subject is able to remember.Williamsen, J. A., Johnson, H. J., & Eriksen, C. W. (1965). Some characteristics of posthypnotic amnesia. Journal of Abnormal Psychology, 70(2), 123–131.
Recognition amnesia Recognition amnesia equates to an impairment of an individual's recognition memory brought on by amnesia. As event-related potentials have been found to be sensitive to familiar stimuli in the absence of recognition impairments it has been suggested that individuals who report amnesia after hypnosis might not be experiencing post-amnesia recognition impairments. Instead, they may not be accurately describing their experience and confuse having amnesia for a lack of attention during encoding of tested stimuli.Allen, J. J., Iacono, W. G., Laravuso, J. J., & Dunn, L. A. (1995). An event-related potential investigation of posthypnotic recognition amnesia. Journal of Abnormal Psychology, 104(3), 421–430.
Source amnesia Post-hypnotic source amnesia refers to the ability of individuals to correctly recall information learned during hypnosis without the recollection of where the information was acquired. In a typical study examining this type of amnesia, individuals are administered a hypnotic induction procedure which is immediately followed by a series of questions concerning unfamiliar facts, for example "what year was Freud born in?". Subjects who are unable to correctly answer these questions are informed of the correct answers. These individuals are then administered a suggestion to be amnesic for everything that occurred during the period of which they are hypnotized. After the hypnotic session, those who exhibit post-hypnotic source amnesia, when asked the same unfamiliar questions again, will respond with the correct answers but will not be able to state where they learned the answer, or, more commonly, rationalize an incorrect source of their answer.Spanos, P.N., Maxwell, I.G., Malva, D.L., & Bertrand, D.L. (1988). Journal of Abnormal Psychology, 97(3), 322–329.
Models and theoretical explanations
Dissociation Dissociation, a theory originally applied by Pierre Janet, implies the view of the conscious self as being totally suspended. Dissociation, when referring to post-hypnotic amnesia, can be applied when the whole period in which an individual is hypnotized is looked at as an episode that is separated from the rest of that individual's experiences by boundaries of amnesia and suggestion. The failure to recall memory when in a normal, conscious state provides evidence to suggest that some kind of functional barrier is holding the information retained during hypnosis back from conscious recall. However, how sufficient the dissociative barrier in a hypnotic patient's mind is seems to be not all that effective, thus diminishing the credibility of the dissociation theory. This is easily seen in several studies where engaging in a secondary hypnotic session or the suggestion of the hypnotist or therapist to retrieve and recall information and events from the hypnotic session, readily and accurately initiate the retrieval that, according to dissociation, should be blocked from retrieval.White, R. W., & Shevach, B. J. (1942). Hypnosis and the concept of dissociation. The Journal of Abnormal and Social Psychology, 37(3), 309–328.
Disrupted retrieval