Posthypnotic amnesia
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. 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 psychogenic amnesia, as it disrupts the retrieval process of memory. It has been suggested that inconsistencies in methodologies used to study post-hypnotic amnesia cause varying results.
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. Similarly, 19th century French neurologist Jean Martin Charcot focused solely on post-hypnotic amnesia. Charcot introduced three states of hypnosis: fatigue, catalepsy, and somnambulism ; 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 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.Some of the earliest experimental studies on post-hypnotic amnesia were done by Clark Hull. 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.
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.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.
Suggested
Suggested post-hypnotic amnesia involves the suggestion to hypnotized persons that, following hypnosis, they will be unable to accurately recall specific material until they receive a reversibility cue. 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.