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 and 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.

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, or 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 the ideas from the rest of the individual's consciousness. However, dissociation theory was put aside for Freud's psychoanalytic theory and the rise of behaviourism until Ernest Hilgard renewed its study in the 1970s.

...
Wikipedia