Post-hypnotic amnesia is the inability of hypnotized individuals to recall events that occurred while under hypnosis. This can be achieved by giving individuals a suggestion during hypnosis to forget some material they learned either before or during hypnosis. Individuals with post-hypnotic amnesia cannot retrieve their memories once they are brought back into hypnosis, and thus are not dependent on the state. However, memories may return when presented with a prearranged cue. This makes post-hypnotic amnesia similar to psychogenic amnesia in that it disrupts the process of memory retrieval. It has been suggested that inconsistencies in the methodologies used to study post-hypnotic amnesia cause the mixed results. Post-hypnotic amnesia was first discovered by the Marquis de Puységur in 1784. While working with his assistant Victor, Puységur noticed that when Victor came out of hypnosis, he had amnesia for everything that happened during the session. Realizing the importance of this power, Puységur soon began treating those with induced amnesia. When the French physician Ambroise-Auguste Lepault published a book on hypnosis in 1866, he suggested that post-hypnotic amnesia was a symptom and a varying degree of hypnosis. Similarly, the 19th century French neurologist Jean-Martin Charcot focused solely on post-hypnotic amnesia. Charcot presented three states of hypnosis: fatigue, sleep, and sleepwalking, or somnambulism. It was in this last state that Charcot believed individuals could be communicated with and respond to suggestions. Charcot showed that if someone suggested that they were experiencing a psychological trauma, those who were neurologically sensitive would exhibit symptoms of psychological trauma. He hypothesized that this was due to a dissociation of thoughts from the rest of the individual's consciousness. However, the dissociation theory was put aside by Freud's psychoanalytic theory and the rise of behaviorism until Ernst Hilgard renewed its study in the 1970s. Clark Hull made some of the first experimental studies of post-hypnotic amnesia. Hull's work demonstrated that there is a dissociation between explicit and implicit memory through studies of proactive interference, retroactive interference, pair associations, and complex mental addition. In the mid-1960s, Evan and Thorne produced studies of source amnesia. In one study, hypnotized subjects were taught answers to ambiguous facts and when they emerged from their hypnotic states, a third of the subjects were able to provide the correct answers. However, these subjects had no conscious memory of where they learned the material. Spontaneous and suggested amnesia can occur after hypnosis or in the individual. For most of the 19th century, researchers reported that post-hypnotic amnesia only occurred spontaneously because scientific knowledge of this type of amnesia was scant. Spontaneous post-hypnotic amnesia is a mild impairment of memory that results from exposure to hypnosis or testing. This type of amnesia can also be experienced across susceptibility groups, but to a much lesser extent and magnitude than post-hypnotic amnesia. Spontaneous amnesia has also been difficult to define as research bias has been found to influence many cases. In one study, participants were divided into two groups. One group received instructions to lose memory and half did not. The next day, the groups were reversed. The results showed that there was little spontaneous amnesia among all participants, leading to doubts about the actual occurrence of amnesia. It was later found that those who were more susceptible to hypnosis were more likely to have suggested amnesia after hypnosis than spontaneous amnesia. These results suggest that spontaneous amnesia is less common than suggested amnesia and that when high spontaneous amnesia scores are reported, some incidents may be false.