Posthypnotic amnesia is a condition that occurs when, with or without
explicit 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.