High hypnotizable people with PHA typically show impaired explicit memory, or difficulty consciously recalling events or material targeted by the suggestion, and a dissociation between implicit and explicit memory, so that even though they can’t recall the forgotten information it continues to influence their behavior, thoughts and actions. The forgetting is reversible—when the suggestion is cancelled, their memories come flooding back. These last two features—the dissociation and reversibility—confirm that PHA is not the result of poor encoding of the memories or of normal forgetting, because the memories return as soon as PHA is cancelled. Rather, PHA reflects a temporary inability to retrieve information that is safely stored in memory. That makes it a useful tool for research.
After hypnosis, participants’ memories were tested twice while the fMRI scanner recorded their brain activity. For Test 1, they were asked 40 questions about the content of the movie (for example, the actress knocked on her neighbor’s door on the way home) and 20 questions about the context in which they saw the movie (for instance, during the movie, the door to the study room was closed). These questions required a “yes” or “no” response. For Test 2, participants were asked the same 60 recognition questions, but first they heard the cue to cancel PHA. So Test 1 measured memory performance and brain activity while the PHA suggestion was in effect and Test 2 measured memory performance and brain activity after it was cancelled.
Before long, hypnotism started finding its way into the world of modern medicine. The use of hypnotism in the medical field was made popular by surgeons and physicians like Elliotson and James Esdaille and researchers like James Braid who helped to reveal the biological and physical benefits of hypnotism. According to his writings, Braid began to hear reports concerning various Oriental meditative practices soon after the release of his first publication on hypnotism, Neurypnology (1843). He first discussed some of these oriental practices in a series of articles entitled Magic, Mesmerism, Hypnotism, etc., Historically & Physiologically Considered. He drew analogies between his own practice of hypnotism and various forms of Hindu yoga meditation and other ancient spiritual practices, especially those involving voluntary burial and apparent human hibernation. Braid's interest in these practices stems from his studies of the Dabistān-i Mazāhib, the "School of Religions", an ancient Persian text describing a wide variety of Oriental religious rituals, beliefs, and practices.
Whereas the older "depth scales" tried to infer the level of "hypnotic trance" from supposed observable signs such as spontaneous amnesia, most subsequent scales have measured the degree of observed or self-evaluated responsiveness to specific suggestion tests such as direct suggestions of arm rigidity (catalepsy). The Stanford, Harvard, HIP, and most other susceptibility scales convert numbers into an assessment of a person's susceptibility as "high", "medium", or "low". Approximately 80% of the population are medium, 10% are high, and 10% are low. There is some controversy as to whether this is distributed on a "normal" bell-shaped curve or whether it is bi-modal with a small "blip" of people at the high end. Hypnotizability Scores are highly stable over a person's lifetime. Research by Deirdre Barrett has found that there are two distinct types of highly susceptible subjects, which she terms fantasizers and dissociaters. Fantasizers score high on absorption scales, find it easy to block out real-world stimuli without hypnosis, spend much time daydreaming, report imaginary companions as a child, and grew up with parents who encouraged imaginary play. Dissociaters often have a history of childhood abuse or other trauma, learned to escape into numbness, and to forget unpleasant events. Their association to "daydreaming" was often going blank rather than creating vividly recalled fantasies. Both score equally high on formal scales of hypnotic susceptibility.
Hypnosis is not a psychotherapeutic treatment or a form of psychotherapy, but rather a tool or procedure that helps facilitate various types of therapies and medical or psychological treatments. Only trained health care providers certified in clinical hypnosis can decide, with their patient, if hypnosis should be used along with other treatments. As with psychotherapy, the length of hypnosis treatment varies, depending on the complexity of the problem.