At its core, the concept of hypnotic susceptibility addresses the ability of a person to be hypnotized and to what degree that individual experiences the process. Historically, this inclination has been measured using several standardized tests relying on clinical observation and subject feedback. More recently, however, studies have shown that it may be possible to determine responsiveness using diagnostic equipment.
The term hypnotic susceptibility not only refers to a person’s ability to be hypnotized but also to the extent of which one may respond. For example, most individuals will experience the increased relaxation that is common to the first phase of hypnosis. The suggestion of altered physical sensations, which is frequently the next step in hypnosis, is experienced by fewer subjects. With each step in the process, a person becomes statistically less likely to respond. Therefore, the deepest effects of hypnosis, including age regression and insensitivity to pain, are felt by the smallest percentage of people.
The two most commonly used tests for determining hypnotic susceptibility are the Harvard Group Scale of Hypnotic Susceptibility (HGSHS) and the Stanford Hypnotic Susceptibility Scale (SHSS). Of the two, the SHSS is most useful in determining hypnotic inclination in an individual, while the HGSHS is best for comparative analysis in populations. Numerous other tests may be used to judge the depth of the hypnotic state, but these are generally informal.
In 1989, researchers at Pennsylvania State University published a study theorizing that hypnotic susceptibility could be measured by electroencephalography (EEG). Basically, the brain activity of individuals who were ranked as having clearly high or low scores on traditional susceptibility tests was measured by recording electrical activity of the brain through probes on the scalp. The conclusion of the study was that these individuals showed markedly different patterns of brain activity, especially within the cerebral cortical area. These tests gave psychologists a concrete way to study a concept that had been previously largely subjective.
In addition to providing a way to measure it, the study at Penn State also gave some insight into the biological factors that may influence hypnotic susceptibility. Previously poor susceptibility had been attributed to psychological components, such as defensiveness and mistrust. Focus on brain function, however, has given rise to new developments. One such discovery, reported in 1996 study by Washington University, is a strong correlation between the speed of natural blinking and the ease of which a subject may be hypnotized.