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