Due to hypnosis-as-product being such a highly subjective experience, there has been developed several semi-objective and objective measurement techniques in order to study these subjective experiences.
The measurement techniques may be divided in two main groups: (1) hypnotic susceptibility scales, and (2) functional neurologic imagery.
6.1. HYPNOTIC SUSCEPTIBILITY SCALES
Hypnotic susceptibility scales, or “hypnotic depth scales”, are tests designed to define the ability of a subject to experience hypnosis-as-product, as well as to elucidate to which extent hypnosis-as-product is experienced by the subject. All of these tests are based on the conveyance of progressively harder hypnotic suggestions to the subject, or subjects, and following ranking of the subjects according to which suggestions, if any, they respond to.
There are 2 main hypnotic susceptibility scales: (1) Harvard group scale of hypnotic susceptibility, form A, and (2) Stanford hypnotic susceptibility scale, form C [Woody and Barnier (2008)].
6.1.1. Harvard group scale of hypnotic susceptibility, form A
The Harvard group scale of hypnotic susceptibility, form A, or “HGSHS:A”, is a standardized test designed to rank groups of subjects simultaneously. The test consists of 12 progressively harder hypnotic suggestions, and following self-ranking of the subjects as to how these suggestions were experienced once hypnosis-as-procedure has been terminated. This provides a highly effective method for selecting quantities of seemingly suitable subjects in a fairly short amount of time [Shor and Orne (1962)].
6.1.2. Stanford hypnotic susceptibility scale, form C
In opposition to the HGSHS:A, the Stanford hypnotic susceptibility scale, form C, or “SHSS:C”, is designed to rank individual subjects with greater detail. As with the HGSHS:A, the SHSS:C consists of 12 progressively harder hypnotic suggestions, but, unlike the HGSGS:A, the responses are instead validated by the hypnotist while hypnosis-as-procedure is in process. This provides a more detailed method for verifying the suitability of subjects on an individual basis [Weitzenhoffer and Hilgard (1962)].
A natural sequela of these hypnotic susceptibility scales is the division of the subjects into 2 groups: (1) high hypnotizable persons, and (2) low hypnotizable persons [Barnier and McConkey (2004)].
6.1.3. High hypnotizable persons
High hypnotizable persons, or “highs”, are subjects that readily and favorably respond to hypnotic suggestions, and, thus, are ranked high on the hypnotic susceptibility scales. As such, high hypnotizable subjects are well suited for both hypnotherapy as well as for the further study of hypnotic phenomena.
6.1.4. Low hypnotizable persons
Conversely, low hypnotizable persons, or “lows”, do not readily respond to hypnotic suggestions, and, thus, are ranked low on the hypnotic susceptibility scales. However, low hypnotizable subjects may, to a certain extent, be trained to respond favorably to hypnotic suggestions and, as such, attain the status of high hypnotizable subjects [Gorassini and Spanos (1999)].
6.2. FUNCTIONAL NEUROLOGIC IMAGERY
While hypnotic susceptibility scales focus on the visible and perceived responses to hypnotic suggestion, functional neurologic imagery focuses on the changes in brain activity that occurs during the experience of hypnosis-as-product.
There are three main types of functional neurologic imagery: (1) functional magnetic resonance imaging, (2) positron emission tomography, and (3) electroencephalography [Oakley (2008)].
6.2.1. Functional magnetic resonance imaging
Functional magnetic resonance imaging, or “fMRI”, locates the areas within the brain with increased activity by identifying the focal increases in cerebral blood flow in response to different stimuli. fMRI does this by measuring the relative concentration of oxyhemoglobin and deoxyhemoglobin at different points in time.
6.2.2. Positron emission tomography
As with fMRI, positron emission tomography, or “PET”, also locates the areas within the brain with increased activity by identifying the focal increase in cerebral blood flow in response to different stimuli. However, PET does this by measuring the relative accumulation of a radioactive positron-emitting isotope, previously injected into the blood stream, at different points in time.
6.2.3. Electroencephalography
On the other hand, electroencephalography, or “EEG”, locates the areas within the brain with increased activity by identifying the focal increases of cerebral neuronal firing in response to different stimuli. EEG does that by measuring the cumulative electrical signals emitted from the cerebral neurons through the surface of the skull at different points in time.
Functional neurologic imagery studies have been performed during hypnosis-as-product while offering all three groups of hypnotic suggestions, including both hypnotic suggestions for action [Blakemore et al. (2003)], perception [Derbyshire et al. (2004)], and memory [Szechtman et al. (1998)].
First and foremost, these studies show that the areas within the brain activated in response to hypnotic suggestions are radically different from those activated in response to imaginative, “non-hypnotic”, or “placebo”, suggestions. As such, responses to hypnotic suggestions truly are functionally different from those of non-hypnotic suggestions.
Furthermore, the areas within the brain activated in response to hypnotic suggestions are virtually identical to those activated in response to the actual involuntary counterpart. As such, the subject experiencing hypnosis-as-product is functionally unable to differentiate between the response to a truly involuntary event or the same event called for by a hypnotic suggestion.
Finally, a single area within the brain is consistently activated in response to all three groups of suggestion, namely the anterior cingulate cortex of the frontal lobes. This suggests that the anterior cingulate cortex is responsible for the mediation and modification of the hypnotic suggestions, and that, in fact, the anterior cingulate cortex represents the executive monitoring, if not also the executive control hypothesized by dissociation theories [Raz et al. (2005)].
