In numerous studies, researchers have compared the physical "body signs" of hypnotic subjects with those of unhypnotized people. In most of these studies, the researchers found no significant physical change associated with the trance state of hypnosis. The subject's heart rate and respiration may slow down, but this is due to the relaxation involved in the hypnotism process, not the hypnotic state itself.
There does seem to be changed activity in the brain, however. The most notable data comes from electroencephalographs (EEGs), measurements of the electrical activity of the brain. Extensive EEG research has demonstrated that brains produce different brain waves, rhythms of electrical voltage, depending on their mental state. Deep sleep has a different rhythm than dreaming, for example, and full alertness has a different rhythm than relaxation.
In some studies, EEGs from subjects under hypnosis showed a boost in the lower frequency waves associated with dreaming and sleep, and a drop in the higher frequency waves associated with full wakefulness. Brain-wave information is not a definitive indicator of how the mind is operating, but this pattern does fit the hypothesis that the conscious mind backs off during hypnosis and the subconscious mind takes a more active role.
Researchers have also studied patterns in the brain's cerebral cortex that occur during hypnosis. In these studies, hypnotic subjects showed reduced activity in the left hemisphere of the cerebral cortex, while activity in the right hemisphere often increased. Neurologists believe that the left hemisphere of the cortex is the logical control center of the brain; it operates on deduction, reasoning and convention. The right hemisphere, in contrast, controls imagination and creativity. A decrease in left-hemisphere activity fits with the hypothesis that hypnosis subdues the conscious mind's inhibitory influence. Conversely, an increase in right-brain activity supports the idea that the creative, impulsive subconscious mind takes the reigns. This is by no means conclusive evidence, but it does lend credence to the idea that hypnotism opens up the subconscious mind.
Whether or not hypnosis is actually a physiological phenomenon, millions of people do practice hypnotism regularly, and millions of subjects report that it has worked on them. In the next section, we'll look at the most common methods of inducing a hypnotic trance.