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.
Source: How Hypnosis Works at How Stuff Works