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Abstract:
This study investigated hypnosis-related changes in cortical
activity in subjects (N=16) classified as having either a high or
low susceptibility to hypnotic induction procedures. Subjects'
electroencephalograms (EEGs) were recorded before, during and after
hypnosis. In two, one minute, eyes-closed baseline conditions
recorded prior to an induction procedure, subjects were instructed
to focus either on their breathing or to generate visual images.
Two further conditions were recorded: first, subsequent to a
hypnosis induction procedure and second after a further hypnotic
'deepening' condition. Instructions to subjects during these
hypnosis conditions were identical to those given during the
baseline conditions. A final post-hypnosis condition (subjects were
instructed to concentrate on their breathing) was also recorded.
Activity was recorded over midline sites and anterior and posterior
sites bilaterally. An examination of broad band theta (3.5-7.5 Hz)
showed that there was no difference in power between high and low
susceptibles. Furthermore, no change in theta power levels were
observed between the conditions in highs, though in lows an
increase in power was observed at all sites during the hypnosis
conditions. In broad band alpha (7.5-13.5 Hz) no condition or group
effects were observed. An examination of more narrowly defined
bands showed a more complex picture of hypnosis related changes in
the EEG. In both lower (3-5.5Hz) and upper theta (4.5-7.5) results
were similar to the broad band theta. In lower alpha (7.5-10.5 Hz),
highs had greater power over anterior but not posterior sites
during all conditions. They also had greater power during the
hypnosis imagery condition compared to a baseline condition. In
higher alpha (10.5-13.5 Hz) no consistent differences in power were
found between the groups. We suggest that region specific changes
in these alpha and theta bands reflect changes in relaxation and
hypnosis in the low and high susceptibles respectively.
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