Expectancy theory of hypnosis posits that any procedure can serve as a hypnotic induction provided it is labelled as “hypnosis”. The present study explored this hypothesis by contrasting the effects of two conventional and two unconventional (sham) hypnotic inductions on hypnotic experiences and electrophysiological correlates. In a 2x2 balanced placebo design, all participants were exposed to four conditions: conventional induction labeled as “hypnosis”, conventional induction labeled as “control”, unconventional induction labeled as “hypnosis”, and unconventional induction labeled as “control”. EEG was recorded from 128 channels. We computed EEG features that were identified in previous studies as correlates of hypnosis or hypnotizability. Consistent with the predictions of expectancy theory, we found that one of the unconventional (sham) inductions, “white noise hypnosis”, evoked comparable hypnosis depth to the conventional hypnotic inductions. However, contrary to its predictions, “embedded hypnosis”, another unconventional induction, evoked smaller hypnosis depth reports than the other three inductions. Most EEG features we explored did not differ between conventional and unconventional induction conditions. A possible exception is frontal theta activity, which appeared to increase more in conventional induction trials. The change in frontal gamma power negatively correlated with hypnosis depth, and occipital theta activity positively correlated with hypnotizability in both conventional and unconventional inductions. Overall, our results provide partial support for the expectancy theory of hypnosis. However, our findings should be considered exploratory. Confirmatory research is required to strengthen our confidence in these effects.
