Eliminating stroop effects with post-hypnotic instructions: Brain mechanisms inferred from EEG.

The classic Stroop task demonstrates the persistent and automatic effects of the meaning of color words that are very hard to inhibit when the task is to name the word color. Post-hypnotic instructions may enable highly-hypnotizable participants to inhibit the automatic access to word meaning. Here we compared the consequences of hypnosis alone and hypnosis with post-hypnotic instructions on the Stroop effect and its facilitation and inhibition components. Importantly, we studied the mechanisms of the hypnosis effects at the neural level by analyzing EEG frequencies. Highly hypnotizable participants performed the Stroop task in a counterbalanced design following (1) post-hypnotic suggestions that words had lost their meaning, (2) after hypnosis alone, and (3) in a control condition without hypnosis. The overall Stroop effect and both its facilitation and interference components, were not significant after the post-hypnotic suggestion but in both other conditions. Hypnosis alone neither affected the Stroop effect nor - in contrast to some previous reports and claims - overall performance. EEG recorded during the Stroop task showed a significant increase in both frontal theta and frontal beta power when participants were under the impact of post-hypnotic suggestions, in comparison to the two other sessions. Together, these findings indicate that post-hypnotic suggestions - but not hypnosis alone - are powerful tools for eliciting top down processes. Our EEG findings could be interpreted as clue that this is due to the investment of additional cognitive control.