Participants

We recruited healthy participants at the University of Jena. Participants with neurological, psychological, cardiovascular, or visual diseases were excluded from the experiment. All participants signed informed consent statements. The study was approved by the ethics committee of the Faculty of Social and Behavioral Sciences of the Friedrich Schiller University of Jena and followed the ethics declaration of Helsinki.

The assessment of suggestibility of participants was organized in groups of about 8 participants using the Harvard Group Scale of Hypnotic Susceptibility9. This test consists of 12 suggestions including challenge items (e.g. arm immobilization), ideomotor items (e.g. hand lowering) and cognitive items (e.g. fly hallucination)10. At the end of the test, participants rated whether and how well they responded on each of these 12 tasks. A total number of 0 to 3 successful tasks indicates low suggestibility, 4 to 7 moderate suggestibility and 8 to 12 tasks high suggestibility11.

Our sample consisted of 60 participants (30 female, 30 male) with 20 low suggestible, 20 middle suggestible and 20 high suggestible participants. Mean age of participants was 23.1 years (range 18–44 years). Participants received 6 Euros per hour for participation. On average, participants earned 24 Euros for the whole study.

Visual Oddball task

The three types of stimuli comprising the Oddball paradigm included triangles as frequent stimuli, appearing in 80% of trials while circles and squares served as rare stimuli, each appearing in 10% of trials. Stimuli occupied a visual angle of 4.6 degrees. Each stimulus was displayed for 500 ms at the center of a monitor screen, separated by an inter-stimulus interval of 1000 to 2000 ms. Each Oddball task consisted of 500 stimuli and lasted about 10 minutes. The experiment was performed using Presentation® software (Version 17.1, Neurobehavioral Systems, Inc., Berkeley, CA, www.neurobs.com).

EEG

EEG was recorded from 64 EEG electrodes with 21 electrodes localized according to the 10–20 system on participants’ heads. The remaining electrodes were interspaced equally between these 21 sites. The impedance of each electrode was kept below 10 kOhm. The initial sampling rate was 1000 Hz, but data were down-sampled off-line to 250 Hz. Linked mastoids (TP9 and TP10) served as reference electrodes. The EEG signal was amplified by BrainAmp amplifiers and recorded with the BrainVision Recorder software (both Brain Products, Gilching, Germany). Data analysis was realized with the EEGlab software12. Artifacts related to eye movements were removed with the ICA procedure of EEGlab13. Epochs were selected between −200 ms and 800 ms for each stimulus and condition. In order to obtain event-related brain potentials for each participant in response to stimuli, artifact-free epochs were averaged for each participant, electrode, and experimental condition. The event-related brain potentials (ERP) were baseline-corrected by the average EEG activity between −200 and 0 ms for each electrode. N1 and P2 amplitudes were assessed at electrode Fz and the P3b amplitude was evaluated at electrode Pz. Time windows for amplitudes were: 80–168 ms for the N1, 168–272 ms for the P2 and 320–472 ms for the P3b. Statistical analyses were computed with R14. For between-group t-tests, we used the Welch unequal variances t-test implemented in R that corrects the degrees of freedom in case of unequal variances. For ANOVA effects, we report generalized eta squared values as effect size. All reported correlations are Spearman correlations.

Procedure

Each participant completed the hypnosis condition and the control condition in one experimental session. The order of conditions was counterbalanced across participants. An illustrative movie of the hypnosis condition in this experiment can be requested from the authors. Prior to the hypnosis condition, participants were handed a real wooden board providing a model for the imagined blockade. Hypnosis was induced by a series of verbal instructions via an in-ear microphone by a trained hypnotist from outside the experimental chamber. She mainly instructed the participants to calm down, relax, and breathe smoothly. Furthermore, suggestions also included an instruction for a motor task of the Harvard Group Scale of Hypnotic Susceptibility9 that served as a test whether participants were hypnotized properly.

Then, the suggestion of the perceptual blockade followed (see supplemental material). Participants were told that the board would block their perception of the screen. While they imagined the blockade, the visual Oddball paradigm was presented. Triangles were used as frequent stimuli, appearing in 80% of trials and circles and squares served as rare stimuli. The rare stimuli appeared in 10% of trials each. Thus, two kinds of rare stimuli were presented, but only one of them, the squares, had to be counted. All stimuli occupied a visual angle of 4.6 degrees and were displayed for 500 ms at the center of the screen. The interval between stimuli varied between 1000 and 2000 ms. In the control condition, participants were presented the same visual stimuli but without the suggestion of a wooden board in front of their eyes. During both conditions participants’ EEG was recorded. To make sure that participants’ eyes were open during the presentation of the visual stimuli, we used an eye tracker (SensoMotoric Instruments, Berlin) and observed a video showing participants’ eyes.

After each condition, participants were asked how many squares they counted. Based on their answers, we computed the counting performance as the percentage of correctly counted squares in each condition. At the end of the experiment, participants rated their experience of the suggested perceptual blockade on a 5 point Likert-Scale (1: “I saw all stimuli with unchanged intensity”, 5: “I did not see the stimuli any more”).