EEG correlates of multimodal ganzfeld induced hallucinatory imagery
Introduction
Stimulation with homogenous light and sound, also known as multimodal ganzfeld (MMGF), induces a state of perceptual deprivation. Sustained exposure to MMGF can lead to an altered state of consciousness (ASC) that is characterised by dreamlike imagery. This imagery is subjectively comparable in quality and intensity to hypnagogic imagery occurring during sleep onset (Witkin and Lewis, 1963).
In an earlier study, comparing electrophysiological signatures of sleep onset, waking mentation and ganzfeld, it was established that ganzfeld imagery occurs in a brain functional state which is distinctly different from that occurring at sleep onset (Wackermann et al., 2002). Frequency spectra of ganzfeld EEG data indicated an activated waking state; thus, contrary to the common belief, ganzfeld imagery is related to a different brain functional state than that associated with hypnagogic imagery. This finding has been confirmed independently by source localisation methods (Faber et al., 2002). Despite these new insights, much of the physiological and psychological effects of MMGF has yet to be examined. In particular, little is known about the specific electrophysiological signatures of ganzfeld-induced imagery.
Ganzfeld usually does not induce a continuous stream of imagery. Subjective experience in MMGF is a dynamically changing state, characterised primarily by adaptive processes in the respective sensory systems with variation of colour saturation, brightness, loudness, etc., intermingled with episodes of typical ganzfeld imagery. Spontaneous thoughts, ideations, fantasies, associations, sometimes even distortions of body scheme and orientation in space and time are reported (Holt, 1964, Schacter, 1976). Reports of ganzfeld experience thus reveal a true potpourri of mentation of various origins.
The dynamic nature of ganzfeld experience cannot be adequately assessed by the method of ‘reports on demand’ used in our earlier study. Frequency of imagery episodes considerably vary between individuals and some participants do not experience any imagery. Therefore, for the purpose of the present study, we opted for the method of ‘self-initiated reports’. Subjects were instructed to report the moments of maximally developed imagery and this enabled us to separate episodes of ganzfeld-induced hallucinations from the no-imagery background.
A similar approach was successfully used to study the relationship between subjective experiences and objective brain electrical activity. In an experiment studying the relationship of long-term sensory deprivation and hallucinatory experiences (Hayashi and Hiroshima, 1992), the participants signaled occurrences of hallucinatory experiences by button presses. In a study by Line et al. (1998), that attempted to localise regions of brain activity associated with the onset of schizophrenic auditory hallucinations, subjects also signaled the onset of auditory hallucinations by button pressing. Germain and Nielson (2001) studied EEG power changes associated with sleep onset images. In this case, subjects marked the onset of hypnagogic imagery by pressing a micro-switch. These studies have shown that ‘self-initiated reporting’ using a button press is a viable and reliable method applicable with subjects who are about to fall asleep, or even in the case of those who experience genuine psychotic hallucinations (Line et al., 1998).
Our previous study indicated that ganzfeld induced an activated waking state characterised by the acceleration of alpha activity. These results, and the results of other authors (Klimesch, 1997, Ray and Cole, 1985, Line et al., 1998, Hayashi and Hiroshima, 1992, Cooper et al., 2003), indicate the importance of the alpha frequency band with respect to research on voluntary, induced, or spontaneously occurring mental imagery. The aim of the present study was to investigate the electrophysiological signatures of episodes of ganzfeld-induced imagery, and to study correlations between objective electrophysiological measures and dimensions of subjective experience.
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Subjects
Forty paid volunteers, 28 female and 12 male, in the age range 20–59 years (mean 39.0 years), recruited by local newspaper ads, participated in the experiment. Thirteen subjects had already participated in another study and were thus familiar with the laboratory facilities. One subject was excluded because he was using anti-depressant medication. The remaining 39 participants were in good health with no neurological or psychiatric history.
Experimental design
The study consisted of two phases. The purpose of the
EEG data
The subsequent analyses relied on intra-individual differences of imagery-related EEG against no-imagery EEG (baseline). For each session, all artefact-free 2-s EEG epochs from all stimulation periods from the start of stimulation (marked as RE in Fig. 1b) up to 120 s preceding the subject's report (SR) were collected and combined to a single dataset which was used for computation of the individual baseline (Fig. 1b). EEG data from the 30-s period preceding SR was processed separately, using a
Report frequency
A total of 82 reports with simultaneous EEG recordings were obtained during the second phase of the study. The number of reports per subject and session varied from zero to nine (mean = 3.90). Report frequency was thus comparable to the screening phase, although one subject showed a very reduced response rate (cf. Table 3).
To assess increasing or decreasing frequency of reports during sessions, time intervals between subjects' reports (pre-response intervals, PRI) were normalised (divided by
Discussion
Our results can be grouped into three categories.
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nonspecific effects of the multimodal ganzfeld (MMGF) stimulation (i.e., not directly related to the hallucinatory imagery);
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effects related to the active task of the participants (button pressing);
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specific effects related to emergence or formation of hallucinatory imagery.
The multimodal ganzfeld induced an activated waking state with pronounced and accelerated α activity. This supports our earlier finding that the multimodal ganzfeld does not
Acknowledgments
We would like to thank Dr. Frauke Schmitz-Gropengießer, Jakub Späti and Daniel Riewe for technical assistance in the experiments. Dr. Frauke Schmitz Gropengießer also transcribed the mentation reports.
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