EEG correlates of multimodal ganzfeld induced hallucinatory imagery.

Multimodal ganzfeld (MMGF) frequently induces dreamlike, pseudo-hallucinatory imagery. The aim of the study was to explore EEG correlates of MMGF-induced imagery. In a screening phase, seven 'high-responders' were selected by frequency and quality of their reported hallucinatory experience in MMGF. Each of these subjects then participated in three MMGF sessions (45 min) with simultaneous 19 channel EEG recordings and indicated occurrences of imagery by pressing a button. Relative spectral power changes during percept formation (30 s preceding subjects' reports) with respect to intra-individual baselines (no-imagery EEG) were analysed. At the beginning of the 30-s 'image formation' period alpha was slightly reduced than in the 'no-imagery' periods. This was followed by increased power in the higher alpha frequency band (10-12 Hz) which then declined in a monotonic fashion. This decline in higher alpha power was accompanied by increased power in the beta frequency bands. Throughout the image formation period there was a steady decline in power of low frequency alpha (8-10 Hz). Correlations between descriptors of subjective experience and EEG power changes were evaluated in terms of their global average magnitude and variability in time. Results indicate that the acceleration of alpha activity is a nonspecific effect of MMGF. In contrast, the tri-phasic profile of faster alpha activity seems to be a specific correlate of the retrieval and transformation of memory content in ganzfeld imagery.