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Hinterberger, Thilo; Kamei, Tsutomu; Walach, Harald . (2011). Psychophysiological classification and staging of mental states during meditative practice. Biomedizinische Technik/Biomedical Engineering, 56(6), 341–350. doi:10.1515/BMT.2011.021 
10.1515/BMT.2011.021