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Interest in the therapeutic potential of psychedelic substances has recently resumed. During an early phase of human psychedelic research, their therapeutic application in different pathologies had been suggested, and the first evidence for efficacy was provided. The range of recent clinical applications of psychedelics spans from cluster headaches and obsessive-compulsive disorder to addiction and the treatment of fear and anxiety in patients suffering from terminal illness, indicating potentially different therapeutic mechanisms. A variety of approaches in psychotherapy emphasize subjective experiences, such as so-called peak experiences or afterglow phenomena, as differentially mediating therapeutic action. This review aims to re-evaluate earlier and recent concepts of how psychedelic substances may exert beneficial effects. After a short outline of neurophenomenological aspects, we discuss different approaches to how psychedelics are used in psychotherapy. Finally, we summarize evidence for the relationship between subjective experiences and therapeutic success. While the distinction between pharmacological and psychological action obviously cannot be clear-cut, they do appear to contribute differently from each other when their effects are compared with regard to pathologies.

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Building on evidence for working memory (WM) coding of vibrotactile frequency information in monkey prefrontal cortex, recent electroencephalography studies found frequency processing in human WM to be reflected by quantitative modulations of prefrontal upper beta activity (20-30 Hz) as a function of the to-be-maintained stimulus attribute. This kind of stimulus-dependent activity has been observed across different sensory modalities, suggesting a generalized role of prefrontal beta during abstract WM processing of quantitative magnitude information. However, until now the available empirical evidence for such quantitative WM representation remains critically limited to the retention of periodic stimulus frequencies. In the present experiment, we used retrospective cueing to examine the quantitative WM processing of stationary (intensity) and temporal (duration) attributes of a previously presented tactile stimulus. We found parametric modulations of prefrontal beta activity during cued WM processing of each type of quantitative information, in a very similar manner as had before been observed only for periodic frequency information. In particular, delayed prefrontal beta modulations systematically reflected the magnitude of the retrospectively selected stimulus attribute and were functionally linked to successful behavioral task performance. Together, these findings converge on a generalized role of stimulus-dependent prefrontal beta-band oscillations during abstract scaling of analog quantity information in human WM.

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In the Ganzfeld technique, the visual and auditory perceptual fields are homogenized. After a short exposure to completely unstructured sensory input, participants transit into an altered state of consciousness. Visual homogenization is typically accomplished by a combination of goggles and bright light; auditory homogenization is accomplished by the presentation of unstructured auditory noise via headphones. The induced state is phenomenologically similar to a transition state between wakefulness and sleep, characterized by alterations in attentiveness, perception, and awareness, as well as by a compressed sense of time. Due to these replicable features of the Ganzfeld-induced state, it can be used within empirical research on the neuronal underpinnings of altered states phenomena. After a historic overview, here, we present data from a study on the stability of the subjectively experienced effects induced under different auditory homogenization conditions. In a fully randomized within-subject design (n = 24), we tested for the effects of three different auditory noise conditions: (1) violet, (2) white, and (3) brown noise. The combination of a standardized psychometric assessment, ratings on subjective time perception, as well as open reports prove the Ganzfeld-induced effects as being stable and effects within each participant as highly replicable, and therefore well suited for experimental purposes. Finally, the subjective experiences elucidated by the Ganzfeld technique are discussed within the framework of predictive coding and how changes in the interaction of top-down and bottom-up brain mechanisms could lead to the observed phenomenology.

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