While we replicated previous findings concerning decreased power in the alpha band [4–11], including work showing that this depends on 5-HT2A receptor stimulation [9], some discrepancies appeared in the time course of alpha power changes after DMT inhalation in the present study. Reduced alpha power manifested most strongly immediately after DMT administration, and gradually receded towards baseline values after approximately 7 minutes. In contrast, previous work by Timmermann and colleagues reported that reports of subjective effect intensity were reduced to half of their peak value after 7 minutes and that alpha rhythm amplitude did not completely recover until approximately 15 minutes after the injection of doses ranging between 7 mg and 20 mg DMT fumarate [11]. These differences in the time course of alpha power reductions could be attributed to different pharmacokinetics between inhaled and intravenous administration routes, with some reports suggesting an equally rapid onset but shorter duration in the case of inhaled DMT [64]. Another possibility is that the effective dose in our study was lower than the doses employed by Timmermann and colleagues. Even though we did not quantify DMT content in the smoked samples, participants and facilitators declared a typical dose of 40 mg, in all cases extracted from Mimosa hostilis bark following a standard acid-base extraction procedure [81]. Previous analysis of similar DMT samples showed an average purity of 85% [82], which would imply an average dose of 35 mg in our study, comparable to the maximum dose used in Timmermann, et al. (2019) when one considers brain availability from an inhaled dose vs. intravenous administration. While smoked and intravenous DMT are both more potent than intramuscular injections [83], more research is required to establish a more precise correspondence between these two routes of administrations. A third possibility is that subjects systematically underestimated the subjective effects at around 7 minutes compared with the peak of the experience, and erroneously declared a return to baseline in spite of still significant subjective and neurophysiological effects; however, the measured EEG power spectra appear to be incompatible with this possibility.
We observed more salient divergences with the previous report by Timmermann and colleagues at other frequency bands. The power of delta (1 – 4 Hz) oscillations increased compared with the eyes-closed baseline, while this increase only appeared as a trend in the report by Timmermann and colleagues. However, our results are consistent with an EEG study of the effects of ayahuasca, which found that delta power increases correlated with DMT plasma concentration, but did not correlate with the concentration of several psychoactive beta-carbolines also found in the brew (harmine, harmol, harmaline, harmalol and tetrahydroharmine) [7]. Importantly, an independent EEG study of ayahuasca found small decreases in delta power that vanished at 120 min after ingestion, which coincides with the peak DMT plasma concentration [4]. Psilocybin was found to increase lagged phase synchronization of delta oscillations [84], and hypersynchronous delta activity has been reported in rodents after administration of DMT [85] and 5-MeO-DMT [86], a close structural analogue of DMT [43]; however, LSD decreased delta power in humans [10]. This heterogeneity could be in part explained by the different neurochemical profile of these drugs [76], especially by activity at other serotonin receptor subtypes (e.g. 5-HT1A, 5-HT1B) as suggested by 5-MeO-DMT-increased delta power in 5-HT2A knockout mice [86]. Also in contrast with the results reported by Timmermann and colleagues, we did not find that DMT increased power in the theta band, in agreement with other studies of ayahuasca, psilocybin and LSD. This discrepancy, however, could be caused by assessments of different contributions (oscillatory vs. fractal components) to the theta power [11].
Perhaps our most salient result consists of increased gamma power under DMT, correlating with multiple items from the 5D-ASC, NDE and MEQ-30 scales that reflect aspects of mystical-type experiences. Gamma power increases did not correlate with the number of discarded components/electrodes/channels, and were preserved after conservative EEG processing criteria which led us to discard 6 out of 35 participants. The role played by gamma oscillations in the neurobiology of psychedelic experiences remains unclear, mainly because gamma activity can result from muscle tension, jaw clenching and microsaccades [87]. In contrast to the results reported by Timmermann et al. intravenous DMT, an early EEG field study of ayahuasca found hyper-coherence in the gamma band [38], consistent with the results from Schenberg and colleagues. However, we note that Timmermann et al. observed differences in the gamma band comparing baseline vs. DMT, but not when comparing placebo vs. DMT. In a study with 50 healthy participants, Kometer et al. found that psilocybin administration results in increased high frequency (55 – 100 Hz) oscillations within the high gamma range [84] that correlated with the reported intensity of mystical experiences, also in line with a preliminary report that also found increased gamma global synchrony [88]. MEG recordings acquired during the acute effects of LSD and psilocybin systematically failed to find gamma power increases compared with placebo [5, 10]. Invasive recordings performed in animals support that psychedelic-induced gamma increases are at least partly mediated by 5-HT2A activation [89]. While past discussions on these discrepancies have focused on the possibility of EEG confounds within the gamma range, we believe that at least some of these discrepancies could also be attributed to differences in contextual factors. Variables related to set and setting could facilitate certain kinds of experiences whose neural correlates lie in the gamma range; moreover, anxious states may result in tension and movement, confounding activity in this band.
One of the most interesting characteristics of the psychedelic drugs is their capacity to induce peak experiences of unity and transcendence, combined with feelings of sacredness, deep meaning, positive mood, ineffability and paradoxicality [30, 31, 63]. These mystical-type experiences correlate with positive outcomes in therapeutic applications of psilocybin, occur with dose-dependent frequency, and are usually placed by research participants among the most meaningful experiences of their lives, even decades after the event [31, 90–92]. A possible neurophysiological mechanism underlying these experiences is increased local and long-range synchrony of gamma oscillations. Supporting this possibility, multiple studies have shown that states achieved by long-term meditators following certain traditions (e.g. non-dual awareness) [93–95] share several defining features with the phenomenology of psychedelic-induced mystical-type experiences [96]. It has also been proposed that surge of gamma band coherence near death is a correlate of NDE [97], an altered state of consciousness that has been compared with the DMT experience in terms of subjective experience [55–57]. In spite of their obvious relevance, very little is known about the neurobiological underpinnings of psychedelic-induced mystical-type experiences and is difficult to know to what extent contextual priming and framing plays a role in these experiences. Field studies seem especially apt to investigate the neurobiological basis of ‘mystical-type’, ‘peak’, or ‘unitive’ experiences, especially in the case of participants who approach psychedelic use from a spiritual perspective. We established gamma increases as a correlate of several 5D-ASC, NDE and MEQ-30 items related to the phenomenology of mystical experiences. As shown in Fig. 5, these correlations tended to manifest most strongly near the peak of the experience (3 min) and then receded gradually. Previous DMT research did not find correlations between gamma band oscillations and these items [56]; however, this work assessed subjective experience using visual analogue scales (VAS) which might not be comparable with the results of well-validated psychometric questionnaires.
We replicated previous findings of increased signal diversity (as measured using the Lempel-Ziv algorithm) under the acute effects of serotonergic psychedelics, including DMT [8, 56]. We also demonstrated that the collective properties of EEG oscillations differ under DMT in comparison to the baseline condition: alpha oscillations decreased coherence and metastability, while gamma oscillations showed the opposite behaviour. As hypothesized previously by Stuckey and colleagues [38], and as supported by the discussion in the previous paragraph, gamma hyper-synchrony could be a manifestation of increased information binding underlying psychedelic-induced unitive experiences. Recently, we found increased low gamma band coherence and signal diversity in the EEG of expert meditators belonging to multiple different traditions [61]. These results are also consistent with a potential involvement of gamma oscillations in certain aspects of the psychedelic-experience that are both facilitated by natural settings and common to other non-pharmacological altered states of consciousness.
Doesn't seem like overall activity decreased a lot. Interesting read
