The present experimental procedure consisted of 3 phases. Phase 1: baseline, before psilocybin administration, phase 2: 30 min after psilocybin administration, and phase 3: 60 min after psilocybin administration (Fig. 1A–F). Changes in FC during each phase were investigated by dividing each phase into 4 intervals (with a duration of 3.5 min each) during which the subject was asked to close and open his eyes, respectively. We observed changes in bilateral frontal FC (BF-FC) as well as right and left fronto-occipital FC (R-FO-FC, L-FO-FC) with respect to phases 1–3 of the experiment. In particular, during phase 2 the coupling strength (CS) in BF-FC, R-FO-FC, and L-FO-FC showed changes with respect to phase 1 (baseline) (Fig. 1C, E, F). BF-FC showed a statistically significant (p = 0.0181) increase compared to baseline (phase 1) at interval 3 in phase 2 (Fig. 1C). R-FO-FC and L-FO-FC first decreased significantly below baseline (p = 0.0393 and p = 0.0072, respectively) before increasing above baseline with the increase in R-FO-FC in interval 4 being statistically significant (p = 0.0449) (Fig. 1E). Psilocybin-induced changes in FC thus occurred 30–45 min (phase 2) after administration in BF-FC, R-FO-FC, and L-FO-FC. No significant change was detected in bilateral occipital FC (BO-FC) (Fig. 1D), but an increasing tendency when analyzing the global FC (Fig. 1B).
The ANOVA revealed no main effects of condition (before drug administration, 30 min afterward and 60 min afterwards) nor time (4-time intervals) for all FC types investigated, but a significant interaction effect between time and condition for the CS values linked to the BF-FC (F = 2.663, p = 0.017) and L-FO-FC (F = 2.479, p = 0.026). This highlights the relatively fast changes in RSFC dynamics induced by psilocybin.
Pulse rate (PR) significantly dropped during the experiment (p <0.01) in phase 3 compared to phase 1 (1.452 ± 0.033 Hz vs. 1.534 ± 0.046 Hz, d = 2.041) and phase 2 (1.452 ± 0.033 Hz vs. 1.530 ± 0.068 Hz, d = 1.431). During phase 2, PR increased and decreased intermittently (Fig. 1G).
We noticed that the recorded fNIRS signals had a different signal-to-noise ratio (SNR) values, quantified by the light-tissue coupling index (LTCI). Only 3 of the 16 channels (18.75 %) had a LTCI value associated with a SNR that would enable reliable detection of small changes in cerebral hemodynamics and oxygenation. Based on this finding, it has to be concluded that our inability to detect strong changes in FC elicited by the psilocybin administration is more likely to be caused by an insufficient SNR of the fNIRS data instead of the absence of a change in FC. The 13 channels without optimal LTCI could still be used in the present analysis since the signals were properly filtered and the signals contained valuable information in the low-frequency domain, which was used in the FC analysis.
Our observation that the most pronounced changes in FC occurred in phase 2 (30–45 min after administration) suggests that this time-frame may also include the strongest neurophysiological effects. Previous studies (involving psilocybin with doses of 10 and 25 mg) concluded that “acute psychedelic effects typically become detectable 30–60 min after dosing, peak 2–3 h after dosing, and subside to negligible levels at least 6 h after dosing”. Our fNIRS FC measures thus responded most likely to the initial phase of the psilocybin effect.
Functional brain connectivity patterns during the resting-state can be separated into the default mode network (DMN), executive control network (ECN), salience network, dorsal attention network (DAN), auditory network, sensorimotor network and the visual network (VN). The observation of our study that the R-FO-FC and L-FO-FC showed a change with respect to baseline can be interpreted as a change in the coupling of the ECN or DAN (covering the frontal cortex) with the VN (covering the occipital cortex).
How do our results compare to PET, ASL and fMRI RSFC findings reported in previous psilocybin studies? A direct comparison is difficult since (i) we used a new technique that has its own sensitivity and specificity with respect to changes in hemodynamics and oxygenation associated with brain activity, and since (ii) available RSFC investigations based on fMRI applied different experimental designs than the one used in our study; for example, previous RSFC fMRI studies administered psilocybin intravenously (i.v.), whereas oral administration was used in our case. An i.v. administration of psilocybin causes a fast onset of psychological symptoms and neurophysiological alternations, peaking after approx. 4 min with a steady decline in the following 20 min (i.v. administration of 2 mg psilocybin). In the first study reported on psilocybin, Carhart-Harris et al. used fMRI and ASL to investigate regional changes of cerebral blood flow (CBF) and FC. A decrease in the blood-oxygen-level-dependent (BOLD) signal and in CBF was found in regions such as the medial prefrontal cortex (mPFC), a central part of the DMN, associated with a decrease in FC between the mPFC and the posterior cingulate cortex. A change in FO-FC as observed in our study has not been reported in this study. Several other publications re-analyzed the study of Carhart-Harris et al. with respect to additional enquiries. These studies found a psilocybin-induced increased FC between the DMN and the right frontoparietal network, auditory network and salience network, changes in the complexity of FC, increase in BOLD signal variance in the bilateral hippocampi and anterior cingulate cortex, and a general increase in the coupling between some of the resting-state networks (with a decrease in coupling in only a few). Our observation of a change in the right and left FO-FC was also detected in the study by Roseman et al. who noticed an increase in the coupling between the VN and the DAN which covers parts of the region measured in the frontal cortex in the present study. Our observation of a change in the coupling between the right and left PFC has not been reported so far, to the best of our knowledge.
Other physiological responses to psilocybin have been reported such as an increase in the heart rate (HR), in parallel with an increase in body temperature, respiration rate, and blood pressure. Transient increases in heart rate and blood pressure (∼15 bpm and systolic blood pressure (SBP) increases of ∼20 mm Hg) have been found as acute effects of i.v. psilocybin administration. However, no significant changes in heart rate or blood pressure have been previously reported after the intake of 10 or 25 mg of oral administration psilocybin. Our finding of an increased HR during phase 2 (30–45 min after administration) and a decreased HR (with respect to baseline) during phase 3 (60–75 min after administration) is thus partially in agreement with these findings. A direct comparison, however, cannot be made, since the studies investigating HR changes after psilocybin intake did not report the exact time point of HR assessment. It might be also the case that the HR decrease in phase 3 was caused by increases in tiredness and relaxation of the subject.
The changes in PR suggest a possible change in vascular tone in the cerebral as well as extracerebral tissue due to changes in the autonomic nervous system activity linked to changes in cardiorespiratory activity. Indeed, the vasoactive properties of psilocybin have been reported. While 5-HT receptor-mediated vasoconstriction appears obvious, there are indications that this mechanism is not the only one. If the vasculature was affected by psilocybin during our experiment, the fNIRS signals (as well as BOLD signals recorded with fMRI) would have been affected by this. This aspect should be considered when interpreting the FC fNIRS data and for future fNIRS experiments investigating the physiological effects of psilocybin.