Procedure

The experiment was conducted during a microdosing event organized by the Psychedelic Society of the Netherlands (PSN), who provided us with the opportunity to ask participants to take part in the experiment by means of an on-stage presentation. Interested participants were presented with envelopes containing the informed consents. We asked participants to read the provided information carefully, to sign it if they gave permission for their participation and use of their anonymized, coded data, and to subsequently return it back in the envelope. The envelope also included the experimental tasks for the first session as a booklet. However, we stressed participants to not open the booklet until one of the experimenters asked them to turn the first page to prevent premature exposure to the tasks. The experimenters also kept a close eye on the attendees to further ensure participant compliance. Next, all participants were carefully guided through the experimental tasks. Each task was explained in detail by one of the experimenters with the aid of example items, before participants were allowed to perform the task themselves. This was repeated for each of the three tasks. The protocol was approved by the local ethics committee (Leiden University, Institute of Psychology). The experiment consisted of a baseline session before participants had consumed any psychedelics and a second session carried out while participants were under the influence of a microdose of psychedelic truffles. The tasks were conducted in a group setting free from outside distraction during both sessions. Participants’ responses were assessed in paper-and-pencil version. The test battery consisted of the Picture Concept Task (PCT) (Wechsler 2003; Hurks et al. 2010) to assess convergent thinking, the Alternate Uses Task (AUT) (Guilford 1967) to assess divergent thinking, and a validated short 12-item version of the Raven’s Progressive Matrices Task (RPM) (Bilker et al. 2012) to test fluid intelligence. While AUT and RPM were presented fully in paper version, PCT stimuli were presented by PowerPoint to insure precise timing of stimuli presentation (further details about the contents of the psychometric tasks are provided under the “Materials” section). Pre- and post-microdosing performance was assessed by administering two different versions of each task, to reduce potential learning effects. Task versions were counterbalanced across sessions and participants.

After finishing the experimental tasks in the first session, participants consumed a microdose of pre-measured psychedelic truffles made available by the PSN. Participants of the workshop agreed to take psychedelic truffles upon their own risk. The PSN did not adhere to any strict guidelines regarding the dosage given to attendees. Nonetheless, they did take the attendees self-reported approximate weight into account in their recommended dosage, by means of subjective evaluations regarding low, average, and high body weight criteria. Attendees judged to have a low body weight were recommended to take 0.22 g of truffles, those with an average body weight 0.33 g, and those with a high body weight were recommended 0.44 g of dried truffles. Moreover, it should be noted that attendees did not have to adhere to the PSN member’s recommendation and were free to choose a dose. However, with reference to the existing microdosing guidelines (Fadiman 2011), the doses provided to participants appeared to be in a meaningful range of a microdose. Following the publication of The Psychedelic Explorer’s Guide (Fadiman 2011), a microdose should lie around one tenth to one sixteenth of a regular dose. Considering that a recreational dose of truffles is about 10 g of fresh truffles, a microdose would equal 1 g of fresh truffles. As fresh truffles consist of two thirds of water, this results in a weight of 0.33 g of dried truffles. Participants consumed on average 0.37 of dried truffles which is an appropriate amount given the calculation. Additionally, data on participants’ height, body weight, and ingested dose of truffles were independently collected by the researchers in order to examine potential dose-dependent effects in the analysis.

Approximately 1.5 h after attendees had consumed the truffles, participants were asked to take part in the second session of the experiment. The 1.5-h time interval was chosen as the effects of truffles are reported to peak around 30–90 min followed by a few hours long plateau of effects before rapidly subsiding back to baseline (Erowid 2017). By choosing this time interval, we could be certain that all participants were tested while the effects of the truffles were still plateauing during the second session. The procedure during the first session was repeated at the second session. Lastly, participants filled in a questionnaire on medical health, psychedelic and general drug use, and general personal information (e.g., gender, age, first language). After testing, we thanked participants for their participation and debriefed them about the purpose of our experiment.

Sample

Out of all 80 attendees at the microdosing event, 38 volunteered in our experiment. Participants indicated that they were healthy, spoke the required languages for performing the experimental tasks, and whether they had prior experience with the use of psychedelic substances. All 38 participants completed the RPM during both sessions. Regarding the PCT, 11 participants had to be excluded from further analysis, either due to incorrect interpretation of the task instructions or because they did not complete the task either for the first or second session. For the AUT, two participants had to be excluded as the experimenters were unable to read the individuals’ writing: two due to missing data on the second session and one due to misinterpretation of the task instructions. Importantly, the individuals that were excluded for the AUT and PCT analysis did not overlap. This suggests that exclusion was random and did not depend on a shared trait or state among those who were excluded from the two analyses. Nonetheless, in order to preserve power, we decided to analyze the data separately for each task. This yielded a sample of 38 participants for the analyses of the RPM data, 27 subjects for the PCT analyses, and 33 for the analyses of the AUT data. Table 1 provides an overview of additional descriptive information regarding the sample.

Table 1 Descriptive statistics for the samples used in the three separate analyses. Numbers indicate mean (SD), unless otherwise specified (gender and prior experience) Full size table

Truffle analysis

Dried truffle samples of what participants used during the microdosing event (corresponding to 0.22, 0.33, or 0.44 g dry weight) were post hoc analyzed at the University of Chemistry and Technology Prague (UCT; Laboratory of Biologically Active Substances and Forensic Analysis) to determine the exact amount of active substances potentially leading to the found effects. Analytical standards of psilocin, psilocybin, norbaeocystin, and baeocystin were synthesized in-house (purity ≥ 95%) at UCT. The evaluation of the developed analytical method (Hajkova et al. 2016) encompassed the determination of Limit of Quantitation (LOQ) and the determination of the applicable concentration range for each compound studied. LOQ was determined as 10 times the ratio of signal to noise. Concentration was determined for each of the truffle samples separately, and each sample was measured twice. The level of the alkaloids was almost identical in all three samples, however, and the differences were lower than the estimated measurement errors when the results from the different samples are averaged. As such, we report the concentrations for the four alkaloids collapsed across the three samples in the results. For the alkaloid concentrations for the three samples separately and further details regarding the methods underlying these analyses, please see Online Resource 1.

Despite psilocybin usually being the most abundant alkaloid in psychedelic truffle psilocin is commonly considered to be responsible for the psychedelic truffles’ characterizing psychoactive effects (Gartz et al. 1994; Tylš et al. 2014). Psilocybin is quickly metabolized into psilocin in the human body through dephosphorylization, thereby limiting psilocybin’s direct contribution to the psychoactive effects in humans. Furthermore, we also report the concentrations of baeocystin and norbaeocystin as it has recently been suggested that they could quickly metabolize into corresponding N-demethylated psilocin derivatives upon consumption through similar processes as are at play in the metabolism of psilocybin (Tylš et al. 2014).

Instruments

Picture concept task

The PCT (Wechsler 2003; Hurks et al. 2010) is a visual creativity task that involves finding a common association between several images. Each trial consists of a matrix of between 2 × 3 to 3 × 4 pictures (see Fig. 1 for an example). The correct solution is a common association between one picture from each row. Thus, a response indicating an association between two pictures from the same row would be incorrect. Because the task presumes there is only one correct solution to each item, the task has been previously used to measure convergent thinking (Hurks et al. 2010). Hence, to complete the task, one should converge on the correct solution, while inhibiting inappropriate or less obvious associations and previously attempted yet incorrect solutions.

Fig. 1 Example trial from the Picture Concept Task. The subject has to identify the common association between one item from each row Full size image

Participants had 30 s per item to find the solution. Because precise time limit is essential in this task, it was impractical to present the task on paper. Instead, we used a PowerPoint presentation in which the slides (i.e., items) transitioned every 30 s. Participants were instructed to mark and name the common association between the pictures on the PCT response sheet in the booklet (see Fig. 2 for a corresponding answer sheet belonging to the item in Fig. 1). The PCT was scored by summing the number of correct responses.

Fig. 2 Illustration of a filled in answer sheet for the items from Fig. 1 Full size image

Alternate Uses Task

The AUT is commonly used in research on creativity to measure divergent thinking performance (Guilford 1967). During the AUT, subjects are presented with a common household object and asked to think of as many possible uses for the object as they can within a limited amount of time. Within each session of our experiment, participants were either presented with the word Pen or the word Towel and given 5 min (per session) to write down as many possible uses for the object. As common, the AUT was rated according to four different variables:

Fluency: the total number of responses

Flexibility: the number of different categories of responses

Elaboration: how much the person elaborates on their response. Each “elaboration” receives one point. For instance, the response “using a brick to prevent a door from slamming shut (1), when it is windy (2)” earns two elaboration points.

Originality: the uniqueness of a response. Originality is calculated by dividing the total number of responses by all subjects, once by 5% and once by 1%. Responses that have also been mentioned by 1% or less of the other participants receive two points for originality, while responses that have been mentioned by 1–5% of the participants receive one point for originality. All other responses receive no originality points.

Out of these four, flexibility is the most reliable and theoretically most transparent index of divergent thinking (Akbari Chermahini and Hommel 2010), while fluency neglects the quality of responses and the originality score is highly sample-dependent. As the scoring of AUT variables can be highly subjective, we used the mean score for each of the four measures obtained from two independent raters to increase the reliability of the dependent measures. Reliability scores (Cohen’s κ) were very high for fluency (session 1, κ = .970; session 2, κ = .935), fair for flexibility (session 1, κ = .342, session 2, κ = .252), fair to moderate for elaboration (session 1, κ = .231; session 2, κ = .547), and fair for originality (session 1, κ = .254; session 2, κ = .318).

Raven’s Progressive Matrices

The RPM was developed by Raven (1938) in order to measure fluid intelligence. We used a shortened 12-item version of the RPM to reduce testing time and participant burden. The 12-item version was developed and validated by Bilker and colleagues (2015) showing high correlations with the full RPM (r = .80 for version A and r = .77 for version B).

In our experiment, the RPM consisted of a series of 2 × 2 or 3 × 3 matrices of pictures in which the lower right picture was always missing. Both horizontally and vertically, a pattern is present in the matrix of pictures allowing the participant to deduce what the missing picture should look like. Underneath each item, six possible solutions were presented and participants marked the correct solution by circling it on paper. While there was no time limit per item, the task had a total time limit of 5 min. Throughout, the RPM items increased in difficulty, but participants were allowed to skip an item in case they felt like they were stuck. However, once they had advanced to a next item (either by skipping or answering), they were no longer allowed to go back to correct earlier responses. The RPM was also scored by summing the number of correct responses.

Analyses

First, separate analyses were run to test for possible interactions between time-point (pre- versus post-truffle ingestion) and participants’ body weight, ingested dose, and prior experience with psychedelic substances on the dependent measures. As we did not find any significant interactions, we dropped these factors from any subsequent analyses. To assess changes in fluid intelligence, we performed a paired samples t tests comparing RPM scores at baseline versus RPM scores post ingestion of the truffles in the entire sample. Next, we performed a paired sample t test to compare convergent thinking performance before and after ingestion for the 27 participants for whom we obtained valid data on the PCT for both sessions. To assess microdosing-induced changes in divergent thinking performance, we analyzed the variables obtained from the AUT. Although of main interest were the flexibility and fluency scores, for completeness, we entered all four variables (i.e., fluency, flexibility, elaboration, and originality) as separate dependent measures in one multivariate Repeated Measures ANOVA with time (pre- versus post-ingestion) as the within subject factor. Significant effects in the multivariate ANOVA were followed-up by the appropriate univariate tests. The significance level for all analyses was set to α = .05.