The aim of the present study was to further investigate the psychological mechanisms underlying the ayahuasca experience, and specifically whether ayahuasca acutely affects creative thinking. It was shown that during the acute inebriation, ayahuasca caused a decrease in conventional convergent thinking and enhanced creative divergent thinking, as measured by the PCT. All dependent variables associated with the PLMT remained unaffected.

The ayahuasca-induced enhancement of divergent thinking could potentially be linked to the effects ayahuasca exerts on brain regions involved in creativity. Three core networks, i.e., the default mode network (DMN), including the ventromedial prefrontal cortex and posterior cingulate cortices, the central executive network (CEN), including dorsolateral prefrontal cortex and posterior parietal cortices, and the salience network (SN), including dorsal anterior cingulate cortex and anterior insula, interact during divergent thinking via corticostriatal-thalamocortical loops (Chávez-Eakle et al. 2007; de Manzano et al. 2010; Fink et al. 2009; Geyer and Vollenweider 2008; Jung et al. 2013; Pinho et al. 2015). The thalamus feeds information into the SN which in turn coordinates the other mentioned networks (Beaty et al. 2016; Uddin 2015). The SN monitors events occurring outside of the body as well as internal consciousness and is able to direct attention to whatever is more important at a certain moment in time. It is suggested that especially these shifts between these externally (CEN) and internally (DMN) oriented cognitive networks are very important in creative divergent thinking (Jung et al. 2013; Perlovsky and Levine 2012). Interestingly, previous research has shown that ayahuasca reduced thalamic gating of sensory and cognitive information (Riba et al. 2002). Taking into account the neuronal pathways previously described, this could lead to an increase in information fed into the salience network. Consistent with this line of thinking is the fact that Riba and colleagues (2006) showed an increase in blood perfusion in the SN after ayahuasca ingestion. Previously, other imaging studies with psychedelics also showed increased blood perfusion or a higher metabolic rate of glucose in the frontal and paralimbic areas during (e.g., anterior insula, anterior cingulate) psilocybin and mescaline (Gouzoulis-Mayfrank et al. 1999a, b; Hermle et al. 1992; Riba et al. 2006; Vollenweider et al. 1997). In addition, Alonso et al. (2015) found broad-band power decrements in the EEG signal after ayahuasca compared to placebo. Based on the knowledge about negative correlations between EEG and BOLD (Moosmann et al. 2003), this was interpreted as reflecting increased activation of areas involved in visual processing and in the cognitive-emotional processing anterior cingulate cortex (ACC), an area that is central to the SN.

Alonso et al. (2015) also showed a temporary induced disruption of neural hierarchies by ayahuasca, i.e., by reducing top-down control and increasing bottom-up information transfer in the human brain (Alonso et al. 2015). It was suggested that the higher excitability of posterior regions in combination with the loosening of the cognitive grip exerted by frontal regions responsible for executive control may underlie the associations and insights that emerge during the experience (Alonso et al. 2015). Palhano-Fontes and colleagues (2015) also found other parts of these networks to be influenced by ayahuasca, i.e., they showed a decrement in the functional connectivity in parts of the DMN after ayahuasca ingestion (Palhano-Fontes et al. 2015). It was suggested that this could result in more cognitive flexibility and consequently potentially enhanced divergent thinking (Carhart-Harris et al. 2014). However, Petri et al. (2014) suggest that the picture is more complex as they showed that the brain does not become a random system after psilocybin administration but still has stable connections which are different from the placebo state and only present in the psychedelic state (Petri et al. 2014). Likewise, Roseman et al. (2014) showed an increase in between-network resting state functional connectivity under psilocybin across normally distinct brain networks (Roseman et al. 2014). This increased integration between cortical areas could give rise to more associations (Petri et al. 2014) and an increased influence of imagination on visual perception (Roseman et al. 2014). Still, the precise neurobiological underpinning of how psychedelics may enhance divergent creative thinking remains largely unknown, and further research is warranted.

Convergent thinking can be seen as the second phase in the creative thinking process, i.e., focused on narrowing possibilities to a workable solution after the ideas have been generated through divergent thinking (Hennessey and Amabile 2009). Studies have shown this phase to be associated with an increase in CEN activity (Sowden et al. 2015). In the present study, ayahuasca caused deterioration in convergent thinking. Palhano-Fontes and colleagues (2015) found that ayahuasca only influenced activity in the DMN without changing the connection between DMN and CEN (Palhano-Fontes et al. 2015). The absence of ayahuasca enhancing effects on the CEN, together with the decrease in thalamic gating and loosened cognitive control described by Alonso and colleagues (2015), could explain the negative effect of ayahuasca on convergent thinking (Alonso et al. 2015). However, research with other psychedelics, e.g., psilocybin, showed an increase in functional connectivity between the DMN and the task-positive network or CEN (Carhart-Harris et al. 2013; Roseman et al. 2014).

Ayahuasca selectively affected performance in the PCT and not in the PLMT. Anecdotal reports from participants suggest that the stimuli of the PCT elicited more novel thoughts due to their more complex and colorful nature. In contrast, the stimuli in the PLMT contrast were very simple black-and-white line drawings. The latter probably gave less input into the system involved in the generation of new ideas. Another point to be mentioned is the quasi-experimental design of this study which potentially limits the conclusions that can be drawn from it. It could be argued that because ayahuasca sessions always followed the baseline session, and as it is known that ideas can get more creative over time (Beaty and Silvia 2012), a potential order effect could have influenced the results. However, besides the fact that parallel versions of tasks were used in a randomized order to counter potential order effects, the double dissociation, i.e., improvement of divergent thinking and impairment of convergent thinking, suggests that results were not subjected to the serial order effect.

Previously, it has been shown that the connection between divergent thinking and mood is particularly strong and positive (Baas et al. 2008; Davis 2009), i.e., more positive mood improves divergent thinking. In contrast, convergent thinking and mood are related in a negative way: more positive mood lowers convergent thinking. Mood of participants in the present study was generally very positive, which may have contributed to their openness to creative ideas. Mood ratings were only taken after drinking ayahuasca and could not be compared to mood states before drinking; nonetheless, they differed statistically from 0. In the future, placebo-controlled studies including measures of mood could test whether mood changes are a moderator in the effects of ayahuasca. Another point which could be addressed in future research is the usefulness of ideas generated in the divergent thinking task as this is part of the definition of divergent creative thinking. By including for example a task in which this quality can be assessed, e.g., the “alternate uses task,” it can be tested whether ayahuasca also improves this aspect of divergent thinking.

It would be interesting and important in the light of potential clinical applications of ayahuasca to investigate whether the effects are stable or also vary in time. It is known that ayahuasca induces an intense modified state of consciousness, starting between 35 and 40 min after administration and lasting approximately 4 h (Frecska et al. 2016). We showed that during this period, divergent thinking was enhanced and convergent thinking distorted. Soler et al. (2015) demonstrated that 24 h after ayahuasca intake, mindfulness-related capacities were enhanced (Soler et al. 2015). Mindfulness, a state of nonjudgmental, sustained, and alert awareness which improves people’s cognitive, emotional, and interpersonal functioning, has been shown to be linked with convergent thinking and less with divergent thinking (Lebuda et al. 2016). Based on these findings, it could be interesting to see whether convergent thinking is recovered 24 h after ayahuasca administration. It is suggested, based on a study of Bouso et al. (2008) on MDMA-assisted psychotherapy in post-traumatic stress disorder, that the potential effect pattern of ayahuasca would make it suited for psychedelic-assisted psychotherapy. The increase in divergent thinking during the acute phase could help patients relive events, recalling various associations without feeling inhibited (Bouso et al. 2008; Frecska et al. 2012, 2016). The sub-acute effects could then be suited in a second, “integration” session in which patients discuss the experiences they had on ayahuasca and find strategies that help them cope with intensive emotions. Future studies should therefore not only focus on either the acute or sub-acute phase but also take both stages into account.

In the past decade, a renewed interest in the therapeutic potential of psychedelics has emerged (Anderson 2012; McKenna 2004; Sessa and Johnson 2015). The present study has shown that ayahuasca promotes divergent thinking, an ability which has been shown to be an important aspect in cognitive therapy (Forgeard and Elstein 2014). It can therefore be suggested that ayahuasca possesses qualities that can promote a therapeutic process. However, since convergent thinking is also a critical aspect in therapy, and the current findings show that ayahuasca impairs this facet during the acute phase, future studies have to investigate whether this effect profile changes over time. Additional research utilizing a placebo-controlled experimental design, including additional creativity measures, is warranted, before results can be generalized.