The 19 interview transcriptions were on average 1048 words long each (standard error = 133). Coding analyses identified a total of four separate groups, each including different clusters with related themes. These four different groups were (1) “welcome influences,” including all influences of music on subjective experience that were described as welcome, wanted, accepted, or appreciated (see Fig. 1, identified in 18 out of 19 patients, i.e. 95% of total); (2) “unwelcome influences,” including all experienced influences of music that were described as unwelcome, unwanted, rejected, or unappreciated (see Fig. 1, identified in ten out of 19 patients, i.e. 53% of total); (3) “appreciated music styles and playlist features,” including all themes related to the liking and appreciating of music genres, styles, and playlist design (see Fig. 2, identified in all 19 patients, i.e. 100% of total); and (4) “unappreciated music styles and playlist design,” including all themes related to the disliking and not appreciating of music genres, styles, and playlist design (see Fig. 2, identified in 11 out of 19 patients, i.e. 58% of total). Here, the term “music styles” refers broadly to the instrumentation, compositional, genre, and acoustic features of the music. The term “playlist design” refers to all aspects related to the selection and structuring of the music into the full music playlist.

Fig.1 Welcome and unwelcome influences of the music. Welcomed influences are displayed on the left in green, and unwelcomed influences are displayed on the right in red. All clusters and themes that are defined as an accepted or welcomed influence of the music on subjective experience. The figure displays cluster present in more than 30% of all participants and per cluster the themes that were present in more than 30% of the cluster. The numbers below the group-, cluster-, or theme-name refers to the total number of patients that referred to this. The size of the circle is proportional to the percentage of patients referring to the group, cluster, or theme Full size image

Fig.2 Appreciated and unappreciated music styles and playlist features. Appreciated music styles and features are displayed on the left, in green, and un-appreciated influences are displayed on the right, in red. The figure only displays cluster present in more than 30% of all participants and per cluster the themes that were present in more than 30% of the cluster. The numbers below the group-, cluster-, or theme-name refers to the total number of patients that referred to this. The size of the circle is proportional to the percentage of patients referring to the group, cluster, or theme Full size image

The figures displaying the four groups (Figs. 1 and 2) include the clusters present in more than 30% of the respective groupand the themes present in more than 30% of the respective cluster. This threshold was chosen for display purposes and emphasisesthe most dominant themes. However, all themes are discussed, and all associated patient quotes are presented in separate tables in Supplementary materials (Tables 1–11). It is important to emphasise that the identification of a theme in a patient’s experience, and subsequently the including of that theme in counting its presence in the total population, does not enable to make any statements on the duration that this theme was present in the patient’s total experience. For example, one patient may have experienced a sense of irritation in response to one particular song, and therefore the theme “irritation” under the cluster “intensification” in the group unwelcomed influences is present. But this does not imply that the patient experienced persistent feelings of irritation during his or her experience: it may simply refer to one short but memorable moment. In addition, the measure also only allows the capturing of spontaneous mentioning and elaborations on the subjective experience of the music in response to the open questions, as opposed to the questions targeting (and biasing) specific facets of the experience. The only bias present within the interview that is important to acknowledge was the inquiry of both “positive” and “negative” influences of the music, leading to the subsequent “welcome” and “unwelcome” groups.

Welcome influences: intensification

The most prominent cluster in the group welcome influences, including 17 out of 19 patients (89% of total), refers to themes that describe an intensification of the subjective experience by the music. Within this cluster, themes that describe an “intensification of emotion” were identified in 15 out of 17 (82% of cluster), including descriptions of music enhancing or changing emotions. Importantly, the emotion-evoking effects that were welcomed showed diverse emotional valence and included descriptions of the music facilitating “happiness” or strong “ecstatic” experiences, as well experiences of the music intensifying “tearfulness.”

Themes describing an “intensification of imagination” were identified in nine out of 17 (53% of cluster). This included statements of the music-evoking vivid and complex mental imagery and of the concrete imagery relating to specific characteristics of the music, such as ethnic “Indian” style of the music being associated with “seeing an Indian temple.”Eight out of 17 patients (47% of cluster) mentioned a “general intensification” effect of the music, without specifically referring to this being an intensification of emotionality, imagery, or others. Other themes, present below 30% in the cluster intensification, include effects of music on “personal thoughts or memories” (2/17, 12% of cluster), music facilitating a “sense of transcendence” (2/17, 12% of cluster), and music enhancing “ego dissolution” (2/17, 12% of cluster) (Fig. 1). See Table 1 (Supplementary materials) for a listing of all themes present in the cluster intensification.

Welcome influences of the music: guidance

The second most prominent cluster of welcome influences includes themes that depict the music as a source of “guidance.” This cluster was mentioned by 15 out of 19 patients (79% of total). Within this cluster, statements that the music provided a “sense of being on a journey” were identified in 11 out of 15 (73% of cluster). This included descriptions of the music being experienced as a “vehicle” that “transports” or “carries” the listener forward, providing a sensation of “travelling” to different psychological “places.”

Themes describing the music as a source for psychological “support” were identified in 11 out of 15 (73% of cluster). This includes various statements of the music providing a sense of “grounding,” “help,” and “reassurance.”Descriptions of the music being in tune with, or in resonance with the person’s intrinsic emotional state, were identified in six out of 15 (40% of cluster). Rather than describing the music as evoking emotion, this theme is defined by statements of the music being experienced as “fitting,” “following,” or “matching” present emotional states.

Finally, five out of 15 of patients (33% of cluster) referred to the music as providing a “sense of continuity and direction,” this included statements of music providing a sense of connection between different parts in the experience, making the experience feel “driven” by the music and “flowing” into a certain direction (Fig. 1). See Table 2(Supplementary materials) for a listing of all themes present in the cluster guidance.

Welcome influence of the music: calming

Ten out of 19 patients (53% of total) described calming effects of the music. From this cluster, nine out ten(90% of cluster) described “general calming” effects, whereas five out of ten patients (50% of cluster) described the music as providing “mental calming” effects, including sensations of peacefulness and of the music calming and “slowing the mind.” One out of ten (10% of cluster) described that the music helped them to feel more physically relaxed. Calming effects of music often referred to ambient music by Brian Eno, Harold Budd, and Stars of the lid. See Table 3 (Supplementary materials) for a listing of all themes present in the cluster “calming.”

Welcome influences of the music: openness to music-evoked experience

Seven out of 19 patients (37% of total) made statements about their own attitude of openness towards the influences of the music and in addition, about the effects of music on their attitude of openness. From this cluster, six out of seven (86% of cluster) referred to the “importance” and the “purpose” of being open to “challenging experience” evoked by the music, and that this felt like an important part of the therapeutic process. This included statements of accepting being deeply emotionally moved by the music and the music helping to “face” or “connect with” the listener’s “unresolved” inner conflicts. Four out of seven (57% of cluster) described that some music specifically helped to enhance their attitude of openness, such as statements that “the music opened (him/her) up” or that because of the music was “well-chosen,” the listener “felt open to it all” (Fig. 1). See Table 4 (Supplementary materials) for a listing of all themes present in the cluster “openness to music-evoked experience.”

Unwelcome influences of the music: intensification

The most prominent cluster, including five out of ten patients (50% of cluster), described music to “intensify” emotions they did not want to feel, such as increased “fearfulness,” “sadness,” or “fear.” In addition, five out of ten (50% of cluster) made statements about the music creating a sense of “discomfort,” including “unpleasant” or “uncomfortable” experiences, and four out of ten (40% of cluster) described irritation as a consequence of the music. In less than 30% of the cluster, the music was described as bringing mental imagery, thoughts or memories that were unwelcome, a sense of puzzlement, inner conflict, tension, or a “dark atmosphere.” This cluster of unwelcome intensification influences forms a contrast with the cluster of themes describing intensification as a welcomed influence (Fig. 1 and Table 1 (Supplementary materials)). See Table 5 (Supplementary materials) for a listing of all themes present in the cluster unwelcomed intensification.

Unwelcome influences of the music: resistance to music-evoked experience

Nine out of 19 patients (47% of total) described feelings of “resistance to the music-evoked experience.” This includes statements of “not liking” or “not wanting” the subjective effects of the music. This cluster of unwelcomed influences contrasts the cluster of themes describing an openness to music-evoked experience, as a welcomed influence (see Table 4 (Supplementary materials) and Fig. 1). See Table 6 (Supplementary materials) for a full list of all themes in the cluster intensification.

Unwelcome influences of the music: misguidance

Six out of 19 (32% of total) made statements about the music providing a sense of “misguidance”; this cluster primarily includes descriptions of the music being a “mismatch” or being incongruent with the unfolding subjective experience. This cluster, named “dissonance,” was present in four out of six (67% of cluster) and forms a contrast with the welcome influence resonance, when the music was experienced as harmonious, or a good match, with the subjective experience. Other themes of misguidance, present in less than 30%, include descriptions of the “music feeling intrusive,” the music being “unable to positively influence a challenging experience,” the music giving a “sense of being manipulated,” the music giving a “sense of unmet potential,” or the music giving a sense of “foreboding,” as if something “bad” was going to happen. This cluster of unwelcome influence contrasts the cluster of themes describing a sense of “supportive” and “helpful” guidance, as a welcome influence (see Table 2 (Supplementary materials) and Fig. 1). See Table 7 (Supplementary materials) for a full list of all themes in the cluster misguidance.

Appreciated music styles and playlist features: music styles

All 19 patients referred to some music styles within the music playlist that they especially appreciated (Fig. 2). Most frequent were positive statements about “ethnic music,” present in eight out of 19 patients (42% of cluster), such as Indian, “Spanish,” or “African” music styles (e.g. Jon Hassel, Ry Cooder, and Ronu majumdar). Positive statements about music with human voice were mentioned by seven out of 19 patients (37% of cluster). Importantly, this refers to vocal music either without lyrics or music with lyrics in a foreign language (e.g. The Journey by Ludovico Enaudi and Enya’s sumiregusa). One other music style that was frequently appreciated by seven out of 19 (37% of cluster) was neo-classical music (e.g. Max Richter or Olafur Arnalds) or classical music (e.g. Henryk Gorecki or Arvo Part). Apart from these styles, the appreciated music styles showed a noticeable diversity. In less than 30%, positive statements were directed to “music with crescendo” (five out of 19, 26% of total), “powerful music” (four out of 19, 21% of total), and only one to two out of 19 made explicit statements about their appreciation for specific instruments, such as violin, guitar, piano, or “music with a solid drone.” See Table 8 (Supplementary materials) for a full listing of all themes referring to music styles that were explicitly appreciated.

Appreciated music styles and playlist features: playlist design

Seventeen out of 19 patients (89% of total) made statements reflecting appreciation for the design of the playlist (Fig. 2). Most prominent were positive descriptions of the “music selection,” described by 12 out of 17 patients (71% of cluster), including descriptions of the music “working well” or being “well-selected.” Secondly, nine out of 17 patients (53% of cluster) provided positive descriptions on the way the music was structured into the full playlist. This theme, named “music order,” is defined by statements of the “structure” and the “ordering” of the music playlist, “aligned” well with the drug effects. The third most prominent theme, present in six out of 17 (35%), corresponds to the “music presence,” meaning the mere presence of the music itself. This includes descriptions from the music being present as helpful, to statements that it could not be imaginable doing the sessions without it and that the music presence felt “necessary.” Finally, other themes include appreciation for “calming music” to be played mainly during onset, ascent, and return phases, whereas more emotive music (i.e. “sentimental” or “cinematic” music) to be better reserved for late in the ascent phase and during peak phase. See Table 9 (Supplementary materials) for a full listing of all themes describing playlist design features that were appreciated.

Unappreciated music styles and playlist features: music styles

Eleven out of 19 patients (58% of total) referred to musical styles that were not appreciated. These responses reflected different degrees of the individual’s disliking of the music and were highly diverse, making no theme present in more than 30% of this cluster (Fig. 2). Some examples of themes in this cluster refer to “music with lyrics,” “vocal music,” “piano music,” “classical or neo-classical music,” and “cheesy music.”Often, vocal music and cheesy music referred to one particular song played during the final return phase by Buffy Saint Mary, up where we belong. See Table 10 (Supplementary materials) for a list of all themes present in the cluster of un-appreciated music styles.

Unappreciated music styles and playlist features: playlist design

Six out of 19 patients (32% of total) referred to aspects of the playlist design that were not appreciated. In 2 out of 6 (33% of cluster), a clear disliking of the music selection was present, and a preference for “own music selection” was expressed (Fig. 2). See Table 11 for a complete list of all themes present in the cluster of un-appreciated playlist design features.

Predictors in music experience for psilocybin experience and therapy outcomes

PCA reduced the dimensions of the 11-ASC to five factors, explaining more than 95% of total variance. These PCs are (1) “mystical experience” (loadings from “experiences of unity,” “spiritual experience,” and “blissful state”), (2) “impaired cognition” (loadings from “disembodiment,”impaired cognition, and “new meanings”), (3) “audiovisual perception” (loadings from “audio/visual synaesthesia” and “elementary imagery”), (4)” anxiety” (primarily loaded by anxiety), and (5) “insightfulness” (loadings from insightfulness and “complex imagery”) (see Fig. 3). Subsequently, music experience (liking, resonance, and openness) and drug intensity scores were correlated with these five factors and ratings for reductions in depression (1 week after psilocybin, defined by % reduction in QIDS score).

Fig.3 Principle component analysis (PCA) of variables from the 11D-ASC. Loadings of the 11 dimensions of the ASC (y-axis), on the first five PCs obtained from PCA followed by varimax rotation explained more than 95% of the variance. The x-axis shows the ordering of principal components, with the components ordered by explained variance (from left to right). The colour bar corresponds to the strength of the loading for each acoustic feature for that components: warm colours indicatea positive loading and cold colours a negative loading Full size image

Reductions in depression 1 week after psilocybin were significantly predicted by the musicexperience variables,liking (r = 0.60, p = .006), resonance (r = 0.59, p = .008), and openness (r = 0.57, p = .001), but not by drug intensity (r = 0.004, p = 0.98). Mystical experience during the psilocybin sessions was significantly predicted by music variables, liking (r = 0.61, p = .006), resonance (r = 0.67, p = .002), openness (r = 0.70, p = .0008), and by drug intensity (r = 0.58, p = 0.009). Insightfulness was predicted by music variables resonance (r = 0.53, p = .016) and openness (r = 0.59, p = .007), as well as by drug intensity (r = 0.65, p = 0.002), but not by music liking (r = 0.44, p = .06). Impaired cognition (r = 0.55,p = 0.01) and audiovisual perception changes (r = 0.71, p = 0.0006) were only predicted by drug intensity and not by any of the music variables. Anxiety was not predicted by any of the variables. All reported significant pvalues refer to FDR-adjusted threshold for significance of 0.016. See Fig. 4.

Fig.4 Correlations between music experience and therapy experience and outcomes. Outcomes of Pearson correlation tests of drug intensity ratings and music experience variables (on y-axis), with decreased depression (1 week after psilocybin) and the acute psilocybin experience (five PCs from ASC) (on x-axis). * = p < 0.05 and ** = p < 0.001, after FDR correction for multiple comparisons Full size image

Inter-rating reliability and discriminative validity of musicexperience variables

Pearson correlation tests between the scores of all researchers (n = 4), who rated the three musicexperience variables (liking, resonance, and openness), demonstrated good inter-rater reliability (average r = 0.6 ± 0.1, from total of 18 correlations). Pearson correlation tests between the three music experience variables showed significant correlations (r = 0.9, r = 0.96, and r = 0.91). Drug intensity did not correlate with any of the music experience variables.

Discussion

Via an analysis of patient interviews, this study identified a number of ways in which music influenced the subjective experiences of patients receiving psilocybin with psychological support for treatment-resistant depression. The most frequently reported themes relate to an intensification of emotions and mental imagery by music under psilocybin, complementing previous studies that demonstrated modulatory effects of LSD on music-evoked emotion (Kaelen et al. 2015, 2017) and music-evoked mental imagery (Kaelen et al. 2016) in healthy volunteers. By focussing on the phenomenology of the acute experience, the present study provided new insights into the role and importance of music in the context of psychedelic therapy. For example, the music appeared to be a significant source of guidance, creating a sense of grounding, as well as a sense of carrying the listener into different psychological places. Specific examples of this can be found in the following two excerpts:

The sad songs would bring painful memories on, more happy songs would make me think of a really good period in my life. Every new song could bring a different image. (#4)

I feel the music in large part drove a lot of the experience. Under the influence of psilocybin, the music absolutely takes over. Normally when I hear a piece of sad music, or happy music I respond through choice… but under psilocybin I felt almost that I had no choice but to go with the music. […] I did feel I was being held. And it did feel like the music opened [me] up to grief, and I just was very happy for that to happen. It wasn’t particularly pleasant in any way, but extraordinarily powerful. It took my thinking and my experience to uncomfortable places, but I was kind of reassured in the experience. There was something there that meant “I’m going to take you on a ride here, but I promise I won’t abandon you. It’s just going to be tough, and you know, you’re going through the grinder here, but you won’t be left in pieces.” That seemed to be… what the music was saying to me. (#14).

In contrast to the sense of guidance by the music were descriptions of the music providing a sense of misguidance. In these situations, the music was most often described as being dissonant with the patient’s emotions and thoughts. One example of the experience of misguidance and dissonance can be found in the following excerpt:

The light music at one point took me to a place where I thought I was safe, and it became unsafe, and the music was playing a trick with me, you know, sort of giving me a false sense of security. I can remember thinking “this is beautiful music, why am I going to this dark place?” It didn’t line up with what had gone on before. I just felt as I was being manipulated, being duped almost. The music lured me to this beautiful place, and then things started to become dark even with this beautiful music still playing. (#16)

One important observation is that effects of the music that were welcomed, included emotions such as increased grieving or tearfulness, and that an attitude of openness towards negative music-evoked emotions was frequently described as helpful in bringing to expression inner psychological conflicts that might then be resolved (Watts et al. 2017). These experiences were grouped under the theme “openness to challenging experience feels therapeutic” and show similarities with recent qualitative research showing perceived therapeutic meaning in transient psychological struggle during psychedelic therapy (Belser et al. 2017; Swift et al. 2017). One example of this attitude of openness towards the music can be found in the following excerpt:

I can even view the negative moments as positive in a way because they served a purpose. The purpose was to sort of let me face the darkness, and my demons, I guess. It was beautiful at times, but also… yeah, the darker moments really helped to reflect on and connect with your unresolved shadows. (#19)

Contrasting such an attitude of openness to challenging experience is an attitude of resistance to the intensification effects of the music. This experience was characterised by not wanting the music or its effects and was named “resistance to intensification.” An example of this can be found in the following excerpt:

I worried that I let [the music] shape this sort of melancholy. There was resistance, massively, to everything, every sort of sensory input, I had a fearful response. I was afraid to open my eyes, I was afraid to do anything, I was afraid that this sort of music was the last thing I’d ever hear. (#5)

Music styles and playlist design

The study also shed light on how different musical styles and the design of the music playlist were experienced. The choice of the music and the design of the music playlist were overall well-appreciated, with the most frequently appreciated musical genres being ethnic-, vocal-, and (neo-) classical music. Appreciation was also expressed for the design of the playlist, in particular for the calming (ambient) music, which was particularly present during the early (pre-onset and early ascent) and the final (return) phases, and at periods during peak, while more emotionally evocative music being reserved for the peak phase. This indirectly supports the therapists’ views that that an optimal playlist design is characterised by a music genre selection that is structured to match the different phases of drug experience (Barrett et al. 2017; Bonny and Pahnke 1972; Grof 1980; Richards 2015).

Strong disliking of the music selection was rare, but when this did occur it proved insightful about the possible functions of music selection: Typically, disliking of the music seemed to be associated with either a “diminishment” of psilocybin’s subjective effects, accompanied by unpleasant feelings (such as discomfort and irritation), and with an attitude of resistance, characterised by an attempt to psychologically reject and distance oneself from the music, such as detailed in the following excerpt:

The music blocked my experience and feelings. A sense of irritation, frustration, and sense of lowering mood. The majority of the songs were not my kind of music, I can’t sit with that music … I have to leave the room. I was sort of feeling bad, because I wanted to work with it. I sensed the potential for a really profound experience. I couldn’t meet that potential with music that I felt was quite mediocre. To me it didn’t feel real, so I felt quite torn. (#6)

Music experience predicts experience and therapy outcomes

As outlined above, notable polarities were observed in the music experience, such as the music being either liked or disliked, the music being either resonant or dissonant with the patient’s experience, and the patient being either open or resistant to the influence of the music. These variables (liking, resonance, and openness) positively predicted the extent to which patients reported having mystical experiences (a factor defined as the experience of unity, blissful emotionality, and spirituality). In addition, resonance and openness, but not liking, predicted the extent to which people reported insightfulness (a factor defined by having inventive ideas, feelings of profoundness, insights, and the experience of vivid personal memories or mental images). Drug intensity, on the other hand, also correlated with other aspects of the psilocybinexperience, such as impaired cognition and audio-visual perception changes. It must be noted that liking, resonance, and openness were highly correlated and thus likely represent one construct. The absence of a significant correlation between music liking and reported insightfulness may therefore be due to a lack of statistical power.

The selective association of the music experience with mystical experience and insightfulness, and not with other subjective experiences, supports the original motivations to include music in psychedelictherapy, i.e. to promote the occurrence of therapeutically meaningful experiences. Modern studies have confirmed that psilocybin can reliably facilitate mystical experiences (Griffiths et al. 2011, 2016), and these experiences have been associated with sustained positive changes in behaviour and personality (MacLean et al. 2011) and with positive therapy outcomes (Garcia-Romeu et al. 2014; Griffiths et al. 2016; Roseman et al. 2017; Ross et al. 2016). Although these studies incorporated music-listening in combination with psilocybin, this study is the first to demonstrate that the music experience during these sessions relates to the occurrence of mystical experiences. A positive relationship was also found between the music experience and reductions in depression 1 week after the psilocybin experience. Importantly, reductions in depression were not related to the intensity of the drug effects. This finding indicates that it is not merely the drug effect in isolation, but an interaction between the drug and the music on subjective experience that promotes positive therapeutic outcomes.

Possible therapeutic mechanisms of music in psychedelic therapy

A principal effect of psychedelics is that they temporarily dysregulate brain mechanisms that normally regulate emotion(Carhart-Harris et al. 2012a, 2016b; Muthukumaraswamy et al. 2013; Tagliazucchi et al. 2016), and this could underlie the enhanced emotional responsiveness to emotionally evocative stimuli reported here as elsewhere (Carhart-Harris et al. 2012b; Kaelen et al. 2015, 2017; Quednow et al. 2012; Vollenweider et al. 2007). The notion that accepting and moving through challenging emotions are important for psychotherapeutic change is central to many psychotherapeutic models (Greenberg and Pascual-Leone 2006), has empirical support (Whelton 2004), and been noted by other psychedelic therapy studies (Belser et al. 2017; Swift et al. 2017; Watts et al. 2017). In psychedelic therapy, the function of psychedelics may be to ease the relinquishment of psychological control (i.e. ego dissolution and enhanced suggestibility (Carhart-Harris et al. 2014)), thereby allowing a fuller and freer (i.e. less inhibited) expression of emotionality. The enhanced receptivity to music, in turn, may play the important function of activating emotionality, thoughts, and memories that are most personally salient. Thereby, music can guide the patient’s experience into directions that are most therapeutically significant. One key difference between psychedelic therapy and other forms of psychotherapy (and conventional pharmacotherapy) may be the capacity of psychedelics and music to rapidly facilitate deeply felt and personally meaningful emotionality (Carhart-Harris et al. 2016a; Gasser et al. 2014; Griffiths et al. 2016; Grob et al. 2011; Johnson et al. 2014; Ross et al. 2016).

It is worth considering that these findings show a remarkable congruency with the theoretical frameworks and patient experiences of “introspective” forms of music therapy, where music is utilised as the means to provide an experience that is thought to help the listener examine and change his/her relationship with themselves (Abbott 2005; Albornoz 2013; Summer 1992, 2011). This includes the use of music to evoke intense emotional experiences (Albornoz 2013), as well as a way to provide a “holding environment,”which feels “safe and secure” to express and experience new aspects of oneself (Carroll 2011; Schulberg 1999). Therapeutic effects of music are widely reported in literature and utilised across different health care disciplines (Finch and Moscovitch 2016; Mondanaro et al. 2017; Pavlov et al. 2017). The present findings therefore engender the view that psychedelic therapy utilises therapeutic effects of music that are enhanced via an interaction between the drug and the music.

Implications for the use of music in psychedelic therapy

Due to the prominence of music-listening in psychedelic therapy, increasing the knowledge of the appropriate therapeutic use of music in psychedelic therapy is important. This becomes particularly critical when psychedelic therapy is implemented on increasingly larger scales. The therapeutic influence of music has been referred to as being of “profound significance”(Bonny and Pahnke 1972), and several authors emphasised the care needed in selecting appropriate music, playing this music at the right circumstances, and within a personalised patient-centred format (Grof 1980; Hoffer 1965). The present study provides support for these views, by showing that when the music was experienced as dissonant with the unfolding experience, disliked, and rejected (resistance), therapeutic outcomes suffered. In contrast, when the music was in resonance with the patient’s experience, liked, and accepted (openness), therapeutic outcomes were most positive.

These music experience variables in this study (resonance, liking, and openness) correlated with each other, suggesting that they represent a single construct within the music experience that is associated with positive therapy outcomes. Liking of music is usually characterised as a mixture of genre appreciation and aesthetic judgements (Juslin 2013; Juslin and Västfjäll 2008; Juslin et al. 2016; North and Hargreaves 1997), and music liking may represent a basic pre-requisite for music to evoke personally meaningful emotionality. In addition, some music styles and acoustic properties may be more suitable for the conscious states induced by psychedelics than others. The patient’s attitude, in turn, appears to require a sufficient degree of openness to the music-evoked experience, and this may imply not only a state of surrender but also a pro-active and curious engagement with the therapeutic content that emerges.

This hypothetical framework holds that an optimal music experience (style liking, music’s resonance, and openness to music) creates an optimal climate for the expression of meaningful therapeutic content, characterised by the sensation of being on a personal journey, with a spontaneous and often intense emergence of personally meaningful imagery, thoughts, and emotionality. This optimal music experience construct may be a critical pre-requisite, and when it is not met adequately, is likely to result in the patient to distance from the music experience (resistance), characterised by feelings of discomfort, and a diminishment of personally meaningful imagery, thoughts, and emotionality (i.e. the absence of the sense of being on a journey). Given the patient’s experience is highly individual and dynamic, this finding suggests that the adaptation of the music during psychedelic therapy sessions may be critical at times, in order to provide adequate therapeutic support conditions, or prevent possible counter-therapeutic experiences: an idea that was often emphasised by early pioneers of psychedelic therapy (Bonny and Pahnke 1972; Grof 1980; Hoffer 1965).

In this framework, the experience of resistance and dislike by the listener may be regarded as an important indicator for the therapist of music’s failure to act therapeutically, and the type of intervention needed to restore music’s therapeutic function may be determined by one central question the therapists may need to clarify, i.e. what is the source of the resistance or dislike? The therapists bear a responsibility to ensure the music styles are sufficiently liked, via thoughtful music selection, and that resonance is maximised by providing an attunement of the music to the patient’s personal and dynamically unfolding experience, via thoughtful playlistdesign and adaptation of the music when needed. However, in addition, it may occur that the music-evoked experience is rich with therapeutically meaningful content, yet the experience may be emotionally challenging, resulting in similar expressions of resistance. In these scenarios, the therapists may instead need to provide adequate therapeutic support for the patient to feel safe and motivated to engage in exploring and expressing the present challenging feeling states, of which the meanings may not always be immediately clear.

Limitations and future directions

This study has a number of limitations. First of all, the data was acquired without a placebocondition, making causal inferences about the nature of the effects problematic. Secondly, the main body of data used for this study was qualitative in nature. Therefore, the experiment did not allow studying the magnitude of the observed themes in the music experience. It should therefore be emphasised that the primary objective of this study was to provide a patient perspective on the influence of music. We hope that this work inspires new hypotheses for future studies, and that it assists therapists and researchers in their use of music in psychedelic therapy. Examples of future directions include testing whether maximization of resonance could improve therapy outcomes, and whether the variables liking, resonance, and openness represent one single factor or separate factors when larger sample sizes and more precise measurements are employed.

A significant body of empirical work is required to advance the therapeutic use of music in psychedelic therapy. One important focus of such work will be the establishing of baseline measures that can reliably predict individual music experiences during psychedelic therapy sessions. Such predictive measures can range from personality traits (e.g. openness to experience, absorption, or suggestibility) to measures of personal music preferences. Furthermore, research that focuses on identifying reliable indicators of positive (welcome/supportive) and negative (unwelcome/unsupportive) influences of music on the therapeutic processes during psychedelic therapy sessions may help therapists adapt music to individual patients.