Why do trance states play a central role across shamanic traditions? In his cultural evolutionary theory of shamanism, Singh proposes that shamanic practices such as trance evolved to convince observers of the shaman's superhuman abilities. By adapting practices to the cognitive dispositions of their audiences, he suggests, shamans consolidated their powerful positions within their societies.

We agree that some cultural aspects, such as convincing observers, played a role in the evolution of shamanic practices and that certain practices are primarily theatrical. Singh's theory, however, focuses on the cognitive dispositions of the observers and neglects important cognitive states of the shaman. We argue that the evolution of shamanism is deeply intertwined with the biology of our brains, namely, the ability to experience altered states of consciousness. Altered states in the form of trance are the foundation of shamanism, and shamans enter trance to provide services and promote healing (Singh, target article; Winkelman Reference Winkelman2010c). Recent experimental evidence supports the biological role of trance in facilitating insight and promoting healing through means such as psychoneuroimmunology. A comprehensive theory of shamanism must consider the biological underpinnings of trance and how altered states can confer beneficial effects in the domains of insight and healing.

Many techniques – including chanting, dancing, taking psychoactive substances, and drumming – are used to induce trance states (Vaitl et al. Reference Vaitl, Birbaumer, Gruzelier, Jamieson, Kotchoubey, Kübler, Lehmann, Miltner, Ott, Pütz, Sammer, Strauch, Strehl, Wackermann and Weiss2005); each technique may induce its own state (Walsh Reference Walsh2007). The drum is the central emblem of the shaman's practice. Monotonous drumming is used to induce trance across cultures, and the commonality suggests a common biological mechanism (Harner Reference Harner1990; Winkelman Reference Winkelman2010c). (Note that some traditions use complex polyrhythms, wherein the master drummer induces trance with sudden rhythmic disruptions; this parallels the sudden induction techniques in hypnosis.) Drumming and music in general can have powerful effects on the body and brain (e.g., on heart rate, blood pressure, muscle tension, mood, biochemical responses for arousal, reward, immunity, and local and large-scale brain dynamics; e.g., Altenmüller & Schlaug Reference Altenmüller, Schlaug, Macdonald, Kreutz and Mitchell2012). Little research, however, has examined the neural mechanisms of rhythm-induced trance. The few early electroencephalography (EEG) studies (e.g., Neher Reference Neher1962) were deeply flawed (Achterberg Reference Achterberg1985).

We recently performed two studies on the neural underpinnings of rhythm-induced trance in experienced shamanic practitioners. In a functional magnetic resonance imaging (fMRI) study, we observed clear differences between trance and non-trance states in functional connectivity of large-scale brain systems. The observed network configuration in trance (e.g., coactive default and control network regions) likely enables an extended internal train of thought and perceptual decoupling (Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016). In a follow-up EEG study, shamanic practitioners had reduced brain responses (i.e., lower amplitude event-related potential components) to sounds during trance, which suggests decreased sensory encoding and elaboration during trance (Hove et al. Reference Hove, Habibi, Stelzer and Cahn2017). Together, these fMRI and EEG studies suggest that monotonous drumming helps the shaman disengage from the sensory environment and enables an extended internal stream of thought.

These neural correlates of rhythm-induced trance closely parallel research findings on creativity and insight. Insights are associated with a shift away from sensory processing toward internal processing (Kounios & Beeman Reference Kounios and Beeman2014). Additionally, insights are largely the product of unconscious processing and can be aided by imagery (Finke Reference Finke1996; Kounios & Beeman Reference Kounios and Beeman2014), which resonates with shamanic trance as an exploratory journey filled with vivid imagery and symbols (Achterberg Reference Achterberg1985). Shamans report that their journeys are a major source of insight (Winkelman Reference Winkelman2010c), and these recent neuroscience findings provide a plausible neural mechanism for how trance promotes insight. Rhythm-induced trance possibly evolved as a technology to increase insight – the shaman can enter an altered state to gain insight into high-dimensional complex problems such as food acquisition and for healing (physical, psychological, or social).

Altered states of consciousness can influence healing by other means. Trance may promote health by psychophysiological or psychoneuroimmunological means (Sidky Reference Sidky2009). The theatrical and ritual aspects of shamanic practices can strengthen the patient's belief in the shaman's power, as stressed by Singh, and in turn maximize psychoneuroimmunological effects for healing. Additionally, altered states can promote healing by fundamentally changing the individual's awareness of reality and the underlying disease state (de Rios & Winkelman Reference de Rios and Winkelman1989). Altered states can provide access to subconscious content and psychosomatic aspects that accompany or cause the disease, which then can be targeted.

The therapeutic value of altered states of consciousness has been rediscovered recently in psychiatry. Many recent studies show benefits of therapies that include psychoactive substances. For example, psilocybin, LSD, MDMA, and ayahuasca show promise for treating depression, anxiety, posttraumatic stress disorder, and drug addiction (Carhart-Harris et al. Reference Carhart-Harris, Bolstridge, Rucker, Day, Erritzoe, Kaelen, Bloomfield, Rickard, Forbes, Feilding, Taylor, Pilling, Curran and Nutt2016; Gasser et al. Reference Gasser, Holstein, Michel, Doblin, Yazar-Klosinski, Passie and Brenneisen2014; Labate & Cavnar Reference Labate and Cavnar2013; Oehen et al. Reference Oehen, Traber, Widmer and Schnyder2012). In these therapies, induction of an altered state in the patient aims to provide a different perspective to access and confront psychological issues that are otherwise inaccessible. Thus, one might draw parallels between shamanic healing and the methods of modern psychotherapy and psychiatry.

In sum, we argue that trance emerged across shamanic traditions as a useful adaptation that can promote insight and healing. Trance and other altered states could promote well-being through a combination of facilitating insight, psychoneuroimmunology, changing perspectives, and emotional transformation. More empirical research is needed to evaluate these mechanisms.

We are sympathetic to a cultural evolutionary view of shamanism and the importance of convincing observers for some practices to take hold. The cultural and biological aspects, however, are deeply intertwined in the development of shamanism. A narrow interpretation of the cultural view might erroneously suggest that the biological aspects of trance are irrelevant or secondary. A long history in anthropology and other fields treats trance with circumspection; these views make performing research and publishing on trance especially challenging (Herbert Reference Herbert2011; Penman & Becker Reference Penman and Becker2009).

Times are changing, however. Interest in shamanism is resurging (Walsh Reference Walsh2007), and recent research on trance and altered states shows the viability of neuroscience investigations and promising clinical applications. More rigorous research on trance is needed. With more data and improved understanding, some more theatrical practices will likely disappear, whereas other beneficial practices should grow and spread.