Singh's cultural evolutionary theory successfully accounts for multiple aspects of shamanism as a recurrent phenomenon. When addressing the evolution of shamanism, however, Singh rejects altered states of consciousness (ASC) as a crucial element, considering these less important than other elements, such as behavioral adaptations. He further claims that different methods of inducing ASC have “profoundly different physiological and psychological effects” (sect. 4.2, para. 1). One of Singh's reasons for setting aside ASC as explanatory is that he thinks the ASC used by shamans do not have a common neurophysiological basis. Yet, this claim is unsubstantiated. Here, we suggest that ASC both share certain neurophysiological features and give rise to shared cognitive and behavioral effects.
Altered states of consciousness induced by methods as varied as sensory deprivation, shamanic drumming, trance, meditation, endurance running, hallucinogen consumption, and even epileptic seizures produce shared cognitive and behavioral effects, including hallucinations, out-of-body experiences, ego dissolution, enhanced imagery, and a distorted sense of time (see Castillo Reference Castillo1990; Danielson et al. Reference Danielson, Guo and Blumenfeld2011; Dietrich Reference Dietrich2003; Forgays & Forgays Reference Forgays and Forgays1992; Hayashi et al. Reference Hayashi, Morikawa and Hori1992; Kjellgren et al. Reference Kjellgren2003; Reference Kjellgren, Lyden and Norlander2008; Mason & Brady Reference Mason and Brady2009; Speth et al. Reference Speth, Speth, Kaelen, Schloerscheidt, Feilding, Nutt and Carhart-Harris2016; Suedfeld Reference Suedfeld1980; Suedfeld & Eich Reference Suedfeld and Eich1995; Vaitl et al. Reference Vaitl, Birbaumer, Gruzelier, Jamieson, Kotchoubey, Kübler, Lehmann, Miltner, Ott, Pütz, Sammer, Strauch, Strehl, Wackermann and Weiss2005; Zuckerman & Cohen Reference Zuckerman and Cohen1964). These shared effects are not likely to be coincidental; rather, it is likely that ASC share certain neurophysiological features that correspond to these cognitive and behavioral effects. Although there is as of yet no consensus as to what these shared neurophysiological features are, it is worth reviewing some evidence supporting the view.
One line of evidence comes from using electroencephalography (EEG) to compare ASC with non-altered states. EEG results indicate that ASC, regardless of induction method, correspond to greater activity in the low-frequency bands – delta, theta, and slow alpha (see Takahashi et al. [Reference Takahashi, Murata, Hamada, Omori, Kosaka, Kikuchi, Yoshida and && Wada2005], Batty et al. [Reference Batty, Bonnington, Tang, Hawken and Gruzelier2006], Cahn & Polich [Reference Cahn and Polich2006], and Fox et al. [Reference Fox, Nijeboer, Solomonova, Domhoff and Christoff2013] for meditation and relaxation states; Neher [Reference Neher1962], Oohashi et al. [Reference Oohashi, Kawai, Honda, Nakamura, Morimoto, Nishina and Maekawa2002], and Gingras et al. [Reference Gingras, Pohler and Fitch2014] for trance and shamanic drumming; Hayashi et al. [Reference Hayashi, Morikawa and Hori1992] and Iwata et al. [Reference Iwata, Nakao, Yamamoto and Kimura2001] for sensory deprivation; Muthukumaraswamy et al. [Reference Muthukumaraswamy, Carhart-Harris, Moran, Brookes, Williams, Errtizoe, Sessa, Papadopoulos, Bolstridge, Singh, Feilding, Friston and Nutt2013], Tagliazucchi et al. [Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016], and Carhart-Harris et al. [Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016] for hallucinogens; and Danielson et al. [Reference Danielson, Guo and Blumenfeld2011] for epileptic seizures). These frequency bands, in turn, are associated with internally directed attention and attenuated interaction with the external environment (e.g., Benedek et al. Reference Benedek, Schickel, Jauk, Fink and Neubauer2014). These results suggest that internally directed attention might be a common element of ASC. Supporting this view, practices such as drumming, trance, endurance running, and focused meditation simulate sensory deprivation to varying degrees and cause highly focused internal attention, such that engagement with external stimuli is highly attenuated (Castillo Reference Castillo1990; Dahl et al. Reference Dahl, Lutz and Davidson2015; Dietrich Reference Dietrich2003; Gingras et al. Reference Gingras, Pohler and Fitch2014; Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Lutz et al. Reference Lutz, Slagter, Dunne and Davidson2008). This disengagement of attention from external stimuli, known as perceptual decoupling, helps to sustain internally directed tasks (Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Smallwood et al. Reference Smallwood, McSpadden and Schooler2007; Smallwood et al. Reference Smallwood, Brown, Tipper, Giesbrecht, Franklin, Mrazek, Carlson and Schooler2011; Spreng et al. Reference Spreng, Stevens, Chamberlain, Gilmore and Schacter2010). Similarly, hallucinogens that induce ASC impair reactions to external stimuli, but this occurs because of the failure of sensory gating in filtering out extraneous stimuli, causing a failure to process incoming information adequately (Carter et al. Reference Carter, Burr, Pettigrew, Wallis, Hasler and Vollenweider2005; Geyer & Vollenweider Reference Geyer and Vollenweider2008).
The second line of evidence comes from studies using neuroimaging techniques such as functional magnetic resonance imaging (fMRI). Such studies suggest that one of the most common effects of ASC, known as ego dissolution (the loss of a sense of self), corresponds to disruptions to the default mode network (DMN). The DMN is activated during self-referential processes, such as autobiographical memory retrieval (Boly et al. Reference Boly, Phillips, Tshibanda, Vanhaudenhuyse, Schabus, Dang-Vu, Moonen, Hustinx, Maquet and && Laureys2008; Buckner et al. Reference Buckner, Andrews-Hanna and Schacter2008; Raichle et al. Reference Raichle, MacLeod, Snyder, Powers, Gusnard and Shulman2001). Studies using fMRI have indicated that, during ASC induced by various methods – including focused meditation (Brewer et al. Reference Brewer, Worhunsky, Gray, Tang, Weber and Kober2011), hallucinogen consumption (Carhart-Harris et al. Reference Carhart-Harris, Erritzoe, Williams, Stone, Reed, Colasanti, Tyacke, Leech, Malizia, Murphy and Hobden2012; Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016; Palhano-Fontes et al. Reference Palhano-Fontes, Andrade, Tofoli, Santos, Crippa, Hallak, Ribeiro and de Araujo2015; Tagliazucchi et al. Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016; Speth et al. Reference Speth, Speth, Kaelen, Schloerscheidt, Feilding, Nutt and Carhart-Harris2016), and epileptic seizures (Danielson et al. Reference Danielson, Guo and Blumenfeld2011) – the activation of the DMN decreases, giving rise to ego dissolution. These studies also reveal that, during ASC, regardless of induction method, high functional connectivity is exhibited between three interacting brain networks: default mode, frontoparietal control, and salience (see Brewer et al. Reference Brewer, Worhunsky, Gray, Tang, Weber and Kober2011; Carhart-Harris et al. Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016; Hasenkamp et al. Reference Hasenkamp, Wilson-Mendenhall, Duncan and Barsalou2012; Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Tagliazucchi et al. Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016). The frontoparietal control network is responsible for cognitive control and attention (Cole & Schneider Reference Cole and Schneider2007; Cole et al. Reference Cole, Repovš and Anticevic2014; Vincent et al. Reference Vincent, Kahn, Snyder, Raichle and Buckner2008). The salience network is responsible for detecting salient events (internal or external) and for directing resources to the relevant neural areas (Christoff et al. Reference Christoff, Gordon, Smallwood, Smith and Schooler2009; Menon & Uddin Reference Menon and Uddin2010; Seeley et al. Reference Seeley, Menon, Schatzberg, Keller, Glover, Kenna, Reiss and Greicius2007). Enhanced connectivity among these three large-scale brain networks could explain some of the effects experienced by individuals during ASC, including the perceptual decoupling discussed above.
This evidence supports the idea of a common neurophysiological foundation to ASC. We further claim that a common neurophysiological foundation to ASC can be of significant explanatory value for a cultural evolutionary theory of shamanism. For example, Singh's theory notes but does not currently explain the shaman's insensitivity to pain, which, in our view, would be explained by perceptual decoupling in ASC. Similarly, Singh's theory does not account for the shaman's ritualistic practice of using dark times of the day or dark spaces, such as caves; in our view, this is explained by the relationship between sensory deprivation and ASC. Finally, Singh's theory notes but does not currently explain why shamans believe they are not in control of their experience, which, in our view, is explained by the experience of ego dissolution common to ASC. Although ASC and their underlying mechanisms require further investigation, we argue that it is too soon to reject the idea of a common neurophysiological foundation to ASC. Our argument is consistent with the primary objectives of Singh's cultural evolutionary theory and can be considered an extension to that theory, because it can help explain the universality, ubiquity, and endurance of shamanic activities around the world.
Singh's cultural evolutionary theory successfully accounts for multiple aspects of shamanism as a recurrent phenomenon. When addressing the evolution of shamanism, however, Singh rejects altered states of consciousness (ASC) as a crucial element, considering these less important than other elements, such as behavioral adaptations. He further claims that different methods of inducing ASC have “profoundly different physiological and psychological effects” (sect. 4.2, para. 1). One of Singh's reasons for setting aside ASC as explanatory is that he thinks the ASC used by shamans do not have a common neurophysiological basis. Yet, this claim is unsubstantiated. Here, we suggest that ASC both share certain neurophysiological features and give rise to shared cognitive and behavioral effects.
Altered states of consciousness induced by methods as varied as sensory deprivation, shamanic drumming, trance, meditation, endurance running, hallucinogen consumption, and even epileptic seizures produce shared cognitive and behavioral effects, including hallucinations, out-of-body experiences, ego dissolution, enhanced imagery, and a distorted sense of time (see Castillo Reference Castillo1990; Danielson et al. Reference Danielson, Guo and Blumenfeld2011; Dietrich Reference Dietrich2003; Forgays & Forgays Reference Forgays and Forgays1992; Hayashi et al. Reference Hayashi, Morikawa and Hori1992; Kjellgren et al. Reference Kjellgren2003; Reference Kjellgren, Lyden and Norlander2008; Mason & Brady Reference Mason and Brady2009; Speth et al. Reference Speth, Speth, Kaelen, Schloerscheidt, Feilding, Nutt and Carhart-Harris2016; Suedfeld Reference Suedfeld1980; Suedfeld & Eich Reference Suedfeld and Eich1995; Vaitl et al. Reference Vaitl, Birbaumer, Gruzelier, Jamieson, Kotchoubey, Kübler, Lehmann, Miltner, Ott, Pütz, Sammer, Strauch, Strehl, Wackermann and Weiss2005; Zuckerman & Cohen Reference Zuckerman and Cohen1964). These shared effects are not likely to be coincidental; rather, it is likely that ASC share certain neurophysiological features that correspond to these cognitive and behavioral effects. Although there is as of yet no consensus as to what these shared neurophysiological features are, it is worth reviewing some evidence supporting the view.
One line of evidence comes from using electroencephalography (EEG) to compare ASC with non-altered states. EEG results indicate that ASC, regardless of induction method, correspond to greater activity in the low-frequency bands – delta, theta, and slow alpha (see Takahashi et al. [Reference Takahashi, Murata, Hamada, Omori, Kosaka, Kikuchi, Yoshida and && Wada2005], Batty et al. [Reference Batty, Bonnington, Tang, Hawken and Gruzelier2006], Cahn & Polich [Reference Cahn and Polich2006], and Fox et al. [Reference Fox, Nijeboer, Solomonova, Domhoff and Christoff2013] for meditation and relaxation states; Neher [Reference Neher1962], Oohashi et al. [Reference Oohashi, Kawai, Honda, Nakamura, Morimoto, Nishina and Maekawa2002], and Gingras et al. [Reference Gingras, Pohler and Fitch2014] for trance and shamanic drumming; Hayashi et al. [Reference Hayashi, Morikawa and Hori1992] and Iwata et al. [Reference Iwata, Nakao, Yamamoto and Kimura2001] for sensory deprivation; Muthukumaraswamy et al. [Reference Muthukumaraswamy, Carhart-Harris, Moran, Brookes, Williams, Errtizoe, Sessa, Papadopoulos, Bolstridge, Singh, Feilding, Friston and Nutt2013], Tagliazucchi et al. [Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016], and Carhart-Harris et al. [Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016] for hallucinogens; and Danielson et al. [Reference Danielson, Guo and Blumenfeld2011] for epileptic seizures). These frequency bands, in turn, are associated with internally directed attention and attenuated interaction with the external environment (e.g., Benedek et al. Reference Benedek, Schickel, Jauk, Fink and Neubauer2014). These results suggest that internally directed attention might be a common element of ASC. Supporting this view, practices such as drumming, trance, endurance running, and focused meditation simulate sensory deprivation to varying degrees and cause highly focused internal attention, such that engagement with external stimuli is highly attenuated (Castillo Reference Castillo1990; Dahl et al. Reference Dahl, Lutz and Davidson2015; Dietrich Reference Dietrich2003; Gingras et al. Reference Gingras, Pohler and Fitch2014; Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Lutz et al. Reference Lutz, Slagter, Dunne and Davidson2008). This disengagement of attention from external stimuli, known as perceptual decoupling, helps to sustain internally directed tasks (Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Smallwood et al. Reference Smallwood, McSpadden and Schooler2007; Smallwood et al. Reference Smallwood, Brown, Tipper, Giesbrecht, Franklin, Mrazek, Carlson and Schooler2011; Spreng et al. Reference Spreng, Stevens, Chamberlain, Gilmore and Schacter2010). Similarly, hallucinogens that induce ASC impair reactions to external stimuli, but this occurs because of the failure of sensory gating in filtering out extraneous stimuli, causing a failure to process incoming information adequately (Carter et al. Reference Carter, Burr, Pettigrew, Wallis, Hasler and Vollenweider2005; Geyer & Vollenweider Reference Geyer and Vollenweider2008).
The second line of evidence comes from studies using neuroimaging techniques such as functional magnetic resonance imaging (fMRI). Such studies suggest that one of the most common effects of ASC, known as ego dissolution (the loss of a sense of self), corresponds to disruptions to the default mode network (DMN). The DMN is activated during self-referential processes, such as autobiographical memory retrieval (Boly et al. Reference Boly, Phillips, Tshibanda, Vanhaudenhuyse, Schabus, Dang-Vu, Moonen, Hustinx, Maquet and && Laureys2008; Buckner et al. Reference Buckner, Andrews-Hanna and Schacter2008; Raichle et al. Reference Raichle, MacLeod, Snyder, Powers, Gusnard and Shulman2001). Studies using fMRI have indicated that, during ASC induced by various methods – including focused meditation (Brewer et al. Reference Brewer, Worhunsky, Gray, Tang, Weber and Kober2011), hallucinogen consumption (Carhart-Harris et al. Reference Carhart-Harris, Erritzoe, Williams, Stone, Reed, Colasanti, Tyacke, Leech, Malizia, Murphy and Hobden2012; Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016; Palhano-Fontes et al. Reference Palhano-Fontes, Andrade, Tofoli, Santos, Crippa, Hallak, Ribeiro and de Araujo2015; Tagliazucchi et al. Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016; Speth et al. Reference Speth, Speth, Kaelen, Schloerscheidt, Feilding, Nutt and Carhart-Harris2016), and epileptic seizures (Danielson et al. Reference Danielson, Guo and Blumenfeld2011) – the activation of the DMN decreases, giving rise to ego dissolution. These studies also reveal that, during ASC, regardless of induction method, high functional connectivity is exhibited between three interacting brain networks: default mode, frontoparietal control, and salience (see Brewer et al. Reference Brewer, Worhunsky, Gray, Tang, Weber and Kober2011; Carhart-Harris et al. Reference Carhart-Harris, Muthukumaraswamy, Roseman, Kaelen, Droog, Murphy, Tagliazucchi, Schenberg, Nest, Orban, Leech, Williams, Williams, Bolstridge, Sessa, McGonigle, Sereno, Nichols, Hellyer, Hobden, Evans, Singh, Wise, Curran, Feilding and Nutt2016; Hasenkamp et al. Reference Hasenkamp, Wilson-Mendenhall, Duncan and Barsalou2012; Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Tagliazucchi et al. Reference Tagliazucchi, Roseman, Kaelen, Orban, Muthukumaraswamy, Murphy, Laufs, Leech, McGonigle, Crossley, Bullmore, Williams, Bolstridge, Feilding, Nutt and Carhart-Harris2016). The frontoparietal control network is responsible for cognitive control and attention (Cole & Schneider Reference Cole and Schneider2007; Cole et al. Reference Cole, Repovš and Anticevic2014; Vincent et al. Reference Vincent, Kahn, Snyder, Raichle and Buckner2008). The salience network is responsible for detecting salient events (internal or external) and for directing resources to the relevant neural areas (Christoff et al. Reference Christoff, Gordon, Smallwood, Smith and Schooler2009; Menon & Uddin Reference Menon and Uddin2010; Seeley et al. Reference Seeley, Menon, Schatzberg, Keller, Glover, Kenna, Reiss and Greicius2007). Enhanced connectivity among these three large-scale brain networks could explain some of the effects experienced by individuals during ASC, including the perceptual decoupling discussed above.
This evidence supports the idea of a common neurophysiological foundation to ASC. We further claim that a common neurophysiological foundation to ASC can be of significant explanatory value for a cultural evolutionary theory of shamanism. For example, Singh's theory notes but does not currently explain the shaman's insensitivity to pain, which, in our view, would be explained by perceptual decoupling in ASC. Similarly, Singh's theory does not account for the shaman's ritualistic practice of using dark times of the day or dark spaces, such as caves; in our view, this is explained by the relationship between sensory deprivation and ASC. Finally, Singh's theory notes but does not currently explain why shamans believe they are not in control of their experience, which, in our view, is explained by the experience of ego dissolution common to ASC. Although ASC and their underlying mechanisms require further investigation, we argue that it is too soon to reject the idea of a common neurophysiological foundation to ASC. Our argument is consistent with the primary objectives of Singh's cultural evolutionary theory and can be considered an extension to that theory, because it can help explain the universality, ubiquity, and endurance of shamanic activities around the world.