Psychedelics and Consciousness: Distinctions, Demarcations, and Opportunities

This review (2021) examines the usage and the meaning of the term ‘consciousness’ within psychedelic research and how theories of consciousness are operationalized to explain the effects of psychedelics in turn. Although psychedelics are unlikely to elucidate the biological basis for phenomenal consciousness (i.e. the hard problem), they are useful tools for investigating claims about the contents of consciousness, and their altered states.


“Psychedelic substances produce unusual and compelling changes in conscious experience which have prompted some to propose that psychedelics may provide unique insights explaining the nature of consciousness. At present, psychedelics, like other current scientific tools and methods, seem unlikely to provide information relevant to the so-called “hard problem of consciousness,” which involves explaining how first-person experience can emerge. However, psychedelics bear on multiple “easy problems of consciousness,” which involve relations between subjectivity, brain function, and behaviour. In this review, we discuss common meanings of the term “consciousness” when used with regard to psychedelics and consider some models of the effects of psychedelics on the brain that have also been associated with explanatory claims about consciousness. We conclude by calling for epistemic humility regarding the potential for psychedelic research to aid in explaining the hard problem of consciousness while pointing to ways in which psychedelics may advance the study of many specific aspects of consciousness.”

Authors: David B. Yaden, Matthew W. Johnson, Roland R. Griffiths, Manoj Doss, Albert Garcia-Romeu, Sandeep Nayak, Natalie Gukasayan, Brian N. Mathur & Frederick S. Barrett


Is your perception of the color red the same as mine? And how does our perception change under the influence of psychedelics? The study of/with psychedelics has given us some preliminary answers as to the content of consciousness. Under ‘altered states of consciousness’ we may experience a red rose with increased intensity, link different concepts, and even link multiple senses (synaesthesia).

What psychedelics don’t (yet) tell us much about is the so-called hard problem of consciousness. Although we can measure the level of changes (as we see in the paper above), we haven’t gained answers as to why we experience anything in the first place.

This paper, by top researchers including David Yaden, Matthew Johnson, and Roland Griffiths, argues that we should be clear and humble when discussing consciousness and psychedelics.

Enlightening quotes from this paper

  • “At present, there is little reason to think that psychedelics will bring us any closer to closing the explanatory gap [between subjective experiences of being and objectively observable phenomena such as brain activity].”
  • “Materialist theories, favored by many scientists, regard phenomenal consciousness as identical to brain states, [but] need to explain how physical processes (e.g., the brain) give rise to qualia,” the “what it feels like” quality of being conscious.”
  • “It is possible that psychedelics will allow altered states of consciousness to follow a similar path of scientific inquiry as emotions by providing a reliable means to induce and influence them in controlled settings.”

The authors argue that psychedelics can play an important role in better understanding the ‘easy problem’ of consciousness. At the same time, they argue that psychedelics currently have not helped elucidate the ‘hard problem’ and don’t see how they could at this time.


Psychedelic substances produce unusual and compelling changes in conscious experience. While psychedelics are unlikely to provide information relevant to explaining how first-person experience can emerge, they may advance the study of many specific aspects of consciousness.

The resurgence of psychedelic research has provided tools for researchers to study mental processes such as perception, affect, and cognition. Some have expressed hopes that psychedelics may somehow help to explain consciousness. Psychedelic substances produce unusual and compelling changes in conscious experience, and some have proposed that psychedelics may provide unique insights into the nature of consciousness. However, the relationship between psychedelics and consciousness hinges on what is meant by the term consciousness.

The Hard and Easy Problems of Consciousness

The hard problem of consciousness is concerned with explaining the immediate, subjective experience of being an organism and how it relates to objectively observable phenomena. The hard problem of consciousness is not currently scientifically answered, and it is not clear that a scientific answer is even possible. However, the so-called “easy problems” of consciousness are plausibly explainable, and they can be explained through current scientific methods given sufficient time and effort.

The hard problem of consciousness is difficult to scientifically address because phenomenal consciousness cannot be observed directly by anyone other than a given conscious entity. Even if it were possible to engineer the creation of consciousness, it would be unclear how to prove that consciousness had indeed been created. The identical problem of verifiability presents itself when attempting to assess consciousness in another person. However, the contents of consciousness can be caused and manipulated to some extent within other people, whereas phenomenal consciousness itself cannot be caused.

Psychedelics may impact beliefs about different theories of phenomenal consciousness under some circumstances, though it is not clear to us at present how psychedelics can inform challenges posed by each of the philosophical theories about the nature of consciousness.

Scientists tend to ignore the hard problem of consciousness and focus on the easy problems of consciousness, such as perceptions, thoughts, feelings, and emotions.

There are several theories that attempt to explain how phenomenal consciousness appears unitary in terms of neural correlates. Although it would be interesting to investigate how psychedelic states relate to the global workspace theory of consciousness, it is not clear how this would improve our understanding of consciousness.

Some have claimed that neurological theories of psychedelic effects may illuminate the subject of phenomenal consciousness. However, it is unclear how any of these theories could provide information relevant to the hard problem of consciousness.

Relaxed beliefs under psychedelics (REBUS) has been proposed as an effect of psychedelics and at least part of it has been postulated as a way to explain phenomenal neuroimaging data during the acute effects of psychedelics. However, it is unclear whether these various measures provide support for this hypothesis. Some observations appear to count against the REBUS model, such as decreased prediction capacity, increased surprise, and a lack of specificity in what constitutes “top-down” or “bottom-up” levels of brain functioning.

The specificity of the DMN in neural effects of psychedelics is also in question, and the REBUS model provides some grounding to concepts expressed in the Free Energy Principle. The model suggests that psychedelics may be efficacious in the treatment of disorders of consciousness. The cortico-striato-thalamo-cortical (CSTC) model proposes that 5-HT2A receptor activation in the thalamus impedes sensory gating functions, leading to sensory overload in the cortex and cognitive changes during the acute effects of psychedelics.

Preller and colleagues suggest that the thalamus plays a key role in various neurobiological theories of consciousness, citing Ward’s (2011) and Tononi & Edelman’s (1998) theories of consciousness. LSD increases thalamic connectivity, thalamus-to-cortex connectivity, and effective thalamic connectivity to both the cortex and the striatum, which may help explain how sensory and interoceptive stimuli come into awareness.

The claustro-cortical circuit model (CCC) proposes that the claustrum, a thin brain connected to a large number of cortical regions, may provide a multisensory binding function and thus be a key brain structure for understanding how phenomenal consciousness may arise from brain function. The claustrum is a brain area that has been proposed to play a role in mediating altered states of consciousness. This role is supported by empirical observations showing that psychedelic drugs alter connectivity between the claustrum and cortical cognitive networks.

Each of these models may generate testable hypotheses regarding key aspects of subjective experience, including affect, and elements of cognition. However, no apparent explanation lies in any of these models for the hard problem of consciousness. In addition to neuroscientific models of consciousness, there are operationalizations of consciousness that focus on carefully characterizing particular aspects of conscious awareness. These operationalizations include theories of phenomenal consciousness that involve varying degrees and stages of non-conscious processing preceding conscious awareness.

Psychedelic drugs have been shown to reduce binocular rivalry switching during the binocular rivalry task, but there is little reason to think that they will have a different kind of effect on these and related tasks when compared to states evoked by any other psychoactive substance. Psychedelics have been used in ritual contexts for thousands of years and are usually connected to religio- spiritual beliefs. Scientific research has demonstrated that psychedelics can produce positive experiences with beneficial consequences.

Psychedelics may induce altered states of consciousness, including a heightened sense of connection, complex imagery, synesthesia, and/or other changes to perception and cognition. They may also influence one’s subjective sense of self. Psychedelics can elicit altered states of consciousness that are characterized by feelings of unity, profundity, positive emotions, alterations to the senses of time and space, and ineffability. These experiences have been associated with a number of persisting benefits.

Psychedelics are likely the most important research tools currently available for investigating altered states of consciousness, and understanding the ramifications of such altered states may be important for understanding mental health. Psychedelics elicit altered states of consciousness that are influenced by the specific psychedelic substance as well as the apparent dose, expectations, context, bioavailability of the given drug formulation, and many other biopsychosocical factors specific to the psychedelic experience.

Psychedelic experiences may involve changes to one’s self-consciousness, which may include experiences with the sense of self, autobiographical reflection, or perceiving meaningful eidetic imagery. These insights may be true or false, and the relative frequency of false insights remains an open question. Psychedelic experiences may alter one’s beliefs about the nature of consciousness. This is due, in part, to the noetic sense associated with psychedelic states.

Psychedelic experiences may contribute to statements concerning “levels of consciousness” and the attribution of selfhood and agency to entities/objects. However, a survey of professors of philosophy found no association between psychedelic use and non-materialist views of consciousness. While we cannot see any clear scientific traction resulting from the intersection of psychedelics and the hard problem of consciousness, we believe it is important to be clear about which sense of the term consciousness is being used at any given time.

The authors thank the National Institutes of Health, the Riverstyx Foundation, and the National Institute on Drug Abuse for funding, and acknowledge that Dr. Barrett is a scientific advisor for WavePaths, LLC, and Dr. Griffiths is on the board of directors of the Heffter Research Institute.

Psilocybin alters the functional connectivity of the claustrum with brain networks that support perception, memory, and attention. Bayne T, Carter O, Bickel S, Parvizi J, Blackburn S, Block N, Carhart-Harris RL et al. (2018) Dimensions of consciousness and the psychedelic state.

Carter, Hasler, Pettigrew, JD, Wallis, GM, Liu, GB, Vollenweider FX (2007) found that psilocybin linked binocular rivalry switch rate to attention and subjective arousal levels in humans. Cerullo, MA (2015) criticized integrated information theory.

In 2001, Geyer MA, Krebs-Thomson K, Braff DL, Swerdlow NR, Vollenweider FX reviewed the pharmacological studies of prepulse inhibition models of sensorimotor gating deficits in schizophrenia, and in 2008, Gray HM, Gray K, Wegner DM reviewed the dimensions of mind perception.

The varieties of religious experience, psilocybin, potential therapeutic effects, and brain network function are all reviewed in Johnson MW, Griffiths RR, Hendricks PS, Barrett FS, Koch C, Massimini M, Boly M, Tononi G, and others.

Letheby C, Levine J, Dolder PC, Schmid Y, Mason NL, Kuypers KPC, Müller F, Reckweg J, Tse DHY, Hutten NRPW, Jansen JFA, Stiers P, Fielding A, Ramaekers JG (2017), Mlot C, Metzinger T, et al. (2000).

LSD induces changes in global and thalamic brain connectivity that are attributable to the 5-HT2A receptor, and these changes contradict the specificity of these alterations for the effects of serotonergic hallucinogens.

Using a functional neuroimaging study with low-resolution electromagnetic tomography, Riba J, Anderer P, Jané F, Saletu B, Barbanoj MJ (2004) studied effects of the South American psychoactive beverage ayahuasca on regional brain electrical activity in humans.

Scott G, Carhart-Harris RL (2019), Psychedelics as a treatment for disorders of consciousness, Seth AK (2018), Singer W (2001), Smigielski L, Kometer M, Scheidegger M, Stress C, Preller KH, Koenig T, Vollenweider FX (2020), Solms M, Friston K (2018), and others have written about consciousness.

Tononi G, Edelman GM, Umbricht D, Vollenweider FX, Schmid L, Grübel C, Skrabo A, Huber T, Koller R (2003) Investigated the effects of psilocybin on mismatch negativity generation and AX-continuous performance task, and discussed the different ways to define consciousness.

The thalamic dynamic core theory of conscious experience is based on the hypothesis that the cingulate cortex input to the claustrum is required for top-down action control.

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Authors associated with this publication with profiles on Blossom

Manoj Doss
Manoj Doss is a researcher at Johns Hopkins University where he studies the cognitive, emotional, and neural mechanisms of psychedelic drugs.

Roland Griffiths
Roland R. Griffiths is one of the strongest voices in psychedelics research. With over 400 journal articles under his belt and as one of the first researchers in the psychedelics renaissance, he has been a vital part of the research community.

Matthew Johnson
Matthew Johnson is an Associate Professor of Psychiatry and Behavioral Sciences at Johns Hopkins University. His research is concerned with addiction medicine, drug abuse, and drug dependence.

Albert Garcia-Romeu
Albert Garcia-Romeu is one of the principal researchers in the renaissance of psychedelics studies. He is doing his research at Johns Hopkins and focuses on psilocybin and how it can help with treating addiction.

Frederick Barrett
Frederick Streeter Barrett is an Assistant Professor of Psychiatry and Behavioral Sciences and works at the Johns Hopkins University Center for Psychedelic and Consciousness Research.

Natalie Gukasyan
Dr. Natalie Gukasyan is a psychiatrist and post-doctoral research fellow at Johns Hopkins University.


Institutes associated with this publication

Johns Hopkins University
Johns Hopkins University (Medicine) is host to the Center for Psychedelic and Consciousness Research, which is one of the leading research institutes into psychedelics. The center is led by Roland Griffiths and Matthew Johnson.

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