The multi-dimensional approach to drug-induced
states a commentary on Bayne and Carter’s
“dimensions of consciousness and the
and Raphae¨l Millie`re
EHESS/ENS, Institut Jean-Nicod, 29 rue d’Ulm, Paris 75005, France and
Faculty of Philosophy, University of
Oxford, Radcliffe Observatory Quarter 555, Woodstock Road, Oxford OX2 6GG, UK
Both authors contributed equally to this manuscript.
*Correspondence address. Faculty of Philosophy, University of Oxford, Radcliffe Observatory Quarter 555, Woodstock Road, Oxford OX2 6GG, UK.
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Bayne and Carter argue that the mode of consciousness induced by psychedelic drugs does not ﬁt squarely within the tradi-
tional account of modes as levels of consciousness, and favors instead a multi-dimensional account according to which
modes of consciousness differ along several dimensions—none of which warrants a linear ordering of modes. We discuss
the assumption that psychedelic drugs induce a single or paradigmatic mode of consciousness, as well as conceptual issues
related to Bayne and Carter’s main argument against the traditional account. Finally, we raise a set of questions about the
individuation of dimensions selected to differentiate modes of consciousness that could be addressed in future discussions
of the multi-dimensional account.
Key words: states of consciousness; pharmacology; philosophy; contents of consciousness; theories and models
In recent years, a debate has emerged regarding the adequate
characterization and taxonomy of global states of conscious-
ness, a notion that loosely refers to “ways of being conscious”
by contrast with specific conscious contents (Bayne and Hohwy
2016). Examples of global states of consciousness include the or-
dinary wakeful state, post-comatose disorders of conscious-
ness, and the dreaming state associated with rapid eye
The traditional view in clinical neuropsychology is that
global states of consciousness can be ranked on a scale corre-
sponding to levels of consciousness, from the “least conscious”
to the “most conscious” state (Laureys 2005). This view has re-
cently been criticized by Bayne et al. (2016), on the grounds that
global states of consciousness differ from each other in more
than one respect, and thus cannot be easily ranked from least
to most conscious. Bayne et al. argue that consciousness is not a
1D construct measured by levels of consciousness, but a multi-
dimensional construct measured across several distinct
In a rich and thought-provoking article, Bayne and Carter
(2018) propose to extend this analysis to the states of conscious-
ness induced by psychedelic drugs such as psilocybin or lysergic
acid diethylamide (LSD). According to Bayne and Carter (hence-
forth B&C), available evidence from controlled studies does not
Received: 24 September 2019; Revised: 4 March 2020. Accepted: 7 March 2020
CThe Author(s) 2020. Published by Oxford University Press.
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warrant the claim that the kind of global state of consciousness
induced by psychedelics might constitute a higher level of con-
sciousness than ordinary waking awareness. Indeed, they argue
that the “psychedelic state” differs from ordinary wakefulness
in several respects, some of which qualify as “enhancements”
and others as “impairments.” They conclude that their analysis
further undermines the plausibility of a 1D account of global
states of consciousness in terms of levels of consciousness.
While we are very sympathetic to B&C’s multi-dimensional
framework and share the view that the effects of psychoactive
drugs challenge the level-based approach to global states of
consciousness, we emphasize here some potential limitations
of their account and offer some suggestions to address them.
Throughout their article, B&C refer to global states of conscious-
ness as states of consciousness or conscious states. It may be
useful to draw an explicit distinction between three notions
that this lexical choice might obscure for some readers:
1. Speciﬁc experiences individuated by their phenomenal char-
acter (“what it is like” to have them), often called “conscious
contents” or “local states of consciousness” (Chalmers 2010;
Bayne et al. 2016). In that sense, the visual experience of a
dog and the auditory experience of barking are two distinct
local states of consciousness, because what it is like to see a
dog differs to what it is like to hear a dog barking.
2. The subject’s overall phenomenology at a given time, i.e., the
totality of what it is like to be that subject at that time. This
notion has been called by different names, such as “total
experience” (Dainton 2000), “total phenomenal state”
(Gertler 2001;Bayne and Chalmers 2003), and “phenomenal
ﬁeld” (Bayne 2010).
3. The subject’s “way of being conscious,” individuated by disposi-
tional properties such as the ability to be conscious of certain
kinds of contents, and the availability of those contents for the
control of cognition and behavior. In previous works, Bayne et al.
have variously referred to this notion as “background states of
consciousness” (Chalmers 2000), “modes of consciousness”
(Bayne and Hohwy 2016), and “global states of consciousness”
(Bayne et al. 2016).
In our opinion, it is better to avoid the expressions “state of
consciousness” and “conscious state” altogether, as they might
inadvertently prompt readers to conflate these three notions.
Furthermore, the expression “global state of consciousness” it-
self might lead some readers to confuse the second notion and
the third notion, insofar as they both refer to global features of
conscious subjects. In order to avoid potential confusions, we
propose to refer to these three notions as “local phenomenal
state”, “global phenomenal state”, and “global mode of con-
sciousness”, respectively. (“Global modes of consciousness”
seems more adequate than “modes of consciousness” simpli-
citer, to avoid confusion with modes of presentation or sensory
modalities, which both pertain to features of local phenomenal
states. We thank the anonymous reviewer for bringing this po-
tential confusion to our attention.) This choice of terminology
clearly distinguishes (i) local and global properties of conscious
subjects and (ii) categorical and dispositional properties of con-
scious subjects (phenomenal states vs. modes of
The Heterogeneity of Psychedelic Modes of
Throughout their article, B&C follow many other authors in re-
ferring to the “psychedelic state” as if it were a unified kind.
Although they briefly acknowledge that different psychedelic
drugs may have different effects on consciousness, they settle
on this expression “as a general term to refer to the paradig-
matic states of consciousness associated with the consumption
of psilocybin and LSD” (Bayne and Carter 2018, p. 2).
For authors who take the uni-dimensional account of global
modes of consciousness for granted, it might make sense to re-
fer to a unique global mode of consciousness induced by psy-
chedelic drugs. For example, if putative levels of consciousness
are indexed by the Lempel-Ziv complexity of spontaneous neu-
ral activity, as has been recently suggested (Schartner et al.
2017), then in so far as all psychedelic drugs increase spontane-
ous signal complexity in a similar way one can meaningfully re-
fer to the single mode of consciousness induced by such drugs.
(Let us emphasize again that we side with B&C’s criticism of
uni-dimensional accounts; we simply want to point out that
talking of a single psychedelic mode of consciousness makes
sense within such accounts, even if they are inadequate for dis-
However, within B&C’s multi-dimensional account it might
not be helpful to refer to the paradigmatic global mode of con-
sciousness induced by psychedelic drugs in general, or even by
psilocybin and LSD in particular. Indeed, the effects of psyche-
delic drugs can vary dramatically on account of three main
•Dosage: the same substance (e.g. LSD) can induce very different
kinds of effects depending on the dose (Shulgin and Shulgin
1997), and the relationship between dosage and subjective
effects is not always linear. For example, generally, (i) at a low
dose of LSD (between 25 mg and 75 mg), the world feel strange and
surprising (Nelson and Sass 2008) but no visual distortion is ex-
perienced (no illusions and no hallucinations); (ii) at a medium
dose of LSD (between 75 mg and 150 mg), real objects of the world
start morphing and illusory properties are projected onto them
(illusions with no hallucinations); (iii) at a high dose of LSD (150
mg and higher), genuine hallucinations (seeing objects which are
not in the real world) are experienced (Masters and Houston
•Context and personal predispositions: the same dose and the
same compound taken in different settings or with different
mindsets can also induce somewhat distinct experiences
•Provides a critical discussion of an inﬂuential new
model of global states of consciousness such as post-
comatose disorders of consciousness and drug-induced
•Offers terminological and conceptual distinctions to ad-
vance the discussion of multi-dimensional accounts of
global states of consciousness.
•Underlines empirical evidence challenging the claim
that there is a single paradigmatic global state of con-
sciousness associated with the effects of psychedelic
•Raises a set of outstanding questions regarding the in-
dividuation of dimensions in multi-dimensional
accounts of global states of consciousness.
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(Hartogsohn 2016). Personality traits (such as absorption;
Studerus et al. 2012) and genetic factors (Ott et al. 2005) can also
signiﬁcantly shape the effects of psychedelics.
•Temporal dynamics: the effects of the same dose of the same
compound can change considerably across time (Preller and
Vollenweider 2018). Furthermore, there is a non-linear relation-
ship between the duration of drug intoxication and the associ-
ated subjective effects. For example, a high dose of N,N-
Dimethyltryptamine can ﬁrst induce very salient bodily sensa-
tions, immediately followed by an abrupt loss of all bodily sensa-
tions (Timmermann et al. 2019).
According to B&C, the paradigmatic global mode of con-
sciousness induced by psychedelics is characterized by (i) per-
ceptual enhancements (increased bandwidth and vividness of
perceptual experience); (ii) cognitive impairments (impaired
decision-making, memory, attention, and abstract thinking);
and (iii) alterations of the experience of space, time, and self-
hood. (Note that talking of “impairment” and “enhancement”
generally makes sense by reference to a baseline performance
on a specific task. But, it is unclear whether such reference is
available for every dimension discussed by B&C, especially the
third one.) However, the evidence reviewed above suggests that
(at least) some of these dimensions could be “impaired” in one
psychedelic mode of consciousness and “enhanced” in another.
For example, psilocybin and LSD might increase the
“bandwidth” of sensory experience at low and medium to high
doses, thus scoring higher than the ordinary wakeful mode of
consciousness on the relevant dimension. However, there is
also evidence that high to very high doses of psilocybin and LSD
can lead to extremely impoverished sensory experiences.
(Consider for example the following anecdotal report describing
the effects of around 500 mg of LSD: “everything in my external
environment [was] turning black and red. I could barely see any-
thing, my vision was almost completely shot” (retrieved from
erowid.org/exp/98623).) As for the dimension related to the
sense of self in B&C’s account, there is evidence that low doses
of psilocybin and LSD can induce introspective moods with an
increased frequency of self-related thoughts, while medium to
high doses can lead to the temporary cessation of self-related
thoughts (Millie`re et al. 2018). Moreover, awareness of one’s
body can be “enhanced” at one time and “impaired” a few
minutes later, for the same subject, with the same drug and
dose (Timmermann et al. 2019).
This does not merely suggest that psychedelic drugs can in-
duce different global phenomenal states—which is trivially true
insofar as what it is like to be intoxicated by some psychedelic
drug at one time may differs from what it is like to be intoxicated
by some psychedelic drug at another time. Rather, available evi-
dence suggests that conscious subjects intoxicated by psyche-
delic drugs in general, and psilocybin and LSD in particular, can
vary significantly with respect to the dispositional properties
(e.g. dimensions) that individuate global modes of conscious-
ness, and that such variation does not always correlate linearly
with dosage. (Interestingly, a recent positron-emission tomogra-
phy imaging study also found that 5-HT2AR occupancy and
plasma concentration of psilocin—the active metabolite of psilo-
cybin—correlates non-linearly with the self-reported intensity of
subjective effects (Madsen et al. 2019).) Consequently, it might be
misleading to speak of a paradigmatic global mode of conscious-
ness induced by psychedelic drugs; even at a coarse-grained
level of analysis limited to B&C’s three dimensions, there is
some evidence that psychedelics can yield distinct global modes
of consciousness. We note that this is a strength of B&C’s
account rather than a limitation: unlike uni-dimensional
accounts, it allows us to capture the rich diversity of global
modes of consciousness that psychedelic drugs can induce.
Dimensions and Levels of Consciousness
In statistics, it is common to distinguish between quantitative
and categorical variables; the former can vary on an ordered
scale (like height), while the latter corresponds to nominal cate-
gories (like blood types). B&C seem to favor the claim that all
dimensions of consciousness are quantitative variables that
can be gradually “enhanced” or “impaired” compared to the
baseline of ordinary wakefulness.
Against the idea that the psychedelics can induce a “higher”
level of consciousness, they suggest that the global mode of
consciousness induced by psychedelic drugs scores higher than
ordinary wakefulness on one dimension and lower on another
(Fig. 1A). This argument relies upon the implicit assumption
that the global mode of consciousness induced by psychedelic
drugs could be considered to be a “higher” level of conscious-
ness than ordinary wakefulness only if it scored higher on all
dimensions (Fig. 1B).
It is worth noting that this assumption would be misguided if
the notion of levels of consciousness was incoherent anyway as a
matter of conceptual analysis—as Bayne et al. (2016) previously
suggested—because consciousness is not a gradable predicate like
height or temperature.(Consider by analogy the predicate being a
dog. Either an entity is a dog or it is not; even if a dog scored higher
than another on a number of dog-related dimensions—furriness,
speed, etc.—it would make little sense to say that such a creature
instantiates a higher level of dogginess, as it were.) Thus, we can
read Bayne et al. (2016) and Bayne and Carter (2018) as providing a
two-pronged strategy against levels of consciousness: they offer a
conceptual argument according to which the predicate “being con-
scious” is not gradable, and an empirical argument that comes into
play if the conceptual argument is judged unsatisfactory.
However, for the implicit assumption of the empirical argu-
ment to be sound, it must be the case that the three dimensions
of B&C’s account can be meaningfully compared and related;
otherwise, the fact that the psychedelic mode of consciousness
scores higher than ordinary wakefulness on all of these dimen-
sions would not be relevant to the debate on levels of con-
sciousness. Consider by analogy two individuals, Alice and Bob,
such that Alice scores higher than Bob on three dimensions re-
spectively related to wealth, height, and blood pressure. These
dimensions are not commensurable: there is no meaningful
way to aggregate the quantities of wealth, height, and blood
pressure instantiated by Alice and Bob in order to claim that
Alice scores higher than Bob on some overarching dimension.
If, on the other hand, all the dimensions discussed by B&C
can be meaningfully compared and related, then it should be
possible to aggregate scores on these dimensions even if—as
B&C argue—the psychedelic mode of consciousness scores
higher than ordinary wakefulness on a dimension related to
perceptual bandwidth, and lower on another dimension related
to cognitive function. But the availability of a meaningful aggre-
gation procedure of this kind would ultimately vindicate the no-
tion of degrees of consciousness. (Suppose that you want to
compare Alice and Bob on three dimensions related to well-
being, corresponding respectively to their happiness, mental
health, and sense of meaning—this example is adapted from
Alexandrova (2017). Insofar as these dimensions can be quanti-
fied and compared, it makes sense to say that if Alice scores
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higher than Bob on all three dimensions, then Alice has a higher
level of well-being than Bob. But it is also intelligible to say that
if Alice scores significantly higher than Bob on the happiness
and mental health dimensions, but slightly lower than Bob on
the sense of meaning dimension, then Alice still has a higher
level of well-being than Bob overall.)
Individuating the Dimensions of Psychedelic
Although B&C’s model only contains three dimensions, addi-
tional variables might be relevant to characterize modes of con-
sciousness induced by psychedelics. For example, it has been
argued that the “sense of reality” is a central dimension of al-
tered states of consciousness and can crucially help classifying
them (Dokic and Martin 2012;Farkas 2014). In particular, includ-
ing a dimension related to the sense of reality could be helpful
to differentiate modes of consciousness induced by serotonergic
drugs from those induced by other hallucinogens—e.g. anticho-
linergics (Shanon 2002;Fortier 2018).
This final point raises broader questions: what are the individ-
uation criteria of the dimensions of conscious modes? On what
grounds should additional dimensions be introduced, or existing
dimensions subdivided? Is the dimensionality of the proposed
model merely dependent upon the desired fineness of grain of the
resulting taxonomy of conscious modes, or is there a optimal
number of dimensions that carves out modes of consciousness at
their natural joints? Such questions could be addressed in future
work building upon B&C’s important contribution.
Conflict of interest statement. None declared.
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