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ORIGINAL INVESTIGATION
Cannabis and creativity: highly potent cannabis impairs
divergent thinking in regular cannabis users
Mikael A. Kowal &Arno Hazekamp &Lorenza S. Colzato &
Henk van Steenbergen &Nic J. A. van der Wee &
Jeffrey Durieux &Meriem Manai &Bernhard Hommel
Received: 9 May 2014 /Accepted: 15 September 2014 /Published online: 7 October 2014
#The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract
Rationale Cannabis users often claim that cannabis has the
potential to enhance their creativity. Research suggests that
aspects of creative performance might be improved when
intoxicated with cannabis; however, the evidence is not
conclusive.
Objective The aim of this study was to investigate the acute
effects of cannabis on creativity.
Methods We examined the effects of administering a low
(5.5 mg delta-9-tetrahydrocannabinol [THC]) or high
(22 mg THC) dose of vaporized cannabis vs. placebo on
creativity tasks tapping into divergent (Alternate Uses Task)
and convergent (Remote Associates Task) thinking, in a pop-
ulation of regular cannabis users. The study used a random-
ized, double-blind, between-groups design.
Results Participants in the high-dose group (n=18) displayed
significantly worse performance on the divergent thinking
task, compared to individuals in both the low-dose (n=18)
and placebo (n=18) groups.
Conclusions The findings suggest that cannabis with low
potency does not have any impact on creativity, while highly
potent cannabis actually impairs divergent thinking.
Keywords Cannabis .Creativity .Divergent thinking .
Convergent thinking
Introduction
Anecdotal evidence suggests that cannabis intoxication en-
hances human creativity. In line with that, Steve Jobs, an
undeniably creative mind, once stated: “The best way I could
describe the effect of the marijuana and hashish is that it would
make me relaxed and creative.”Other regular users claim that
cannabis induces a state in which they experience unusual and
original thoughts (Tart 1970). In a more recent review, over
50 % of users reported heightened creativity during cannabis
intoxication (Green et al. 2003). This widespread perception
of cannabis as a creativity-enhancer makes it important to
verify whether cannabis actually induces these supposed ef-
fects. Delta-9-tetrahydrocannabinol (THC), the main psycho-
active compound present in the Cannabis sativa plant, has
been found to reduce inhibitory control (McDonald et al.
2003) and stimulate striatal dopamine (DA) release (Bossong
et al. 2009; Kuepper et al. 2013). These features of THC
intoxication, in turn, are expected to play a role in particular
aspects of creative thinking (Akbari Chermahini and Hommel
2010; Hommel 2012). On the other hand, THC has been
linked to the emergence of psychotic symptoms both due to
acute administration (D’Souza et al. 2004), as well as in the
long-term (Kuepper et al. 2010). As a result, the possible
beneficialeffects of using cannabis, ifany, might not outweigh
the potential risks associated with its abuse.
The concept of creativity is not very well defined, and there
is no agreement on one particular measure how to assess it.
M. A. Kowal (*):L. S. Colzato :H. van Steenbergen :
J. Durieux :M. Manai :B. Hommel
Cognitive Psychology Unit, Institute of Psychology, Leiden
University, PO Box 9555, 2300 RB Leiden, The Netherlands
e-mail: m.a.kowal@fsw.leidenuniv.nl
M. A. Kowal :L. S. Colzato :H. van Steenbergen :
N. J. A. van der Wee :B. Hommel
Leiden Institute for Brain and Cognition, Leiden, The Netherlands
A. Hazekamp
Bedrocan BV, Veendam, The Netherlands
N. J. A. van der Wee
Department of Psychiatry, Leiden University Medical Center,
Leiden, The Netherlands
Psychopharmacology (2015) 232:1123–1134
DOI 10.1007/s00213-014-3749-1
While some authors consider the concept to refer to the
product of creative activities, others take it to reflect the
personality of the product’s creator (for an overview, see
Runco 2007). To circumvent these difficulties, we restricted
our analyses to two well-established creative processes and
the respective classical assessment methods: divergent and
convergent thinking (Guilford 1967). Divergent thinking
takes place when people try to find as many solutions to a
loosely defined problem as possible—a process often referred
to as “brainstorming.”It is often assessed by means of
Guilford’s(1967) Alternate Uses Task (AUT), which requires
individuals to generate as many as possible uses for a common
household item (such as a pen or book) as they can think of
(e.g., reading it, using it as a doorstop, etc.). In contrast,
convergent thinking takes place when trying to find the one
possible solution to a very well-defined problem. This process
is often assessed by means of Mednick’s(1962) Remote
Associates Task (RAT), in which people are presented with
three supposedly unrelated concepts (e.g., “time,”“hair,”
“stretch”) and are requested to identify the one concept that
can be related to all three of them (“long”). Research indicates
that performance in AUT and RAT is not (strongly) correlated
(Akbari Chermahini and Hommel 2010;AkbariChermahini
et al. 2012). Moreover, there is evidence that the two types of
creative thinking are differently related to subcortical DA
levels: While divergent thinking performance relates to
markers of DA levels in the form of an inverted U-shape,
convergent thinking performance displays a linear, negative
correlation with DA markers (Akbari Chermahini and
Hommel 2010). In addition, this dissociation of human
creativity seems to correspond to the dual pathway to
creativity model (De Dreu et al. 2008; Nijstad et al.
2010) suggesting that creative performance emerges from
the balance between cognitive flexibility and cognitive
persistence—two dissociable cognitive control functions
(De Dreu et al. 2012).
With regard to the neural effects of THC, the link between
creative thinking and DA appears to be particularly interest-
ing. Administration of THC has been shown to indirectly
induce DA release in the striatum (Bossong et al. 2009;
Kuepper et al. 2013), and there is evidence that its chronic
application can lead to dopaminergic hypoactivity in the long-
term, especially if the onset of cannabis use is at a young age
(Hoffman et al. 2003; Urban et al. 2012; Bloomfield et al.
2014). As divergent thinking performance is expected to be
optimal with medium subcortical DA levels (Akbari
Chermahini and Hommel 2010), one may suspect that THC
can have a beneficial effect on this creative process, particu-
larly in individuals with low dopaminergic functioning. This
assumption is further supported by the fact that the reduction
in inhibitory control, as observed in response to stimulation by
pure THC (McDonald et al. 2003) and cannabis (Ramaekers
et al. 2006; Ramaekers et al. 2009), has been related to
dopaminergic functioning as well (Mink 1996). Reduced in-
hibitory control can be considered to reflect a cognitive con-
trol state with weak top-down guidance. Such a state should
affect convergent and divergent thinking differently (Hommel
2012). As pointed out by Bogacz (2007), human decision-
making and the retrieval of possible alternatives can be con-
sidered a process that emerges from the interaction of top-
down guidance and low-level competition between alterna-
tives. If so, convergent thinking, with its many top-down
constraints targeting one single solution, would seem to re-
quire a control state that provides strong top-down guidance
and strong local competition. In contrast, divergent thinking,
with its loosely defined problem and its many solutions, seems
to require a control state that provides weak top-down guid-
ance and only little local competition (Hommel 2012). To the
degree that THC indeed induces a control state with weak top-
down guidance and local competition, it might thus be ex-
pected to improve divergent thinking, interfere with conver-
gent thinking, or both (Hommel 2012; Colzato et al. 2012).
Unfortunately, the available research on the link between
cannabis and creativity allows only for partial verification of
these expectations. With respect to divergent thinking, one
study showed that subjects intoxicated with joints (cannabis
cigarettes) containing a low dose of THC (3 mg in total)
displayed significantly enhanced performance on two diver-
gent production tasks, compared to a group that received a
higher THC dose (6 mg in total; Weckowicz et al. 1975).
Curran et al. (2002) showed that, as compared to placebo, oral
THC (7.5 and 15 mg) dose-dependently improved verbal
fluency—an important aspect of divergent thinking (Guilford
1967), at least as assessed by the AUT. Improved verbal
fluency performance was also found in a naturalistic study
that showed beneficial effect of smoked cannabis (10 % THC
on average) on divergent thinking to be restricted to users low
in trait creativity (i.e., individuals that obtained a low score on
a self-assessment questionnaire about achievements in differ-
ent creative domains; Schafer et al. 2012). In addition to
fluency, cannabis administration (joints containing 19 mg of
THC) has also been shown to increase the number of original
responses on a test of associative processes, in comparison to
placebo (Block et al. 1992). In contrast, Tinklenberg et al.
(1978) did not observe any improvement in performance
during the Torrance Tests of Creative Thinking (TTCT; Tor-
rance 1966), which is often assumed to tap into divergent
thinking, after oral consumption of THC (a biscuit containing
0.3 mg/kg body weight of THC). Another study found de-
creased TTCT scores for fluency, flexibility, and elaboration
after smoking a cannabis joint (containing 10 mg of THC) in
regular cannabis users but not in first-time users (Bourassa and
Vaugeois 2001). In summary, the methodological differences
between the various studies aside, many but not all findings
suggest that THC may induce a cognitive control state with
weak top-down guidance, thus efficiently decreasing the
1124 Psychopharmacology (2015) 232:1123–1134
competition between cognitive representations and enhancing
divergent thinking (Hommel 2012; Colzato et al. 2012).
For convergent thinking, the evidence is even more limited.
Weckowicz et al. (1975) observed a trend toward less efficient
convergent thinking task after smoking joints containinga low
dose of THC (3 mg in total) or a higher dose (6 mg in total), in
comparison to both a placebo and a pure control group.
However, the same study also found impaired convergent
thinking but only for the high-dose condition. The most recent
investigation found potentially detrimental effects of smoking
cannabis (10 % THC on average) on RAT performance in a
group of cannabis users assumed to be high in trait creativity
(Schafer et al. 2012). Although the naturalistic approach of
this study makes it difficult to account for specific dose-related
differences, the results of the research of both Schafer et al.
(2012)andWeckowiczetal.(1975) suggest that THC can
disrupt the process of searching and converging on a single
solution to a problem.
A number of the observed inconsistencies between studies
might be due to differences with respect to THC dosage and
method of administration, which, in turn, affects the bioavail-
ability and the onset of action of the compound (Hazekamp
et al. 2006). Moreover, an individual’s history of cannabis use
needs to be identified before cognitive changes in response to
THC can be predicted. Administration of joints (containing up
to 39 mg of THC) to regular cannabis users has been found to
produce no accuracy impairments on a test battery assessing
several cognitive functions (Hart et al. 2001)and,morespe-
cifically, on tasks related to episodic and working memory
(Hart et al. 2010). Furthermore, after smoking a cannabis joint
(containing 500 μg/kg body weight THC), chronic users did
not display any behavioral deficiencies on tasks assessing
tracking performance and divided attention (Ramaekers
et al. 2009) or changes in an event-related potential (ERP)
reflecting early attentional processes (Theunissen et al. 2012),
compared to infrequent users. In addition, regular cannabis
users were shown to display reduced sensitivity to the psy-
chotomimetic effects of THC (administered as an intravenous
dose up to 5 mg; D’Souza et al. 2008). In contrast, inhibitory
control has been found to be similarly impaired among both
occasional and chronic users when intoxicated with cannabis
(Ramaekers et al. 2009).
Accordingly, since research points to reduced cannabinoid
receptor type 1 (CB
1
) density in the brains of regular cannabis
users (Hirvonen et al. 2012), one may suspect that the toler-
ance of chronic users to some of the detrimental effects of
THC is, to some extent, related to their dopaminergic func-
tioning. Specifically, due to the concentration of CB
1
recep-
tors at gamma-aminobutyric acid (GABA) and glutamate
neurons, CB
1
receptor downregulation can influence the ac-
tivity of these neurotransmitters (Hoffman et al. 2003). Be-
cause DA neurons are frequently co-localized with
GABAergic and glutamatergic terminals, the dopaminergic
deficiencies observed in chronic cannabis users may be ex-
plained by lasting, maladaptive modulation of DA by GABA
and glutamate (Fattore et al. 2010; Fernández-Ruiz et al.
2010). If so, keeping in mind the inverted U-shape relation-
ship between subcortical DA levels and divergent thinking
performance (Akbari Chermahini and Hommel 2010) and the
effect of THC on striatal DA release (Bossong et al. 2009;
Kuepper et al. 2013), it may be expected that individuals with
a relatively low level of dopaminergic functioning, such as
regular cannabis users, are more likely to demonstrate en-
hanced performance on a divergent thinking task, provided
that the THC dose is not excessively high. In contrast, in a
population without long-term dopaminergic imbalances, such
as healthy drug-naïve individuals, even a reasonably low dose
of THC could stimulate DA production to the level that it
exceeds the threshold for optimal performance. In the case of
convergent thinking performance, which is best with low
subcortical DA levels (Akbari Chermahini and Hommel
2010), it may be predicted that it will deteriorate in response
to THC, irrespective of the dose and cannabis use history of
the individual.
In order to examine these possibilities, we investigated the
effect of two different doses of vaporized cannabis (containing
5.5 or 22 mg THC; see “Study drugs”section) and placebo on
convergent and divergent thinking in a sample of chronic
cannabis users, using a between-groups design. On the basis
of the assumption that a low dose of cannabis can remove
potential impairments caused by regular use (Weckowicz et al.
1975; Kelleher et al. 2004), we expected that participants
intoxicated with a low dose of cannabis should display higher
scores on a divergent thinking task, compared to placebo.
Conversely, we predicted impairment of performance in the
high-dose condition, in contrast to the low-dose and placebo
conditions. In the case of convergent thinking, we expected
that both doses of cannabis should impair this process, com-
pared to placebo. In addition, since divergent thinking perfor-
mance has been found to be related to an individual’smood
(Zenasni and Lubart 2011), we assessed perceived mood as a
possible modulating factor.
Materials and methods
The current study was part of a larger study which involved
additional tasks and measurements.
Participants
Power analysis was performed to assess the approximate
number of subjects required for detecting medium (d=0.5)
or large effect sizes (d=0.8). Consequently, with an expected
sample size of 60, three conditions, and a set alpha of 0.05, the
power to detect main effects with a medium or large effect size
Psychopharmacology (2015) 232:1123–1134 1125
for a between-group ANOVA is0.679 and 0.979, respectively.
Calculations were made using the analysis program fpower
(Friendly 2014).
Fifty-nine healthy regular cannabis users (52 males and 7
females) participated in the study in exchange for a small
financial compensation. Subjects were recruited through ad-
vertisements on the internet, on community bulletin boards,
and in coffee shops (outlets in which Dutch law permits the
sale of small quantities of cannabis to consumers) and by word
of mouth. Detailed demographic and substance use informa-
tion is presented in Table 1. Written informed consent was
obtained from all participants, after a complete explanation of
the nature of the study. The study was approved by the
Medical Ethics Committee of the Leiden University Medical
Center.
The participants were randomly assigned to one out of
three experimental conditions: placebo, 5.5 mg or 22 mg
THC. The groups were comparable in terms of age, substance
use characteristics, and IQ test score. All subjects were re-
quired to be regular users (use cannabis at least four times a
week, for a minimum of 2 years) and to be native Dutch
speakers. The exclusion criteria were as follows: (1) history
or presence of an axis I psychiatric disorder (DSM-IV;
assessed with the use of the Mini International Neuropsychi-
atric Interview (M.I.N.I.); Lecrubier et al. 1997), (2) clinically
significant medical disease, (3) use of psychotropic medica-
tion, (4) current or previous regular use of other drugs except
cannabis (regular use defined as having used a drug more than
four times in a lifetime), and (5) abuse of alcohol (more than
14 units a week). Compliance with the inclusion and exclu-
sion criteria was assessed by means of self-report. Addition-
ally, subjects were asked to refrain from caffeine, chocolate,
and alcohol 12 h before the experimental session and not to
use nicotine2 h before the study. It was also not allowed to use
cannabis within 2 days before the experiment. Participants’
compliance with these criteria was evaluated by means of a
personal interview and the use of a saliva drug test, which
detected the recent use of cannabis, morphine, or cocaine
(Oral-View™Saliva Multi-Drug of Abuse Test; Alfa Scien-
tific Designs, Inc., Poway, CA, U.S.A.).
From the initial sample of 59 subjects, two male partici-
pants withdrew from the study before completing the two
creativity tasks—one stated personal issues, while the other
did not provide any explanation. Another subject experienced
anxiety before cannabis administration and had to abort the
experiment. In the case of adverse events related to drug
administration, one participant reported anxiety, combined
with fatigue and nausea, which prevented him from complet-
ing the tasks. Moreover, one female subject was excluded
from the analysis due to lack of compliance to task require-
ments (i.e., she refused completing the tasks due to not liking
their nature). This left 54 subjects for the final analysis (48
males and 6 females), except for the convergent thinking task
(RAT). In this case, one male participant (in the 22 mg THC
condition) requested to abort the study due to personal reasons
before being able to complete the task, which left only 53 data
sets for the RAT analysis.
Study drugs
The active drug substance consisted of the dried, milled,
and homogenized flowers of the plant C. sativa (variety
“Bedrocan”®; 19 % THC). It was obtained from
Bedrocan BV (Veendam, The Netherlands) where it
was cultivated under standardized conditions according
to the requirements of Good Agricultural Practice
(GAP). The placebo (variety “Bedrocan”®; <0.5 %
THC) used in the study had a moisture content and
terpenoid profile (providing the typical smell and taste
of cannabis) identical to the active drug. Study
Tabl e 1 Demographic and substance use data for each experimental group
Placebo 5.5 mg THC 22 mg THC Significance level
N(male/female) 18 (18:0) 18 (17:1) 18 (13:5) p=0.019
Age 21.1 (2.4) 21.1 (2.1) 22 (2.5) n.s.
IQ test score 7.8 (2.6) 7.3 (2.7) 7.4 (2.3) n.s.
Monthly cannabis use 42.8 (31.3) 51.3 (52.6) 39.3 (27.8) n.s.
Years of cannabis exposure 6 (3.1) 4.8 (1.9) 6.2 (2.6) n.s.
Monthly alcohol use 26.2 (17.8) 23.7 (19.8) 18.8 (13.5) n.s.
Years of alcohol exposure 5.3 (2.6) 4.8 (2.5) 6.9 (2.7) n.s.
Monthly nicotine use 214.4 (207.7) 121.3 (140) 156 (185.3) n.s.
Years of nicotine exposure 4.6 (3.8) 3.5 (4.2) 4.3 (4) n.s.
Standard deviations in parentheses
n.s. non-significant difference, Age reported in years, IQ test score measured by a shortened version of Raven’s Standard Progressive Matrices, Monthly
cannabis use consumption of cannabis cigarettes (joints), Monthly alcohol use consumption of alcohol units, Monthly nicotine use consumption of
cigarettes
1126 Psychopharmacology (2015) 232:1123–1134
medication was prepared by ACE Pharmaceuticals BV
(Zeewolde, The Netherlands). For each individual dose,
exact amounts of active cannabis and placebo were
mixed so that each dose was equal to 250 mg total
weight but with varying concentrations of THC (place-
bo/5.5 mg/22 mg THC). Study medication was stored in
a refrigerator (2–8 °C) in triple-layer laminated foil
pouches (Lamigrip). Shelf life stability under these con-
ditions was determined to be at least 1 year.
On the study day, each subject received a randomized
single dose of cannabis by means of a Volcano® vaporizer
(Storz & Bickel GmbH, Tüttlingen, Germany)—a reliable and
safe method of intrapulmonary administration of THC
(Hazekamp et al. 2006; Zuurman et al. 2008). Cannabis was
vaporized at a temperature of 230 °C into a standard Volcano
balloon as supplied with the vaporizer. For blinding purposes,
the Volcano balloon was covered with a non-transparent plas-
tic bag so that no differences in density of the vapor were
visible between dosages.
When administrating THC by means of vaporizing, it
should be taken into account that only part of the dose present
in the plant material is vaporized into the balloon (Hazekamp
et al. 2006) and that a portion of the THC inhaled from the
balloon is not absorbed by the lungs but is exhaled again
(Zuurman et al. 2008). Therefore, in order to achieve an
absorbed dose of approx. 2 and 8 mg THC, we loaded the
Volcano vaporizer with 5.5 and 22 mg of THC, respectively.
Moreover, since the THC delivery of the Volcano vaporizer
and cannabis joints is comparable (Abrams et al. 2007), the
loaded vs. absorbed dose distinction can be applied to smoked
cannabis as well.
During administration, subjects were instructed to inhale
deeply and hold their breath for 10 s after each inhalation.
They were not allowed to speak during the inhalation period
and were required to empty the balloon within 5 min. Subjects
had the opportunity to practice the inhalation procedure using
an empty balloon before cannabis administration.
Shortened Raven’s Standard Progressive Matrices (measure
of intelligence)
Individual IQ test scores were determined by means of a
reasoning-based intelligence test (Raven et al. 1988). Each
item of this test consists of a pattern or sequence of a diagram-
matic puzzle with one piece missing, the task being to com-
plete the pattern or sequence by choosing the correct missing
piece from a list of options. The items are getting more
difficult as the test taker proceeds through the test. The Stan-
dard Progressive Matrices (SPM) test assesses the individual’s
ability to create perceptual relations and to reason by analogy
independent of language and formal schooling. The version of
the test used in the study consisted of 14 items.
Alternate Uses Task (divergent thinking)
In this task (Guilford 1967), participants were asked to list as
many possible uses for two common household items (i.e.,
pen, shoe) as they could. The scoring had four components:
fluency (the total of all responses), flexibility (the number of
different categories used; e.g., “household uses”), originality
(where each response was compared to the responses from the
other subjects, responses given by only 5 % of the participants
being counted unusual [1 point] and responses given by only
1 % as unique [2 points]), and elaboration (referring to the
amount of detail; e.g., while a book used as “adoorstop”
would count 0, “a doorstop to prevent a door slamming shut
in a strong wind”would count 2: 1 point for explanation of
door slamming and 1 point for additional detail about the
wind). Of these four criteria, the component flexibility has
been found to be the theoretically most transparent and the
empirically most consistent and reliable score (Akbari
Chermahini and Hommel 2010).
Remote Associates Task (convergent thinking)
In this task (developed by Mednick 1962), participants were
presented with three unrelated words (e.g., time, hair, and
stretch) and asked to find a common associate (long). The test
consisted of 14 items, which were taken from Dutch version
of the RAT from Akbari Chermahini et al. (2012).
Affect grid (subjective measure of mood)
As in Colzato et al. (2013), the current mood of participants
was assessed by means of a 9×9 Pleasure ×Arousal grid
(Russell et al. 1989).
Visual analogue scales (subjective measure of drug effects)
The subjective effects of cannabis were assessed by means of
three scales (horizontal 100-mm lines, the left pole labeled
“not at all”and the right “extremely”)referringto“(feeling)
high,”“good drug effect,”and “bad drug effect.”Participants
were to mark a point at the continuous line to indicate their
experience.
Design and procedure
The study used a randomized, double-blind, placebo-con-
trolled, between-groups (placebo vs. 5.5 vs. 22 mg THC)
design. All participants were tested individually, and the order
of the two creativity tasks—AUT and RAT—was
counterbalanced. Upon arrival, the subjects were asked to
complete the SPM test within 10 min. Afterward, the study
drug was administered. Six minutes after cannabis adminis-
tration, participants were required to indicate the subjective
Psychopharmacology (2015) 232:1123–1134 1127
effects of the drugs by means of the visual analogue scales
(VAS). This assessment of the effects of the drugs was then
repeated twice—before and after the completion of the two
creativity tasks (35 and 60 min after administration). Partici-
pants were provided with both the AUT and RAT in printed
form (in the time window between 35 and 60 min after
administration) and had 10 min to complete each task. In
addition, in order to evaluate the subjective perception of
mood, subjects were required to rate their mood on the Affect
grid after the completion of each creativity task (at 48 and
60 min after administration).
Statistical analysis
Scores from mood assessments and VAS, together with the
five measures from the two creativity tasks (fluency, flexibil-
ity, originality, and elaboration scores from the AUT; the
number of correct items from the RAT) were calculated for
each subject. The results of the AUT were rated by two
independent readers, blinded to the conditions (Cronbach’s
alpha=1.00 [fluency]; 0.87 [flexibility]; 0.94 [originality]; 0.9
[elaboration]). The final scores were the means of both ratings.
All measures were analyzed separately. In the case of the
AUT, RAT, and IQ test scores, age, and substance use data,
between-groups ANOVAs were run with condition (placebo
vs. 5.5 vs. 22 mg THC) as between-groups factor. Data
regarding sex was analyzed with the use of a Pearson’schi-
squared test. Mood and VAS scores were analyzed by means
of repeated-measures ANOVAs with time after cannabis
administration (48 vs. 60 min. for mood; 6 vs. 35 vs.
60 min. for VAS) as a within-subjects factor and condition
as a between-groups factor. Post hoc multiple comparisons
ttests were applied with Bonferroni correction. A signifi-
cance level of p<0.05 was adopted for all tests.
Results
Demographic and substance use data
No significant main effects of condition were found in the case
of age (F(2, 51) = 0.74, p=0.482), IQ test score (F(2, 51)=
0.159, p=0.854), monthly cannabis use (F(2, 51)=0.453, p=
0.639), years of cannabis exposure (F(2, 51)=1.433, p=
0.248), monthly alcohol use (F(2, 51)=0.855, p=0.431),
years of alcohol exposure (F(2, 51)= 3.027, p=0.057), month-
ly nicotine use (F(2, 51)= 1.231, p=0.3), and years of nicotine
exposure (F(2, 51)= 0.383, p=0.684). However, the experi-
mental conditions significantly differed by sex (χ
2
(2, N=54)=
7.875, p=0.019); see Table 1.
Creativity tasks
Overall task performance in the AUT and RAT was
comparable to studies without pharmacological interven-
tions (e.g., Akbari Chermahini and Hommel 2010); see
Fig. 1and Table 2.
Divergent thinking
Significant main effects of condition were found on fluen-
cy (F(2, 51)=7.378, p=0.002), flexibility (F(2, 51)=7.708,
Fig. 1 Bar graphs showing mean
scores for the four components of
the Alternate Uses Task (AUT;
fluency,flexibility,originality,and
elaboration) for each
experimental group. The asterisk
indicates a significant (p<0.05)
difference between the 5.5 and
22 mg THC conditions and
between the placebo and 22 mg
THC conditions. Error bars
represent SE of the mean
1128 Psychopharmacology (2015) 232:1123–1134
p=0.001), and originality (F(2, 51)=8.952, p<0.001), but
not on elaboration (p>0.05).
As expected, post hoc multiple comparisons revealed that
participants in the 22 mg THC condition showed significantly
reduced scores, as compared to placebo and 5.5 mg THC,
respectively, for fluency (t(34)= 3.072, p=0.01; t(34)=3.582,
p=0.003), flexibility (t(34)=3.061, p=0.011; t(34)= 3.367,
p=0.002), and originality (t(34) =2.584, p=0.045; t(34)=
4.021, p<0.001). However, contrary to expectations, subjects
in the 5.5 mg THC condition did not display any significant
increases over placebo, on any of the AUT components
(p>0.05).
Moreover, in order to test whether sex differences had
impact on the observed results and match the groups for sex,
we repeated the analysis after the exclusion of all female
subjects. Significant main effects were retained for fluency
(F(2, 45) = 5.774, p= 0.006), flexibility (F(2, 45)=6.325, p=
0.004), and originality (F(2, 45) =7.641, p=0.001).
Convergent thinking
Contrary to expectations, there was no main effect of condi-
tion on the number of correct items from the RAT (p>0.05).
Subjective measures of drug effects and mood
Drug effects
Overall, only the rating of “high”showed a main effect of time
after cannabis administration (with Huynh-Feldt correction;
F(1.862, 93.109)= 15.777, p< 0.001). However, significant
main effects of condition were found on all three scores: high
(F(2, 50)= 11.656, p<0.001), good drug effect (F(2, 50)=
8.701, p=0.001), and bad drug effect (F(2, 50) = 6.507, p=
0.003). There were no significant interaction effects (p>0.05).
Post hoc multiple comparisons revealed that subjects in the
placebo condition showed significantly lower ratings of high,
compared to the 5.5 mg (t(34)= 2.95, p=0.006) and 22 mg
THC groups (t(34)= 4.49, p<0.001); see Fig. 2. Moreover, the
ratings of good drug effect in the placebo condition were
significantly lower than in the 5.5 mg (t(34)=3.535,
p<0.001) and 22 mg THC groups (t(34)= 2.365, p=0.023);
see Fig. 3. In the case of both high and good drug effect, no
significant differences were found between the scores in the
5.5 mg and 22 mg THC groups (p>0.05). Conversely, in the
case of the ratings of bad drug effect, participants in the 22 mg
THC condition demonstrated significantly increased scores,
compared to placebo (t(34)=3.48, p=0.006) and 5.5 mg THC
(t(34)=3.141, p=0.012); see Fig. 4. In addition, the ratings of
bad drug effect did not significantly differ between the place-
bo and 5.5 mg THC conditions (p>0.05).
Mood
There were no main effects of time after cannabis administra-
tion on the ratings of pleasure or arousal (p>0.05). Moreover,
mood ratings in the placebo (6.3 vs. 6.2 for pleasure; 5.1 vs. 5
for arousal), 5.5 mg (7.1 vs. 7 for pleasure; 5.5 vs. 5.2 for
arousal), and 22 mg THC (6.1 vs. 6.4 for pleasure; 4.8 vs. 4.7
for arousal) conditions did not show significant main effects of
condition on pleasure or arousal (p>0.05). There were no
significant interaction effects (p>0.05).
Discussion
Our findings demonstrate that a high dose of vaporized can-
nabis (22 mg THC) impairs divergent thinking in regular
cannabis users, in comparison to a low dose (5.5 mg THC)
and placebo cannabis preparation. This is reflected in the
decreased scores for fluency, flexibility, and originality of
responses ofparticipants inthe high-dose condition. However,
contrary to expectations, a low dose of cannabis did not
enhance divergent thinking in chronic cannabis users: Indi-
viduals in the low-dose group did not significantly outperform
subjects in the placebo group on any of the components of the
AUT. Moreover, convergent thinkingappears to be unaffected
Tabl e 2 Means, SD, and ANOVA results for the four components of the Alternate Uses Task (AUT; fluency,flexibility,originality,andelaboration)and
the number of correct items from the Remote Associates Task (RAT), for each experimental group
Placebo 5.5 mg THC 22 mg THC Fvalue pvalue η
2
p
MSE
AUT
Fluency* 29.2 (9.5) 30.6 (9.2) 19.6 (9) 7.378 0.002 0.224 86.615
Flexibility* 22.3 (4.9) 23.6 (6.2) 16 (7.2) 7.708 0.001 0.232 38.683
Originality* 21.2 (8.4) 27.5 (11.5) 14.1 (8.1) 8.952 <0.001 0.26 90.63
Elaboration 2.5 (2.8) 1.2 (1.6) 1.2 (1.6) 2.152 0.127 0.078 4.552
RAT 4.8 (2.3) 4.5 (2.8) 4.9 (3.6) 0.116 0.891 0.005 8.904
*p<0.05 (significant difference between 5.5 and 22 mg THC, and between placebo and 22 mg THC)
Psychopharmacology (2015) 232:1123–1134 1129
by either a low or high dose of cannabis, as condition had no
impact on the numbers of correct RAT items.
Although the conclusions are limited by a between-groups
design, the finding that administration of a high, but not low,
dose of cannabis impairs divergent thinking performance of
regular cannabis users may suggest that DA release in the
striata of participants in the high-dose condition (Bossong
et al. 2009; Kuepper et al. 2013) exceeded the threshold for
optimal performance (Akbari Chermahini and Hommel
2010). This is in line with neuroscientific considerations that
point to a homeostatic function of DA in regulating the
balance between opposing cognitive control states—flexibil-
ity and stability (Cools et al. 2009;CoolsandD’Esposito
2011). Flexibility refers to the ability to effectively switch
between cognitive representations for the purpose of choosing
the best alternatives, while the function of stability is to
promote constancy of representations in spite of interference
(Cools and D’Esposito 2011). Consequently, keeping in mind
the effect of cannabis on inhibition (Ramaekers et al. 2006;
Ramaekers et al. 2009), it is safe to assume that individuals in
the high-dose condition experienced a reduction in inhibitory
control after cannabis administration. Although this should
Fig. 3 Mean subjective good
drug effect (rated as a percentage)
experienced in each experimental
group as a function of time after
cannabis administration. Symbols
indicate a significant (p<0.05)
difference between the 22 mg
THC and placebo conditions (*)
and between the 5.5 mg THC and
placebo conditions (**). Error
bars represent SE of the mean
Fig. 2 Mean subjective high
(rated as a percentage)
experienced in each experimental
group as a function of time after
cannabis administration. Symbols
indicate a significant (p<0.01)
difference between the 22 mg
THC and placebo conditions (*)
and between the 5.5 mg THC and
placebo conditions (**). Error
bars represent SE of the mean
1130 Psychopharmacology (2015) 232:1123–1134
promote a control state with weak top-down guidance
allowing for flexible updating of information (Hommel
2012; Colzato et al. 2012), supra-optimal levels of DA in the
striatum have been found to stimulate flexibility to the point
that it surpasses the threshold for optimal performance, induc-
ing distractibility as a result (see Cools and D’Esposito 2011).
Accordingly, it is possible that the observed impairment of
divergent thinking in the high-dose condition was the result of
this process. Presumably, induction of a control state with
weak top-down guidance is a necessary, but not sufficient,
prerequisite for enhanced divergent thinking performance.
Conversely, excessively potent cannabis may disturb the del-
icate balance between stability and flexibility by stimulating
flexibility to its extreme, hence impairing divergent thinking.
In addition, from a more motivational perspective, it is
possible that a high dose of cannabis induces the phe-
nomenon of “ego-depletion”(i.e., exhausts the limited
cognitive resources and motivation required for cognitive
control operations; Baumeister et al. 1998;Inzlichtand
Schmeichel 2012). This seems probable taking into ac-
count the observation that participants in the high-dose
condition experienced more intense unpleasant subjective
effects of cannabis, than those in the low-dose and place-
bo groups. In line with that, research points to anxiety,
paranoia, delusions, and mental disorganization as fre-
quent adverse effects of cannabis intoxication (Green
et al. 2003;D’Souza et al. 2004). Therefore, the various
undesirable forms of distraction induced by cannabis
could have drained the control resources of individuals
in the high-dose condition. In other words, it is possible
that the need to exert self-control over the adverse effects
of cannabis leads to a reduction in motivation and
available cognitive resources required for subsequent op-
timal divergent thinking performance (Inzlicht and
Schmeichel 2012).
In the case of the low-dose group, the lack of enhance-
ment of divergent thinking does not provide support for
the idea that a low dose of cannabis can eliminate cogni-
tive impairments caused by regular use (Weckowicz et al.
1975;Kelleheretal.2004). Nevertheless, since the per-
formance of subjects in the low-dose and placebo groups
was comparable in the case of the AUT, it may be as-
sumed that the lack of cannabis-induced cognitive deteri-
oration in the low-dose condition was indicative of the
tolerance of regular cannabis users to the effects of the
drug (Hart et al. 2001; Ramaekers et al. 2009;Hartetal.
2010; Theunissen et al. 2012). Furthermore, it is possible
that the similar level of performance of both groups re-
flects their maximal potential for divergent thinking. Re-
search indicates that placebo effects are able to stimulate
subcortical DA release (Scott et al. 2007; Scott et al.
2008). Possibly, administration of a low dose of cannabis
resulted in a comparable dopaminergic response as in the
case of the placebo (Bossong et al. 2009; Kuepper et al.
2013). This seems plausible considering the fact that the
placebo cannabis preparation used in the study was iden-
tical in terms of smell and taste to actual cannabis. As
such, it had more potential to produce a placebo effect. In
addition, the minimal amount of THC present in the
placebo might have also affected DA release to some
extent. Consequently, the subcortical DA levels of indi-
viduals in both the low-dose and placebo conditions could
have been within the range for optimal divergent thinking
performance (Akbari Chermahini and Hommel 2010).
Fig. 4 Mean subjective bad drug
effect (rated as a percentage)
experienced in each experimental
group as a function of time after
cannabis administration. Symbols
indicate a significant (p<0.05)
difference between the placebo
and 22 mg THC conditions (*)
and between the 5.5 and 22 mg
THC conditions (**). Error bars
represent SE of the mean
Psychopharmacology (2015) 232:1123–1134 1131
Limitations
Although the most recent investigation into the link between
cannabis and convergent thinking suggested a potentially
detrimental effect of cannabis intoxication on this process
(Schafer et al. 2012), our study failed to detect any impact
on RAT performance. Perhaps, our version of the task with 14
items was not sensitive enough to identify potential cannabis-
induced impairments. Moreover, an important limitation is the
between-groups design of the study. Consequently, it is pos-
sible that particular characteristics of the subject sample could
have altered the effects of the drug. Specifically, the difference
in sex between the conditions seems as a likely candidate in
this regard (Crane et al. 2013). In addition, research points to
genetic predispositions like polymorphism of the CB
1
recep-
tor gene (Beng-Choon et al. 2011; Stadelman et al. 2011), or
the catechol-O-methyltransferase (COMT) gene (Schulz et al.
2012), as other factors which might modulate the cognitive
effects of cannabis intoxication.
Another issue is related to the causal relation between the
observed results and THC. In spite of the fact that application
of cannabis, instead of pure THC, provides the benefit of a
higher ecological validity of the study, the use of plant mate-
rial could have influenced the findings. Specifically, terpe-
noids, which are the compounds responsible for the charac-
teristic smell and taste of cannabis, have been shown to
interact with cannabinoids to produce various synergistic ef-
fects (see Russo 2011). However, even if that was the case in
our experiment, the terpenoid profile was comparable between
the different doses, including the placebo cannabis prepara-
tion. Consequently, any potential terpenoid-cannabinoid inter-
actions were controlled for. Unfortunately, the study lacked a
measurement of THC blood plasma levels, which would allow
for evaluating the relation between THC in the bloodstream
and task performance. Furthermore, since the number of in-
halations from the Volcano balloon and the duration of inha-
lations were not standardized, it is likely that this resulted in
large differences in absorbed THC between subjects. In addi-
tion, the saliva test used in our experiment provided only an
estimate of recent use. Possibly, the compliance of subjects
with no-consumption criteria should instead be verified by
examining the urinary levels of THC metabolites (11-COOH-
THC), which is capable of detecting intoxication over a longer
period of time. Moreover, the lack of testing for alcohol
intoxication can be considered another limitation in evaluating
the compliance of participants with no-consumption criteria.
Conclusion
The findings indicate that administration of cannabis with a
high THC content to regular cannabis users is detrimental for
divergent thinking, while less potent cannabis does not seem
to enhance this important component of creativity. The
available evidence allows only for a speculation about the
presence of these effects in a group of drug-naïve individuals,
or occasional cannabis users. In any case, it can be claimed
that the phenomenological experience of a person intoxicated
with cannabis might not necessarily reflect his or her actual
performance. In particular, the frequently reported feeling of
heightened creativity could be an illusion. In other words,
smoking a joint may not be the best choice when in need of
breaking the “writer’s block,”or overcoming other artistic
inhibitions, and smoking several of them might actually be
counter-productive.
Acknowledgments We would like to thank Ms. Linda van der Hulst
from the Central Pharmacy of the Leiden University Medical Center for
her assistance in preparing the study and our subjects for participation.
Conflict of interest Dr. Arno Hazekamp receives a salary from
Bedrocan BV—the company which provided the study drugs. The au-
thors have full control of all primary data, and they agree to allow the
journal to review their data if requested.
Open Access This article is distributed under the terms of the Creative
Commons Attribution License which permits any use, distribution, and
reproduction in any medium, provided the original author(s) and the
source are credited.
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