Modulation of risk-taking in marijuana users by transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC)

ArticleinDrug and alcohol dependence 112(3):220-5 · December 2010with115 Reads
Impact Factor: 3.42 · DOI: 10.1016/j.drugalcdep.2010.06.019 · Source: PubMed
Abstract

Cognitive deficits that are reported in heavy marijuana users (attention, memory, affect perception, decision-making) appear to be completely reversible after a prolonged abstinence period of about 28 days. However, it remains unclear whether the reversibility of these cognitive deficits indicates that (1) chronic marijuana use is not associated with long-lasting changes in cortical networks or (2) that such changes occur but the brain adapts to and compensates for the drug-induced changes. Therefore, we examined whether chronic marijuana smokers would demonstrate a differential pattern of response in comparison to healthy volunteers on a decision-making paradigm (Risk Task) while undergoing sham or active transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC). Twenty-five chronic marijuana users who were abstinent for at least 24h were randomly assigned to receive left anodal/right cathodal tDCS of DLPFC (n=8), right anodal/left cathodal tDCS of DLPFC (n=9), or sham stimulation (n=8); results on Risk Task during sham/active tDCS were compared to healthy volunteers from a previously published dataset. Chronic marijuana users demonstrated more conservative (i.e. less risky) decision-making during sham stimulation. While right anodal stimulation of the DLPFC enhanced conservative decision-making in healthy volunteers, both right anodal and left anodal DLPFC stimulation increased the propensity for risk-taking in marijuana users. These findings reveal alterations in the decision-making neural networks among chronic marijuana users. Finally, we also assessed the effects of tDCS on marijuana craving and observed that right anodal/left cathodal tDCS of DLPFC is significantly associated with a diminished craving for marijuana.

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Available from: Alvaro Pascual-Leone, Dec 19, 2013
Drug and Alcohol Dependence 112 (2010) 220–225
Contents lists available at ScienceDirect
Drug and Alcohol Dependence
journal homepage: www.elsevier.com/locate/drugalcdep
Modulation of risk-taking in marijuana users by transcranial direct current
stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC)
Paulo S. Boggio
a,,1
, Soroush Zaghi
b,1
, Ana Beatriz Villani
a
, Shirley Fecteau
b
,
Alvaro Pascual-Leone
b
, Felipe Fregni
b,c
a
Cognitive Neuroscience Laboratory and Developmental Disorders Program, Center for Health and Biological Sciences, Mackenzie Presbyterian University,
Rua Piaui, 181, 10 Andar, Sao Paulo, SP 01241-001, Brazil
b
Berenson-Allen Center for Noninvasive Brain Stimulation, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, USA
c
Laboratory of Neuromodulation, Spaulding Rehabilitation Hospital, Harvard Medical School, Boston, USA
article info
Article history:
Received 18 December 2009
Received in revised form 14 June 2010
Accepted 18 June 2010
Available online 21 August 2010
Keywords:
Transcranial direct current stimulation
Non-invasive brain stimulation
Marijuana
Cognitive effects
Decision-making
Risk
abstract
Cognitive deficits that are reported in heavy marijuana users (attention, memory, affect perception,
decision-making) appear to be completely reversible after a prolonged abstinence period of about 28
days. However, it remains unclear whether the reversibility of these cognitive deficits indicates that (1)
chronic marijuana use is not associated with long-lasting changes in cortical networks or (2) that such
changes occur but the brain adapts to and compensates for the drug-induced changes. Therefore, we
examined whether chronic marijuana smokers would demonstrate a differential pattern of response in
comparison to healthy volunteers on a decision-making paradigm (Risk Task) while undergoing sham
or active transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC).
Twenty-five chronic marijuana users who were abstinent for at least 24 h were randomly assigned to
receive left anodal/right cathodal tDCS of DLPFC (n = 8), right anodal/left cathodal tDCS of DLPFC (n = 9),
or sham stimulation (n = 8); results on Risk Task during sham/active tDCS were compared to healthy
volunteers from a previously published dataset. Chronic marijuana users demonstrated more conser-
vative (i.e. less risky) decision-making during sham stimulation. While right anodal stimulation of the
DLPFC enhanced conservative decision-making in healthy volunteers, both right anodal and left anodal
DLPFC stimulation increased the propensity for risk-taking in marijuana users. These findings reveal alter-
ations in the decision-making neural networks among chronic marijuana users. Finally, we also assessed
the effects of tDCS on marijuana craving and observed that right anodal/left cathodal tDCS of DLPFC is
significantly associated with a diminished craving for marijuana.
© 2010 Elsevier Ireland Ltd. All rights reserved.
1. Introduction
Although marijuana is the most widely used illicit drug in the
world, the long-term cognitive effects of chronic marijuana use
remain poorly understood. In a study of 63 heavy marijuana users
who had smoked at least 5000 times in their lives and were smoking
daily at study entry, neuropsychological tests showed some impair-
ments in cognition relative to former users and controls for up to
7 days after heavy use, but notably these impairments were virtu-
ally eliminated after 28 days of marijuana abstinence (Pope et al.,
2001). Other studies support the finding that attention and mem-
ory deficits that are reported in heavy marijuana users appear to be
reversible after prolonged abstinence (Harrison et al., 2002). How-
Corresponding author. Tel.: +55 11 21148001; fax: +55 1121148563.
E-mail addresses: boggio@mackenzie.br, psboggio@gmail.com (P.S. Boggio).
1
Equally contributing authors.
ever, it remains unclear whether the reversibility of these cognitive
deficits indicates that (1) chronic marijuana use does not alter cor-
tical networks or (2) that such changes occur but the brain adapts to
and compensates for the drug-induced changes. Indeed, functional
MRI studies show changes in the functional activation of various
brain areas in active and abstinent marijuana users compared to
controls despite similar task and cognitive test performance (Chang
et al., 2006; Gruber et al., 2009). Here we use a method of non-
invasive brain stimulation to further explore the decision-making
neural network response to brain stimulation among chronic mar-
ijuana users.
Methods of non-invasive brain stimulation, e.g. repetitive
transcranial magnetic stimulation (rTMS) and transcranial direct
current stimulation (tDCS), have the capacity to induce remark-
able effects on real-time neuropsychological executive functioning
and are valuable in studying the effect of neuromodulation on var-
ious neural networks. For example, the application of tDCS to the
dorsolateral prefrontal cortex (DLFPC) can modulate the percep-
0376-8716/$ see front matter © 2010 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.drugalcdep.2010.06.019
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P.S. Boggio et al. / Drug and Alcohol Dependence 112 (2010) 220–225 221
Table 1
Demographic data.
Sham tDCS Right anodal/left cathodal Left anodal/right cathodal p-value
Marijuana subjects
Number 8 9 8
Sex (male) 4 5 6 0.8
Age (years ± SD) 22.4 ± 2.2 23.1 ± 3.5 22.9 ± 2.0 0.6
Length of marijuana use (years ± SD) 5.9 ± 2.8 5.2 ± 2.9 6.3 ± 2.3 0.7
Frequency of use (per week mean ± SD) 5.1 ± 1.5 5.4 ± 1.9 6.1 ± 1.2 0.5
Healthy subjects 12 12 12
Number 12 12 12
Sex (male) 4 3 4 p > 0.17
*
Age (years ± SD) 21.3 ± 2.1 19.5 ± 1.0 20.0 ± 1.4 p = 0.1
**
General groups Marijuana Healthy
Sex (male/total) 15/25 11/36 p < 0.05
Age (years ± SD) 22.9 ± 2.5 20.3 ± 1.7 p < 0.05
*
p-value when comparing marijuana subjects and healthy subjects for each subgroup (Fisher’s exact test).
**
Values for the interaction test group × subject (healthy and marijuana).
We show measures of dispersion as standard deviation (±SD).
tion of somatic (Boggio et al., 2008c) and emotional pain (Boggio
et al., 2008b), alter the pattern of cravings (Fregni et al., 2008a,b;
Boggio et al., 2008a) and mood (Fregni et al., 2006), and even affect
learning and memory (Floel et al., 2008; Fregni et al., 2005; Boggio
et al., 2009). One area of particular interest has been the effect
of these neuromodulatory methods on decision-making processes
and risk-taking behavior.
The Risk Task (Rogers et al., 1999) is a binary decision-making
exercise that can offer a useful measure of impulse control and
risk-taking behavior. In this task, subjects choose between two
mutually exclusive low-risk or high-risk selections. Because the
largest reward is always associated with the less likely of the
two options, this gambling paradigm weighs the propensity of an
individual to take risks in favor of large short-term gains at the
likely expense of overall long-term losses. The decision-making
system required to solve this binary choice involves a complex
process that weighs converging neural inputs that assign and rep-
resent a relative preference for each of the two options (Rolls and
Grabenhorst, 2008). In this process, the orbitofrontal cortex rep-
resents the expected reward value of an abstract stimulus (such
as potential monetary reward) by associating these abstract stim-
uli with the affective value of primary reinforcers such as taste,
touch, texture, and facial expression. Notably, however, a recent
functional MRI study among chronic marijuana users suggests that
marijuana users may process emotional information differently
from those who do not use marijuana; the study demonstrated
alterations in the fronto-limbic circuitry that regulates affective
perception and impulse behavior (Gruber et al., 2009). This finding
suggests that chronic marijuana users may also have differences in
risk and decision-making neural networks.
Because it remains unknown whether chronic marijuana users
process decision-making tasks differently from non-using controls,
we assessed the performance of chronic marijuana users on the
Risk Task while undergoing sham and active tDCS of the DLPFC.
We set out to examine whether chronic marijuana smokers would
demonstrate a differential pattern of response in comparison to
non-marijuana smoking healthy volunteers from a previously pub-
lished dataset.
2. Methods
2.1. Study design
We conducted a single-center, doubled-blinded, randomized, and sham-
controlled trial to investigate the effect of a single-session of tDCS on marijuana
craving and performance on a decision-making task (Risk Task, Rogers, 1999) in
chronic marijuana users. This study conformed to the ethical standards of the Dec-
laration of Helsinki and was approved by the institutional ethics committee from
Mackenzie Presbyterian University, Brazil.
2.2. Participants
Twenty-five marijuana users (15 males, 10 females; all right-handed; mean age
22.8 ± 2.6 years; mean history of use 5.8 ± 2.7 years; frequency 5.5 ± 1.9 episodes
of use/week) were recruited from Mackenzie Presbyterian University to partic-
ipate in this study. Written advertisements were posted around campus and
interested subjects contacted the study coordinator to enroll; the study coordi-
nator explained the risk/benefits of the study and screened interested individuals
for eligibility. Subjects were regarded as suitable to participate in this study if
they fulfilled the following criteria: (1) age between 18 and 32 years, (2) right-
handedness, (3) self-reported marijuana use of frequency at least 3 occasions each
week for at least 3 years duration, (4) no other drug use or alcohol dependence,
(5) no clinically significant neuropsychiatric disorder; (7) no use of central nervous
system-effective medication, other than marijuana; and (8) no history of epilepsy,
brain surgery, tumor, intracranial metal implantation, or clinically significant head
trauma.
All subjects were naive to tDCS and the Risk Task. subjects were required to
abstain from marijuana use for at least 24 h prior to participation in the experiments;
the abstinent period was measured by self-report. All study participants provided
written, informed consent. Demographic characteristics are summarized in Table 1.
2.3. Transcranial direct current stimulation (tDCS)
tDCS is based on the application of a weak direct current to the scalp via two
saline-soaked surface sponge electrodes (35 cm
2
) and delivered by a battery-driven,
constant current stimulator. The device used, developed by our group, is particularly
reliable for double-blind studies: a switch can be activated to interrupt the electrical
current while maintaining the ON display and showing the stimulation parameters
throughout the procedure to the experimenter and participant. Although there is
significant shunting of current in the scalp, sufficient current penetrates the brain
to modify the transmembrane neuronal potential (Miranda et al., 2006; Wagner
et al., 2007), and thus, influence the level of excitability and modulate the firing
rate of individual neurons. The effects on cortical excitability depend on current
orientation, such that anodal stimulation generally increases cortical excitability,
while cathodal stimulation decreases it (Nitsche and Paulus, 2000).
The electrodes montage was the same used in a previous study (Fecteau et al.,
2007) where young, healthy, drug-naïve volunteers performed the Risk Task during
prefrontal tDCS. We also followed a similar study paradigm. In this way, we became
able to compare our present results to those of the previous study. Thus, partici-
pants were randomly assigned to receive left anodal/right cathodal tDCS (n = 8), right
anodal/left cathodal tDCS (n = 9), or sham stimulation (n = 8). For left anodal/right
cathodal tDCS, the anode electrode was placed over the left F3 (international EEG
10/20 system) and the cathode electrode was placed over the right F4. For stimula-
tion right anodal/left cathodal, the polarity was reversed: the anode electrode was
placed over F4 and the cathode electrode was placed over F3. For active stimula-
tion, subjects received a constant current of 2 mA intensity with 10 s of ramp up and
down. The tDCS started 5 min before the task began and was delivered during the
entire course of the risk task, which lasted 10 min. The same procedure was used for
sham stimulation, but current was applied only for the first 30 s, a method that has
been shown to be reliable for blinding subjects with respect to stimulation condition
(Gandiga et al., 2006).
2.4. Marijuana craving
The level of marijuana craving in each group was assessed immediately before
and after the stimulation period. Participants were asked to rate their level of craving
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222 P.S. Boggio et al. / Drug and Alcohol Dependence 112 (2010) 220–225
on a visual analogue scale (VAS, 0–10), where 0 represents absolutely no craving for
marijuana and 10 represents the greatest craving possible.
2.5. Risk task
The Risk Task was administered one time following either sham or active stim-
ulation. In the risk task, subjects are presented with six horizontally arranged boxes
colored as pink or blue. The ratio of pink and blue boxes varies from trial to trial as
5:1, 4:2, or 3:3. In each of 100 trials, the participants are asked to choose the color
of the box they believe may contain the winning token. They are told that the token
has an equal probability of being hidden in any of the six boxes. Thus, for each trial,
the ratio of pink to blue boxes (referred to as level of risk) effectively determines the
probability of finding the winning token. For instance, if the ratio of blue: pink is 5:1,
this would mean that if participant chooses the blue boxes, he or she would have a
5/6 probability of finding the winning token. In this way, the participant’s choice of
pink vs. blue reflects the level of risk they are willing to endure.
Participants are rewarded with a gain of points when they correctly guess the
color of the box that is hiding the winning token. However, they are punished with a
loss of points when they select the wrong color. The amount of reward (or penalty)
points associated with any scenario varies. The reward ratio or balance of reward is
clearly indicated on the screen and varies as 90:10, 80:20, 70:30, or 60:40. Impor-
tantly, there is always an inherent conflict between level of risk and balance of
reward; the largest reward is always associated with the less likely of the two out-
comes (i.e. the most risky option). For example, in a trial with five blue boxes and
one pink box, the winning token is more likely to be one of the blue boxes (five in
six probability); however, choosing blue, in this case, would be associated with a
smaller reward. However, if the participant picks the wrong color, he loses the same
amount of points that he would otherwise have gained. The participants’ objective
is to earn as many points as possible.
In this way, the Risk Task measures the propensity for risk-taking within a
decision-making task that otherwise entails little strategy and working memory.
The task requires participants to weigh the immediate benefit vs. long-term cost
of their choices. Participants who consistently choose the lowest risk/lowest gain
option will be consistently choosing the box with the highest probability of win-
ning but least attractive reward; such a strategy would result in small short-term
gains and losses but is most likely to achieve a long-term gain. On the other hand,
participants who choose the high-risk–high gain option would be demonstrating a
preference for the possibility of high immediate gain at the likely detriment of large
long-term losses, a disadvantageous long-term strategy.
2.6. Statistical analysis
We performed a similar analysis as with our previous study in healthy drug-
naïve volunteers (Fecteau et al., 2007). Thus, the outcome measures in the present
study were: (1) percentage of instances in which the participant chose the high-
probability/low-risk option (percentage low-risk choice, a binary variable, 0–100%),
and (2) the amount of time it took for the participants to enter a selection (response
time, a continuous variable, measured in milliseconds). Performance on all 100 trials
of the task were analyzed except for the neutral conditions in which there were an
equal number of pink and blue boxes.
Results were then combined and compared for the three tDCS groups: (1)
those receiving left anodal/right cathodal tDCS of DLPFC (n = 8), (2) right anodal/left
cathodal tDCS of DLPFC (n = 9), and (3) sham stimulation (n = 8). Analyses were per-
formed using STATA (College Station, Texas, USA). We used a mixed linear model
to analyze decision time difference across the groups. We modeled decision time
change using the covariates of tDCS group (left anodal/right cathodal stimulation,
right anodal/left cathodal stimulation, sham stimulation), balance of reward (90:10,
80:20, 70:30, 60:40), level of risk (low-risk, high-risk), and interaction terms tDCS
group × balance of reward × level of risk. For the outcome considering the percent-
age choice of low-risk versus high-risk (binary outcome), we performed a logistic
regression model in which the dependent variable was the percentage of low-risk
choice [# low-risk/(#low-risk + #high-risk)] and the independent variables were
group (left anodal/right cathodal stimulation, right anodal/left cathodal stimulation,
sham stimulation), balance of reward (90:10, 80:20, 70:30, 60:40), and interaction
tDCS group × balance of reward. As we performed multiple tests, we used Bonferroni
adjustments for multiple comparisons. Finally we performed a comparison analysis
with previous data from our healthy subjects study (Fecteau et al., 2007). Data from
this study were combined into the full model and we assessed whether subjects
group (healthy vs. marijuana subjects) was a significant variable in this model.
3. Results
3.1. Sample groups
There were no significant differences among the demographic
data between marijuana users divided among the three groups
(right anodal/left cathodal tDCS, left anodal/right anodal tDCS,
sham stimulation); however when data were combined, there was
a small but significant difference between the two groups (see
Table 1 for statistics and absolute values). However, baseline differ-
ences in risk-taking were indeed present between healthy controls
and marijuana users (see below).
3.2. Adverse effects
None of the volunteers experienced adverse effects during or
after tDCS. Some of the participants reported a slight itching sen-
sation under the electrodes during approximately the first 30 s of
stimulation.
3.3. Low-risk choice during sham stimulation
Marijuana users randomized to sham stimulation chose the
lower-risk option more frequently than healthy subjects undergo-
ing sham stimulation. Marijuana users chose the low-risk prospect
for an average of 87.3% of cases, in comparison to the healthy
controls who chose the low-risk prospect 82% of the time (this
difference was statistically significant (p < 0.001).
3.4. Effect of transcranial stimulation on low-risk choice
Our main a-priori hypothesis based on our previous findings
(Fecteau et al., 2007), was that participants receiving bifrontal tDCS
(either anodal tDCS to the right DLPFC coupled with cathodal tDCS
to the left DLPFC (referred as “right anodal”) or anodal tDCS to the
left DLPFC coupled with cathodal tDCS to the right DLPFC (referred
as “left anodal”)) would change risk-averse behavior on the Risk
Task. To test this hypothesis, we used a specific logistic regression
model using percentage low-risk choice as the dependent variable.
Results revealed a main effect of group of stimulation (p = 0.0025).
Interestingly, however, the direction of the hypothesized behav-
ior change was opposite to the findings in healthy controls.
2
In fact
we conducted a full model combining data from marijuana subjects
with data from healthy subjects and showed a significant difference
between these two groups regarding the main outcome (choice of
low vs. high-risk) (p < 0.001 (z = 11.65) for the variable subjects con-
dition and p < 0.001 (z = 9.06) for the interaction subjects condition
vs. treatment). In the marijuana group, participants receiving either
condition of active stimulation (right anodal or left anodal stimu-
lation) demonstrated a lower percentage low-risk choice; that is,
both groups of active stimulation chose the high-risk prospects
more often as compared to participants receiving sham stimulation.
In fact, pair-wise analysis demonstrated significant differences for
the comparison of anodal left vs. sham stimulation (OR = 1.29 95%
CI 1.11–1.51, p = 0.001) and comparison of anodal right vs. sham
stimulation (OR = 1.50 95% CI 1.11–2.03, p = 0.008). Finally, there
was no difference between anodal left and anodal right (OR = 1.11
95% CI 0.86–1.4) (Fig. 1). Also, there was no significant differ-
ence between women and men in their choices–the term gender in
the model was not significant (p > 0.05).
3.5. Effect of reward ratio on low-risk choice
An important issue then is the balance of reward whether
decision-making (in this case choosing the low vs. high-risk
prospect) is associated with the balance of reward (i.e. 90:10,
80:20, 70:30, or 60:40 reward ratio) (Rogers et al., 1999; Knoch
et al., 2006). Results revealed a significant main effect of balance
2
Choice between low-risk and high-risk prospect (in percentages) from our
healthy subject study during different tDCS conditions (same as in the current study
in subjects users of marijuana) note that this study has been published (Fecteau
et al., 2007) are provided with the online version of this article.
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P.S. Boggio et al. / Drug and Alcohol Dependence 112 (2010) 220–225 223
Fig. 1. During both right anodal/left cathodal and left anodal/right cathodal tDCS of
the DLPFC, the marijuana users demonstrated a significant increase in choice of the
more-risky prospect. The figure shows that these subjects chose the low-risk choice
with decreased frequency.
of reward (p < 0.001): Participants randomized to sham stimula-
tion tended to choose the low-risk prospect less often when its
associated reward was diminished a similar findings as in our
healthy subjects study. We then investigated whether the trends
with regards to the balance of reward was similar across groups:
we found a significant interaction for the term group × balance of
reward (p < 0.001). However, the direction of this trend was inverted
as compared to the results from our study with healthy subjects.
Whereas in our previous study, healthy subjects undergoing right
anodal stimulation were so conservative in their choice of the low-
risk prospect as to be nearly unaffected by the balance of reward,
here, in this study, marijuana users undergoing active stimulation
demonstrated a trend towards the high-risk prospect in the setting
of a large balance of reward (e.g. 90:10, 80:20 reward ratio). The
marijuana users demonstrate an inverted tendency during active
stimulation as compared to sham to select the more superficially
attractive option, the choice with the largest reward ratio (Fig. 2).
3.6. Total points earned
As participants gained or lost points according to their individual
decisions, we then tested whether group assignment had an inter-
action with the total points earned. Although there was a difference
in the strategy during the trial, ANOVA showed no significant dif-
ferences in the total of points earned (F(2,22) = 0.23, p = 0.79) during
active and sham stimulation.
3.7. Response time
We then tested whether differences in risk-taking were due
to changes in decision time, a potential confounder. Our analysis
showed that the main effect of group was not significant (p > 0.05),
therefore suggesting that response time was similar across groups
of stimulation. We also examined whether the decision times were
longer when participants were confronted to a 4:2 vs. a 5:1 scenario,
as found in Rogers et al. (1999), Knoch et al. (2006) and Fecteau et
al. (2007). There was no main effect of level of risk (p > 0.05).
3.8. Effect of stimulation on marijuana craving
Finally, we tested the effect of stimulation on marijuana crav-
ing. ANOVA showed a significant interaction of craving scores (VAS,
0–10) and time (before/after stimulation), F(2,22) = 10.9, p = 0.0005.
The results show that subjects reported significantly reduced crav-
ing for marijuana after right anodal/left cathodal DLPFC stimulation
as compared to sham stimulation.
Fig. 2. Marijuana users reported a significant decrease in marijuana craving (visual analogue scale, 0–10) after right anodal/left cathodal tDCS of DLPFC (dotted line). Left
anodal/right cathodal tDCS of DLPFC resulted in a non-significant increase in marijuana craving (dashed line). Sham stimulation resulted in no changes to craving scores
(solid line).
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224 P.S. Boggio et al. / Drug and Alcohol Dependence 112 (2010) 220–225
4. Discussion
We employed the Risk Task to provide insight to the decision-
making neural network of chronic marijuana users during sham
and active transcranial stimulation. In our study, there were
no significant differences in the total of points earned between
groups. However, the task does offer insight to the strategies
utilized by participants, particularly the propensity for risk-
taking.
Surprisingly, marijuana users chose the lower-risk option more
frequently than healthy subjects during sham stimulation. This
finding is interesting because whereas in our study marijuana users
tended toward the lower-risk option, in a study by Whitlow et al.
(2004) chronic marijuana users abstinent for 10–18 h displayed a
propensity for more risky decisions—participants rendered deci-
sions in the Gambling Task that led to larger immediate gains but
higher overall losses. Marijuana use in that study was associated
with deficits in the ability to balance rewards and punishments.
Extrapolating the results from that study might have suggested
that, in our study, abstinent marijuana users would have displayed
a greater propensity for risk-taking at baseline, but there are a num-
ber of reasons for this difference. First of all, it should be noted that
high performance on the Gambling Task requires participants to
apply deductive reasoning in their on-going experience with the
task to determine which deck contains advantageous vs. disad-
vantageous outcomes. In this respect, the Gambling Task is a more
cognitively complex decision-making paradigm as compared to the
Risk Task, which offers a more specific measure of propensity for
risk-taking. Indeed, poor performance in the Gambling Task among
marijuana users may be the result of impulsivity, an inability to
learn from experience, insensitivity to gains/losses, or high levels
of risk-taking.
Secondly, it is possible that chronic marijuana use may con-
tribute to the development of compensatory mechanisms that
promote more judicious risk-taking during abstinence. Lane et al.
(2005) showed that acute marijuana administration among occa-
sional users does increase selection of risky response options.
However, Vadhan et al. showed that acute marijuana intoxication
among highly experienced marijuana smokers does not interfere
with weighing of risky options or advantageous decision-making.
These results suggest that the effect of acute marijuana adminis-
tration on risk-taking may differ between occasional and chronic
users (Vadhan et al., 2007). Chronic marijuana use may contribute
to plastic changes that alter the cognitive effects of the drug, while
the same altering neural processing might persist in its absence.
Changes to the distribution of endocannabanoid receptors may
underlie this effect (Lichtman and Martin, 2005).
In our study, marijuana users demonstrated an increase in
choice of the more-risky prospect during tDCS of the DLPFC. These
results are interesting as they contrast with the result in healthy
volunteers in which anodal right tDCS/cathodal left tDCS has been
shown to promote conservative decision-making by upregulating
the capacity of the right DLPFC to suppress superficially seductive
options. Here among marijuana users, however, DLPFC stimulation
renders an opposite function in that it increased the propensity for
risk-taking. The result that anodal right/cathodal left tDCS method
of brain stimulation had a completely opposite effect on mari-
juana users as compared to the pattern observed in non-marijuana
using controls reveals an altered decision-making neural network
among chronic marijuana users. This result is consistent with pre-
vious studies of executive functioning in marijuana users (Bolla
et al., 2002), which suggest that marijuana users may recruit an
alternative neural network as a compensatory mechanism dur-
ing performance on tasks of executive functioning. Functional MRI
studies also support the suggestion that marijuana users may shift
the inter-hemispheric balance of activity across the prefrontal cor-
tex so to overcome an underlying propensity towards sub-optimal
decision-making (Eldreth et al., 2004).
An alternate aim of our study was to determine whether tDCS
of the DLPFC could be used to reduce marijuana craving. Prelim-
inary studies have shown that activation of the right DLPFC can
reduce food (Uher et al., 2005), alcohol (Boggio et al.), and cocaine
cravings among addicts (Camprodon et al., 2007). Moreover, acti-
vation of the left DLPFC with high-frequency TMS has been shown
to reduce nicotine consumption and cigarette smoking craving
(Amiaz et al., 2009; Eichhammer et al., 2003; Fregni et al., 2008a).
This study shows that right anodal/left cathodal DLPFC stimulation
(i.e. right DLPFC activation) reduces marijuana cravings. Indeed,
subjects reported significantly reduced craving for marijuana after
right anodal/left cathodal DLPFC stimulation. This study confirms
the role of DLPFC as a potent target for neuromodulation of craving
perception.
One limitation of this study is that chronic marijuana users were
restricted from smoking marijuana for a period of only 24 h prior
to the study, and this was measured only by self-report (similarly
to our healthy study (no marijuana users) (Fecteau et al., 2007)).
An assessment of long-term effects would best be achieved by
examination of marijuana users who were abstinent for at least a
7–28 day period as verified by either hair or urine sample. Indeed,
certain functional MRI changes noted in frontal and medial cere-
bellar regions of marijuana users have been shown to normalize
with increasing duration of abstinence in the abstinent users. Even
so, other fMRI changes in the right prefrontal, medial and dorsal
parietal, and occipital brain regions persist in spite of long-term
abstinence (Chang et al., 2006). Thus, it is indeed possible that our
findings here may suggest a neuroadaptive state that is limited
to active or acutely abstinent marijuana use. Further tDCS stud-
ies should investigate the role of tDCS in subjects with prolonged
abstinence. Another limitation is the small sample size of this study
that therefore may decrease the external validity of our findings.
However the aim of this exploratory study was to generate initial
data to be used in further confirmatory studies. Finally although
we compared the data from this study with our previous study
with healthy subjects (Fecteau et al., 2007) and showed a signif-
icant difference on the effects of tDCS across the two studies and
no demographic differences between the two groups when ana-
lyzing the small subgroups; there was a small (mean difference of
2.6 years), but significant difference (as the variance was small)
in age and gender between the two groups when data from sub-
groups are combined. Subjects in the marijuana group are slightly
older and this may explain risk-taking behavior differences in base-
line between these two groups of subjects. Also it needs to be
considered that data comes from two different studies; therefore
alternative mechanisms to explain the differences are possible.
5. Summary
In healthy volunteers, right anodal stimulation of the DLPFC
decreases the propensity for risk-taking. Acutely abstinent mari-
juana users have a lower propensity for risk-taking at baseline, but
this study shows that any polarity of DLPFC stimulation increases
their propensity for risk-taking. The differential effect of transcra-
nial brain stimulation reveals an altered decision-making neural
network among chronic marijuana users. In addition, this study
shows a role for right anodal/left cathodal DLPFC stimulation in
reducing marijuana craving.
Role of funding source
Nothing to declare.
Page 5
P.S. Boggio et al. / Drug and Alcohol Dependence 112 (2010) 220–225 225
Contributors
Conceived and designed the experiments: PSB ABV. Conducted
the experiments: PSB ABV. Analyzed the data: SZ PSB FF SF.
Interpreted the results: APL, PSB, FF. Wrote the first draft of the
manuscript: SZ PSB FF. Revised and approved the manuscript: All
the authors.
Conflict of interest
The authors have no conflicts of interest.
Acknowledgment
Paulo S. Boggio is supported by a CNPq researcher grant
(305718/2009-6).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.drugalcdep.2010.06.019.
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    [Show abstract] [Hide abstract] ABSTRACT: transcranial Direct Current Stimulation (tDCS) over the dorsolateral prefrontal cortex (dlPFC) has been shown to be clinically useful in the treatment of drug addiction. We conducted a double-blind randomized clinical trial aiming to assess the effects of bilateral dlPFC tDCS (left cathodal/right anodal) on crack-cocaine addiction. We defined craving as the primary outcome, and other clinical measurements, including depressive and anxiety symtoms, and quality of life, as secondary outcomes. 17 male crack-cocaine users (mean age 30.4 ± 9.8 SD) were randomized to receive five sessions of active tDCS (2 mA, 35 cm(2), for 20 minutes), every other day, and 19 males (mean age 30.3 ± 8.4 SD) to receive sham-tDCS (placebo), as control group. Craving scores were significantly reduced in the tDCS group after treatment when compared to sham-tDCS (p = 0.028) and to baseline values (p = 0.003), and decreased linearly over four weeks (before, during and after treatment) in the tDCS group only (p = 0.047). Changes of anxiety scores towards increase in the sham-tDCS and decrease in the tDCS group (p = 0.03), and of the overall perception of quality of life (p = 0.031) and of health (p = 0.048) towards decrease in the sham-tDCS group and increase in the tDCS group, differed significantly between groups. Repetitive bilateral tDCS over the dlPFC reduced craving to crack-cocaine use, decreased anxiety and improved quality of life. We hypothesize that tDCS effects may be associated with increased pre-frontal processing and regulation of craving behavior. © The Author 2015. Published by Oxford University Press on behalf of CINP.
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    • "6. Defining objectively the monitoring measures: As it can be seen in Tables 2 and 3, drug craving is the most common cognitive target and assessment measure in addiction and NIBS studies. However, there is an increasing trend for targeting other components including risk-taking behavior (Boggio et al., 2010;Fecteau et al., 2014;Gorini et al., 2014), relapse (daSilva et al., 2013;Klauss et al., 2014), drug consumption (Boggio et al., 2009;Fecteau et al., 2014;Meng et al., 2014), and attentional bias (Meng et al., 2014). Self-report questionnaires are subjective, and therefore prone to risk of assessment bias. "
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    • "Several studies have shown that tDCS can improve motor performance and motor learning in healthy subjects, both during (Galea et al., 2011) and after stimulation (Boggio et al., 2006 ). In addition, tDCS holds promise as a therapeutic tool in neurologic diseases such as stroke (Hummel and Cohen, 2006; Zimerman et al., 2012) or epilepsy (Fregni et al., 2006); psychiatric diseases such as depression (Boggio et al., 2008) or drug addiction (Boggio et al., 2010); and in chronic pain (Lefaucheur et al., 2008). However, a source of increasing concern has been that despite initially promising results, a number of studies attempting to replicate findings of prior tDCS studies have not found the same effects (Meesen et al., 2014; O'Connell et al., 2014; Polanowska et al., 2013; Wrigley et al., 2013). "
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