Recreational cocaine polydrug use impairs cognitive flexibility but not working memory
Lorenza S. Colzato
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Maritte Huizinga
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Bernhard Hommel
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M. Huizinga Department of Psychology, University of Amsterdam
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Amsterdam, The Netherlands
Rationale Chronic use of cocaine is associated with dysfunctions in frontal brain regions and dopamine D2 receptors, with poorer mental flexibility and a reduced ability to inhibit manual and attentional responses. Little is known, however, about cognitive impairments in the upcoming type of recreational cocaine polydrug user (14 g monthly consumption). Objective We studied whether recreational cocaine polydrug users, who do not meet the criteria for abuse or dependence, showed impairments in working memory (WM) and cognitive flexibility. Methods Controls and recreational cocaine polydrug users (who abstained from cocaine and other substances more than 1 week) were matched by sex, age, alcohol consumption, and IQ (Raven's Standard Progressive Matrices). Groups were tested by using two cognitive tasks measuring cognitive flexibility and three tasks investigating the maintenance and monitoring of information in WM. Results Recreational cocaine polydrug users performed significantly worse than controls on tasks tapping cognitive flexibility, but show comparable performance in the active maintenance and monitoring of information in WM. Conclusions The findings suggest that recreational use of cocaine selectively impairs cognitive flexibility but not the maintenance of information in WM. The inability to adjust behavior rapidly and flexibly may have repercussions for daily life activities.
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Since 2007, cocaine is Europes second preferred
recreational drug after cannabis (European Monitoring Centre for
Drugs & Drug Addiction 2007). Taking cocaine by snorting
route is not a privileged habit anymore, as it was in the
80s, but now is affordable for everyone, in particular for
recreational purposes. Cocaine has, thus, become a
common street drug. At long term, chronic (i.e., daily) use of
cocaine is associated with reduced functioning of dopamine
D2 (DAD2) receptors (Volkow et al. 1999) and
dysfunctions in lateral prefrontal cortex (LPFC), orbitofrontal
cortex (OFC; Bolla et al. 1998; Bolla et al. 2004), anterior
cingulate, and cerebellum (Hester and Garavan 2004).
Given that all these areas have been shown to play major
roles in the control of goal-directed action (Miller 2000), it
is not surprising that cocaine dependence is correlated with
deficiencies in cognitive control functions (Hester and
Garavan 2004; Verdejo-Garcia et al. 2006). Indeed, a
majority of studies on chronic cocaine users (see Bolla
et al. 1998; Jovanovski et al. 2005, for a review) points in
that direction: Chronic users, compared to nonusers, are
less able to inhibit overt responses (Fillmore and Rush
2002), perform worse on tasks measuring mental flexibility
(Verdejo-Garcia et al. 2006; Verdejo-Garcia and
PerezGarcia 2007) and conflict control (Franken et al. 2007), are
less able to control visual attention (Kbler et al. 2005), and
choose more disadvantageously in a decision-making task
(Monterosso et al. 2001).
So far, however, only few studies have systematically
looked into cognitive impairments in the upcoming type of
the recreational user, who does not meet the criteria for
abuse or dependence but takes cocaine (preferably by
snorting route) on a monthly frequency (1 to 4 g1,
commonly consumed in only a few sessions). Very
recently, Colzato et al. (2008b) found that the spontaneous
eyeblink rate, a marker of striatal dopaminergic
functioning, is significantly lowered in recreational users,
suggesting that even the recreational use of cocaine is associated
with a hypoactivity of the subcortical dopamine system.
Consistent with this consideration, recreational users
exhibit impairments in the inhibition of overt manual
responses (Colzato et al. 2007) and of covert attentional
responses (Colzato and Hommel 2009), which both are
assumed to relying on proper dopaminergic functioning.
However, even though the empirical connection between
inhibitory control functions and cocaine use is
considerable, the causal relation between the two is not
straightforward. Indeed, there are some hints to preexisting
neurodevelopmental factors that may play a mediating
role. For instance, monkeys that have preexisting lowered
D2 receptor densities run a higher risks to use cocaine and
to become addicted (Nader et al. 2006), and chronic
human users may suffer preexisting problems in inhibitory
control (Bechara 2005) and impulsivity (Verdejo-Garcia et
al. 2008).
Previous studies on recreational use of cocaine have
focused on inhibitory control. The ability to inhibit
unwanted thoughts and actions is commonly considered
an important part of executive control, but it represents just
one of a larger set of control functions. In an attempt to
categorize the available concepts and measures in a
coherent fashion, Miyake and colleagues have investigated
the psychometric relationships between the tests and tasks
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