With great interest, we read the article by Huddy et al. (2013) recently published by Psychological Medicine. The authors aimed to shed empirical light on the co-morbidity of first episode psychosis (FEP) and cannabis consumption with respect to two subdomains of impulsivity – response inhibition and reflection impulsivity. It was shown that patients with FEP had significantly greater impairment in response inhibition but not in reflection impulsivity compared to healthy controls. By contrast, patients with current cannabis use had greater reflection impulsivity but no impairments in response inhibition.
Over the past years an increasing number of studies have shown that impaired aspects of impulsivity like response inhibition are found across multiple substances with different pharmacological effects (Verdejo-Garcia et al. 2008; Solowij et al. 2012) and across several neuropsychiatric diseases (Fontanelle et al. 2011). In particular, there is a growing body of evidence for impaired impulsivity FEP on the one hand and complex associations of different aspects of impulsivity with cannabis consumption on the other. Interestingly, the study by Huddy et al. (2013) fails to show differences in response inhibition in cannabis users compared to drug-naive and discontinued users. It was concluded that abnormal reflection impulsivity is associated with substance use in psychosis but not psychosis itself; the opposite relationship may hold for response inhibition.
The findings by Huddy et al. are in line with previous evidence of non-acute studies with different abstinence periods starting from 17 hours to more than a year. No impairments of cognitive inhibition among recreational (Griffith-Lendering et al. 2012) and long-term users (Pope et al. 2002) have been shown. Interestingly, performance of the Stroop task was affected by marijuana use only in individuals with lower cognitive reserves (Bolla et al. 2002) while cannabis users in another study did not differ significantly from controls, but were vulnerable to task complexity with increasing demands creating more sources of interference (Solowij et al. 2002). Moreno et al. (2012) did find significantly different inhibitory control in recreational users with Stroop and Go/No-Go; and heavy users in Pope & Yurgelund-Todd (1996) exhibited more errors of inhibition than light users. In contrast, clear impairments among cannabis users of this aspect of impulsivity have been shown after acute administration (McDonald et al. 2003; Metrik et al. 2012).
While reflection inhibition in FEP in Huddy et al. (2013) is not significantly impaired compared to healthy controls, this is the case for cannabis users compared to non-users. Co-morbid continued usage in FEP shows earlier onset of psychosis and cannabis consumption, increased daily usage, more abuse of other drugs and tends to increase positive symptoms, interpretable as being more prone to jumping to conclusions. Reflection inhibition is a failure of pre-decisional information sampling and evaluation of situations (Solowij et al. 2012). Thus, it putatively influences decision making, which is impaired among acutely intoxicated cannabis users (Ramaekers et al. 2006; Vadhan et al. 2007) as well as after a time of abstinence (Griffith-Lendering et al. 2012; Moreno et al. 2012). However, whether impulsivity is the consequence of prolonged drug abuse or simply a personality trait leading to discontinuation difficulties, is still open to debate.
Although neuroimaging studies have provided compelling evidence for cannabis-related effects on brain structure and functioning (Martín-Santos et al. 2010), it still remains to be seen whether there are any specific effects on response inhibition. In conclusion, we suggest more studies like Huddy and colleagues (2013) as well as longitudinal and prospective designs are needed to investigate whether specific aspects of impulsivity could be considered neuropsychiatric endophenotypes of comorbid psychiatric disorders.
Declaration of Interest