This preliminary study examined neuropsychological function among adolescent marijuana users without comorbid psychiatric disorders compared to non-using community control teens at three time points over three weeks of abstinence. Marijuana users performed worse than controls on measures of verbal learning and verbal working memory, above and beyond the effects attributable to age or SES. Specifically, adolescent marijuana users learned fewer words after approximately 3 days of abstinence, but they performed similarly to controls after 2 and 3 weeks without substance use. Marijuana users had poorer verbal working memory at 2 weeks of abstinence but were similar to controls after 3 weeks. Finally, although they had similar attention processing speeds, marijuana users were less accurate than controls on an attention/vigilance task throughout the 3-week abstinence period. No group differences were found on time estimation. Overall, these results suggest decrements in verbal learning and verbal working memory among marijuana-using youth that may resolve with abstinence, while attention accuracy deficits appear to persist.
To our knowledge, this is the first study of adolescent marijuana users to examine neurocognitive recovery throughout the first three weeks of abstinence. The presence of memory, attention, and working memory deficits among adolescent marijuana users is in concordance with several previous studies of youth (Fried et al., 2005
; Harvey et al., 2007
; Medina, Hanson et al., 2007
; Millsaps et al., 1994
; Schwartz et al., 1989
; Tapert et al., 2002
). The current findings also concur with animal studies reporting learning and memory deficits in adolescents after protracted cannabis exposure (Cha et al., 2006
; Schneider & Koch, 2003
; Schneider et al., 2008
; Stiglick & Kalant, 1982
), and with human adult reports of attention, memory, and executive deficits (Bolla et al., 2002
; Grant et al., 2003
; Pope & Yurgelun-Todd, 1996
; Solowij et al., 2002
Previously, our group reported memory, attention, psychomotor, and sequencing deficits among marijuana-using youth after approximately one month of abstinence (Medina, Hanson et al., 2007
). The previous report included a large comprehensive neuropsychological battery that was summarized with composite scores, while the current study used an abbreviated battery that tested a few key areas of functioning. The complex attention composite score used in Medina, Hanson et al. (2007)
mirrors the current measures of first trial verbal word list learning, verbal working memory, and attention accuracy; deficits in complex attention after a month of abstinence were primarily driven by poorer performance among marijuana users on several working memory tasks, including first trial word list learning. The current study found that deficits in verbal learning and working memory improved by 2 or 3 weeks of abstinence. The relatively intact performances after 3 weeks of abstinence here may be due to differences in measure sensitivity and sample size (N
=65 in the previous study).
We did not find reduced accuracy of time estimation in this sample of adolescent marijuana users. Further, a recent study of adolescent marijuana users did not find an association between time estimation and cerebellar volumes (Medina et al., 2010
). Although several adult studies have reported deficits in the accurate estimation of time (Chait & Pierri, 1992
; Lieving et al., 2006
; Perez-Reyes et al., 1991
), these studies were conducted during acute marijuana intoxication, whereas the current study was conducted after several days to weeks of abstinence. It is possible that marijuana’s purported effects on time estimation may only occur during intoxication with the drug. Further, previous reports used alternate approaches to measure time estimation. Overall, deficits in time estimation do not appear to persist after acute intoxication, but further research is warranted.
Deficits in learning and working memory are consistent with adult research that marijuana use is associated with hippocampal and prefrontal cortex abnormalities (Block et al., 2002
; Eldreth et al., 2004
; Gruber & Yurgelun Todd, 2005
; Loeber & Yurgelun-Todd, 1999
; Lundqvist et al., 2001
). Attention accuracy deficits also implicate dysfunction within attention networks (Eldreth et al., 2004
; Gruber & Yurgelun Todd, 2005
). Marijuana use during adolescence may disrupt the normal maturational processes that take place during this period, including pruning of gray matter and myelination of white matter (Benes et al., 1994
; Gogtay et al., 2004
; Jernigan & Gamst, 2005
; Pfefferbaum et al., 1994
), resulting in altered structure and function of the brain (Jacobsen et al., 2007
; Medina, Nagel et al., 2007
; Medina et al., 2010
; Medina, Schweinsburg et al., 2007
; Padula et al., 2007
; Rubino et al., 2009
; Schweinsburg et al., 2008
; Schweinsburg et al., 2005
; Tapert et al., 2007
). It is also important to emphasize that in some functional imaging studies, marijuana-using youth demonstrated aberrant patterns of activation despite similar performance to non-users (Padula et al., 2007
; Schweinsburg et al., 2008
; Schweinsburg et al., 2005
), suggesting that even once cognitive deficits are no longer detectable, brain function may remain affected. That is, marijuana users may use alternate strategies or may need to “work harder” to obtain the same result as teens who do not use drugs. More research is needed to determine the nature, extent, and duration of any cognitive or neurological abnormalities related to marijuana use.
The clinical implications of the current findings should also be discussed. The current deficits detected in verbal learning, working memory, and attention were less than one standard deviation worse than controls (small to medium effect sizes) and do not indicate a clinical impairment. However, these marijuana-using high school and college students may have more difficulty learning and processing new information and accurately completing their schoolwork or other tasks, and as a consequence, may not perform to expected levels (Lynskey & Hall, 2000
). Although the grades achieved in school were not statistically different between groups in the current study, our previous larger study indicated that marijuana users had lower grades than non-users (Medina, Hanson et al., 2007
), and they may need to work harder to achieve this grade level (Tapert et al., 2007
). Furthermore, participants may be considered high functioning among the general population of adolescent cannabis users, given their high average estimated intelligence, middle to upper class socioeconomic status, and lack of comorbid psychiatric or medical disorders. Thus, if relatively advantaged young marijuana users manifest some deficits, it is possible that those with fewer protective factors may be more vulnerable to cognitive, academic, and occupational consequences. Finally, as significant deficits were detected after several days to 3 weeks of abstinence, youth with frequent use patterns are likely to manifest similar deficits on an ongoing basis.
Some limitations of this study should be considered. This preliminary study used a brief cognitive assessment at three time points, but additional deficits may have been detected with a more comprehensive battery. Although the groups were generally well-matched, marijuana users were slightly older and had higher parental SES than controls; however, this difference was counter to the direction of our hypotheses. Further, the sample consisted mostly of males, so confirmation of observed effects among female users is needed. As with most studies of substance users, it is difficult to determine whether the deficits reported here were the consequences of marijuana use. For example, attention accuracy deficits may predate substance use, may take longer to recover, or may be due to other substance use. However, the relatively small sample size restricted our ability to examine relationships between cognition and the extent of marijuana or other substance use. Because alcohol was used at relatively high levels among users, we cannot attribute any deficits solely to the effects of marijuana use. Nevertheless, regular marijuana use in the context of moderate to heavy alcohol use may be detrimental to cognitive function in teens.
In summary, this preliminary study found poorer verbal learning and verbal working memory among recent marijuana-using adolescents that improved with continued abstinence, and deficits in attention accuracy that persisted at least through 3 weeks of abstinence. These results implicate possible hippocampal and prefrontal cortex abnormalities, as well as dysfunction within attention networks. Adolescent marijuana users could be unaware of any cognitive difficulties, yet academic, behavioral, and occupational functioning may be negatively affected. Additional research is needed to determine whether marijuana use during adolescence results in alterations to neurological or cognitive functions that persist into adulthood.