The NEO Five-Factor Inventory ascertained characteristics according to the five factor model of personality: degree of neuroticism, extraversion, openness, agreeableness, and conscientiousness. Parents were given the Child Behavior Checklist to assess level of psychopathological syndromes. The SWM task consisted of 18 21-second blocks that alternated between resting fixation, baseline vigilance, and working memory conditions. Each block began with a one-second word cue that indicated the type of upcoming block. In the SWM condition, the word “WHERE” cued subjects to remember the locations of abstract line drawings that were individually presented in one of eight spatial locations on a screen. Subjects were instructed to press a button every time a figure appeared in the same location as a previous design within that block, regardless of the shape. Unbeknownst to subjects, repeat location stimuli were 2-back, and 3 of 10 trials in each block were targets. The baseline vigilance condition began with the word “DOTS”, followed by presentation of the same abstract stimuli shown in the same possible spatial locations as in the SWM condition; subjects were to press a button every time a figure appeared with a dot above it . Resting blocks displayed the word “LOOK” followed by presentation of a fixation cross in the center of the screen. For both the vigilance and working memory conditions, stimuli were presented for 1000 ms with an interstimulus interval of 1000 ms . All teens were trained with a 4-minute version of the task and monitored to ensure comprehension of task instructions prior to scanning. Responses were collected with a fiber optic button box. The toxicology procedure was designed to minimize the possibility that participants used substances in the 28 days prior to fMRI assessment. Cannabinoid metabolites remain detectable in urine for at least four days and 27 days on average in heavy users .
Urine samples were collected 2 – 3 times per week during the 28 days prior to the fMRI session to detect metabolites indicating recent use of cannabis grow systems,amphetamines, methamphetamines, benzodiazepines, cocaine, barbiturates, codeine, morphine, phencyclidine, and ethanol. Samples were analyzed in the VA Medical Center laboratory using cloned enzyme donor immunoassay assay kits . Observed sample collection reduced the possibility of participant tampering. Quantitative indices from samples were tracked to determine if cannabinoid metabolite levels decreased over the 28 days. Youths with initial samples positive for cannabis remained eligible if the values continued to decrease. If levels increased, the participant was given one chance to restart the 28-day toxicology screening process. Three quarters of marijuana users successfully completed this toxicology screening indicating abstinence for 28 days before scanning, and only these 15 subjects were included in analyses. Participants who were unable to complete 28 days of abstinence were not scanned, and were more likely to be male and slightly heavier users than those who remained abstinent. Imaging data from each teen were processed and analyzed using Analysis of Functional NeuroImages . Prior to statistical analyses, the time series data were corrected for motion by registering each acquisition to a selected repetition with an iterated least squares algorithm , creating an output file specifying adjustments made for three rotational and three displacement parameters for each participant. Using deconvolution processing , the time series data were correlated with a reference function coding the hypothesized BOLD signal across the task and modeling anticipated delays in hemodynamic response . This multiple linear regression approach yielded a fit coefficient for each subject in each voxel, representing the relationship between the observed and hypothesized signal change while controlling for linear trends and degree of motion correction applied. Fit coefficients were obtained for contrasts between SWM and vigilance conditions, SWM and fixation, and vigilance and fixation. Anatomical and functional datasets were warped into standard space , and functional data were resampled into 3.0 mm 3 voxels and smoothed with a 5.0 mm full-width half-maximum Gaussian filter. Group differences in BOLD response contrast to SWM relative to vigilance were evaluated using independent samples t-tests in each brain voxel.
Single sample t-tests identified regions of task-related brain response in each group separately. Tocontrol for Type I error, significant group difference clusters consisted of contiguous significant voxels that exceeded 1328 µl in volume, yielding an overall clusterwise α = 0.05. To understand the nature of group differences between SWM and vigilance brain response, we performed follow-up analyses examining SWM relative to fixation, and vigilance relative to fixation, in each significant cluster. Exploratory follow-up regressions among MJ teens determined the influence of substance use and behavioral characteristics on BOLD response in brain regions demonstrating significant group differences. This study examined fMRI brain activation during a spatial working memory task among marijuana using teens and controls after 28 days of monitored abstinence, verified by biweekly urine toxicology screens. Despite similar overall patterns of brain response to SWM, group differences were observed in right dorsolateral prefrontal cortex, right posterior parietal cortex, medial superior occipital cortex, and medial inferior occipital cortex. MJ teens displayed reduced SWM BOLD response relative to control teens in right dorsolateral prefrontal cortex, which is consistently implicated in spatial working memory . In contrast, our previous work revealed increased SWM response in this region among heavy alcohol and marijuana usingteens who had been abstinent an average of just 8 days . Kanayama and colleagues also observed greater dorsolateral prefrontal response among adult heavy marijuana users on a similar SWM task 6 – 36 hours after marijuana use. However, after 25 days of abstinence, adult marijuana users showed decreased left dorsolateral prefrontal blood flow during a modified Stroop task . Moreover, during visual attention, active marijuana users with positive urine toxicology screens evidenced greater reductions in right prefrontal fMRI response than abstinent users . Considered together with the results of the current study, these findings suggest a change in neural recruitment throughout the course of abstinence. This could relate to residual drug effects or withdrawal symptoms during early abstinence, less need for neural compensation, or a change in neurocognitive strategy as the brain adapts to different stages of sobriety.
We did not observe a correlation between brain response and recency of marijuana use in this sample, but most neuropsychological recovery appears to occur during the first week of abstinence . Thus, there may be little change in neurocognitive functioning after 28 days of abstinence, or such an effect may be too subtle to detect with a relatively small sample. Careful examination of neural response within the first month of abstinence may better clarify this relationship. Compared to controls, MJ teens demonstrated increased SWM activation in right posterior parietal cortex,ebb and flow cannabis a region involved in SWM and attentional processes . Research on parietal functioning during working memory among individuals with substance use disorders has been somewhat inconsistent. Using the same SWM task, our previous work failed to observe parietal abnormalities among adolescent users of alcohol and marijuana , though teens with alcohol use disorders alone showed increased posterior parietal brain response compared to controls despite similar task performance between groups. Greater fMRI activation in posterior parietal cortex has also been observed during SWM among adult marijuana users as well as during verbal working memory among adolescent marijuana users experiencing nicotine withdrawal . Heightened activation among individuals with substance use disorders may be associated with compensatory neural responding to perform well on a task . MJ teens in the current study displayed increased response in parietal cortex, yet diminished activation in prefrontal cortex, both of which play important roles in SWM . Frontal cortex may be primarily involved in general executive functioning components of working memory tasks, while superior parietal cortex may more specifically sub-serve attentional allocation and visuospatial rehearsal demands of SWM . Thus, abstinent MJ teens may rely more on spatial rehearsal and attention rather than general executive abilities to perform the task, resulting in increased recruitment of posterior parietal cortex, but decreased right dorsolateral prefrontal activity. This altered pattern is consistent with previous evidence of reorganized attention networks in MJ users . Further, estimated typical blood alcohol concentration achieved was negatively associated with parietal response among MJ teens, which could indicate that heavier drinking MJ teens may demonstrate less neural compensation or be less likely to utilize spatial strategies as those with lighter alcohol use histories. This is consistent with previous findings of diminished parietal activation during SWM among alcohol use disordered young adults , and suggests a potential interaction between heavy alcohol and marijuana use in youth . We previously characterized the relationship between age and fMRI response among 49 typically developing teens ages 12 – 17 using the same SWM task . Younger teens evidenced increased response in superior portions of posterior parietal cortex, while older teens utilized more inferior aspects of posterior parietal cortex. This shift in localization of parietal response across adolescence indicates a change in strategy, with younger teens relying on rote spatial rehearsal and older teens implementing more spatial storage. MJ teens in the current study demonstrated increased SWM activity relative to controls in superior portions of right parietal cortex, paralleling response patterns of younger adolescents. Thus, MJ teens may employ spatial rehearsal strategies more consistent with those used by younger youths. This may suggest the possibility of altered neuromaturation among adolescent marijuana users, which could implicate an adverse influence of marijuana on the developing brain or preexisting neural differences that may have contributed to the initiation of substance use.
MJ teens demonstrated increased vigilance response compared to controls in two regions of medial occipital cortex: superior portions of the cuneus, and lingual gyrus/inferior cuneus. Occipital cortex has been associated with visual attention, and may become more active as attentional capacity is reached yet less active during practiced tasks . Greater occipital response among MJ teens may indicate less efficient processing and greater attentional demand during vigilance blocks. Such occipital hyper activation among MJ teens was not observed during SWM blocks, during which attentional resources were not focused solely on visual selective attention, but allocated to accommodate working memory processing. Previous studies have implicated diminished attention capacity in heavy marijuana using adults and adolescents . During SWM, MJ teens may allocate limited attentional resources to spatial processing, depriving attentional input to executive systems, resulting in increased parietal and decreased frontal activation. A SWM task with greater executive demand may require more attention input to frontal systems, diminishing response capability in parietal cortices. Among adolescent marijuana users, those who began regular use earlier in adolescence demonstrated greater abnormalities in occipital brain response.Similarly, adults who began using during early adolescence showed greater neural dysfunction during spatial attention and poorer functioning on tests of attention , and verbal abilities and short term memory . Animal models also indicate that cannabinoid exposure during adolescence is associated with greater impairments in working memory and spatial learning than adult exposure . Thus, evidence suggests that marijuana use during adolescence may influence the course of brain development, and those who begin using at a younger age may be more susceptible to dysfunction with continued use. Brain response differences between groups may also relate to aberrant cerebral blood flow among MJ teens. Adult marijuana users have demonstrated reduced resting frontal and cerebellar blood flow during short-term abstinence . Further, elevated cerebrovascular resistance and systolic blood flow remained high after a month of abstinence, suggesting lasting blood flow abnormalities . These blood flow abnormalities could affect the magnitude of the observed BOLD response . Specifically, reductions in frontal blood flow may contribute to diminished frontal SWM activation among MJ teens. Future investigations could more closely account for resting perfusion when examining BOLD response among marijuana users.This study raises several questions. First, most MJ users in this study were moderate to heavy drinkers. Though this is representative of adolescent marijuana users, the functional impact of marijuana use alone is difficult to determine. Studies with larger samples and variability in drinking patterns will help elucidate the substance-specific neurocognitive effects. MJ and control teens were comparable on demographics, behavior, personality and intellectual functioning, and differed slightly on mood, but none of these measures accounted for group differences in brain response. However, abnormal activation patterns among MJ teens may relate to preexisting traits that were not measured.