Numerous studies have reported neurocognitive impairments associated with chronic marijuana use. Given that the hippocampus contains a high density of cannabinoid receptors, hippocampal-mediated cognitive functions, including visuospatial memory, may have increased vulnerability to chronic marijuana use. Thus, the current study examined brain activation during the performance of a virtual analogue of the classic Morris water maze task in 10 chronic marijuana (MJ) users compared to 18 nonusing (NU) comparison subjects. Imaging data were acquired using blood oxygen level-dependent (BOLD) functional MRI at 3.0 Tesla during retrieval (hidden platform) and motor control (visible platform) conditions. While task performance on learning trials was similar between groups, MJ users demonstrated a deficit in memory retrieval. For BOLD fMRI data, NU subjects exhibited greater activation in the right parahippocampal gyrus and cingulate gyrus compared to the MJ group for the Retrieval-Motor Control contrast (NU > MJ). These findings suggest that hypoactivation in MJ users may be due to differences in the efficient utilization of neuronal resources during the retrieval of memory. Given the paucity of data on visuospatial memory function in MJ users, these findings may help elucidate the neurobiological effects of marijuana on brain activation during memory retrieval.
Research on marijuana (MJ) use continues to be a major area of investigation, since MJ remains the most widely used illicit drug in several countries, including the United States [
Short-term effects of MJ have been reported across a number cognitive domains (for review [
The hippocampus may be notably vulnerable to the effects of MJ, given the high density of cannabinoid receptors in this area [
The hippocampus is necessary for processing spatial layout and configural representation of an environment [
The frontal cortex, specifically, the dorsolateral prefrontal cortex, has been implicated in spatial working memory tasks [
The study sample consisted of ten chronic marijuana (MJ) users (8 males) and eighteen nonusing (NU) comparison subjects (11 males). Participants were recruited through local advertisement and screened by telephone interview to ensure they met criteria for inclusion in the study. All aspects of the clinical research protocol were reviewed and approved by the Institutional Review Board of McLean Hospital (Belmont, MA, USA). After a complete description of the study, participants provided written informed consent. All participants received monetary compensation ($100) for study completion. Participant demographics are presented in Table
Demographic and marijuana use data.
MJ ( |
NU ( | |
---|---|---|
Age (years) | 20.3 ± 3.6 | 22.8 ± 5.0 |
(Age range: 18–30) | (Age range: 18–33) | |
Education (years) | 13.4 ± 1.5 | 15.5 ± 2.4 |
Ethnicity (Caucasian/non-Caucasian)a | 9/0 | 11/7 |
SESb | 43.4 ± 10.6 | 49.1 ± 12.2 |
Age of MJ onset | 15.6 ± 1.2 | — |
Smokes per week | 10.7 ± 5.5 | — |
Grams per weekc | 4.8 ± 4.9 | — |
Duration of use (yrs) | 4.0 ± 2.4 | — |
ASI (MJ use out of 30 days) | 25.5 ± 4.0 | — |
MWC | 2.0 ± 2.1 | — |
THC (ng/mL) | 193.5 ± 219.2 | — |
Data represent mean ± standard deviation. MJ: marijuana; NU: nonusers; SES: Socioeconomic status; ASI: Addiction Severity Index; MWC: Marijuana
Withdrawal Check List. aOne missing data point for MJ user. Non-Caucasian
classification consisted of Asian, African American, and others.
bOne missing data point for NU. cTwo missing data points for grams per week.
To qualify for study entry, MJ smokers had to have smoked MJ a minimum of 2500 times, used MJ at least five of the last seven days prior to the study visit, test positive for urinary cannabinoids, and meet DSM-IV criteria for MJ abuse on the day of scanning. MJ users were asked to refrain from smoking for 12 hours immediately preceding the study visit and were told a urine sample would be collected at the initiation of the study visit, in order to improve compliance. The NU participants reported fewer than 5 lifetime episodes of MJ use and did not use any other illicit substances. Exclusion criteria for all subjects included history of head injury, loss of consciousness, history of organic mental disorder, seizure disorder or central nervous system disease, and contraindications to MR scanning (e.g., pacemaker, aneurysm clips, metallic implants, pregnancy, or claustrophobia). MJ users reported consuming
The Barratt Simplified Measure of Social Status (BSMSS) was used to measure socioeconomic status (SES) [
All participants provided a urine sample to be tested for amphetamines, barbiturates, benzodiazepines, cocaine, opiates, phencyclidine, and tetrahydrocannabinol (THC) (Triage Drugs of Abuse Panel: Immediate Response Diagnostics, Biosite, San Diego, CA, USA). A positive result for THC confirmed recent MJ use in the MJ group, while a negative result was required for the NU group. Standard laboratory urinalysis assessed an aliquot of the urine sample, which included gas chromatography-mass spectroscopy in order to quantify the level of 11-nor-9-carboxy-delta 9-tetrahydrocannabinol (THC-COOH) and creatinine (Quest Diagnostics, Cambridge, MA, USA). To allow for differences in urinary concentration among the participants, levels of THC-COOH were normalized to urinary creatinine levels.
A measure of general intellectual ability (IQ) was derived using two of the four subtests (vocabulary and matrix reasoning) from the Wechsler Abbreviated Scale of Intelligence (WASI, [
A PC-compatible laptop was used for testing and operating the virtual water maze program (NeuroInvestigations, Inc., Lethbridge, Canada). The virtual environment was comprised of a circular pool located in the center of a square room, with four large abstract pictures positioned on the walls, which served as landmarks (Figure
(a) Screen captures of the water maze task during the Learning and Retrieval conditions (hidden trials) (left) and the Motor Control condition (visible trials) (right). (b) BOLD fMRI scanning sequence used during Retrieval and Motor Control conditions.
Prior to the start of the experiment (nonscanning and scanning conditions), participants completed a training phase outside of the MR suite, which consisted of two trials with the platform visible in the NE quadrant, in order to familiarize them with the task and the use of the joystick. The virtual environment used for training had landmarks that were unique from those in the virtual environment presented during nonscanning and during fMRI. The experimental phase consisted of three conditions: Learning (Hidden Trials—conducted outside the magnet/nonscanning); Retrieval (Hidden Trials—conducted in the magnet); Motor Control (Visible Trials—conducted in the magnet). During the Learning condition, each participant completed 4 blocks of hidden platform trials (4 trials per block, each trial beginning from a different location), in which the platform was hidden under the surface of the water and the participants were instructed to navigate to the platform as quickly as possible. The platform was always located in the same position. Once the participant successfully navigated to the area where the platform was located, a message on the computer displayed “Platform found.” If the platform was not located within 60 sec, the platform became visible and the following message was displayed on the screen: “The platform is visible, swim to it.” The next trial began 1 sec after the previous trial ended (1 sec intertrial interval (ITI)). After completion of the Learning trials, the Probe trial began, in which the platform was removed from the virtual environment unbeknownst to participants. The probe trial ended after participants navigated around the environment for 30 sec.
The Retrieval Condition was similar to the Learning condition and participants were instructed to navigate to the hidden platform as quickly as they could. The platform was always located in the same location as during the learning condition. During the Motor Control condition, each participant completed 2 blocks of visible platform trials (4 trials per block, each trial beginning from a different location). The platform was visibly above the surface of the water and participants were instructed to navigate to the platform as quickly as possible, without paying attention to environmental landmarks. The location of the platform was the same as in the hidden condition, thereby minimizing the potential for encoding novel information during navigation in the environment. The scanning sequence lasted for 360 sec and consisted of alternating “on” (4 active “on” periods) and “off” periods (5 rest “off” periods). During the “on” periods participants navigated through the virtual environment and completed as many trials as possible within each of the four 60 sec “on” periods. Therefore, the number of completed trials varied between participants. During “off” periods, participants viewed a black screen that displayed the message “please wait for instructions” (Figure
Dependent measures for Learning, Retrieval, and Motor Control conditions on the WMT included path length, navigation latency, and first movement latency. Path length (relative to pool diameter) was measured as the distance to reach the platform. Navigation latency was measured as the total time (sec) to complete the task minus the total elapsed time (sec) prior to the first movement. Latency to first movement was measured as the time (sec) before the participant initiated navigation in the pool. Path length (distance to the platform), navigation latency, and latency to first movement measures were averaged across trials per block for the Learning, Retrieval, and Motor Control conditions. For the Probe trial, dependent measures included percent of total distance traversed within the correct platform quadrant (NE), reflecting as an index of spatial learning, and heading error towards the platform, calculated as the angular deviation from a straight path to the center of the platform from the starting position. Heading error was measured at the first occurrence that participant distance was greater than 25% of the pool diameter from the starting position. The number of trials completed during the Retrieval and Motor Control conditions was also recorded.
Two independent raters blind to participant diagnosis rated navigation strategies used by participants during the Probe trial. The strategy chosen to solve the water maze could affect behavioral performance [
Sample strategies used by participants during the WMT Probe trial. For the direct strategy (left), participants navigated directly to the platform, and for the nondirect strategy (right), participants navigated in a circuitous or random route that was not in the direction of the platform quadrant.
Functional MRI scanning was performed on a 3T Siemens Trio whole-body MR scanner (Siemens Healthcare, Erlangen, Germany), using a birdcage quadrature RF head coil for acquisition of echo planar imaging (EPI) blood oxygen level-dependent (BOLD) fMRI. Sagittal scout images were first acquired for alignment and localization using a fast spin echo sequence (FSE) with the following parameters: repetition time (TR) = 3 msec, echo time (TE) = 40 msec, field of view (FOV) = 20 cm, matrix size
SPM5 (Wellcome Department of Imaging Neuroscience, University College, London, UK) was run in Matlab (MathWorks, Natick, MA, USA) for analysis of functional MRI data. To correct for motion in BOLD fMRI data, an intrarun realignment algorithm was utilized, which uses the first image as a reference. An exclusionary criterion of 2 mm of head motion in any direction was used. An EPI template in Montreal Neurological Institute (MNI) stereotactic space was employed to normalize the realigned images, which were resampled into 2 mm cubic voxels. To spatially smooth the normalized images, an isotropic Gaussian filter (full width half maximum [FWHM] = 10 mm) was then applied [
The motor control condition of the water maze paradigm was used as a control condition since there was no learning or memory component (i.e., subjects simply navigate to the visible platform). Age was entered as a covariate into the analysis. To identify brain areas activated during hidden conditions, predetermined condition effects were calculated at each voxel by the fixed model and a single image of mean activation for Retrieval-Motor Control was created for each subject. The group data were then analyzed using a random-effects model on a second level to account for interindividual variance. Comparisons between groups (NU > MJ; MJ > NU) were performed using a two-sample
One-way analyses of variance (ANOVAs) were used to compare MJ users and NU on demographic, clinical measures, cognitive measures, and behavioral measures. SPSS 18.0 (SPSS, Chicago, IL) was used for all statistical analyses
As illustrated in Table
No significant differences were observed between the groups for IQ, as measured by WASI, or for mood, as measured by the HAM-A, or on PANAS (positive or negative affect subscales) (Table
Clinical and cognitive measures.
MJ ( |
NU ( |
|
|
|
---|---|---|---|---|
PANAS | ||||
Positive affect | 30.9 ± 6.6 | 33.9 ± 7.5 | 1.1 | 0.29 |
Negative affect | 12.2 ± 3.2 | 11.7 ± 2.4 | 0.2 | 0.66 |
HAM-A | 2.7 ± 2.3 | 1.5 ± 1.7 | 2.5 | 0.12 |
Mental rotation total score | 16.6 ± 4.6 | 14.7 ± 4.9 | 1.0 | 0.33 |
SBSOD total scorea | 4.8 ± 0.7 | 4.6 ± 1.0 | 0.2 | 0.68 |
WASI IQ | 121.3 ± 8.3 | 120.2 ± 8.9 | 0.1 | 0.74 |
Data represent mean ± standard deviation. MJ: marijuana; NU: nonusers.
PANAS: positive and negative affect scale; HAM-A: Hamilton Anxiety
Scale; SBSOD: Santa Barbara Sense-of-Direction Scale; WASI: Wechsler
Abbreviated Scale of Intelligence. aMJ = 8; NU = 14.
There was a significant effect of Block (
Average path lengths on WMT hidden trials during the Learning condition in MJ (open circles) and NU (closed squares) groups across trial blocks.
The Group effect for Retention trended towards significance, with MJ users displaying a lower percentage of total navigation distance within the NE (correct) platform quadrant on the Probe trial relative to NU (
The number of hidden platform trials completed during fMRI did not differ between groups, with MJ users completing
The number of visible platform trials completed also did not differ between groups, with MJ users completing
There was a significant preference observed for the use of a direct versus a nondirect strategy during the Probe Trial in the NU group, with 78% (
Foci of maximally activated brain regions—Retrieval-Motor Control.
Whole brain | MNI coordinates | Cluster size ( |
|
Voxel |
Cohen’s |
|||
---|---|---|---|---|---|---|---|---|
Region | BA |
|
|
|
||||
NU > MJ | ||||||||
L inferior frontal pars triangularis | 45 | −46 | 20 | 12 | 44 | 4.46 | <.001 | 1.78 |
L inferior frontal pars opercularis | ||||||||
L superior frontal pars orbitalis | 47 | −26 | 46 | −2 | 36 | 3.78 | <.001 | 1.51 |
L superior frontal gyrus | ||||||||
L middle frontal gyrus | 46 | −42 | 40 | 28 | 201 | 3.67 | .001 | 1.47 |
L inferior frontal pars triangularis | ||||||||
R pallidum | 48 | 18 | 6 | −4 | 25 | 3.42 | .001 | 1.37 |
R putamen | ||||||||
R inferior frontal pars triangularis | 47 | 40 | 42 | 0 | 25 | 3.24 | .002 | 1.30 |
R inferior frontal pars orbitalis | ||||||||
R middle frontal gyrus | ||||||||
MJ > NU | — | — | — | — | n.s. |
L: left hemisphere; R: right hemisphere. BA: Brodmann area.
Region of interest analysis of bold activation—Retrieval-Motor Control.
Region | MNI coordinates | Cluster size ( |
|
Voxel |
Cohen’s |
||
---|---|---|---|---|---|---|---|
|
|
|
|||||
NU > MJ | |||||||
Hippocampus | — | — | — | — | n.s. | ||
R. parahippocampal gyrus | 24 | −4 | −28 | 26 | 2.20 | .018 | 0.88 |
Anterior cingulate gyrus | 10 | 38 | −4 | 316 | 3.11 | .002 | 1.24 |
Midcingulate gyrus | 12 | −12 | 40 | 563 | 2.97 | .003 | 1.19 |
MJ > NU | |||||||
Hippocampus | — | — | — | — | n.s. | ||
Parahippocampal gyrus | — | — | — | — | n.s. | ||
Cingulate gyrus | — | — | — | — | n.s. |
L: left hemisphere; R: right hemisphere.
Representative SPM images depicting significant activation for the Retrieval-Motor Control contrast for NU > MJ contrast. (a) NU displayed greater activation in the right parahippocampal gyrus relative to the MJ group. (b) NU displayed greater activation in the bilateral anterior cingulate gyrus and bilateral midcingulate gyrus relative to the MJ group. For the MJ > NU contrast, the MJ group showed no regions of greater BOLD activation relative to NU group. All images are in neurological orientation, that is, left = left and right = right; the color bar at the right reflects the suprathreshold value of the SPM (
This pilot study compared current, chronic MJ users and NU during performance of a virtual water maze task of spatial learning and memory. While task performance on learning trials was similar between groups, there was a trend for MJ users to display a lower percentage of total navigation distance within the correct quadrant during the probe trial relative to NU, suggesting a subtle difference in memory retention. During performance on hidden trials during fMRI, although the number of completed trials did not differ between groups, MJ users exhibited significantly longer path lengths and shorter latencies to first movement, which also indicates a deficit in memory retrieval. Performance on visible trials did not differ between groups, however, suggesting that groups had comparable motor abilities. Visuospatial perception (i.e., mental rotation) and environmental spatial ability (i.e., Santa Barbara Sense of Direction Scale) also did not differ between groups, which is consistent with a previous investigation that failed to find differences in orientation skills
The parahippocampal area is necessary for the processing associations between landmark objects and the environment (landmark-based memory), or contextual memory [
In the present study, exploratory analysis of the cingulate gyrus revealed greater activation of the anterior cingulate and the midcingulate gyrus during memory retrieval in the NU group compared to the MJ group. The anterior cingulate cortex is an executive region that plays a critical role in modulation of attention, with reciprocal connections with the amygdala, providing support for its role in arousal and motivation [
Brain activation changes associated with MJ use appear to be task specific, with some studies demonstrating hyperactivation, suggestive of increased recruitment, and other studies demonstrating hypoactivation, suggestive of inefficient neural networking [
At least in the current study, the strategy chosen to solve the water maze could have affected behavioral performance [
There are a number of strengths and weakness associated with this pilot study. A strength of this study is that it is the first investigation of hippocampal brain activation during the performance of a virtual analogue of the well-established Morris water maze task in MJ users, who were well characterized, clinically diagnosed with marijuana abuse, and who did not meet criteria for any other substance abuse disorders. Furthermore, self-report of marijuana use was confirmed by urine drug toxicology screen and results from the SCID and the MWC suggest that in this sample of chronic, heavy MJ smokers, they were not experiencing significant withdrawal symptoms at the time of assessment. In terms of limitations, only a modest number of subjects were examined in this pilot study, which limits generalizability and precludes the ability to examine sex differences within the groups. Performance measures of spatial ability typically have large variability and, therefore, the current investigation should be replicated using a larger sample of subjects. However, despite the modest sample size, significant BOLD activation differences in the parahippocampal and cingulate region were detected. Although differences in age may have been a potential confound, fMRI analyses were corrected for age and differences remained significant between groups. The MJ group also endorsed more alcohol and tobacco use than the NU comparison group, albeit at modest, nonclinical levels. Nonetheless, effects on BOLD activation cannot be ruled out, particularly since nicotinic alpha-7 acetylcholine receptors are highly distributed in the hippocampus [
The length of the period of abstinence from marijuana use has been shown to impact performance on memory tasks and brain function [
In summary, data from this pilot study demonstrate significant differences in BOLD activation in MJ users compared to a NU comparison group during memory retrieval on a spatial navigation task. These data suggest that MJ users utilize neuronal resources in a manner that differs from NU, as suggested by the observed hypoactivation of the/parahippocampal area during navigation, but perhaps also from frontal hypoactivation due to the attentional demands of the task. Further research is warranted to determine the potential mechanism of action by which MJ use may affect brain activation during memory retrieval. Nevertheless, the current findings demonstrate that MJ use exerts a significant effect on neural activity, which is relevant to public health concerns associated with understanding the long-term consequences of chronic marijuana use on brain function in young adults. Indeed, altered brain function in the absence of gross behavioral performance differences may be an early indicator of future long-term consequences associated with continued use, particularly given that relatively short history of MJ use in the current study sample. Early alterations in neuronal function may potentially be related to the later manifestation of MJ-related cognitive impairments, as well as an increased risk for psychiatric conditions [
This work was supported by NIDA Grant no. R03 DA022482 (J. T. Sneider). The authors wish to thank Dr. Derek Hamilton for his technical assistance with the virtual water maze task. In addition, the authors thank Dr. William D. S. Killgore for programming the virtual water maze for use with fMRI as well as his helpful feedback.