The study was approved by the Joint Institute of Psychiatry and Maudsley Hospital Ethics Committee. All subjects provided written informed consent. Safety protocols have previously been described (Morrison et al, 2009
A randomized, double-blind, placebo-controlled crossover study in healthy volunteers of the effects of IV THC (1.25
mg) on WM performance, psychopathology, and concurrent EEG activity.
A total of 20 healthy participants were recruited via the King's College e-mail lists. Inclusion criteria were: age between 21 and 50 years, previous cannabis use
1; a score of <15 on the General Health Questionnaire (GHQ-12, Goldberg and Blackwell, 1970
). Exclusion criteria included: current pregnancy, a history of mental illness, drug or alcohol dependence (excluding nicotine), current or past severe medical disorders, or a history of major mental illness in a first-degree family member. Previous alcohol and drug use were recorded and a urine drug screen was carried out. Participants were asked to avoid alcohol and drugs for 24
h before, and to abstain from driving for 24
h after experiments. Sessions were performed at least 2 weeks apart and started between 0900 and 1400 hours. Placebo and THC were administered under double-blind conditions, in a randomized counterbalanced order. Subjects received remuneration for their participation.
Synthetic THC was supplied by THC Pharm GmbH (Frankfurt am Main, Germany) and prepared as 1
mg/ml vials for IV injection, by Bichsel Laboratories (Interlaken, Switzerland) as previously described (Naef et al, 2004
). After dilution in normal saline, preparations for injection contained 1.25% (v/v) ethanol absolute. Sterile cannulae were inserted into veins in the antecubital fossa of both arms: one for administration of pharmaceuticals and one for plasma sampling. THC was administered in 1
ml/min pulses over a period of 5
min (total dose=1.25
mg). Blood samples were taken at baseline and at 1, 5, 15, 60, and 120
min after dosing, for analysis of THC as previously described (Morrison et al, 2009
). The plasma concentration of THC following IV delivery follows a similar time course to that observed following inhalation (Naef et al, 2004
). The dose (1.25
mg) was selected on the basis of previous studies, and roughly corresponds to one standard ‘joint' (D'Souza et al, 2004
; Morrison et al, 2009
; Bhattacharyya et al, 2010
Psychopathological and Cognitive Measures
Psychotic symptoms were rated using the positive and negative syndrome scale (PANSS; Kay et al, 1987
). A completely independent PANSS-trained senior psychiatrist rated 3 × 10-min periods—at baseline, at 30-min post-pharmaceutical, and finally at 90-min post-pharmaceutical. Apart from these 3 periods, the psychiatrist and participant were kept separate to minimize potential bias. The PANSS consists of a positive subscale (7 items: delusions, conceptual disorganization, hallucinations, hyperactivity, grandiosity, suspiciousness/persecution, and hostility), a negative subscale (7 items: blunted affect, emotional withdrawal, poor rapport, passive/apathetic social withdrawal, difficulty in abstract thinking, lack of spontaneity and flow of conversation, stereotyped thinking) and a general psychopathological subscale (14 items). Items are rated from 1–7 (absent–severe), thus the range of scores on the positive subscale is 7–49. It is recognized that there is wide inter-individual variation in PANSS scores following IV THC (D'Souza et al, 2004
; Morrison et al, 2009
). Previously, we found that investigator-rated (PANSS) and participant-rated (Community Assessment of Psychic Experiences) measures of THC-elicited positive symptoms were in agreement (rho=0.62, p
<0.001), and that both measures of positive symptoms were distinct from anxiety (Morrison et al, 2009
). Considered as a group, however, positive symptoms following THC are modest and short-lived. Overall, in earlier studies ~35–50% of healthy participants showed changes of
3–4 points on the PANSS positive subscale under THC conditions (D'Souza et al, 2004
; Morrison et al, 2009
Immediately after the psychiatric assessment at 30-min post-pharmaceutical, participants were administered a standard computerized version of the n-back task. The n-back procedure has been used extensively to measure human WM performance (Owen et al, 2005
). Participants were required to monitor a series of letters and report when the current letter matched the letter n
integers back, where n
=1 (1-back) or n
=2 (2-back), the latter being more difficult. The task requires continuous updating of information stores. In contrast, in the 0-back condition (which does not require manipulation of material in WM), participants responded to the appearance of a prespecified letter. Overall, the task consisted of alternating 30-s blocks of 0-back, 1-back, and 2-back conditions, and lasted 6
min in total. Within blocks, letters were displayed every 2
s for 1
s. Written instructions were read out and participants were given a practice run to demonstrate their understanding of the rules. Subjects were seated ~66
cm from a CRT monitor and instructed to report correct answers as rapidly as possible by pressing a joy-pad button with their R-index finger. Accuracy of responses and reaction times were measured and stored digitally.
All data recording and signal processing were performed in Neuroscan 4.3. EEG activity was recorded from 63 electrode sites using a Quik-Cap system (Compumedics), with a linked mastoid reference and ground at AFz. All impedances were maintained below 10
kΩ. Additional electrodes were placed at the outer canthi to measure horizontal electrooculographic (EOG) activity (monopolar with linked mastoid reference). Vertical EOG was measured using a bipolar recording with electrodes above and below the left eye. The EEG was sampled at 2000
Hz and corrected for eye-blinks using a regression approach. The corrected EEG was epoched, using a 10% Hanning window, into 2048
ms segments (−24 to 2024
ms with respect to each n-back letter stimulus). Epochs were baseline corrected. For each of the three n-back conditions, average power within the frequency bands delta (1–3.5
Hz), theta (3.5–7
Hz), alpha (8–13
Hz), beta (14–25
Hz), low-gamma (30–40
Hz), and high-gamma (60–70
Hz) bands was calculated using Fast Fourier Transform.
For the power analysis, individual electrodes were grouped as left frontal, LF (F1, F3, F5, F7, AF3); right frontal, RF (F2, F4, F6, F8, AF4); left central, LC (C1, C3, FC1, FC3); right central, RC (C2, C4, FC2, FC4); left temporal, LT (FT7, T7, TP7, CP5, P7); right temporal, RT (FT8, T8, TP8, CP6, P8); left occipito-parietal, LOP (O1, PO5, PO3, P3, P1); and right occipito-parietal, ROP (O2, PO6, PO4, P4, P2). The mean value from each group of electrodes was used for statistical analysis. The midline electrodes FZ, CZ, and PZ were analyzed individually.
For the coherence analysis, the data were transformed to bipolar derivations. These derivations consist of pairs of neighboring electrodes at different scalp locations to eliminate the contribution of activity from a common reference to the coherence estimate. Bipolar channels were derived for left and right frontal and parietal regions (F3/F5; PO3/PO5; F4/F6; PO4/PO6). The measure of coherence used is equivalent to a Pearson's correlation performed with complex numbers. It measures the correlation (a value between 0 and 1) of EEG activity in a specific frequency band between two scalp locations. For each of the three n-back conditions, coherence measures were calculated between three prespecified inter-regions, left frontal–left parietal F3/F5-PO3/PO5, right frontal–right parietal F4/F6-PO4/PO6, and left frontal–right frontal F3/F5-F4/F6. Laplacian derivations were also derived from frontal sites over the left hemisphere (LpF3, LpF5, and LpFT7) and right hemisphere (LpF4, LpF6, and LpFT8). In Laplacian derivations, the influence of deep brain sources is minimized by referencing each site to the average of its four surrounding neighboring electrodes (see results).
All analyses were conducted in SPSS 15.0. Distributions were checked for normality using Kolmogorov–Smrnov statistics. Non-parametric tests were used to analyze PANSS scores, because of floor effects under placebo conditions. Thus, differences between THC and placebo sessions were assessed using Friedman's test and relationships between PANSS scores and EEG measures were analyzed using Spearman's rho. Accuracy and speed of performance in the n-back were analyzed by repeated-measures ANOVA, with Task Difficulty (0-back, 1-back, and 2-back) and THC Treatment (placebo, THC) as within-subject factors. Relationships between accuracy/speed of processing in the most challenging (2-back) condition of the n-back and EEG measures were analyzed by Pearson's correlation coefficient.
A repeated-measures ANOVA was used to analyze EEG power in each frequency band (delta, theta, alpha, beta, low-gamma, and high-gamma). Within-subject factors were: Region (LF, RF, LC, RC, LT, RT, LOP, ROP, Fz, Cz, and Pz), Task Difficulty (0-back, 1-back, and 2-back), and THC Treatment (placebo and THC). A repeated-measures ANOVA was used to analyze EEG coherence. Factors (4) were frequency (delta, theta, alpha, beta, low-gamma, and high-gamma), inter-Region (left frontal–left parietal, right frontal–right parietal, and left frontal–right frontal), Task Difficulty (0-back, 1-back, and 2-back) and THC Treatment (placebo and THC). Separate ANOVAs were also conducted for each frequency band (delta to high-gamma), in which factors were inter-Region, Task Difficulty, and THC Treatment. Where sphericity assumptions were violated, Huynh–Feldt corrected statistics were used. Post hoc t-tests were carried out where appropriate.
Correlations between psychological outcomes and EEG measures were Bonferroni corrected to adjust for multiple comparisons. Otherwise significance was accepted at p<0.05. All analyses were two-tailed.