Psychopharmacology (2012) 219:981–990 Table 1 Demographic characteristics and patterns of drug use Subject characteristics (n=11) Age IQ Nicotine use last year (cigarettes/week) Alcohol use last year (units/week) Cannabis use last year (no. occasions) Illicit use lifetime (no. occasions) Mean±SD 21.7±2.3 104.5±6.0 3.0±8.4 16.7±8.7 17.9±13.4 1.3±1.6 Range 18–27 94–111 0–28 2–40 5–52 0–4 familiarized with the scan protocol in a mock scanner to reduce stress effects on the following test days. The actual study consisted of two test days, separated by at least 2 weeks to allow for complete clearance of drugs. A standard breakfast or lunch was provided at the beginning of each test day, to ensure equal states of metabolism on both test days. Subjects were instructed not to use cannabis from 2 weeks before the first test day until study completion. Clearance of drugs was tested by means of a urine sample at the beginning of each test day. Additionally, no alcohol was permitted in the 48 h preceding a test day, and subjects needed to refrain from smoking, eating, and drinking during 4 h preceding each session. Drug administration On test days, subjects received THC or placebo by means of a Volcano® vaporizer (Van Hell et al. 2011; Zuurman et al. 2008) at four time points. The first dose consisted of 6 mg THC or placebo. To maintain average stable levels of intoxicating effects throughout the experiment, upload dosages of 1 mg were used, 30 min apart, as predicted from previously described dose–effect relationships (Strougo et al. 2008). After the first three administrations of THC or placebo, subjects performed several cognitive tasks during which fMRI scans were obtained. After the last dose of THC or placebo, a battery of neuropsychological tasks was performed (see also Van Hell et al. 2011). Here, results are reported for the monetary incentive delay (MID) task, results of other assessments are reported elsewhere. Drug effects Venous blood samples were collected to determine plasma concentrations of THC and its two most important metabo- lites, 11-OH-THC and 11-nor-9-carboxy-THC. Blood samples were processed according to Zuurman et al. (2008). Subjective effects were measured at baseline and before and after each task and throughout the test day using self- reported visual analogue scales (VAS) (Bond and Lader 1974; Bowdle et al. 1998). Heart rate and respiratory 983 function were monitored continuously during scanning. Heart rate was assessed by measuring the electrocardiogram using four electrodes attached to the subject's chest, and respiratory function was assessed by measuring the expan- sion of a respiration band around the subject's abdomen. Task The MID task was based on the paradigm described by Knutson et al. (2001a) (see also Van Hell et al. (2010) and Fig. 1). The task consisted of 48 trials, each lasting 8 s on average (range 6–12 s). At the beginning of each trial, a cue was presented signaling a trial in which a reward could be won (“reward trial”, a circle) or a trial that was never rewarded (“neutral trial”, a square). After the cue, a target was presented for a very short time, and subjects had to press a button before the target disappeared. After each reward trial, feedback was given which indicated a successful (“hit”) or unsuccessful response (“miss”) with the amount of money won (respec- tively “2 euros” or “0 euro”), as well as the total reward. Anticipation time (the time between cue and target) and inter trial interval were varied (4.3–10.3 s; mean 6.6 s, and 0–30 s; mean 4.2 s, respectively). Prior to the experiment, ten practice trials were presented to familiarize subjects with the task. From the practice data, the shortest reaction time to a target was used to determine an individual threshold. Half of the targets in reward trials were presented 200 ms longer than the individual threshold, and half of the trials 150 ms shorter to ensure a close to equal number of correct and incorrect responses, to achieve optimal statistical power, as well as a similar total monetary reward for all subjects. Neutral trials were presented with an identical distribution as reward trials. Scanning parameters Image acquisition was performed on a Philips Achieva 3.0 Tesla MR scanner with a Quasar dual gradient set. Fig. 1 MID paradigm (see also Van Hell et al. (2010))