540 Pain screening, weighing and the calibration procedure, all test days were similar. After each laser stimulus, subjects reported pain intensity on a NRS where 0 represented “no sensation”, 4 represented “barely painful” (i.e. pain threshold) and 10 “unbearable pain” (Brown et al. 2008; GarciaLarrea et al. 1997; Leandri et al. 2006; Watson et al. 2007). Arousal and stress Subjective stress and arousal were reported 30 s before the pre- and posttest by four adjective pairs from the Short Adjective Check List (SACL; O'Neill and Parrott 1992) in Norwegian translation. Subjective stress was measured by two NRSs anchored by the two adjective pairs tense– relaxed and nervous–calm, where a score of zero indicated completely relaxation/calmness and a score of 10 indicated maximum tension/nervousness. Subjective arousal was measured by two NRSs anchored by the two adjective pairs energetic–tired and alert–drowsy, where a score of zero indicated completely tiredness/drowsiness and score of 10 indicated maximum alertness/energetic. Stress and arousal were expressed as the mean of the two NRSs used for reporting of subjective stress and arousal. The items from the SACL were chosen for their high factor loadings on the stress and arousal factors on the SACL, similar to earlier research on stress and arousal (O'Neill and Parrott 1992; Parrott 1993). The SACL stress scale was converted to NRS scales. Expectations Reporting of expectations before the posttest could bias pain report during the posttest, so expectancies were measured after the posttest. Two types of expectations were recorded: certainty of pain relief and expected pain reduction. On the days the subjects were told they received the drug, certainty of pain relief was measured by asking “Answer as exact as possible what your expectations were just before the posttest. On a scale from 0–100 were 0 means ‘not certain at all’ and 100 means ‘completely certain’, how certain were you that the pain relieving drug you received today would be efficient in relieving the pain” (Arntz and Lousberg 1990). Certainty/uncertainty about pain intensity has been found to mediate top down influences on pain perception (Arntz and Lousberg 1990; Brown et al. 2008). Expected pain reduction was measured on all days by asking: “Did you expect the pain to decrease in the posttest compared to the pretest? YES/NO. If you were expecting a Psychopharmacology (2011) 215:537–548 decrease: on a scale from 0–100 were 0 means ‘no reduction’ and 100 means ‘total reduction’, how much reduction in pain did you expect?”. If subjects answered “NO” to the first question, a value of zero was entered. Recording and analysis of laser-evoked potentials Electroencephalographic (EEG) measurements were done from 32 electrodes (Fp1, Fp2, F7, F3, Fz, F4, F8, FC5, FC1, FC2, FC6, T7, C3, Cz, C4, T8, TP9, CP5, CP1, CP2, CP6, TP10, P7, P3, Pz, P4, P8, PO9, O1, Oz, O2, PO10) in accordance with the international 10–20 system. An electrode cap with isolated electrodes (actiCap, Brain Products, Germany) was used. Conductive gel (SuperVisc, EasyCap, Herrshing, Germany) was applied to all electrode sites by a syringe with a blunt needle. Impedance was below 5 kΩ in all channels. Common reference was employed. The evoked potentials were recorded on a Quick Amp EEG system (Brain Products, Germany) and analysed off-line with Analyzer 2.0 software (Brain Products, Germany). Data were sampled within a 0.3- and 70-Hz bandpass with a sampling rate of 500 Hz. Electrooculog- raphy was measured from vertical eye electrodes to control for artefacts related to blinks and eye movements. The raw files were re-referenced using linked ear mastoids as the reference. A notch filter set to 50 Hz was applied off-line. The data were segmented into epochs of 1,250 ms including the baseline period of 250 ms. Ocular correction was performed by an algorithm in Vision Analyser 2.0 (Gratton et al. 1983). Baseline correction was done using the last 250 ms prior to the laser stimulus as the baseline, before averaging the 20 pretest and 20 posttest stimuli. Automatic peak detection was applied using 200–280 ms as the latency window for the N2 component and 300–380 ms for the P2 component. Finally, the first author, without knowing to which experimental condition the files belonged, did a visual inspection of the peaks detected in order to rule out any errors. Only data from the Cz (vertex) electrode were reported. Design and statistics The statistical analysis was performed using repeated measures ANOVA. The design was a two information (got drug, got no drug) by two drug (caffeine, no caffeine) by two gender mixed design, with the first two factors as within-subjects factors and the last factor as a between subjects factor. Posttest minus pretest difference scores were computed for pain intensity, LEPs, arousal and stress and used as outcome measures. Significant interactions between drug and information were followed up by planned comparisons between the control and all other conditions and between the caffeine and active placebo conditions.