Prior to the fMRI session, participants practiced the tasks outside of the MRI scanner for about 30 min. Once in the scanner, subjects were instructed to refrain from moving even if they felt startled by the pictures. Compliance with this request was high, and no data were excluded because of excessive head motion (defined as maximal absolute displacement larger than 1.7 mm). During scanning, participants viewed a series of the IAPS pictures and pressed a button using their right index finger when an image was displayed. Participants were instructed to view each picture even if the picture was aversive. Each picture appeared for 1 s and was followed by a fixation cross; the intertrial interval was 2.5 s. The onset of each picture was randomly sampled from one of three jitters (100, 300, or 500 ms), to minimize anticipation of the stimuli. The 250 trials (200 images and 50 visual-fixation trials that did not require a response) were randomly mixed according to the scripts for stimulus timing simulation using programs of the AFNI (Analysis of Functional Neuro-Images) software package (http://afni.nimh.nih.gov).
