For nearly all contrasts, clusters were defined as 10 or more contiguous voxels (Xiong, Gao, Lancaster, & Fox, 1995) with z scores above 3.29 (p < .001, two-tailed); the one exception was that in the contrast of negative versus positive high-arousal items, the z-score cutoff was 2.81 (p < .005, two-tailed). Local peaks within each cluster were identified (Mintun, Fox, & Raichle, 1989). For larger clusters that covered adjacent but distinct anatomical regions (e.g., hippocampus, amygdala, and insula), anatomical masks were derived from the anatomical automatic label image packaged with MRIcro (Rorden & Brett, 2000). Customized MATLAB (Mathworks Inc., Natick, MA) codes were used to selectively average raw image intensity values elicited in each of the arousal conditions with regard to the fixation baseline. Raw intensity values were computed over the range of the hemodynamic response function (20 s) within the clusters (one average for each 2.5-s time window).
