EMG data were sampled at 1000 Hz and filtered (30–500Hz). Data were rectified and smoothed with a 20-ms time constant. Startle responses were defined by the peak magnitude of the blink reflex that occurred 20–100 ms after stimulus onset, relative to a 50-ms average baseline that immediately preceded the probe onset. Exclusion of trials based on large baseline artifacts resulted in the removal of less than one percent of trials. For each subject, peak eyeblink magnitudes were T-scored (based on all conditions) and subsequently averaged within each condition. T-score transformation was used to attenuate large inter-individual differences in reflex magnitude. Retrospective subjective ratings were averaged across runs. Performance on trials that preceded or followed shocks, and those that preceded or followed probes were analyzed separately from those that did not contain a probe or shock. Accuracy did not differ as a result of shock or probe administration, and thus all trials were included in the final analysis. Trials where participants did not respond before the next letter appeared on the screen (i.e., within 2500 ms) were omitted. However, omissions were very infrequent and they did not occur systematically across conditions. Performance, startle magnitude, and subjective ratings were analyzed with repeated measures analyses of variance (ANOVA), paired-sample t-tests, and Pearson product-moment correlation coefficients (two-tailed tests) in order to assess within-subjects effects. Alpha was set at 0.05 for all statistical tests. Greenhouse-Geisser corrections (GG-ε) were used in all repeated-measures ANOVAs involving factors with more than two levels; uncorrected degrees of freedom, corrected p-values, and ε values are reported in these cases.