The acoustic startle stimuli were generated and amplified using a PC with Psychology Tools’ E-Prime software. The noise burst was presented binaurally using Sennheiser HD500 (Wedemark, Germany) headphones. The acoustic startle stimulus consisted of a 50-ms 100-dB(A) presentation of white noise with instantaneous rise time. A startle probe was presented during 32 of the 48 slides, at a random time of 2.5–5 seconds after slide onset, such that a probe was presented during 8 of the 12 slides within each valence condition. Counterbalancing for slide order was arranged so that, across subjects, each slide would be associated with a startle probe equally often. To enhance unpredictability of the startle presentation, we presented 16 startle probes during ITI, but these were not included in our analyses.
