The acoustic startle stimulus consisted of a 50-ms 100-dB presentation of white noise with instantaneous rise time. The stimuli were generated and amplified by Pentium III PC using Psychology Tools’ (Pittsburgh, PA) Eprime software. The noise burst was presented binaurally using Sennheiser HD500 (Wedemark, Germany) headphones. A startle probe was presented during 16 of the 24 slides in each block, at a random time of 2.5-5 s after slide onset, such that a probe was presented during 4 of the 6 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. In addition, to enhance unpredictability of the startle presentation, 8 startle probes were presented during interslide intervals in each block.
