Skin conductance level was measured simultaneously with BOLD response using two disposable carbon fiber electrodes attached between the first and second phalanges of the second and third digits of the left hand (EL509, BIOPAC Systems Inc., Goleta, CA). An MRI-compatible BIOPAC Systems skin conductance module (EDA100C-MRI) was used to sample skin conductance at 1000 Hz and amplify this data. Data was stored for offline analysis using AcqKnowledge 4.2 software (BIOPAC Systems, Inc.). Waveforms were low-pass filtered using a Blackman window with a cutoff of 40Hz and mean-valued smoothed over 100 adjacent data points.
To assess the level of conditioned fear responding separate from unconditioned responses to the US, we included only non-reinforced trials of the CS+ in the analyses. SCR for each CS was calculated by subtracting the mean skin conductance level during the first second of CS presentation from the highest skin conductance level occurring in the 1-7 seconds following CS onset [33 (link), see also 34 ,35 (link) on SCR onset latency]. Our use of non-reinforced CS+ trials permitted examination of SCR after CS offset. Raw SCRs were square root transformed to normalize distributions [32 (link),36 (link)]. SCRs to the CS+ versus CS− were compared using a paired t-test.
To assess the level of conditioned fear responding separate from unconditioned responses to the US, we included only non-reinforced trials of the CS+ in the analyses. SCR for each CS was calculated by subtracting the mean skin conductance level during the first second of CS presentation from the highest skin conductance level occurring in the 1-7 seconds following CS onset [33 (link), see also 34 ,35 (link) on SCR onset latency]. Our use of non-reinforced CS+ trials permitted examination of SCR after CS offset. Raw SCRs were square root transformed to normalize distributions [32 (link),36 (link)]. SCRs to the CS+ versus CS− were compared using a paired t-test.