Measuring Physiological Arousal via SCRs

To quantify within-session changes in physiological arousal, even-related SCRs were measured using two Ag–AgCl electrodes (EL509 Biopac electrodes) filled with isotonic electrode gel (GEL101 Biopac gel) attached to the palmar surface of the hypothenar eminence of the left hand. SCRs were amplified and recorded with a sampling rate of 1000 Hz using the Biopac MP150 system and Acqknowledge software, version 4.2 (BIOPAC Systems, Goleta, CA, USA). Subsequently, a low-pass filter of 0.5 Hz was applied to the skin conductance signal in order to filter out high frequency artefacts. Then, similar to previous studies^{21 (link)–23 (link)}, event-related SCRs were extracted using a local baseline approach, by deducting the mean value of the skin conductance signal 1.2–1.5 s after stimulus onset from the peak value 1.5–4.5 s after stimulus onset. To reduce between subject variability, SCRs were then range-corrected by dividing each response by the largest stimulus elicited SCR for each individual subject^{24 (link)}. For comparability, within-session changes in SCRs was analyzed in the same way as brain responses (see below).