Hemispheric Asymmetry in Processing Pleasant and Unpleasant Stimuli - Emotional Valence Processing

Abstract

Differences in electrophysiological response between pleasant and unpleasant emotional stimuli were examined in this study. Recording of event related potentials (ERP) was organized in such a way that the participants, 24 female students from the University of Zagreb, were first exposed to a priming stimulus (emotional pictures) followed by a target stimulus (emotional pictures). Adjusted pictures from the IAPS database (The International Affective Picture System) were used both for primes and target stimuli. Intensity and dominance of the pictures were controlled by using only pictures with medium values, while emotional valence was manipulated through three categories of pictures, unpleasant, neutral and pleasant. In order to detect differences in processing different emotional valence stimuli, measurement of the ERP signal was carried out with 180 emotional pictures (60 of each emotional valence) combined with three sets of priming stimuli, which made a total of 540 pairs of presented stimuli. The signal was recorded over frontal electrodes, and the results were analysed in the time window of 300-400 ms. The goal of the study was to test hemispheric asymmetry in processing emotional versus neutral stimuli by controlling gaze to emotional content of the picture, intensity and dominance of the emotional pictures as well by enhancing possible differences in processing emotional stimuli by introducing preceding priming emotional stimuli. The priming effect was not found to be a significant contributor to the processing of emotional target stimuli. A difference was found on the left hemisphere between processing pleasant and unpleasant target stimuli, as well as a difference between processing unpleasant and neutral target stimuli. Regardless of priming, a difference was found on the left and the right hemisphere between processing pleasant and unpleasant target stimuli, whereas a difference between processing unpleasant and neutral stimuli was found on the left hemisphere as well as difference between processing pleasant and neutral target stimuli on the right hemisphere. Keywords: IAPS (International Affective Picture System), ERP (Event Related Potential), emotional valence, frontal asymmetry