Nonemotional features suppress early and enhance late emotional electrocortical responses to negative pictures

Neural processing of emotional pictures is often indexed by two electrocortical responses: the early posterior negativity (EPN) and the late positive potential (LPP). Because emotional pictures vary in nonemotional features (e.g., composition, human content, and spatial frequency), researchers often match pictures on nonemotional features to avoid their confounding effects on the EPN and LPP. However, this matching is tedious and might be unnecessary if the confounding effects could be shown to be negligible. In an item-analysis of mean amplitudes to 400 negative to neutral pictures from the International Affective Picture System (IAPS), nonemotional features had larger effects on EPN than LPP. Picture composition suppressed the relationship between emotion and EPN. Further, data simulations showed that for small picture sets, nonemotional features inflated the correlation between emotion and LPP. Therefore, nonemotional features suppress the EPN and enhance the LPP, particularly so in small picture sets.