Abstract
The International Affective Picture System (IAPS; Lang, Bradley, & Cuthbert, 2008) is a stimulus database that is frequently used to investigate various aspects of emotional processing. Despite its extensive use, selecting IAPS stimuli for a research project is not usually done according to an established strategy, but rather is tailored to individual studies. Here we propose a standard, replicable method for stimulus selection based on cluster analysis, which re-creates the group structure that is most likely to have produced the valence arousal, and dominance norms associated with the IAPS images. Our method includes screening the database for outliers, identifying a suitable clustering solution, and then extracting the desired number of stimuli on the basis of their level of certainty of belonging to the cluster they were assigned to. Our method preserves statistical power in studies by maximizing the likelihood that the stimuli belong to the cluster structure fitted to them, and by filtering stimuli according to their certainty of cluster membership. In addition, although our cluster-based method is illustrated using the IAPS, it can be extended to other stimulus databases.
Highlights
It is widely accepted that emotion plays a critical role in human psychology and is inextricably entwined with behavior and cognition
We argue that clustering methods constitute a valuable means for creating experimental stimulus groups based on the International Affective Picture System (IAPS) normative data, by ensuring that group formation is optimized according to various measures
Outliers Given the variety of emotional material included within the IAPS database, we employed a form of outlier identification as an objective means to filter out images exceeding the emotional intensity of stimuli expected in daily life, which could prove overly stressful for participants
Summary
It is widely accepted that emotion plays a critical role in human psychology and is inextricably entwined with behavior and cognition. Perhaps the most widely used tool in this pursuit is the International Affective Picture System (IAPS; Lang, Bradley, & Cuthbert, 2008), which consists of 1,182 images and is designed for the experimental study of affective processing. It is based on the PAD model, involving pleasure/valence arousal, and dominance—a threedimensional framework for measuring emotions (Mehrabian, 1996; Russell & Mehrabian, 1977). To exploit the flexibility offered by such a large number of stimuli, several typical approaches for image selection have been used, with some of the most common
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