Spatial frequencies or emotional effects? A systematic measure of spatial frequencies for IAPS pictures by a discrete wavelet analysis.

The influence of the emotional attributes of a visual scene on early perception processes remains a key question in contemporary affective neurosciences. The International Affective Picture System (IAPS; Lang et al., 2005) was developed to provide a set of standardized stimuli for experimental investigations of emotional processes. These stimuli have been widely used in brain activity investigations to study the influence of valence and/or arousal on visual processing. However, visual perception is strongly influenced by the physical properties of the images shown, especially their spatial frequency content, an aspect that has been unexpectedly neglected at large. In this study, we examine the complete set of IAPS with a discrete wavelet transform to highlight relations between the energy in different spatial frequencies and the emotional features of the pictures. Our results showed that these associations are weak when the complete dataset is considered, but for selected subsets of pictures, clear differences are present in both affective and spatial frequency content. The IAPS remains a powerful tool to explore emotional processing, but we strongly suggest that researchers use subsets of images that are controlled for the energy of their spatial frequencies when investigating emotional influence on visual processing.