Affective auditory stimuli: Characterization of the International Affective Digitized Sounds (IADS) by discrete emotional categories

primarily in the visual realm. In recent years though, there have been a growing number of experiments using audio stimuli as a means to study emotion, both as unisensory stimuli and as part of multisensory stimuli. Because this is a more recent trend than using a vision-only approach, there is a significant gap between the availability of wellcharacterized audio and visual stimuli in the scientific community. Many visual stimulus sets including the International Affective Picture Set (IAPS; Lang, Bradley, & Cuthbert, 2005), the Affective Norms for English Words (ANEW; Bradley & Lang, 1999a), and the Pictures of Facial Affect (POFA; Ekman & Friesen, 1975) have all been characterized according to both of the two predominant theories used to describe emotion, the dimensional and discrete category approaches (Mikels et al., 2005; Stevenson & James, 2007; Stevenson, Mikels, & James, 2007). This is not the case with auditory stimuli. A few auditory stimuli sets have been standardized according to the dimensional theories of emotion independent of emotional category. One of these is the International Affective Digitized Sounds (IADS), a set of 111 standardized, emotionally evocative sounds that cover a wide range of semantic categories. This system was created with three goals in mind: better experimental control of emotional stimuli, increasing the ability of cross-study comparisons of results, and increased ability to directly replicate studies (Bradley & Lang, 1999b). To achieve these goals, the IADS were originally normalized using the Self-Assessment Manikin (SAM), a scale that assesses valence, arousal, and dominance as dimensions describing emotion (Bradley & Lang, 1994). The dimensional theories of emotion propose that affective meaning can be well characterized by a small number of dimensions. Dimensions are chosen on their ability to statistically characterize subjective emotional ratings with the least number of dimensions possible (Bradley & Lang, 1994). These dimensions generally include one bipolar or two unipolar dimensions that represent positivity and negativity, and have been labeled in various ways, such as valence or pleasure. Also usually included is a dimension that captures intensity, arousal, or energy level. The IADS, as well as numerous other sound collections, have been used successfully with characterizations of valence and arousal, two dimensions that have been shown to make dimensional theories of affect most powerful (Mehrabian & Russell, 1974; Smith & Ellsworth, 1985; Yik, Russell, & Barrett, 1999), as well as dominance. Neural activity has been identified as responding preferentially with positive or negative sounds as opposed to neutral sounds (Frey, Kostopoulos, & Petrides, 2000; Royet et al., 2000) as well as evoking different psychophysiological responses, including heart rate, skin conductance, and respiration rate (Gomez & Danuser, 2004). Differences in these responses can be seen in psychopathic individuals who have abnormal reactions to both positive and negative emotional sounds