Asymmetrical characteristics of emotional responses to pictures and sounds: Evidence from pupillometry

In daily life, our emotions are often elicited by a multimodal environment, mainly visual and auditory stimuli. Therefore, it is crucial to investigate the symmetrical characteristics of emotional responses to pictures and sounds. In this study, we aimed to elucidate the relationship of attentional states to emotional unimodal stimuli (pictures or sounds) and emotional responses by measuring the pupil diameter, which reflects the emotional arousal associated with increased sympathetic activity. Our hypothesis was that the emotional responses to both the image and sound stimuli are symmetrical: emotion might be suppressed when attentional resources are allocated to another stimulus of the same modality as the emotional stimulus—such as a dot presented at the same time as an emotional image, and a beep sound presented at the same time as an emotional sound. In our two experiments, data for 24 participants were analyzed for a pupillary response. In experiment 1, we investigated the relationship of the attentional state with emotional visual stimuli (International Affective Picture System) and emotional responses by using pupillometry. We set four task conditions to modulate the attentional state (emotional task, no task, visual detection task, and auditory detection task). We observed that the velocity of pupillary dilation was faster during the presentation of emotionally arousing pictures compared to that of neutral ones, regardless of the valence of the pictures. Importantly, this effect was not dependent on the task condition. In experiment 2, we investigated the relationship of the attentional state with emotional auditory sounds (International Affective Digitized Sounds) and emotional responses. We observed a trend towards a significant interaction between the stimulus and the task conditions with regard to the velocity of pupillary dilation. In the emotional and auditory detection tasks, the velocity of pupillary dilation was faster with positive and neutral sounds than negative sounds. However, there were no significant differences between the no task and visual detection task conditions. Taken together, the current data reveal that different pupillary responses were elicited to emotional visual and auditory stimuli, at least in the point that there is no attentional effect to emotional responses to visual stimuli, despite both experiments being sufficiently controlled to be of symmetrical experimental design.