Lateralisation of emotions: evidence from pupil size measurement

The way our brain processes emotional stimuli has been studied intensively. One of the main issues still under debate is the laterality of valence processing. Herein, we employed the fact that pupil size increases under conditions of higher mental effort and during emotional processing, in order to contrast three proposed hypotheses in the field. We used different manual response mapping for emotional stimuli: Participants responded with their right hand for positive and with their left hand for negative facial expressions, or vice versa. The hands position was either regular (Experiment 1) or crossed (Experiment 2) in order to rule out a "spatial-valence association" alternate explanation. A third experiment was conducted by employing a passive viewing procedure of peripheral emotional stimuli. In the first two experiments, pupil size was larger when participants responded to positive stimuli with their left hand and to negative with their right hand, compared with the opposite mapping. Results of Experiment 3 strengthen the findings of Experiments 1 and 2. These findings provide significant psychophysiological evidence for the valence hypothesis: Processing positive stimuli involves the left hemisphere, while processing negative stimuli involves the right hemisphere. These results are discussed in relation to contemporary theories of emotion processing.