Hemispheric interaction, task complexity, and emotional valence: Evidence from naturalistic images

► Does hemispheric interaction distinguish complexity levels within a task? ► Positive or negative images presented with or without a lateralized distractor. ► Hemispheric interaction costs positive image processing. ► Hemispheric interaction benefits negative image processing. ► Respective cost and benefit mediated by laterality. Two experiments extend the ecological validity of tests of hemispheric interaction in three novel ways. First, we present a broad class of naturalistic stimuli that have not yet been used in tests of hemispheric interaction. Second, we test whether probable differences in complexity within the class of stimuli are supported by outcomes from measures of hemispheric interaction. Third, we use a procedure that presents target stimuli at fixation rather than at a lateralized location in order to more closely approximate normal viewing behavior. Images of positive or negative valence were presented with a lateralized distractor or no distractor at all. Response time and accuracy to determine whether an image was pleasant or unpleasant was measured. Results found that positive images were more quickly and accurately processed by the left hemisphere alone, while negative images were more quickly processed when the hemispheres interacted, and were more accurately processed when the hemispheres interacted than the left hemisphere alone. The findings support the idea that hemispheric interaction costs the performance of a simple task and benefits the performance of a complex task, and that the respective cost or gain is mediated by the pattern of laterality for emotional processing.