Do humans and nonhuman animals share the grouping principles of the iambic–trochaic law?

Abstract

The iambic-trochaic law describes humans' tendency to form trochaic groups over sequences varying in pitch or intensity (i.e., the loudest or highest sounds mark group beginnings), and iambic groups over sequences varying in duration (i.e., the longest sounds mark group endings). The extent to which these perceptual biases are shared by humans and nonhuman animals is yet unclear. In Experiment 1, we trained rats to discriminate pitch-alternating sequences of tones from sequences randomly varying in pitch. In Experiment 2, rats were trained to discriminate duration-alternating sequences of tones from sequences randomly varying in duration. We found that nonhuman animals group sequences based on pitch variations as trochees, but they do not group sequences varying in duration as iambs. Importantly, humans grouped the same stimuli following the principles of the iambic-trochaic law (Exp. 3). These results suggest the early emergence of the trochaic rhythmic grouping bias based on pitch, possibly relying on perceptual abilities shared by humans and other mammals, whereas the iambic rhythmic grouping bias based on duration might depend on language experience.