Paper on the iambic-trochaic law in Psychological Review:
You can try out a version of the experiment at the prosodylab field station!
In a sequence of otherwise equal sounds, listeners tend to hear a series of trochees (groups of two sounds with an initial beat) when every other sound is louder; they tend to hear a series of iambs (groups of two sounds with a final beat) when every other sound is longer. The article presents evidence that this so-called “Iambic–Trochaic Law” (ITL) is a consequence of the way listeners parse the signal along two orthogonal dimensions, grouping (Which tone is first/last?) and prominence (Which tone is prominent?). A production experiment shows that in speech, intensity and duration correlate when encoding prominence, but anticorrelate when encoding grouping. A model of the production data shows that the ITL emerges from the cue distribution based on a listener’s predicted decisions about prominence and grouping respectively. This, and further predictions derived from the model, are then tested in speech and tone perception. The perception results provide evidence that intensity and duration are excellent cues for grouping and prominence, but poor cues for the distinction between iamb and trochee per se. Overall, the findings illustrate how the ITL derives from the way listeners recover two orthogonal perceptual dimensions, grouping and prominence, from a single acoustic stream.