Acoustic Foundations of the Speech-to-Song Illusion

In the speech-to-song illusion, certain spoken phrases are heard as highly song-like when isolated from context and repeated. This phenomenon occurs to a greater degree for some stimuli than for others, suggesting that particular cues prompt listeners to perceive a spoken phrase as song. Here we investigated the nature of these cues across four experiments. In Experiment 1, participants were asked to rate how song- like spoken phrases were after each of eight repetitions. Initial ratings were correlated with the consistency of an underlying beat and within-syllable pitch slope, while rating change was linked to beat consistency, within-syllable pitch slope, and melodic structure. In Experiment 2, the within-syllable pitch slope of the stimuli was manipulated, and this manipulation changed the extent to which participants heard certain stimuli as more musical than others. In Experiment 3, the extent to which the pitch sequences of a phrase fit a computational model of melodic structure was altered, but this manipulation did not have a significant effect on musicality ratings. In Experiment 4, the consistency of intersyllable timing was manipulated, but this manipulation did not have an effect on the change in perceived musicality after repetition. Our methods provide a new way of studying the causal role of specific acoustic features in the speech-to-song illusion via subtle acoustic manipulations of speech, and show that listeners can rapidly (and implicitly) assess the degree to which nonmusical stimuli contain musical structure.