Contrastive adaptation in categorization of speech versus song

Prior research on categorization of a vocalization as speech versus song has focused on how stimulus characteristics are mapped onto the boundary between these domains. It remains an open question, therefore, whether the boundary between speech and song can shift based on the context surrounding a stimulus. In Experiment 1 participants categorized stimuli which varied along a continuum from speech to song. These ambiguous stimuli were preceded by adaptor stimuli which were either unambiguous song or speech. Stimuli preceded by a speech adaptor were rated as more song-like, while stimuli preceded by a song adaptor were rated as more speech-like. In Experiment 2 data from a previous study on transformation from speech to song with repetition were re-analyzed to examine contextual effects. After hearing a stimulus which transformed into song, participants were more likely to rate the next stimulus as sounding more speech-like, compared to after hearing a stimulus which continued to sound like speech when repeated. These findings show that when listeners decide whether a stimulus is speech or song, they do not only use acoustic information but also take into account contextual information. Thus, the perceptual boundary between speech and song is not fixed.