Sentences used in the speech-to-song illusion: Comparisons of acoustic vowel space

In the speech-to-song illusion, certain spoken sentences start sounding like song when repeated several times. This perceptual transformation does not occur for all stimuli, suggesting that acoustic properties of the stimulus may contribute to the illusion. We investigated the contribution of the acoustic properties of vowels to this phenomenon by analyzing the acoustic vowel-space area bounded by formant frequencies of /i/, /æ/, and / ɔ/ in a data set of transforming (illusion) and non-transforming (non-illusion) stimuli. In general, larger vowel-space areas are associated with more acoustic distinctions between vowel categories. We found that the overall vowel space was larger in non-illusion than illusion stimuli. A possible reason for this difference may be that listeners learn to associate large vowel spaces with speech and small vowel spaces with song through exposure to differences between formant frequencies in spoken and sung vowels. We propose that the shifted vowel spaces in which non-illusion sentences may be associated with speech perception thereby activate speech processing circuitry that inhibits the illusion of their transformation into song.