Acoustic invariance and the perception of place of articulation: a selective adaptation study

Two adaptation experiments were conducted exploring the perception of place of articulation in stop consonants. We hope to determine whether in making a phonetic decision about place of articulation, the perceptual system is sensitive to subtle changes in the fine structure of a CV stimulus (i.e. onset formant frequencies and transition motions) when both the adapting and test stimuli shared the same invariant properties. These invariants are defined in terms of the gross spectral shape at stimulus onset. In the first experiment, the effects of adaptors varying in duration (full CV syllables or shortened CV syllables called onsets), vowel context ([a] versus [i]), onset formant frequencies, and presence or absence of moving transitions were tested on CV [ba da ga] test stimuli. All adaptors contained sufficient information to extract the invariant properties for place of articulation based on spectral shape at consonant release. Results showed that the various onsets did not adapt full CV [ba da ga] test stimuli. In the second experiment, the effects of the onset adaptors were tested on onset CV [ba da ga] test stimuli. Results showed that onsets can adapt an onset place of articulation continuum in a manner similar to place of articulation adaptation using full CV syllables. Further, the fine structure of the stimuli significantly affected the adaptation results. Finally, vowel contingency effects seem to reflect differences in onset frequencies of the consonants in CV syllables rather than the steady-state frequencies of the following vowels.