Effects of vowel coloration on perceived directional offsets between single-syllable speech sounds

Abstract

The ability of normal hearing listeners to spatially localize single-syllable speech sounds was examined via a two-interval, two alternative forced choice (2AFC) spatial offset discrimination task. To examine directional biases in horizontal sound localization, this task required listeners to observe the direction of an unseen second talker’s voice immediately after hearing a first talker’s voice in an alternative direction. Listeners reported only the relative frontward versus rearward spatial offset between the two voices heard during each trial. Headphone-based virtual acoustic simulation enabled blind listening tests that were completed without feedback to the listener, revealing the ease with which listeners judged the relative spatial offset of the second speech sound, although those judgments were strongly biased by differences in the source spectra. The single-syllable speech-sound stimuli were processed using individually measured head-related transfer functions (HRTFs) of 15 listeners for whom individually measured earphone transfer functions (ETFs) were also employed. While analysis of the 2AFC response data via receiver operating characteristic (ROC) curves showed a generally high level of directional sensitivity, systematic biases were observed that could be explained via the differences in vowel coloration between the first and second source presented during each trial.