Discrimination and streaming of speech sounds based on differences in lateralization in the horizontal and median planes

Understanding speech in complex backgrounds relies on our ability to perceptually organize competing voices into streams. Differences in fundamental frequency (F0) between voiced sounds are known to enhance stream segregation; less is known about the perceptual organization of unvoiced sounds such as fricative consonants. We showed previously that natural consonant-vowel (CV) pairs can be segregated based on F0 differences, despite lacking F0 cues in the fricative part. This study also used CVs, filtered by head-related impulse responses (HRIR) to simulate different positions in the horizontal and median planes. In the median plane, cues are limited to high frequencies, and so should affect fricatives more than vowels. Both discrimination, using a three-interval forced-choice task, and streaming, using a within- or across-stream repetition-detection task, were tested. The CV pairs were either held constant during a trial, or varied randomly. In the constant condition, any difference in spectrum would indicate a change in location along the median plane; in the variable condition, such differences would require listeners to extract spectral regularities from across widely varying spectra of the speech stimuli. Preliminary results suggest that discrimination and streaming are more challenging in the median than in the horizontal plane. [Work supported by NIH grant R01DC012262.]