Auditory-nerve fiber representation of temporal cues to voicing in word-medial stop consonants

Abstract

The length of the interval between the onset of consonant closure and the onset of voicing in a following vowel is a temporal cue that may distinguish between consonants /d/ and /t/ in word-medial environments; this interval has been called the "consonant duration" [V. W. Zue and M. Laferriere, J. Acoust. Soc. Am. 66, 1039-1050 (1979); S. Davis and W. V. Summers, J. Phon. 17, 339-353 (1989)]. The representation of this cue in the discharge patterns of chinchilla auditory-nerve fibers was measured. The two-syllable utterances /ida/, /ita/, /uda/, and /uta/ were recorded by one male and one female talker. The onset of consonant closure produced discharge rate decreases in nearly all neurons. Either the release of closure or the onset of voicing for the second vowel could elicit an increase in discharge rate. The latencies of these discharge rate changes varied across populations of neurons. A neural measure of consonant duration was extracted from the pattern of latencies. The "encoded duration" was longer for utterances with a medial /t/ than for utterances with a medial /d/. For each utterance the encoded duration increased with increases in characteristic frequency. The variability of the encoded duration measure was small enough to preserve the distinction between utterances with different word-medial consonants. The variability of the encoded duration was large, relative to the acoustic differences between utterances that included the same medial consonant. This pattern of variability could contribute to the formation of perceptual categories by reducing the audibility of within-category acoustic differences.