Workshop on the Auditory Basis of Speech Perception Keele University, UK July 15-19, 1996

Representation of Low-Frequency Vowel Formants in the Auditory Nerve

Tatsuya Hirahara (1), Peter Cariani (2), Bertrand Delgutte (2,3)

(1) NTT Basic Research Laboratories, Astugi, Kanagawa, Japan
(2) Eaton Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, MA, USA
(3) Research Laboratories of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA

We have investigated the auditory representation of vowels with low-frequency formants by recording the activity of auditory-nerve fibers in anesthetized cats in response to Japanese /i/-/e/ synthetic-vowel continua. Vowels having either low (150 Hz) or high (350 Hz) fundamental frequency F0 were varied in either first-formant frequency Fl or the level of a "crucial harmonic" near Fl to span the HI-Id continuum. Two different neural representations of the stimulus spectrum in the Fl region were examined: a population rate-place profile and a population interspike interval distribution. Characteristics of both representations depend on F0. When individual harmonics are resolved by the ear, as for high F0s, first formant frequency does not have explicit correlates in either ANF rate-place patterns or interspike interval distributions. Rather, both representations show clear patterns corresponding to individual harmonics, as well as the amplitude ratios of "crucial harmonics" near Fl that determine vowel identity in psyehophysical tests/When harmonics are not resolved, as for low F0s, both rate-place and population-interval profiles of individual harmonics fuse to form broader, single peaks near Fl, providing an explicit neural representation of formant frequency.

Full Paper

Bibliographic reference.  Hirahara, Tatsuya / Cariani, Peter / Delgutte, Bertrand (1996): "Representation of low-frequency vowel formants in the auditory nerve", In ABSP-1996, 83-86.