Neural representations of vowels and vowel-like sounds

Abstract

Neural representations of speech at every level of the neuraxis have nonlinear features that are not described by spectrograms or linear filter-banks. In this study, recent computational models for populations of cells in the auditory periphery, brainstem, and midbrain were used to explore the implications of vowel features for neural responses. Peripheral neural responses are characterized by strong periodicities, dominant frequencies, that depend upon the distribution of energy across the harmonics and vary across vowels [Fant, 1970]. Strong periodicities related to the fundamental and low-frequency harmonics are observed for peripheral neurons tuned to a wide range of frequencies [Delgutte & Kiang, 1984]. The computational model captures this feature of the physiological responses. These periodicities are interesting because many midbrain neurons are tuned to fluctuations in this frequency range. Single-formant vowels allowed systematic manipulations of the relationship between formant and harmonic frequencies. Larger envelope fluctuations occur when harmonic and formant frequencies are mismatched than when they are aligned. The auditory models suggest that these differences in fluctuation amplitude are significant for responses of higher–order auditory neurons that are tuned to fluctuation rate. The long-term goal is to understand the interrelationship of vowel space and neural responses. Support: NIH-NIDCD-R01-001641(LHC) & NSF-0853929(JMM)