BME Seminar Series: Laurel Carney, Ph.D.
Coding of Vowels in the Auditory Midbrain
From a public health perspective, speech is the sound that we must understand in order to effect significant improvement in the lives of people with communication disorders. The neural code for speech over the wide range of sound and noise levels experienced daily has been elusive due to strong nonlinearities of the inner ear and central auditory neurons. In collaboration with a phonetician, we are studying neural responses to the acoustic features, and their combinations, that are most critical for differentiating speech sounds. Our current work focuses on the neural coding and processing of vowels. We have developed a novel, testable hypothesis for the robust representation in the midbrain of two salient features of vowels: fundamental frequency (F0), or voice pitch, and formant frequencies, the spectral peaks that differentiate vowels. This hypothesis takes into account the facts that i) in addition to having a best frequency (BF), most midbrain neurons are tuned for periodicities in the range of voice pitch, and ii) the strength of the periodicities in the response of the auditory periphery changes systematically depending upon the relation between BF and formant frequency. Because the representation proposed by our vowel-coding hypothesis, and supported by our preliminary data, is fundamentally different from classical models for neural representations of speech sounds, the signal-processing strategies to restore it will differ fundamentally from existing strategies.