Pitch representations in the auditory nerve : two concurrent complex tones

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 39-43).Pitch differences between concurrent sounds are important cues used in auditory scene analysis and also play a major role in music perception. To investigate the neural codes underlying these perceptual abilities, we recorded from single fibers in the cat auditory nerve in response to two concurrent harmonic complex tones with missing fundamentals and equal-amplitude harmonics. We investigated the efficacy of rate-place and interspike-interval codes to represent both pitches of the two tones, which had fundamental frequency (FO) ratios of 15/14 or 11/9. We relied on the principle of scaling invariance in cochlear mechanics to infer the spatiotemporal response patterns to a given stimulus from a series of measurements made in a single fiber as a function of FO. Templates created by a peripheral auditory model were used to estimate the FOs of double complex tones from the inferred distribution of firing rate along the tonotopic axis. This rate-place representation was accurate for FOs above about 900 Hz. Surprisingly, rate-based FO estimates were accurate even when the two-tone mixture contained no resolved harmonics, so long as some harmonics were resolved prior to mixing. We also extended methods used previously for single complex tones to estimate the FOs of concurrent complex tones from interspike-interval distributions pooled over the tonotopic axis. The interval-based representation was accurate for FOs below about 900 Hz, where the two-tone mixture contained no resolved harmonics. Together, the rate-place and interval-based representations allow accurate pitch perception for concurrent sounds over the entire range of human voice and cat vocalizations.by Erik Larsen.S.M

Neural Coding of Sound Envelope in Reverberant Environments

Speech reception depends critically on temporal modulations in the amplitude envelope of the speech signal. Reverberation encountered in everyday environments can substantially attenuate these modulations. To assess the effect of reverberation on the neural coding of amplitude envelope, we recorded from single units in the inferior colliculus (IC) of unanesthetized rabbit using sinusoidally amplitude modulated (AM) broadband noise stimuli presented in simulated anechoic and reverberant environments. Although reverberation degraded both rate and temporal coding of AM in IC neurons, in most neurons, the degradation in temporal coding was smaller than the AM attenuation in the stimulus. This compensation could largely be accounted for by the compressive shape of the modulation input–output function (MIOF), which describes the nonlinear transformation of modulation depth from acoustic stimuli into neural responses. Additionally, in a subset of neurons, the temporal coding of AM was better for reverberant stimuli than for anechoic stimuli having the same modulation depth at the ear. Using hybrid anechoic stimuli that selectively possess certain properties of reverberant sounds, we show that this reverberant advantage is not caused by envelope distortion, static interaural decorrelation, or spectral coloration. Overall, our results suggest that the auditory system may possess dual mechanisms that make the coding of amplitude envelope relatively robust in reverberation: one general mechanism operating for all stimuli with small modulation depths, and another mechanism dependent on very specific properties of reverberant stimuli, possibly the periodic fluctuations in interaural correlation at the modulation frequency.National Institutes of Health (U.S.) (Grant R01DC002258)National Institutes of Health (U.S.) (Grant P30DC0005209)Paul and Daisy Soros Fellowships for New American

Speech Communication

Contains research objectives and summary of research on four research projects.National Institutes of Health (Grant 5 RO1 NS04332-14)National Institutes of Health (Grant 5 T32 NS07040-02)National Institutes of Health (Fellowship 1 F22 NS00796-01)National Institutes of Health (Grant 1 ROI NS13028-01)National Institutes of Health (Grant 5 T3Z NS07040-02)National Institutes of Health (Fellowship 1 F22 MH58258-02)U. S. Army- Maryland Procurement Office (Contract MDA904-76-C-0331

Speech Communication

Contains reports on three research projects.National Institutes of Health (Grant 2 ROI NS04332)National Institutes of Health (Training Grant 5 T32 NS07040)C. J. LeBel FellowshipsNational Institutes of Health (Grant 5 RO1 NS13028)National Science Foundation (Grant BNS76-80278)National Science Foundation (Grant BNS77-26871

Speech Communication

Contains reports on two research projects.National Institutes of Health (Grant 2 ROl1 NS04332)National Institutes of Health (Training Grant 5 T32 NS07040)C.J. LeBel FellowshipsNational Science Foundation (Grant BNS77-26871

Signal Transmission in the Auditory System

Contains table of contents for Section 3 and reports on four research projects.National Institutes of Health Grant R01 DC00194National Institutes of Health Grant P01 DC00119National Science Foundation Grant IBN 96-04642W.M. Keck Foundation Career Development ProfessorshipNational Institutes of Health Grant R01 DC00238Thomas and Gerd Perkins Award ProfessorshipAlfred P Sloan Foundation Instrumentation GrantJohn F. and Virginia B. Taplin Award in Health Sciences and TechnologyNational Institutes of Health/National Institute of Deafness and Other Communication DisordersNational Institutes of Health/National Institute of Deafness and Other Communication Disorders Grant PO1 DC0011

Signal Transmission in the Auditory System

Contains table of contents for Section 3, an introduction, and reports on seven research projects.National Institutes of Health Grant 5 R01 DC00194National Institutes of Health Grant P01 DC00119National Institutes of Health Grant F32 DC00073National Institutes of Health Grant 5 R01 DC00473National Institutes of Health Grant 2 R01 DC00238National Institutes of Health Grant 2 R01 DC00235National Institutes of Health Grant 5 P01 DC00361National Institutes of Health Grant T32 DC00006Whitaker Health Sciences Fun