Neural coding of sound envelope in reverberant environments

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 153-159).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 broadband noise stimuli presented in simulated anechoic and reverberant environments. Consistent with the attenuation of amplitude modulation (AM) in the stimulus, both rate and temporal coding of AM were degraded in IC neurons. However, in most neurons, the degradation in temporal coding was smaller than the degradation in the stimulus. In many neurons, this compensation could be accounted for by the modulation input-output function (MIOF), which describes the nonlinear transformation of modulation depth from the sound stimulus into the neural response. However, in a subset of neurons, the MIOF underestimated the strength of temporal coding, suggesting that reverberant stimuli may have a coding advantage over anechoic stimuli with the same modulation depth. Additional experiments suggest that interaural envelope disparities and interaural decorrelation introduced by reverberation may partly explain this coding advantage. In another set of experiments, we tested the hypothesis that temporal coding of AM is not static, but depends dynamically on the modulation depth statistics of preceding stimulation. In a subset of neurons, preceding stimulation history significantly altered the MIOF. On average, temporal coding of modulation frequency was more robust in conditions when low modulation depths predominate, as in reverberant environments. Overall, our results suggest that the auditory system may possess mechanisms for reverberation compensation, and point to an important role of binaural and dynamic neural processes for robust coding of AM in reverberant environments.by Michaël C. C. Slama.Ph.D

Middle ear pressure gain and cochlear input impedance in the chinchilla

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 35-37).Measurements of middle ear conducted sound pressure in the cochlear vestibule PV have been performed in only a few individuals from a few mammalian species. Simultaneous measurements of sound-induced stapes velocity VS are even more rare. We report simultaneous measurements of VS and PV in chinchillas. The VS measurements were performed using single-beam laser-Doppler vibrometry; PV was measured with fiber optic pressure sensors like those described by Olson [JASA 1998; 103: 3445-63]. Accurate in-vivo measurements of PV are limited by anatomical access to the vestibule, the relative sizes of the sensor and vestibule, and damage to the cochlea when inserting the measurement device. The small size (170 [mu]m diameter) of the fiber-optic pressure sensors helps overcome these three constraints. PV and VS were measured in six animals, and the middle ear pressure gain (ratio of PV to the sound pressure in the ear canal) and the cochlear input impedance (ratio of PV to the product of VS and area of the footplate) computed. Our measurements of middle ear pressure gain are similar to published data in the chinchilla at stimulus frequencies of 500 Hz to 3 kHz, but are different at other frequencies. Our measurements of cochlear input impedance differ somewhat from previous estimates in the chinchilla and show a resistive input impedance up to at least 10 kHz. To our knowledge, these are the first direct measurements of this impedance in the chinchilla. The acoustic power entering the cochlea was computed based on our measurements of input impedance. This quantity was a good predictor for the audiogram at frequencies below 1 kHz.by Michaël C.C. Slama.S.M