Research

Research

Dr. Zeng's research focuses on the encoding of sensory information in normal and impaired auditory systems. We try to understand how simple attributes of sounds, such as loudness and pitch, as well as how complex sounds, such as speech and music, are encoded in the peripheral and central auditory systems. We address these questions by integrating information obtained from normal-hearing listeners, hearing-impaired listeners, and cochlear-implant listeners whose hearing is partially restored by electric stimulation of the auditory nerve. Our previous studies have demonstrated that investigating the performance of listeners with impaired hearing can greatly deepen our understanding of intensity coding (Zeng and Shannon, 1994; Zeng and Shannon, 1995; Zeng, Galvin, and Zhang, 1998) and speech recognition (Shannon et al., 1995; Zeng and Galvin, 1999). We certainly hope that our better understanding of the auditory information encoding can lead us to develop more efficient and cost-effective hearing aids and cochlear implants for hearing-impaired people (Zhang and Zeng, 1997; Wilson et al., 1998).

Dr. Zeng’s current research addresses hearing in noise or the "cocktail party" problem: How does the normal system solve this problem and how can we help hearing-impaired listeners improve their performance in noise? Dr. Zeng is involved in the following projects:

(1) Signal processing in audition (5R01DC002267)
As the only medical device that can restore hearing in deaf people, the cochlear implant has produced good speech recognition in quiet. However, the current implants are seriously limited in speech recognition in noise and in music perception. The long-term objectives of this research program are to understand signal processing in the normal auditory system and to restore functional hearing via auditory prostheses in hearing-impaired persons. Recent work from our laboratory and others has shown that pitch, temporal fine structure, and dynamic acoustic cues are crucial to improve realistic listening performance, but are not adequately encoded in current cochlear implants. Our working hypothesis is that extraction and encoding of these important cues will lead to an overall improvement in cochlear implant performance. We propose three novel methods to test this hypothesis. The three Specific Aims address each of these novel methods: (1) Co-vary stimulation rate and position to encode pitch; (2) Adapt modern vocoder algorithms to encode temporal fine structure; and (3) Use biologically-inspired signal processing to encode dynamic acoustic cues. Our multidisciplinary approach integrates psychophysical, speech coding, and signal processing techniques. A unique feature of this approach is that all algorithms are developed based on rigorous psychophysical and simulation measures, and will be evaluated and perfected in actual implant users with real-time implementations. Successful completion of the proposed research should yield results of high theoretical and practical significance. It will likely advance scientific knowledge on the centuries-old but still unresolved pitch coding question (Aim 1), bridge the technological gap between relatively rudimentary cochlear implants and modern telecommunication (Aim 2), and inspire translational work from basic research to clinical problems (Aim 3).

(2) Interactions between Acoustic and Electric Stimulation (1R01DC008858)
Cochlear implants have restored partial hearing to more than 100,000 hearing-impaired people worldwide. As cochlear implant performance continues to improve, more and more patients with residual acoustic hearing are eligible for, and some have received, a cochlear implant. We have identified three groups of implant patients who possess significant residual acoustic hearing: those who have a hearing aid on the contralateral side, those who use combined electro-acoustic stimulation in the same ear, and those who use the implant to suppress tinnitus and may have normal hearing contralaterally. We propose to systematically study the interactions between acoustic and electric hearing in these subjects. Our three specific aims are: 1. To construct an accurate pitch map in electric hearing 2. To measure psychophysical interactions between acoustic and electric stimulation 3. To probe the mechanisms and optimize the performance of bimodal hearing An accurate pitch map will allow us to reconstruct "electric harmonics" and develop authentic acoustic simulations of the cochlear implant. A systematic characterization of the psychophysical interactions is necessary to understand the mechanisms and maximize the benefit of bimodal hearing. The proposed work is of high clinical relevance because it may help overcome two of the most significant shortcomings of the current cochlear implants, namely, music perception and speech recognition with background noise, particularly when the noise is a competing voice.

(3) Hearing deficits due to auditory neuropathy (5R01DC002618; PI: Arnold Starr)
This proposal will study a recently recognized form of hearing disorder called auditory neuropathy (AN) that is due to a disorder of auditory nerve functions in the presence of normal cochlear receptor activities. AN subjects have normal measures of cochlear outer hair cell activities but abnormal measures of the central auditory pathway functions beginning with auditory nerve. The hearing disorder typically affects speech comprehension out-of-proportion to the pure tone loss, particularly speech recognition in noise. AN is not rare and accounts for 10 percent of newborns identified as having hearing loss and also develops in childhood and in adults. In adults the disorder of auditory nerve is commonly associated with a peripheral neuropathy. In addition, AN and sensory hearing loss occur together. Loss of neural synchrony and decreased auditory nerve input are proposed as cardinal mechanisms underlying the hearing disorder. The sites of abnormality along auditory nerve have recently been identified at auditory ganglion cells and their axons (Type I AN) or distally in the auditory periphery where auditory nerve terminals, inner hair cells, and the synapses between inner hair cells with nerve terminals are in close approximation (Type II AN). Our long-term goals are to understand the underlying mechanisms of AN and the accompanying hearing impairments to provide a scientific basis for alleviating the hearing deficit in AN subjects. We propose four experiments using both psychophysical and electrophysiological techniques that will help us achieve these goals. We will characterize (1) the reorganization of central auditory pathway occurring in response to altered auditory nerve input; (2) the contributions of dysynchronization and decreased auditory input as pathophysiological mechanisms underlying AN; (3) the objective physiological and psychophysical measures of improved auditory temporal processes in cochlear implanted AN subjects, and (4) the improvement of speech recognition using new auditory processing techniques. The results of our studies could have major impact on the diagnosis, classification, understanding, and treatment of auditory neuropathy.

(4) Directional Hearing in Complex Auditory Environments (5R01DC003083; PI: Ruth Litovsky)
Humans spend a majority of their time in auditory environments that are complex and reverberant. An important and ongoing task for the auditory system is the segregation of target signals from interfering sounds and the suppression of echoes. The binaural system is known to be important in accomplishing these goals. The proposed research will investigate directional hearing in multi-source and reverberant acoustic environments. We will study sound localization, suppression of echoes, and speech intelligibility, in human adults with normal hearing, and with bilaterally-implanted cochlear implant users. Aim 1 will investigate the precedence effect (PE) using more everyday paradigms than those previously employed, with speech stimuli and multiple-echo patterns that vary in number, directional properties, intensity and spectral content. In Aim 2 we will investigate the impact of these echo patterns on speech intelligibility in the presence of competing sounds whose content and spatial locations and will be varied. We will test the hypothesis that "informational masking" can be induced when listeners experience uncertainty regarding the number, content and locations of competing sounds. In Aim 3 we will study binaural mechanisms in bilateral CI users through specialized hardware that controls the binaural synchronization between the two ears. Studies will measure localization, echo suppression, speech in intelligibility in complex acoustic environments, and basic binaural processes. This research addressed fundamental issues concerning the ability of humans to function in everyday listening situations. Problems in such environments are commonly reported by hearing impaired individuals. Studies in Aim 3 might contribute to the future design of hearing aids and cochlear implants that take advantage of binaural cues and compensate for the degradation of functioning in noisy and reverberant environments.

(5) UC Irvine Core Center for Hearing and Communication Research (1P30DC008369; PI: Raju Metherate)
Research in NIDCD mission areas is expanding at the University of California, Irvine (UCI). Since 2000, UCI has sponsored interactions and interdisciplinary collaborations among a group of 13 faculty who currently comprise the Center for Hearing Research (CHR). The potential for further interdisciplinary work is substantial, with 20 faculty spanning 11 Departments in 5 Schools at UCI performing research in NIDCO mission areas. For these investigators, CHR will establish a UC Irvine Core Center for Hearing and Communication Research to: 1) facilitate ongoing research by consolidating resources and providing access to state-of-the-art technology, 2) promote interdisciplinary work by educating users about Core facilities and providing expert assistance, and 3) recruit investigators from diverse backgrounds to research in NIDCD mission areas. We propose two Core facilities: 1) The Imaging Core will provide a 2-photon imaging system for visualizing neurons and real-time neural activity in live or fixed tissue. The system will enable new kinds of experiments, e.g., examining activity simultaneously in many neurons, or overtime during development. An imaging specialist will facilitate the integration of this technology into existing research programs and among users from multiple disciplines. 2) The Computing and Engineering Core will provide signal processing and electronic systems support for physiological and behavior research and for device development. The development of common signal processing algorithms will promote interdisciplinary collaborations (e.g., between physiological and behavioral projects) and facilitate data sharing. Core personnel will conduct a lecture series to familiarize new researchers with the engineering technologies used in core users' laboratories. The two Cores also will interact with each other, e.g., software development for image analysis, or for sharing data. These shared facilities will enhance ongoing research at UCI and accelerate the trend towards interdisciplinary research in NIDCD mission areas. The resulting synergy among disciplines is critical for a fuller understanding of communication and communication disorders.

Dr. Zeng previous investigation included: (1) Intensity coding in acoustic and electric hearing; (2) Speech processor for auditory prostheses; and (3) Hearing deficits due to auditory neuropathy.

Dr. Zeng’s laboratory is equipped with state-of-the-art signal processing software and hardware that allows precise generation, control, and presentation of acoustic stimuli including tones, noises, speech, and music sounds. Clinical and experimental programming interfaces are available for cochlear implant related research. Acoustic simulations of various degrees of hearing impairment, cochlear implants, and auditory neuropathy are also available for research purposes and for allowing a normally hearing listener to appreciate hearing loss.