Research


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Otoimmunology of inner ear diseases of unknown causes:   Autoimmune sensorineural hearing loss refers to an entity of inner-ear diseases that the causes may be attributed to autoimmune responses in the inner ear (McCabe, 1979). In a clinical research project carried out in Beijing Tongren Hospital/Beijing Institute of Otorhinolaryngology, China, we performed immunologic tests in a group of patients with Meniere's disease and identified a subgroup of patients in whom abnormal immune reactivities might be associated with the etiology of the disease (Xu et al., 1989; 1990).  In another project carried out in University of Basel, Switzerland, the genetics of a group of 42 Caucasian patients with inner ear diseases was studied using HLA (Human Leukocyte Antigens) typing techniques. Those patients were suspected of an autoimmune etiology because all had positive serum antibodies against in the inner ear structures as detected by an immunofluorescence assay. Our results did not reveal a positive correlation between the HLA types and the inner-ear diseases (Xu et al., 1993).

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Otoacoustic emissions and peripheral processing:   Measuring otoacoustic emissions (OAEs ) is part of standard audiological assessment today. However, we had to struggle to establish the technique of OAE measurements in Beijing many years ago (Xu et al., 1989). Besides, an innovated technique of measuring OAE cochleogram evoked with bone-conducted stimulation was also developed, which had potential clinical applications (Xu et al., 1991). Research in OAEs continued in University of Basel, Switzerland, where we studied suppression of the distortion production 2f1-f2 otoacoustic emission (Harris et al., 1992). In another project, multiple tone-burst were used as stimuli to evoke OAEs. This project helped us to understand the peripheral analysis of frequency in human ears (Xu et al. 1994).

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Cortical mechanisms for sound localization:   In a set of experiments, we have demonstrated that the spike patterns of auditory cortical neurons carry information about sound-source location (Middlebrooks, et al., 1994; 1998). My focus was on testing the sensitivity of cortical neurons to sound location in the median vertical plane, where interaural difference cues are negligible and spectral cues are dominant. The studies involved multichannel extracellular recording of units in the nontonotopic auditory cortex (area AES and A2) of anesthetized cats. Artificial neural network was employed to recognize spike patterns. Fairly accurate performance of the neural network was achieved in classifying patterns elicited by broadband noise (Xu et al., 1998), indicating information about sound location is carried by the spike patterns of cortical neurons. We further explored, for the first time, the physiological basis for spatial illusion elicited by spectrally-manipulated noise. We showed that spike patterns of cortical neurons systematically mislocalized sounds that have been passed through a narrowband filter. Both correct and incorrect locations signaled by the neurons could be predicted quantitatively by a model of spectral processing (Xu et al, 1999; 2000). 

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External-ear transfer function:   As sound passes from a free-field source to the ear canal, its spectrum is transformed by the interaction with the head and external ear. Such an external-ear transfer function varies with the angle of incidence of sound, so the spectrum of the sound in the ear canal carries directional information. Our study indicated that the individual differences in the external-ear transfer function of cats were no smaller than those found in human (Middlebrooks, 1999; Xu and Middlebrooks, 2000). Thus, caution should be exerted in interpreting results obtained with nonindividualized transfer function (e.g., virtual auditory space) in behavioral and physiological studies of sound localization.

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Cochlear implants:   Cochlear implant research has been our recent area of research. The goals of our research are to understand the mechanisms of electrical hearing and to identify ways to improve the function of auditory prostheses. A number of studies have been carried out at Kresge Hearing Research Institute to study various factors that affect electrical hearing from detection to more complicated psychometric functions to speech perception (Pfingst et al., 2001;  Franck, Xu and Pfingst, 2002; Pfingst and Xu, 2004; Pfingst, Xu and Thompson, 2004; Pfingst and Xu, 2005; Xu et al., 2005, Pfingst et al. 2007, 2008). Another approach to study speech perception in cochlear implants is to utilize acoustic simulations of cochlear implants (e.g., Shannon et al., 1995). Using such simulations, we studied tone perception (Xu et al., 2002; ) and English phoneme recognition (Xu, Thompson and Pfingst, 2005; Xu and Zheng, 2007). Tone perception, a crucial aspect of speech perception in tone languages (e.g., Chinese, Thai, etc.), as well as music perception, presents challenges for cochlear implants. In another study using auditory chimera technique, we examined the relative importance of temporal envelope and fne structure for tone perception (Xu and Pfingst, 2003). In collaboration with Beijing Tongren Hospital, we are beginning to study the tone perception and production in native tone-language speaking children implanted with cochlear implants (Xu et al., 2004; 2007; Han et al. 2007; Zhou and Xu, 2008; Zhou et al., 2008).

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Research

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This page is last updated on 5/8/2008.