IBN 98-15056 JOHNSON. Compared to listening with one ear, listening with two ears provides significant advantages to hearing in quiet and noisy rooms. Individuals who are deaf in one ear perform significantly more poorly than normal listeners on tasks such as locating sound sources and identifying spoken words presented in noise. Experiments have shown that differences between the sounds at the two ears (i.e., interaural differences) are used by our brains to provide this "binaural" advantage. Two important brain centers involved in comparing these interaural differences are the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB). MNTB neurons provide the LSO with information about sounds coming from one ear, while cochlear nuclear neurons provide information about sounds coming from the opposite ear. The studies of Dr. Tsuchitani and her collaborators have shown that LSO neurons use this information to compute interaural differences in loudness (ILD) and time-varying ILDs produced by the sound-shadowing effect of the head and ear. Dr. Johnson and collaborators developed a computational model of LSO neurons that permits simulation of LSO responses, and statistical models that provide descriptions and predictions of how the LSO system responds to different types of binaural stimuli. The studies to be undertaken will explore the role of the LSO and MNTB in the neural processing of complex signals akin to those encountered naturally by employing methods developed for virtual acoustics. Transient complex stimuli will be used, presented alone and in background noise. The apparent location of the signal source will be manipulated by applying cat head-related transfer functions supplied by Dr. Joseph Hind and Dr. John Brugge of the University of Wisconsin. Dr. Tsuchitani, an auditory neurophysiologist, will conduct the experiments manipulating the stimuli and recording the electrical responses of single and multiple neurons in the LSO and MNTB to these stimuli. Dr. Johnson, a communication/signal processing theorist, will be applying advanced signal analysis techniques to the recorded data to determine how the ensemble of LSO neurons responding to complex stimuli processes binaural information. This technique will be used to determine what perceptual cues, ILD, time varying ILDs, interaural time differences, and interaural spectral differences, the LSO ensemble response provides. The results of this project will reveal how the brain uses interaural differences to produce the binaural advantage in hearing in quiet and noisy rooms.
