Basic Binaural Sensitivity
Our goal is to measure psychophysical performance under a variety of binaural conditions. Specifically we focus on localization and masked detection tasks with perceptually simple stimuli as well as speech intelligibility under conditions with minimal uncertainty. Further we investigate the differences between monaural and binaural performance in an attempt to differentiate between binaural processing advantages and better ear advantages.
Models of Brainstem and Midbrain Neurons
We are developing computational models for the activity of neurons in the auditory brainstem and midbrain. Biophysical properties of model cells are based on data from in vitro and in vivo recordings in the literature. Our focus is on the binaurally sensitive neurons that respond to interaural time difference (ITD) and/or interaural level difference (ILD), most prominently on the medial and lateral superior olivary nuclei and the nuclei of the lateral lemniscus and the inferior colliculus. Our model simulations are based upon specific hypotheses on the effects of interactions between synaptic excitation and inhibition and of characteristics of membrane properties on ITD and/or ILD sensitivity of single binaural neuron in response to pure tones and amplitude-modulated tones. Recent modeling results suggest that ITD and/or ILD tuning properties of binaural neurons in the brainstem are affected by their intrinsic properties, such that binaural responses of a cell may not be static. Our ongoing modeling projects include neural representations of integrated ITD and ILD information in the inferior colliculus and population encodings of spatial locations of tonal stimuli masked by noises.
Performance of Bilateral Cochlear Implants
The increasing number of listeners who are receiving bilateral cochlear implants (CIs) makes an understanding of capabilities of these listeners important for applications and for theoretical understanding. We are measuring the hearing abilities of bilateral CI users in several types of experiments, including sound localization, parameter discrimination, and speech intelligibility in the presence of interferers. Our primary focus is the use of interaural timing and level information. From these studies we expect to learn about both implant abilities and the mechanisms of interaural processing in normal hearing listeners. Studies include traditional headphone studies, studies of performance in rooms, and studies with virtual acoustic stimulation.
Performance with Complex Stimuli
We are measuring psychophysical performance of normal-hearing and hearing-impaired listeners in several types of tasks in complex acoustical environments. Acoustical environments may be considered complex for several reasons, such as reverberation, echoes, multiple sources in different locations, moving sources, and multiple frequency bands with high uncertainty. We believe that performance in complex environments is particularly important for listeners, particularly those with hearing impairments. Non-speech identification (informational masking or central masking), non-speech discrimination, speech intelligibility, and source localization are used to measure subject performance in these environments.
Signal Processing Models
Based on optimal use of information contained at different levels of the auditory pathway we calculate performance on psychophysical tasks. We are also evaluating and expanding currently available models including cancellation models and correlation models.
Studies of Hearing Impairment
Understanding the difficulties of hearing-impaired listeners is a primary motivation of our research. We are measuring performance in psychophysical tasks for listeners with several types of hearing impairments. We are developing and evaluating models of the impaired auditory system.
Virtual Acoustic Environments
We are developing capabilities for spatializing sound properly in simple and complex environments. We simulate both anechoic and reverberant environments which include single or multiple sources. These sources can be static in space or spatially-dynamic. The simulations allow exploration of what information is perceptually important for the psychophysical performance under these conditions. Components of spatialization we are exploring include simple models of basic binaural cues, empirical Head Related Transfer Functions (HRTFs), spatial interpolation of HRTFs, simple reflections, and room reverberation.