Andrew D. Brown, Ph.D. - US grants

DESCRIPTION (provided by applicant): The ability to localize sound is integral to communication and environmental awareness. Normal hearing listeners localize sound in the horizontal plane by responding to interaural differences in signal timing (ITD) and level (ILD) at the two ears. In ordinary listening environments (e.g., rooms), accurate localization requires that listeners respond to the cues carried by the early-arriving incident sound rather than the spurious cues carried by later-arriving reflected sound. In fact, localization by normal hearing listeners is remarkably unaffected by echoes and reverberation. The various phenomena associated with this observation are known collectively as the precedence effect. Although the adaptive value of the precedence effect in sound localization is clear, important questions about the effect remain. For example, although ITD and ILD are distinct acoustic cues, thought to be processed separately in the auditory brainstem, most precedence findings reflect data from studies employing headphone ITD or free field stimuli where the contributions of ITD and ILD to localization judgments are inseparable: relatively little is known about the contribution of ILD to precedence, despite evidence that ILD is the dominant cue in localization for some listeners, including a growing population of bilateral cochlear implant users. Thus, the broad long-term goal of the proposed research is to elucidate the separate contributions of ITD and ILD to the precedence effect in order to establish a more complete and clinically informative account of sound localization in ordinary listening environments. The specific aims of the proposed research are (1) to examine listeners'sensitivity to "echoes" following repeated presentations of "source-echo" stimuli carrying only ITD or ILD and (2) to examine listeners'sensitivity to "echoes" following repeated presentations of "source-echo" stimuli carrying combinations of ITD and ILD. Recent data from the Applicant's lab suggests that the time course of ITD sensitivity is substantially different from the time course of ILD sensitivity. Thus, toward the first aim, an initial experiment will test the hypothesis that echo suppression is more robust for stimuli carrying ITD than for stimuli carrying ILD. Toward the second aim, an additional experiment will test the hypothesis that "dynamic" echo suppression depends on sensitivity to both ITD and ILD "images." Across experiments, stimuli will be presented via headphones to allow for independent manipulation of ITD and ILD cues, and echo thresholds will be estimated by an adaptive psychophysical procedure. The proposed research will provide valuable new insight on a large body of existing precedence effect literature and on the unique roles of ITD and ILD in spatial hearing. The work will thus inform both basic and clinical understanding of audition, underscoring this proposal's relevance to the mission of NIDCD. PUBLIC HEALTH RELEVANCE: The ability to determine where sounds are coming from is an important part of hearing. Although everyday listening environments introduce echoes and reverberation that might be expected to make it very difficult to determine the location of actual sound sources, normal hearing listeners are remarkably capable of accurate localization under such conditions. The proposed research builds on what is known about this capacity to expand basic understanding of sound localization and to provide insight on the reduced sound localization abilities of certain hearing impaired populations, including bilateral cochlear implant users.

DESCRIPTION (provided by applicant): The ability to localize and segregate sound sources in space is important for communication and environmental awareness. Normal-hearing (NH) human listeners effectively localize and segregate sound sources on the basis of interaural time and level differences (ITD and ILD) - binaural acoustic cues to sound source azimuth. Hearing-impaired individuals who use hearing aids (HAs) and cochlear implants (CIs) are often significantly or profoundly impaired in their abilities to detect these cues, leading to significan communication difficulties in everyday listening environments. Such deficits are believed to arise from a combination of patient factors, such as prolonged auditory deprivation leading to altered central function, and clinical device limitations, such as inadequate temporal precision in signal processing or bilateral asymmetry in automatic gain control algorithms. An improved understanding of signal-related limitations on binaural hearing performance will lead to an enhanced basic understanding of the binaural system and may lead to consequent improvements in clinical device technology and patient outcomes. Therefore, the broad long-term goal of the proposed research is to elucidate the effects of binaural cue degradations on sound localization and segregation, toward improved understanding of normal and impaired binaural hearing. The specific aims of the proposed research are: (1) To characterize the effects of interaural decorrelation and time-varying ILD compression on normal-hearing psychophysical sensitivity to ILD, ITD and virtual sound source location and (2) to characterize the effects of such distortions on the encoding of ILD, ITD and virtual sound source azimuth by neurons of the inferior colliculus. Experiments will integrate (1) behavioral tasks including discrimination, intracranial lateralization, and virtual space localization for characterization of psychophysical sensitivity to control and binaurally degraded stimuli, including simulated device-processed stimuli generated from our recent measurements of actual bilateral HA outputs and (2) in vivo extracellular recording techniques for characterization of physiological (neural) sensitivity to control and degraded stimuli. Behavioral and neural data will be related using Fisher information to compare behavioral and neural binaural discrimination performance. Simultaneous with these experiments, the Applicant will regularly meet with clinicians (including the Co- Sponsor) and observe clinical sessions to gain additional perspective on communicative and other listening difficulties faced by people with hearing loss. Such meetings will enrich the Applicant's training and may provide insights for the design of follow-up of future experiments. The proposed work will support the mission of NIDCD by advancing basic understanding of the binaural auditory system and exogenous (signal-related) limitations on binaural benefit from bilateral clinical devices (especially HAs).

Project Description In normal hearing, sound reaches the cochlea via the tympanic membrane and middle ear, i.e. air conduction (AC). A secondary mode of hearing, known as bone conduction (BC), bypasses the conductive pathway to cause pressure fluctuations within the cochlea primarily via vibration of surrounding bone. While the contributions of BC to audition under normal listening conditions are minimal, BC signals are readily passed to the cochlea using mechanical transducers, and hearing tests employing such transducers have been used for over a century to diagnose conductive hearing pathologies. For individuals with conductive hearing loss, BC stimulation via modern BC hearing aids can improve audibility in the affected ear(s). BC hearing aids may also be worn in cases of single-sided deafness, wherein a BC hearing aid on the deaf side transmits the signal through the bones and tissues of the head to the opposite (hearing) cochlea. Indeed, transcranial cross-talk is a key feature of BC stimulation, and traditionally even in cases of bilateral conductive loss, only a single BC device is worn. Bilateral BC stimulation results in superposition of two signals at each ear, which is expected to limit binaural disparities and associated perceptual benefits (sound localization, speech-in-noise perception). However, non-zero binaural disparities via bilateral BC have been reported in several biophysical studies, and improved sound localization and speech-in-noise perception with bilateral versus unilateral BC have been reported in several psychophysical studies. While efforts to understand and exploit binaural and spatial hearing via BC have thus increased in recent years, basic constraints on performance remain poorly understood. The current proposal aims to fill this knowledge gap. A first set of experiments (Aim 1) will quantify acoustic information conveyed via bilateral BC devices using measurements of intracochlear pressure, the input drive to the auditory system, in cadaveric specimens. A complementary set of measurements will be made during simultaneous AC and contralateral BC stimulation, simulating the signal presented to single-sided deaf BC hearing aid users, who show variable but generally poor outcomes in spatial tasks. A second set of experiments (Aim 2) will determine the limits of normal-hearing psychophysical sensitivity to binaural information conveyed via bilateral BC across key stimulus parameters, including frequency, bandwidth, and spatial cue type. Additional measurements of BC performance in everyday spatial tasks, including sound localization and speech-in-noise perception, will be completed and related to intracochlear and basic psychophysical data. Collectively, the proposed experiments will evaluate the overarching hypotheses that (1) BC signal superposition within the cochlea(e) generates systematic spatial cues, and that (2) such cues can be used to systematically modulate perception. Data will provide critical new insight on factors that constrain spatial hearing via BC, and point to strategies for improvement of BC hearing aid technology and outcomes.