The most pressing issue with hearing aids and cochlear implants is that they function poorly in noisy environments for most users, where even mild hearing loss can make it difficult to ignore background sound. Suppression of unwanted sound is crucial for communication in social settings, such as the workplace. Inability to understand speech in these situations, called masking, can lead to social isolation and reduced employment. Two principal types of masking interfere with optimal function of hearing aids and cochlear implants. The first type, called energetic masking, is well characterized through psychophysics, physiology and modeling. The second type, called informational masking, is currently only defined as a psychological construct and much less understood. Energetic masking occurs when target speech and background sound excite the same auditory nerve fibers at the same time. Even an ideal listener would be mostly unable to recover an energetically masked target. In contrast, informational masking occurs even when target and background sound do not overlap in time and frequency, and when an ideal listener could restore the target information. Informational masking thus holds a key to improved hearing aid and cochlear implant design. Moreover, individual listeners differ dramatically in their ability to suppress informational masking. However, hearing aids and cochlear implants only intend to mitigate energetic masking, ignoring vulnerability to informational masking. Towards improved fitting of hearing aids and cochlear implants, we propose to develop an objective scale of vulnerability to informational masking based on cortical function. We propose to examine cortical mechanisms of informational masking in humans and in an animal model organism of human auditory processing, the Mongolian gerbil (Meriones unguiculatus). First, we will test normal-hearing human listeners as well as gerbils under conditions of informational masking and simultaneously record from auditory cortex. In humans, we will record the hemodynamic response of blood oxygenation, using a quick and robust assessment technique with clinical relevance. In gerbils, we will measure neuronal activity in auditory cortex from trained animals. We will use this data to develop an objective metric of an individual?s vulnerability to informational masking. Second, we will examine the neuronal mechanisms of informational masking by introducing rapid unpredictable changes in background sound and assessing if high vulnerability to informational masking is due to predominant reliance on suppressing background activity (as opposed to enhanced responses to the target) in humans and gerbils. Third, using our animal model, we will test how hearing loss affects susceptibility to informational masking. Collectively, this proposal will functionally define informational masking at both perceptual and cortical processing levels. The results are expected to significantly advance our understanding of the origins and scope of this central auditory processing deficit in common everyday situations with background sound.
Current hearing aids and cochlear implants fail at the critical issue of restoring speech intelligibility in situations with noisy backgrounds, for the majority of individuals. Towards successful rehabilitation, we here examine cortical mechanisms that allow listeners to suppress background sound, with and without hearing loss.
