The Impact of "Offset-Deafness" on Perception and Cortical Processing of Speech Sounds in Noise

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Understanding speech in noisy environments is one of the most difficult --- and important --- tasks of human hearing. Accurate perception of speech in noise involves not only mechanisms of hearing within the ear, but also mechanisms of hearing within the brain. Many children with developmental disorders and elderly adults have abnormal difficulty perceiving speech in noisy environments, even when their hearing and their ability to perceive speech in quiet environments is normal. These difficulties with speech-in-noise perception are thought to arise, at least in part, from developmental and age-related abnormalities in the auditory brain rather than the ear. However, the nature of the auditory brain abnormalities affecting perception of speech in noisy environments remains poorly understood.We have recently discovered, in a mouse model, that auditory brain sensitivity to sound offsets (disappearances) can be impaired while sensitivity to sound onsets (appearances) is not. This previously unrecognised form of hearing difficulty --- "offset-deafness" --- appears to arise in the brain rather than the ear. Standard clinical tests of human hearing focus almost exclusively on sensitivity to sound onsets, and would not even detect a specific deficit in auditory brain sensitivity to sound offsets like that observed in the mouse model. However, sensitivity to sound offsets is thought to be critical for speech-in-noise perception; therefore, it is possible that "offset-deafness" in humans could produce difficulties with speech-in-noise perception despite apparently normal hearing in clinical tests.The main objective of this research is to test the hypothesis that "offset-deafness" can disrupt perception and cortical processing of speech-like sounds in noise. We propose to test this hypothesis using both our mouse model of naturally occurring "offset-deafness", and normal mice in which we will induce "offset-deafness" experimentally by suppressing activity of sound-offset-sensitive cells in the auditory brainstem. We will measure the ability of the mice to detect changes in speech-like sounds in noise, and simultaneously we will measure auditory cortical activity evoked by those sounds. In addition, we will collect similar behavioural and neural measures of brain sensitivity to simpler sounds such as brief gaps in noise and quiet tones in silence. By comparing results between mice with and without "offset-deafness", we will achieve our main objective: to determine whether reduced auditory brain sensitivity to sound offsets impairs perception and cortical processing of speech-like sounds in noise. Additional objectives will be to determine whether abnormalities in speech-in-noise sensitivity can be predicted from sensitivity to brief gaps in noise; and how sound-offset-sensitive cells in the auditory brainstem influence perception.These experiments may help us to understand why some people with apparently normal hearing --- especially children with developmental disorders and elderly adults ---- have abnormal difficulty with perception of speech in noisy environments. If "offset-deafness" can indeed impair perception of speech in noisy environments, then the introduction of simple clinical tests for sound-offset sensitivity could help to identify patients likely to have to difficulty with speech-in-noise perception. Moreover, novel assisted-listening devices that enhance perception of sound offsets might improve the ability of young children to hear effectively in noisy classrooms, and of elderly adults to maintain healthy levels of social interaction into old age.