In a natural listening environment the auditory system must simultaneously process sounds spanning a wide spectral and dynamic range. For example, at a party a listener may simultaneously hear the whisper of a nearby speaker, the performance of a musical group, and the mid-range hum of background speech babble. This creates conditions where understanding a single speaker in such an environment can be a difficult task. Louder competing sounds may mask portions of the signal ultimately leading to degradation of the content. Despite these disruptions listeners'perception of speech is of a continuous source among louder unrelated sounds. The robust perceptual stability listeners experience is maintained through the phenomenon of auditory continuity and is a fundamental component in the ability to parse, organize, and maintain multiple sources in a complex acoustic scene (""""""""Cocktail Party Problem"""""""" [Cherry, 1953]). For hearing-impaired listeners, auditory continuity is particularly important. In conditions where the effective signal-to-noise ratio may be lower the perceptual restoration of occluded information is even more heavily relied upon to maintain a source's stability and contextual relevance. This proposal aims to study the neural correlates of auditory continuity in single neurons in the auditory cortex of the awake behaving common marmoset (Callithrix Jacchus) to understand how patterns of neural response change when a listener is actively tracking a source in competing noise. Saccadic eye-movements will be used as a behavioral response in discrimination tasks to determine the psychometric functions associated with acoustic conditions under which continuity and discontinuity are perceived in this species. This will enable selection of listener specific conditions for neurophysiological testing expected to produce categorically different strengths of perceptual continuity. Neural response will be measured during selected conditions while the animal is both passive and behaving. It is predicted that patterns of neural response to perceptually distinguishable conditions of continuity and discontinuity will be more differential when an animal is behaving and actively tracking a source in a noisy environment in comparison to conditions of passive listening. It is also predicted that active tracking will cause individual cells to show dynamic increases in their response selectivity to sounds following interruptions such that a cell more selectively responds only to sounds that are extrapolations of the sound preceding the interruption. Understanding shifts in patterns of neural response that are dependent on the preceding stimulation and relating their strength to the perceptual condition will clarify an anticipatory process likely integral in the perceptual restoration of dynamic sounds such as speech and music in a noisy environment. Further, this may contribute to the design improvement of both wearable and implanted hearing-aid devices for hearing-impaired listeners who currently rely upon auditory continuity to successfully interact in their acoustic scene. Relevance: Understanding a single speaker in a noisy environment is a difficult task made even more difficult for hearing- impaired listeners. Despite louder background noise obscuring portions of a sound a continuous perception of a speaker is heard through the phenomenon of auditory continuity. This proposal studies the neural correlates of this phenomenon and how listeners are able to track and maintain a sound in a noisy environment, and results may contribute to the design of better hearing-aid devices.
| Crum, Poppy A C; Hafter, Ervin R (2008) Predicting the path of a changing sound: velocity tracking and auditory continuity. J Acoust Soc Am 124:1116-29 |
