Individual differences in processing of supra-threshold sound: an investigation of normal-hearing listeners
Ruggles, Dorea Ruth
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Normal hearing is typically defined by threshold audibility, or the loudness of sounds that an individual is able to hear. This convention exists despite the fact that everyday communication relies on extracting and interpreting features of supra-threshold sound. Many normal-hearing listeners struggle to perform certain supra-threshold auditory tasks, and debate persists as to whether such difficulties originate from deficits in cognitive processing or in peripheral, sensory encoding. In this work, we show that there are large individual differences among normal-hearing adults (18 to 55 years), who were asked to report a stream of digits located directly ahead in a simulated rectangular room. Simultaneous, competing masker digit streams were simulated at locations 15° left and right of center, and the level of reverberation was adjusted to vary task difficulty. Performance was best in the anechoic condition and worst in the high-reverberation condition, but listeners nearly always reported a digit from one of the three competing streams, showing that reverberation did not render the digits unintelligible. Most importantly, inter-subject differences were extremely large. These differences were not significantly correlated with age, memory span, or hearing status. They were, however, correlated with behavioral differences in the ability to detect small frequency modulations in pure tones and with the strength of the frequency following response (FFR), a physiological measure of spectro-temporal detail encoding in supra-threshold sounds early in the auditory pathway. The decomposition of the FFR into envelope and carrier components shows that envelope frequency following is the preferred cue for completing the spatial attention task, but it degrades with early aging. As a result, older listeners depend more on carrier phase locking cues than younger people. These results suggest that differences in peripheral encoding help explain individual differences in the ability to communicate in challenging settings, but that aging also has a separate, dissociable effect on early encoding in the auditory system. Tests like these may help tease apart contributions of peripheral and central deficits to communication impairments, ultimately leading to new approaches for helping listeners cope with complex listening environments.
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