Temporal coding is the ability of the auditory system to detect and transmit fluctuations in time across the acoustic signal. Sensorineural hearing loss and early aging have the potential to reduce the temporal resolving power of the auditory system. The relevant literature, however, shows mixed results from psychophysical studies, speech perception studies, and physiologic studies. The lack of a clear understanding of how hearing loss changes temporal processing (or not) may be the result of several factors including early age-related changes in neural recovery or adaptation and methodological issues (e.g., equivalent presentation levels, stimulus complexity). A series of experiments to measure temporal processing in middle-aged adults with and without high-frequency sensorineural hearing loss (abbreviated as HFL throughout proposal) is proposed. This understudied group has been largely overlooked in the study of auditory aging because most older subjects have some degree of high-frequency hearing loss and research is often conducted in low-frequency regions where pure-tone thresholds are similar to those of younger subjects. Presenescent changes have been found for speech perception (Helfer and Fargo, 2009) and for auditory temporal coding (Grose et al., 2006). Significantly poorer gap detection occurs for stimuli presented in regions of normal pure-tone sensitivity when high-frequency hearing loss is present. The proposed research will improve our understanding of early aging and HFL on auditory temporal coding. The purpose of the primary study is to determine the extent to which early aging and HFL impact temporal coding in regions of normal low-frequency hearing. The current study addresses these purposes with the following three specific questions: (1) Is there a significant difference in low-frequency temporal coding between listeners with normal pure-tone sensitivity and listeners with HFL? (2) Is there a significant difference in temporal coding between young subjects (aged 18 - 30 years) and middle-aged subjects (aged 40 - 60 years) who have normal pure-tone sensitivity? and (3) Are psychophysical or electrophysiologic measures of temporal coding related? Four groups of subjects will be targeted for a series of experiments as follows: (1) young adults with normal pure-tone sensitivity (YNH), (2) middle-aged adults with normal pure-tone sensitivity (MANH), (3) young adults with HFL (YHFL), and (4) middle-aged adults with HFL (MAHFL). Normal hearing will be defined as pure-tone thresholds d25 dB HL for the frequencies from 250- to 8000 Hz. High-frequency hearing will be defined as pure-tone thresholds d25 dB HL from 250 - 2000 Hz and e 35 dB HL from 3000 - 8000 Hz. Participants will be recruited from the Audiology Clinic at the VA at Mountain Home. One short-term objective is to acquire a high-level of technical understanding of signal processing and experimental design as well as insight into parallel experimental designs in multiple domains (e.g., clinical audiology, psychophysics, and electrophysiology). The new skills will provide a bridge between my basic research skills acquired to date and advanced skills necessary to transition into an independent VA investigator. A second short-term goal is to acquire the experience needed to analyze cortical auditory evoked potentials, which inherently are complex. Thus, the focus will be practical and theoretical integration by leveraging the expertise of the CDA-2 mentors (Drs. Murnane, Eddins, and Fowler).
High-frequency sensorineural hearing loss (HFL) affects up to 93% of males in the United States aged 60-69 years and 57% of men aged 40-49 years. Veterans with hearing loss typically have more noise exposure and are older than the general population; these circumstances both contribute to HFL. Between 2008 and 2009 compensable claims for impaired hearing grew 12.4%, which was more than any other body system impairment and middle-aged adults (35-54 years) accounted for 30% of those Veterans (Annual Benefits Report FY2009, p. 9-10). Currently, it is not understood in what way early aging and HFL contribute to reduced temporal processing and speech understanding. The knowledge gained by this research will provide direct benefit to Veterans suffering from HFL by better rehabilitative strategies (e.g., auditory training) or devices (e.g., processors for hearing aids).
