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The incidence of Diabetes Mellitus (DM) is about 9.6% in the US, and its prevalence is increasing rapidly and globally (NIDDKD, 2007). A common, but under-recognized, complication of DM is hearing difficulties. Although epidemiological studies (Bainbridge, et al., 2008) suggests that individuals with diabetes are twice as likely to have hearing loss as non-diabetic individuals, research on DM-related auditory deficits is relatively sparse and have been inconclusive and/or ambiguous regarding the nature of the hearing loss. We tested the hypothesis that the DM-related listening difficulties are manifestations of subclinical deficit(s) in higher-order auditory processing. Following a routine audiological evaluation, we examined a group of adult DM (Type II) patients with tests that assessed peripheral (high-frequencyaudiometry) and central processing (spatial listening, listening in competing noise, temporal processing and contralateral-suppression of OAEs) abilities. Our results indicate elevated high frequency pure-tone (>4 kHz) thresholds, increased difficulty listening in competing noise, poorer spatial listening skills, and poorer temporal processing abilities in the group of DM patients when compared to controls. These results suggest that central auditory processing deficits in patients with DM are more striking than commonly investigated peripheral deficits and thus contribute, and probably exacerbate, the functional listening difficulties experienced by these patients.

Diabetes mellitus (DM) is a systemic disease that affects a number of sensory systems and is a major public health concern. A common, but under-recognized complication of DM is hearing difficulty. However, research on DM-related auditory deficits is relatively sparse. Recent data shows over a two-fold higher prevalence of hearing impairment in diabetic patients compared to non-diabetic individuals, further solidifying this strong association between diabetes and hearing loss. However, the nature of these hearing difficulties has not been clearly elucidated. In the present study, the auditory processing abilities of a group of adults (N = 9; mean age = 53.3) with type 2 DM was compared to an age matched control group (N = 9; mean age= 47.6). In addition, visual processing abilities of the participants were examined to determine if the hearing difficulties associated with DM were a function of a more global sensory deficit. The selection criteria for the DM group included age, duration of diabetes (> 5 years), and recent measures of diabetic control (hemoglobin A1C and blood sugar). Further, all participants in the test and control groups had a comprehensive hearing evaluation and had hearing (measured with pure tones 250-4000 Hz) within normal limits bilaterally. The following test measures were recorded. The Michigan Neuropathy Screening Instrument (MNSI) was administered to screen each subject for peripheral neuropathy symptoms. The Speech, Spatial & Qualities (SSQ12) questionnaire was administered to attain a measure of each individuals subjective hearing complaints. Subtle deficits in the peripheral auditory system were examined using (a) Extended high frequency audiometry, that assessed ultra high frequencies from 9000-16000 Hz bilaterally, (b) The Threshold Equalizing Noise (TEN) test, to assess for potential dead regions within the cochlea, and (c) Distortion product otoacoustic emissions (DPOAE) were collected bilaterally to assess frequency specific cochlear outer hair cell function. Higher-order auditory processing was examined with the Quick Speech in Noise Test (QuickSIN), the Listening in Spatial Noise test-Sentences (LiSN-S) and the Random gap detection, to measure temporal processing abilities. Further, the Text Reception Threshold Test (TRT) was employed to examine visual processing abilities. The results revealed that individuals with DM had significantly (a) Elevated high frequency (> 4 kHz) pure tone thresholds; (b) Increased self-reported hearing difficulties based on SSQ12 scores (c) Poorer spatial listening in noise skills based on LiSN results, (d) Lower DPOAE amplitudes for higher frequencies, and (e) Poorer temporal processing skills based on the average gap detection thresholds, when compared to the age-matched controls. These findings not only support the strong association between diabetes and hearing loss, they suggest that that even individuals with controlled diabetes potentially suffer with subclinical auditory processing deficits that may affect their quality of life. These findings have implications in the management of DM in the future. Further research is needed in order to determine clinically feasible means of evaluating DM subjects.

Age-related declines in hearing abilities are common and can be attributed to changes in the peripheral and central levels of the auditory system. Although central auditory processing is enhanced in younger musicians, the influence of lifelong musicianship on age-related decline in central auditory processing has not yet been investigated. Therefore, the purpose of this dissertation was to investigate whether lifelong musicianship can mitigate age-related decline in central auditory processing. In the first experiment, age-related declines on four hearing assessments were compared between musicians and non-musicians. Speech-in-noise and gap-detection thresholds were found to decline at a slower rate in musicians, providing an increasing advantage with age. Furthermore, musicians had a lifelong advantage in detecting a mistuned harmonic, although the rate of age-related decline was similar for both musicians and non-musicians. Importantly, there was no significant effect of musicianship on pure-tone thresholds, suggesting that lifelong musicianship can mitigate age-related decline in central but not peripheral auditory processing. To test this hypothesis, a second experiment compared auditory evoked responses (AERs) between groups of older and younger musicians and non-musicians. Results indicated that exogenous neural activity was enhanced in musicians, but that age-related changes were similar between musicians and nonmusicians. Furthermore, endogenous, attention-dependent neural activity was enhanced in older adults, suggesting a compensatory cognitive strategy. Importantly, endogenous activity was preferentially enhanced in older musicians, suggesting that lifelong musicianship enhanced cognitive processes related to auditory perception. In the final experiment, the ability to segregate simultaneous sounds was tested in older and younger musicians and non-musicians by using a mistuned harmonic paradigm, where AERs to harmonic complexes were compared to AERs when one of the harmonics was mistuned. Results indicated that musical training in older adults has little effect on early automatic registration of the mistuned harmonic. In contrast, late attention-dependent activity, associated with the perception of the mistuned harmonic as a separate sound, was influenced by musical training in older adults, suggesting that lifelong musicianship preserves or enhances cognitive components of concurrent sound segregation. In summary, musical training was found to reduce age-related decline in hearing abilities due to enhanced central processing of auditory information.

Auditory processing disorder (APD) is characterized by deficits in the auditory modality that are not due to a global processing problem or to deficiencies in the peripheral auditory system. Diagnosis of APD is time-consuming and could benefit from an objective test. Temporal processing ability is often impaired in cases of APD, which includes deficits on gap detection tasks. Previous attempts to correlate gap detection thresholds to electrophysiological responses have used the P1-N1-P2 response, mismatch negativity (MMN), and 40 Hz auditory steady-state response (ASSR), but these attempts have not been successful, especially using between-channel gap detection tasks. The current study used a modulation gap detection task and recorded the above responses to supra- and subthreshold gaps and stimuli with no gap. A significant P2 response and a later positive peak distinguished perception of a suprathreshold gap. Improvements over previous studies, the relation to auditory training, and limitations and directions for future research are discussed.

Older adults with sensorineural hearing loss have greater difficulty understanding speech than younger adults with equivalent hearing (Gates & Mills, 2005). This increased difficulty may be related to the influence of peripheral, central auditory processing or cognitive deficits and although this has been extensively debated the relative contribution to speech understanding is equivocal (Working Group on Speech Understanding and Aging, 1988). Furthermore, changes to the speech mechanism that occur as a result of age lead to natural degradations of signal quality. Studies involving hearing impaired listeners have not examined the influence of such naturally degraded speech signals. The purpose of this study was to determine: (1) whether older hearing impaired listeners demonstrate differences in speech understanding ability or perceived effort of listening on the basis of the age of the speaker and the predictability of the stimulus, and (2) whether any individual differences in speech understanding were related to central auditory processing ability. The participants included nineteen native speakers of New Zealand English ranging in age from 60 to 87 years (mean = 71.4 years) with age-related sensorineural hearing loss. Each participant underwent a full audiological assessment, three measures of central auditory processing (the Dichotic Digits Test, the Random Gap Detection Test and the Staggered Spondaic Words Test), and completed a computer-based listening experiment containing phrases of high and low predictability spoken by two groups: (1) young adults (18 – 30 years) and (2) older adults (70 years and above). Participants were required to repeat stimulus phrases as heard, with the researcher entering orthographic transcriptions into the custom-designed computer programme. An Analysis of Covariance (ANCOVA) was used to determine if significant differences existed in percentage words correct scores as a factor of speaker group (young versus older speakers) and stimulus predictability (high predictability versus low predictability phrases), with level of presentation (dB) as a covariate. Results demonstrated that although there were no significant differences in percentage words correct with regards to speaker group as expected, lower scores were achieved for low predictability phrases. In addition, increased listener effort was required when listening to the speech from the older adult group and during the low predictability phrase condition. Positive correlations were found between word understanding scores and tests of dichotic separation, which suggests that central auditory processing deficits contribute to the speech understanding difficulties of older adults. The implications of these findings for audiological assessment and rehabilitation are explored.

Auditory processing disorder (APD) is a label that describes a variable set of symptoms that share a common feature of difficulty listening to sounds in the absence of an actual audiological deficit (Moore, 2006). Clinical assessment of APD typically involves a test battery consisting of tests designed to examine the integrity of various auditory processes of the central auditory nervous system. Individuals with APD have difficulty recognising speech when parts of the signal are missing. One category of tests used to assess the extent to which this deficit is associated with reduced performance on the task is the low-pass filtered speech test. The University of Canterbury Adaptive Speech Test-Filtered Words (UCAST-FW) is a computer-based adaptive low-pass filtered speech test developed for the assessment of auditory processing skills in adults and children. Earlier studies with the UCAST-FW (McGaffin, 2007; Sincock, 2008; Heidtke, 2010; Abu-Hijleh, 2011) have produced encouraging results. However, there appear to be a number of confounding factors. The UCAST-FW is testing New Zealand listeners using an Australian recording of American test material. The purpose of the current study was to develop a new four-alternative forced choice test to replace the Northwestern University Children’s Perception of Speech (NU-CHIPS) stimuli the UCAST-FW currently utilises. The new test consists of 98 sets of four test items, (one target item and three foil alternatives) designed to be used in a four-alternative forced choice picture-pointing procedure. Phonemic analysis of the new word list and the NU-CHIPS word lists revealed a similar distribution of phonemes for target words of both word lists. The development of the new word list is described and the clinical applicability is explored.

Age-related declines in hearing abilities are common and can be attributed to changes in the peripheral and central levels of the auditory system. Although central auditory processing is enhanced in younger musicians, the influence of lifelong musicianship on age-related decline in central auditory processing has not yet been investigated. Therefore, the purpose of this dissertation was to investigate whether lifelong musicianship can mitigate age-related decline in central auditory processing. In the first experiment, age-related declines on four hearing assessments were compared between musicians and non-musicians. Speech-in-noise and gap-detection thresholds were found to decline at a slower rate in musicians, providing an increasing advantage with age. Furthermore, musicians had a lifelong advantage in detecting a mistuned harmonic, although the rate of age-related decline was similar for both musicians and non-musicians. Importantly, there was no significant effect of musicianship on pure-tone thresholds, suggesting that lifelong musicianship can mitigate age-related decline in central but not peripheral auditory processing. To test this hypothesis, a second experiment compared auditory evoked responses (AERs) between groups of older and younger musicians and non-musicians. Results indicated that exogenous neural activity was enhanced in musicians, but that age-related changes were similar between musicians and nonmusicians. Furthermore, endogenous, attention-dependent neural activity was enhanced in older adults, suggesting a compensatory cognitive strategy. Importantly, endogenous activity was preferentially enhanced in older musicians, suggesting that lifelong musicianship enhanced cognitive processes related to auditory perception. In the final experiment, the ability to segregate simultaneous sounds was tested in older and younger musicians and non-musicians by using a mistuned harmonic paradigm, where AERs to harmonic complexes were compared to AERs when one of the harmonics was mistuned. Results indicated that musical training in older adults has little effect on early automatic registration of the mistuned harmonic. In contrast, late attention-dependent activity, associated with the perception of the mistuned harmonic as a separate sound, was influenced by musical training in older adults, suggesting that lifelong musicianship preserves or enhances cognitive components of concurrent sound segregation. In summary, musical training was found to reduce age-related decline in hearing abilities due to enhanced central processing of auditory information.

Auditory processing is the ability of the brain to manipulate and utilise the neural output of the ear based on the frequency, intensity, and temporal features of the incoming acoustic signal. An auditory processing disorder (APD) is a deficiency in this ability. One category of tests that examine auditory processing ability are the various versions of the "filtered words test" (FWT), whereby a monaural, low-redundancy speech sample is distorted by using filtering to modify its frequency content. Due to the richness of the neural pathways in the auditory system and the redundancy of acoustic information in spoken language, a normal listener is able to recognize speech even when parts of the signal are missing, whereas this ability is often impaired in listeners with APD. One limitation of the various versions of the FWT is that they are carried out using a constant level of low-pass filtering (e.g. a corner frequency of 1000 Hz), which is prone to ceiling and floor effects. The purpose of this study was to counter these effects by modifying the FWT to use a computer-based adaptive procedure, to improve the sensitivity of the test over its constant-level counterparts. The University of Canterbury Monosyllabic Adaptive Speech Test (UC MAST) was performed on 23 normal adults, and 32 normal children (7 to 11 years of age). The child participants also underwent the SCAN-C test for APD in Children (Revised). Findings indicated a significant maturational effect on the UC MAST. Adult participants performed significantly better on the UC MAST in comparison to the child participants. In addition, adult participants performed the UC MAST more reliably than their younger counterparts. No correlation was found between performance on the UC MAST and SCAN-C test. The development of the UC MAST is discussed and the clinical implications of the findings are explored.

Dissertation (Ph.D.)--Auburn University, 2006. / Abstract. Includes bibliographic references.