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Bone-conduction auditory steady-state responses Small, Susan Anne

Abstract

The purpose of this dissertation was to determine whether multiple auditory-state responses (ASSRs) to bone-conduction stimuli can be used to investigate bone-conduction hearing, an essential part of any audiometric assessment. Infant bone-conduction testing methods, maturation of bone-conduction hearing, and ipsilateral/contralateral asymmetries in ASSRs were also assessed. The results show that bone-conduction ASSRs can be used to estimate thresholds in infants and adults with normal hearing. It was also found that choice of electroencephalogram (EEG) conditioning and processing can avoid spurious ASSRs due to aliasing. Non-auditory ASSRs (probably vestibular and indistinguishable from an auditory response) were also identified for high-intensity air-conduction stimuli (problematic when diagnosing residual hearing). Investigation of infant testing methods on bone-conduction threshold shows that: (i) bone-oscillator coupling method (elastic-band vs. hand-held) has no effect on threshold, (ii) use of different oscillator locations on the temporal bone does not affect threshold but a forehead placement results in elevated thresholds, and (iii) infants do not appear to have an occlusion effect (thus one can can assess with or without earphones). Young infants have much better low-frequency bone-conduction hearing compared to adults, which increases with maturation beyond 24 months of age. Infant bone-conduction hearing is slightly poorer in the high frequencies, improving significantly with age only at 2000 Hz. Within all infant groups, low-frequency thresholds are better than high-frequency thresholds; for adults, 500-Hz thresholds are poorer than high frequencies and there is no difference among thresholds above 500 Hz. Bone-conducted signals are much more effective for infants across frequency, especially at low frequencies. Normal levels for bone-conduction hearing in young and older infants are proposed. Ipsilateral/contralateral asymmetries in air- and bone-conduction ASSRs are clearly present more often and are larger in infants compared to adults, and suggest that most infants have 10-30 dB of interaural attenuation. These asymmetries have potential as a clinical tool for isolating the cochlea that is contributing to the response in infants. The results of these studies indicate that infants can now be screened for normal bone-conduction hearing with ASSRs; however, infants with hearing loss must be tested before elevated bone-conduction ASSRs thresholds can be interpreted.

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