Binaural Performance in Normal-hearing Young Adults Influenced by Short-term Induced Unilateral Conductive and Sensory Changes

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There are no data available in the literature that have specifically evaluated differences in adaptation to unilateral conductive or sensory changes. However, based on clinical experience it may be postulated that changes of outer or middle ear function appear to be tolerated more easily than those of cochlear origin. Very often, patients seen in the clinic are unaware of a slight conductive hearing loss. By contrast, patients are immediately disturbed by a minor decline of cochlear function. One of several complaints of these patients is a change in their spatial orientation or difficulties in understanding speech in a noisy environment . The goal of the study was to determine if binaural performance tested psychoacoustically using a lateralization task is influenced differentially by short-term induced unilateral conductive or sensory changes. Lateralization performance was evaluated in seven normal-hearing subjects during induced auditory periphery asymmetry resulting from: l. exposure to noise presented for 5 minutes at 115 dBA SPL or 2. bilateral occlusion with earplugs of unequal attenuation for 48 hrs. An adaptive procedure was used to detenmine hearing thresholds of a 4-kHz narrow-band noise (NBN). In a lateralization task subjects indicated the positions of intracranial images created by the same NBN pr esented binaurally at SO dB SL with interau ral level differences ( I LDs) varying within plus/minus 12 dB. The tests were performed over a one-hour period post-exposure, immediately prior to and following plugging the ears, and at 24 and 48 hrs post-plugging. Immediately after the exposure or after plugging, there was a shift of lateral ization towards an unexposed side or the side blocked by the plug with a smaller attenuation, respectively. After a few minutes post-exposure, signals with I LD=0 were lateralized at midl ine. Within 30 minutes post -plugging, those signals were gradually lateralized closer to midline but remained off center for the rest of the plugging period. Thus, subjects showed fast adaptation to induced unilateral sensorineural changes and incomplete adaptation to induced asymmetrical conductive changes. Those rather unexpected r esults can be explained using a qualitative model assuming that: 1. a conductive impainment reflects a loss of sensitivity and 2. a cochlear impairment reflects both a loss of sensitivity and of the compressive nonlinearity on the basilar membrane. Recently, there has been an increase in the number of psychoacoustical studies on hearing-impaired listeners with a majority of them directed toward revealing deficits in monaural processing. However, in most acoustic environments encountered in everyday life, there are multiple sounds originating from different sources, and hearing-impaired people often display less binaural advantage than do normally hearing persons. The results of the cu rrent study support the view of the lack of a simple relationship between monaural and binaural processing, which is often r eported in studies on hearing-impaired people. This is an important issue in the process of fitting hearing aids binaurally.


Dallas, TX

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