The effects of quinine-induced hearing loss on speech and other psychophysical tasks

Quinine causes a temporary disruption of outer hair cell (OHC) function, and thus can be used to examine the role of OHCs on auditory perception. This technique can avoid confounds that are often present in studies using listeners with permanent hearing loss, including aging and multiple loci of hearing loss. Normal-hearing listeners ingested 5.76-11.43 mg/kg body weight of quinine, resulting in 5-15 dB of hearing loss. The quinine-induced hearing loss (QIHL) was used to address two primary research questions. The first was to examine the effects of hearing loss due to OHC damage on speech recognition, and in particular on the relationship between speech recognition and suprathreshold processing measures of frequency selectivity and temporal resolution. Frequency selectivity was estimated by comparing the level of a noise masker needed to mask a fixed-level, 2-kHz signal when the masker contained a spectral notch at 2 kHz and when it contained no notch. Similarly, temporal resolution was estimated by comparing the masker level needed to mask the 2-kHz signal when the noise masker contained a temporal gap and when it did not. Speech recognition thresholds were measured in quiet and in the presence of a masker (speech-shaped noise or time-reversed speech) fixed at 70 dB SPL. Signal level was varied adaptively to estimate 50% correct recognition. Quinine resulted in reduced frequency selectivity and reduced temporal resolution. Quinine also elevated speech recognition thresholds in quiet (by 7 dB on average), but the thresholds in the presence of the maskers were unaffected. The second research question was to examine the effects of OHC damage on cochlear compression. Compression for a 2-kHz signal was estimated psychophysically using growth-of-masking (GOM) functions, and physiologically using distortion product otoacoustic emission (DPOAE) input-output (I-O) functions. Under normal-hearing conditions, the magnitude of compression was generally stronger when estimated via DPOAE I-O functions than with GOM functions. The magnitude of compression was reduced in the majority of subjects following a QIHL when estimated with GOM functions, but remained relatively unchanged when estimated using DPOAE I-O functions.