Abstract
Beyond reduced audibility, there is convincing evidence that the auditory system adapts according to the principles of homeostatic plasticity in response to a hearing loss. Such compensatory changes include modulation of central auditory gain mechanisms. Earplugging is a common experimental method that has been used to introduce a temporary, reversible hearing loss that induces changes consistent with central gain modulation. In the present study, young, normal-hearing adult participants wore a unilateral earplug for two weeks, during which we measured changes in the acoustic reflex threshold (ART), loudness perception, and cortically-evoked (40 Hz) auditory steady-state response (ASSR) to assess potential modulation in central gain with reduced peripheral input. The ART decreased on average by 8 to 10 dB during the treatment period, with modest increases in loudness perception after one week but not after two weeks of earplug use. Significant changes in both the magnitude and hemispheric laterality of source-localized cortical ASSR measures revealed asymmetrical changes in stimulus-driven cortical activity over time. The ART results following unilateral earplugging are consistent with the literature and suggest that homeostatic plasticity is evident in the brainstem. The novel findings from the cortical ASSR in the present study indicates that reduced peripheral input induces adaptive homeostatic plasticity reflected as both an increase in central gain in the auditory brainstem and reduced cortical activity ipsilateral to the deprived ear. Both the ART and the novel use of the 40-Hz ASSR provide sensitive measures of central gain modulation in the brainstem and cortex of young, normal hearing listeners, and thus may be useful in future studies with other clinical populations.