Adaptation to Unilateral Hearing Loss in Humans: Cortical and Perceptual Effects

The goal of this project is to increase understanding of adaptation in human auditory cortex after peripheral hearing loss in adults. Two types of cortical adaptation have been studied for hearing: expansion of cortical representation for the frequency that is at the edge of a sharply-sloping hearing loss and a reduction in the usual pattern of cortical lateralization (more contralateralthan ipsilateral activity) in cases of unilateral deafness. We have used functional magnetic resonance imaging (fMRI) and psychoacoustictasks to investigate the relationship between cortical adaptation and perceptual ability in two individuals with a sloping hearing loss in one ear and normal hearing in the other ear. Both individuals showed reduced lateralization of activity in auditory cortex in response to sounds heard in their normal ear as compared to their impaired ear. Reduced lateralization after stimulation of the normal ear was attributable to an increase in ipsilateral cortical activation and was observed both at frequencies where hearing was absent in the impaired ear and also where it was fully preserved as indicated by audiometric criteria. Evidence of perceptual changes in the patients that we studied was found in two binaural listening tasks where a signal was presented in one ear and a distractor sound in the other. Performance on signals presented to the impaired ear was more susceptible to interference from the normal ear than vice versa even at frequencies where audiometric sensitivity in the two ears was equivalent. Together, the fMRI results and psychoacoustic results provide a compelling parallel between cortical adaptation and perceptual function in adult humans. This project uses fMRI to determine further the nature and extent of long-term neural adaptation in individuals with unilateral hearing impairment, and psychoacoustic experiments to determine the nature of the perceptual changes that are associatedwith corticaladaptation. Increased understanding of long-term cortical adaptation to sensory loss has the potential to provide insight into how processes of learning are constrained both by patterns of neural connectivity created during development and by the limitations on neural growth in mature animals. Increased understanding of cortical adaptation also has the potential to shed light on how higher-level neural processes influence the benefits and limitations of interventions that restore (or partially restore) lost sensory input.