Mismatch Negativity a Neural Marker of Auditory Spatial Adaptational Plasticity in Unilateral Hearing Loss patients

Overview & Synopsis

This thesis investigates different spatial hearing functions in 3 types of populations: Normal Hearing Subjects (NHS), Unilateral Hearing Loss patients (UHL) and Bilateral Hearing Loss patients ( BHL). To discover the mechanisms underlying the adaptive strategies that are observed in UHL with acquired deafness. The main aim of the thesis is to verify whether spatial Mismatch Negativity (MMN) could be a neuronal marker of spatial auditory plasticity observed in UHL patients, and to verify whether these neural correlates are consistent with the spatial auditory performance. Two types of investigations were applied to 20 NHS, 21 UHL and 14 BHL. The first investigation is a sound source identification task measured by the root mean square error (RMS). The second assessment is an electroencephalography (EEG) study where we analyzed the amplitude and latency of the MMN. MMN is defined as an auditory evoked potential that reflects the brain's ability to detect a change in one physical property of a sound. We used a standard sound in a reference position (50°) with three deviations from the standard (10° , 20°, and 100°), in binaural and monaural conditions. UHL patients were divided into 3 groups according to their spatial performances. The group of good performers (UHL {low rms}) showed better RMS scores in comparison with NHS with earplugs (NHS-mon), with performances similar to those of NHS subjects in binaural condition. A progressive increase of the MMN with the angle of deviation from the standard was noted in all groups. With a significant reduction of MMN amplitude in monaural NHS when the ear plug was applied on the ipsilateral side of the standard. MMN showed consistent variation with the behavioral observations, where UHL {low rms} patients had larger MMN amplitudes than those of monaural NHS and similar to those of binaural NHS. UHL patients have adaptive spatial auditory strategies. Our study was able to demonstrate that spatial auditory plasticity that occurs after deafness can be reflected by the MMN. Neural observations (i.e. the MMN) are correlated with behavioral observations of spatial source identification. This means that the spatial cortical plasticity, that took place in these subjects, is not limited to the functions of identification of the sound source, but exceeds these capacities towards more complex mechanisms such as deviance detection and short-term memory, that are involved in the spatial discrimination function.