Binaural integration for speech in noise and sound localization: impact of brain plasticity following unilateral hearing loss

In patients with unilateral hearing loss (UHLp), binaural processing is obviously disrupted and spatial localization of the sound source is impaired as well as the ability in understanding speech in noisy environments. At the brain level, a limited number of studies have explored the functional reorganisation that occurs in the adult after a unilateral deafness. We conducted an original study aimed at investigating in UHLp the relationships between the severity of unilateral hearing loss, the resulting deficit in binaural processing and the extent of cortical reorganisation across the auditory areas.
We have recruited 14 UHL patients (hearing loss 37-120 dB HL) and aged-matched hearing controls. All subjects were evaluated for free-field sound localization abilities and speech in noise comprehension (French Matrix test). All subjects went through a fMRI protocol to evaluate the activation pattern across auditory areas during a natural sounds discrimination task. First, a the neuronal level in the auditory cortex we observed a lack of suppression/occlusion mechanisms that characterize binaural integration. Second, the brain imaging analysis clearly demonstrated that in non-primary areas (NPAC), UHL induces a shift toward an ipsilateral aural dominance. Such reorganization, absent in the PAC, is correlated to the hearing loss severity and to lower spatial localization ability performances. Second, a regression analysis between brain activity and patient’s performances, clearly demonstrated a link between the spatial ability deficit and a functional alteration that impacts specifically the posterior auditory areas known to process spatial hearing. On the contrary, the core of the auditory cortex appeared relatively preserved and maintains its normal implication in processing non-spatial acoustical information.
Altogether our study adds further evidences of a functional dissociation in the auditory system and shows that binaural deficits induced by UHL affect predominantly the dorsal auditory stream.