Behavioral/Systems/Cognitive Relearning Sound Localization with a New Ear

Humansound localization results primarily fromtheprocessingof binaural differences in sound level andarrival time for locations in the horizontal plane (azimuth) andof spectral shape cues generatedby thehead andpinnae for positions in the vertical plane (elevation). The lattermechanism incorporates twoprocessing stages: a spectral-to-spatialmapping stage and abinauralweighting stage that determines the contribution of each ear to perceived elevation as function of sound azimuth. We demonstrated recently that binaural pinna molds virtually abolish the ability to localize sound-source elevation, but, after several weeks, subjects regained normal localization perfor-mance. It is not clearwhichprocessing stage underlies this remarkable plasticity, because the auditory systemcouldhave learned thenew spectral cues separately for each ear (spatial-mapping adaptation) or for one ear only, while extending its contribution into the contralat-eral hemifield (binaural-weighting adaptation). To dissociate these possibilities, we applied a long-termmonaural spectral perturbation in 13 subjects. Our results show that, in eight experiments, listeners learned to localize accurately with new spectral cues that differed substantially from those provided by their own ears. Interestingly, five subjects, whose spectral cues were not sufficiently perturbed, never yielded stable localization performance. Our findings indicate that the analysis of spectral cues may involve a correlation process between the sensory input and a stored spectral representation of the subject’s ears and that learning acts predominantly at a spectral-to-spatial mapping level rather than at the level of binaural weighting. Key words: human; monaural; spectral cues; pinna; directional hearing; plasticit