Relearning Sound Localization with a New Ear

Abstract

Human sound localization results primarily from the processing of binaural differences in sound level and arrival time for locations in the horizontal plane (azimuth) and of spectral shape cues generated by the head and pinnae for positions in the vertical plane (elevation). The latter mechanism incorporates two processing stages: a spectral-to-spatial mapping stage and a binaural weighting 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 performance. It is not clear which processing stage underlies this remarkable plasticity, because the auditory system could have learned the new spectral cues separately for each ear (spatial-mapping adaptation) or for one ear only, while extending its contribution into the contralateral hemifield (binaural-weighting adaptation). To dissociate these possibilities, we applied a long-term monaural 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 spectralto-spatial mapping level rather than at the level of binaural weighting.

Relearning Auditory Spectral Cues for Locations Inside and Outside the Visual Field

“…While there has been no systematic study of what constitutes 'sufficient' residual cues for adaptation, previous studies have shown that retaining some elevation dependent change in spectral features, such as the mid frequency spectral notch or high (8 to 16 kHz) spectral grains, results in accommodation over a period of weeks of exposure to the mould (Hofman et al 1998). Similar mould effects can be seen in van Wanrooij and van Opstal (2005) where the bare ear on one side of the head was compared to the other ear wearing a mould. In the current study, DTFs measured with the moulds in place clearly indicate that elevation-dependent changes remain in the mid frequency notch and the high frequency gain as does a substantial mid frequency gain in the 3 to 6 kHz region (Fig.…”

“…Previous work has indicated that accommodation does not necessarily proceed in a linear fashion. When a mould was inserted in one ear, Van Wanrooij and van Opstal (2005) reported a sinusoidal pattern in the changes of perceived elevation for the AV region with a period of between 6 and 10 days. While we observed some evidence of nonlinear patterns in some subjects, no clear sinusoidal pattern was evident in any of the performance measures we used here.…”

“…Middlebrooks et al 1989). So as to compare the acoustic effects produced by our moulds with data obtained in their work, we have calculated the SI according to van Wanrooij and van Opstal (2005). In that study, the SI was calculated for six subjects using a selection of frontal midline locations and varied between 0.22 to just under 0.5.…”

“…Van Opstal and colleagues manipulated the spectral cues of one ear (Van Wanrooij and Van Opstal 2005) or both ears (Hofman et al 1998) for periods of 19 to 39 days. Localization performance was tested for locations within the audio-visual region of space (±30°: Hofman et al (1998) and ±75°: Van Wanrooij and Van Opstal (2005) in azimuth and elevation) using eye or nose pointing, respectively.…”

“…Localization performance was tested for locations within the audio-visual region of space (±30°: Hofman et al (1998) and ±75°: Van Wanrooij and Van Opstal (2005) in azimuth and elevation) using eye or nose pointing, respectively. Bilateral moulds produced an initial disruption of vertical, but not horizontal, localization acuity-a finding which is consistent with the view that these moulds substantially disrupted monaural spectral cues but had little effect on binaural cues.…”