INTERNATIONAL CONGRESS ON ACOUSTICS MADRID , 2-7 SEPTEMBER 2007 THE PLASTIC EAR : COPING WITH A LIFETIME OF CHANGE PACS : 43

The shape of the pinna results in spectral cues to sound location. Over a life time the pinna shape changes. Previous work has demonstrated that, over several weeks, subjects can learn new spectral cues for locations in the audio-visual (AV) region of space. In this study we examined the extent and time course of relearning for spatial locations inside and outside the AV region. Silicon moulds were worn in the pinnae of 8 subjects for up to 62 days and auditory localization performance was measured for 76 locations surrounding the listener. There was a significant increase in the cone-of-confusion errors and decreased elevation accuracy when the moulds were first inserted consistent with the disruption of spectral cues. Performance improved significantly for all subjects over accommodation periods of 30 to 62 days but remained significantly above control. During the accommodation, localisation performance did not show a linear improvements but was characterised by cyclic changes with a period of many days. Performance error at the end of accommodation period can be attributed in part to reduced spectral information in the “new” spectral cues and/or to re-learning beyond 5 weeks. INTRODUCTION Our perception of auditory space is dependent on acoustic cues extracted from the sounds at each ear. These cues include binaural difference in the time of arrival and the amplitudes of the sounds at each ear (the binaural cues) and the spectral filtering of the sound by the outer ear (the spectral or monaural cues). The highly convoluted and asymmetrical shape of the ears gives rise to complex variations in the filter properties so that each particular location in space can be associated with specific spectral profiles of the filter. These spectral cues are dependent on the precise morphology of the outer ears and the placement of the ears on the head. Over the course of a life time the outer ear changes subtly in size and shape and this will produce changes in the nature of the spectral cues to location [1]. So as to ensure the fidelity of our perception of auditory space, the auditory system needs to be able to recalibrate to these changes over time by mapping the evolving spectral profiles to specific positions in space. Both behavioural and electrophysiological work has demonstrated that the auditory system is to some extent capable of recalibrating to these changed cues (for recent reviews see [2, 3]). Importantly, this plasticity is not just restricted to a developmental period of rapid change but with the appropriate developmental experience, can occur throughout adulthood [4]. Perceptual adjustments have been demonstrated for short term exposure to modified cues [5] as well as more complete compensation for relatively prolonged changes [6]. There are a number of constraints or limitations on the ability of the auditory system to adapt to modified cues to sound location following both relatively short term or long term exposure. In one experiment that manipulated the interaural time difference (ITD) cue using a hearing aid [7], the introduction of abnormally long interaural delays produced initial perceived displacement of a frontal sound source away from the midline. Over the course of 3 5 days of chronic exposure, the perceived location adapted to half or less of the initial displacement. Incomplete adaptation was also reported for relatively short exposure times combined with a number of different training regimes [5]. In that experiment visual and auditory cues were provided for locations in the frontal hemisphere but were mismatched so that the auditory cues indicated locations more lateral to the visual cues. There was some adaptation of the perceived sound locations over the course of eight 2 hour sessions carried out over a number of weeks. A short acting after-effect was also observed with veridical auditory cues following adaptation to the altered cues. By contrast, in another experiment exploiting hearing aids the cues where