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Sound Localisation And Lateralisation In Bilateral Implant Users Schön, F, Müller, J,
Helms, J Introduction: Bilateral implanted patients benefit from their two CIs in different ways. In this context, the question arises, whether they re-gain the ability of directional hearing and which binaural cues they can use. Methods:Sound localisation abilities were examined in 11 bilaterally implanted patients. 500ms bursts of CCITT noise and of CCITT noise filtered by the head shadow were presented at 70, 75 and 80 dB. The bursts were delivered from one out of seven loudspeakers positioned between –90° to 90° azimuth. The subject had to indicate the loudspeaker from which the stimulus was presented. Sensitivity to interaural level differences (ILD) was assessed by performing localization tests (4 subjects) with the volume on the two speech processors unbalanced to various degrees. To investigate the subjects' sensitivity to interaural time differences (ITD), lateralisation was measured (7 subjects) as a function of the time difference between two Gaussian-like pulses, each presented to one of the subject's ears via headphone. Results: All subjects
showed a significant correlation (10-7 < p < 10-110) between
the position and the judged direction of the loudspeaker. Nevertheless,
subjects differed in their response behaviour. Precision of the judgments
as expressed with the standard deviation of the judgements varied
in a wide range from about 10° to 50°. The response characteristics
showed a central bias to various extent, as is also found in normal
hearing subjects. Unbalanced loudness of the speech processors produced
a mean shift of the judged direction of 1.4 degree/Hel (1 Hel = 1
point on the used loudness scale with 50 subdivisions). Six out of
seven subjects showed a significant sensitivity to ITDs. A mean time
difference of 1.2 ms produced a complete lateralisation. Conclusions: Bilateral
cochlear implantation potentially restores spatial hearing in cochlear
implant users. Similar to normal hearing subjects, CI patients can
use level und time difference cues for sound localization.
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