BINAURAL MEASURES OF COCHLEAR IMPLANT LISTENERS

Bernhard U. Seeber and Ervin Hafter

3210 Tolman Hall, Dept. of Psychology, Univ. of California at
Berkeley, USA 

The Simulated Open Field Environment creates realistic auditory scenes
with simulated sources and echoes inside an anechoic chamber. By using
48 loud-speakers arranged horizontally around the subject the patient
can listen with his own device in the free-field which by definition
assumes the correct HRTFs.  
A visual display is used for localization responses and provides a
simulated 3D-scene. We are in the process of compiling a binaural
hearing test suite for CI listeners consisting so far of the following
experiments: 

1) A 360° horizontal localization test allows for measures of
front/back confusions. Responses are taken on a continuous scale which
provides better resolution that e.g. speaker identification. Synthetic
(low-pass and wide-band noise) as well as natural sounds (noises,
speech, police siren, dog bark) allow predictions of performance in
every day conditions.  
Results: One bilateral CI listener shows no localization ability with
the single, newer implant, but only 26.6° rms-error for both CIs if
front/back confusions are resolved. Front/back confusions are 52%
which is far higher than for a normal hearing subject (2%). Another
patient with CI and hearing aid in combination showed 36.6° rms-error
if 33% confusions were resolved.  

2) Frontal localization: A test for localization of frontal targets
with a light pointer method allows for accurate estimates of
localization performance.  

3) Impact of echoes on localization: Localization is studied for
wide-band and speech sounds in situations of summing localization and
precedence. Previous studies showed that the excellent localization
ability of a  CI-patient was entirely based on the evaluation of
interaural level cues (ILDs) (Seeber and Fastl, DAGA 2003, 2004). As
interaural timing cues play a crucial role in precedence we suppose
that CI-patients do not show the precedence effect. Instead, we assume
that the localized direction is determined by the ILD-sum of source
and echo which would lead to a shift towards the direction of the echo
as in summing localization. In this case echoes would destroy the
localization information for CI patients. Results from a monaural CI
patient with residual hearing on the contralateral side showed no
effect of echo delay time on localization.  

4) The echo threshold was studied by adjusting the delay time between
source and echo pairs to find the time at which the echo becomes
audible and thus disturbing. Results of a bilateral CI patient show
echo thresholds for clicks similar to thresholds of normal hearing
subjects which can be attributed to gap detection. The patient was not
able to give consistent threshold estimates for a CVC word because he
could not parse source and echo apart, suggesting reduced precedence. 

Support provided by NOHR 018750 and NIH RO1 DCD 00087.