Interference of echoes on spatial perception with cochlear implants

Bernhard Seeber and Ervin Hafter
Dept. of Psychology, 3210 Tolman Hall #1650, University of California
at Berkeley, Berkeley, CA 94720-1650, USA

Natural environments provide us with plentiful acoustic information
about the direction and distance of sound sources. In rooms, however,
the directional information is affected by the presence of echoes
interfering with the primary sound. For the information to be
evaluated, both have to be separated from each other. Patients with
cochlear implants (CIs) demonstrate a pronounced difficulty in
understanding speech in multi-talker situations whereas quiet and
acoustically dry environments pose only little problems. This is
especially important for children who learn in classroom environments
which are often not as quiet or acoustically treated as would be
necessary for good speech understanding.  
The present study investigates the impact of echoes on the perceived
location of a primary sound in dependence of echo delay time. Normal
hearing subjects show a suppression of the effect of the lagging
sound on the perceived direction of the leading sound. This is termed
'precedence' or 'the law of the first wavefront' and expresses our
ability to locate sounds in rooms. The effect is mainly based on the
evaluation of interaural temporal cues (ITDs) at low frequencies.
Previous studies showed that bilateral CI-patients do not evaluate
ITDs for localizing sounds but use interaural level cues instead. The
current study focuses on the question if precedence can be obtained
with CIs.  
Subjects localized a wide-band-noise or the speech utterance "shape"
when played from speakers at ±30° with varying interstimulus-delays.
Four subjects with bilateral CIs were tested, two of which showed
some ability in a baseline test of localization. None of them showed
the precedence effect. If a time delay was introduced between lead
and lag, sounds were localized at both loudspeakers separately, even
for short delay times of 0.5ms. This suggests that the leading and
lagging sound are perceptually segregated, and echoes are not
suppressed. Studies with vocoder-simulations of CIs are underway to
determine if the information present in ILDs and envelope-ITDs is
sufficient to evoke the precedence effect in normal hearing subjects.
 
The lecture will highlight how binaural information is altered with
CIs, which perceptual limitations derive from this alteration, and
how the limitations could be overcome in future generations of CIs.
Support provided by NIH and NOHR.