Simulated interaural place mismatch of cochlear implant electrodes 
leads to breakdown of the ability to suppress reflections

Bernhard U. Seeber
MRC Institute of Hearing Research, Nottingham, UK

Background: Cochlear implants (CIs) often restore speech understanding 
in quiet, but most patients complain that the presence of reverberation 
or noise makes understanding of speech more difficult or even 
impossible. The precedence effect is thought to help in those 
situations as it shows the perceptual suppression of sound reflections 
while localization is maintained at the leading, the direct sound. In 
our previous study of the precedence effect about half of the tested 
bilateral CI-patients showed no precedence effect; instead a single 
sound source was localised in-between the leading and the lagging 
source while a few patients showed the precedence effect even for 
temporally overlapping stimuli. Simulations of the precedence effect 
with a noise-band vocoder instead showed a breakdown such that the 
reflection was always audible. 

Aims: (1) To test if the precedence effect can be evoked with 
CI-simulations based on a sinusoidal vocoder. 
(2) To test if a place mismatch of CI-electrodes between the ears 
affects the ability to suppress reflections. 

Methods: The precedence effect was tested in a localization dominance 
paradigm with a sinusoidal vocoder for brief noise bursts and a word. 
The interaural match of the carrier frequencies was varied. 

Results: The precedence effect broke down, i.e. the lag was audible, 
if carrier sinusoids were not matched in frequency. However, for zero 
or small frequency offsets some subjects showed the precedence effect 
and heard one sound at the lead location regardless of the interaural 
time difference (ITD) conveyed in the carrier. 

Conclusions: It is possible to evoke the precedence effect with ongoing 
sounds without encoding ITDs in the carrier. The precedence effect can 
solely be based on interaural level and time differences conveyed in 
the envelope if carrier frequencies have a similar frequency. 
This suggests that accurate place matching of CI-electrodes would 
help the analysis of concurrent sounds.