Binaural hearing in reverberant space with cochlear implants

Bernhard U. Seeber1,2, Stefan Kerber1, Ervin R. Hafter2

1 MRC Institute of Hearing Research, Nottingham, UK
2 Department of Psychology, UC Berkeley, Berkeley, CA, USA

Benefits from bilateral cochlear implants (CIs) have been 
demonstrated for directional speech perception and sound 
localization. Most of these studies took place in laboratory 
settings which did not resemble real-world listening situations. 
There, patients frequently report localization problems caused by 
noise and reverberation. Normal-hearing listeners possess 
mechanisms to locate sources correctly despite the presence of 
sound reflections, but it is questionable if these precedence 
effect mechanisms are available to CI users. We have studied the 
impairment caused by reverberation which also sheds light on the 
involved binaural processes.

Horizontal sound localization was tested in the free-field of our 
“Simulated Open Field Environment”. Sound reflections of a small 
room were calculated and played from individual loudspeakers such 
that their spectral, temporal and spatial properties were 
reproduced. Results of the initial study by Seeber and Hafter with 
ten bilateral CI users showed that localization ability declined 
considerably in moderate reverberation. Whilst eight listeners 
were able to discriminate the side of sound incidence for a single 
word in anechoic space, only five preserved this ability in 
reverberation. Localization was better for repeated than single 
noise bursts, suggesting that binaural information needed 
integration over time. 

In a subsequent study, localization ability was measured at 
varying direct-to-reverberant ratios (DRR). While normal hearing 
participants showed no localization impairment with spoken words 
down to about -8 dB DRR, localization of CI users was already 
affected at positive DRRs between 0 and +10 dB. In many natural 
listening situations, however, DRRs are negative. It is thus 
questionable if patients would receive the full (localization) 
benefit from their second implant when listening in rooms. 

In order to better understand the underlying mechanisms, 
lateralization with interaural level (ILD) and time (ITD) 
differences was investigated in a direct stimulation paradigm. 
Whilst all participants were able to lateralize based on ILDs, 
only some showed adequate sensitivity to envelope ITDs. These also 
performed better in reverberant space, suggesting that envelope 
ITDs were used. Previous studies in anechoic space had instead 
indicated that sound localization with CIs is based mostly on 
ILDs. 

Supported by NIH RO1 DCD 00087, NOHR 018750, the Intramural 
Programme of the MRC, and Cochlear Corp.