Bilateral Cochlear Implant Psychophysics

Bernhard U. Seeber and Stefan Kerber


Bilateral cochlear implantation developed rapidly over the past 10 years and it 
became the norm in many countries when implanting young deaf children. Benefits of 
bilateral implantation are nevertheless under scrutiny, not least because of the 
high costs involved. Cochlear implants were originally developed to transmit only 
the slow rate information present in speech and that basic working principle has 
changed little since. In this talk I will review the benefits obtainable by 
bilateral implantation and contrast them with the limitations likely observed in 
everyday listening situations. 

Patients with bilateral cochlear implants generally report that sound localization 
improved with their second device but localization problems persist in rooms and 
noisy situations. While normal hearing people are able to largely ignore sound 
reflections from room boundaries when localising sounds, our initial studies 
suggested that for cochlear implant users even moderate room reverberation causes 
localization ability to deteriorate. The present study examined localisation 
performance of cochlear implant patients in simulated realistic rooms with 
parametrically varied amounts of reverberation. 

Studies were conducted in the Simulated Open Field Environment, a setup of 48 
loudspeakers in an anechoic chamber to synthesise the sound field of realistic 
rooms. Reflection patterns for sound sources in rooms were calculated and played 
from individual loudspeakers in such a way that spectral, temporal and, most 
importantly, spatial properties of each reflection were like in a real room. Using 
these simulations, sounds were presented at varying levels of reverberation, i.e. 
direct-to-reverberant ratios (DRR).  Participants indicated the perceived direction 
of the sound by positioning a light spot.

Results showed that the presence of reverberation was highly detrimental for 
localization ability in all patients. While normal hearing participants showed no 
impairment of localization of single words down to DRRs of -8 dB, the best cochlear 
implant patient only tolerated reverberation down to 0 dB DRR, and most patients 
showed some localization impairment already at positive DRRs. In many natural 
listening situations, however, DRRs are usually negative and depend, besides on 
room parameters, on the distance of the listener from the sound source. DRRs in the 
simulated room used in this study were -3 dB at a distance of 1.3 m. In most 
classroom situations children with bilateral CIs would sit further from the teacher 
and thus at more negative DRRs. It is thus questionable if those children would 
receive the full benefit from their bilateral implants in listening situations 
where they most need them.