Perception of concurrent sounds in precedence situations with bilateral cochlear
implants 

Bernhard Seeber

Auditory Perception Lab, UC Berkeley, USA
now: MRC Institute of Hearing Research, Nottingham, UK


The analysis of auditory scenes poses a severe problem to many cochlear implant
(CI) subjects: Single sounds can be identified and localized, but the presence of
multiple sounds impairs this ability. The present study investigated the effect of
single and multiple echoes on localization. In normal hearing the precedence
effect allows for unimpaired localization at the leading sound despite the
presence of a delayed sound copy from a different direction. CI subjects, however,
have limited access to temporal, spectral, and binaural cues which form basis for
segregating sounds. With CIs 3 outcomes are conceivable in precedence situations:
(1) Precedence exists as in normal hearing, (2) The reduced temporal resolution
leads to an integration of binaural information from lead and lag and a single
image is heard somewhere, e.g. halfway, between both source locations, (3) The lag
is not suppressed, i.e. lead and lag are both heard separately. The results show
evidence for all 3 possible outcomes. Remarkable is that some patients showed
precedence for temporally overlapping stimuli. This requires access to binaural
cues of the lead despite a strong waveform interaction with the lag. In the other
astonishing case patients were able to segregate lead and lag at very short delays
which must have relied on binaural information from both locations. Alternately,
in the view of auditory scene analysis they were not able to fuse lead and lag
into one auditory image, unlike patients who showed precedence.  

Support was thankfully provided by NIH RO1 DCD 00087 and NOHR grant 018750.