The contribution of temporal fine structure and envelope information to 
performance in multi-source, reverberant environments 

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

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. This is thought to be based on the evaluation of temporal fine 
structure – information which is not accurately transmitted by CIs. Because of 
the strong information transmission limits at the neural interface in CIs, 
knowledge of the essential information to function in complex environments is 
crucial for a targeted development of future CI-strategies. 

We have developed the Simulated Open Field Environment, a loudspeaker setup 
to reproduce the reverberant sound field of multiple sources in the free-field of 
an anechoic chamber, to investigate hearing mechanisms in difficult listening 
situations with normal-hearing participants and patients alike. 

In our 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. This suggests that 
those patients can not distinguish the binaural information in lead and lag, 
resulting in an average. However, selected patients showed the precedence 
effect even for temporally overlapping stimuli while instead the simulation of the 
paradigm with normal hearing listeners using a noise-band vocoder resulted in a 
breakdown such that the reflection was always audible. 

Subsequent studies were aimed at finding the information needed to evoke the 
precedence effect with diminished acoustic information as in CIs. Results show 
that 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. However, carrier 
frequencies need to be similar in both ears, suggesting that accurate place 
matching of CI-electrodes would help the analysis of concurrent sounds. Further 
results suggest that envelope fluctuations are even needed to evoke the 
precedence effect.