The effect of amplitude modulation related grouping on the precedence effect
Bernhard U. Seeber
MRC Institute of Hearing Research
Science Rd., University Park, Nottingham, UK, NG7 2RD

The precedence effect describes our ability to suppress detrimental effects of 
reflections on localisation of sound sources in rooms. We recently demonstrated 
with cochlear implant simulations that the precedence effect can break down such 
that the reflection is audible in addition to the leading sound. It appears that 
the reflection can not be grouped, integrated with the leading sound, hence it is 
heard separately as an echo. How does the auditory system know which reflection 
belongs to a particular direct sound, particularly at long delays? 

    Studies on auditory scene analysis showed that common onsets and coherent 
amplitude modulation (AM) serve as grouping cues. For the precedence effect, where 
lead and lag do not occur simultaneously, this suggests that onsets should 
facilitate segregation of the lag from the lead. On the other hand, AM in the 
envelope could provide information to identify the lag as a copy of the lead 
despite the delay. 

    We investigated the effect of AM on echo thresholds for independent lead-lag 
delays of the fine structure and of the amplitude modulation of the envelope. AM 
had a strong influence on the precedence effect: at some envelope delays between 
lead and lag, no echo thresholds could be determined, while for others, echo 
thresholds were elevated above the unmodulated baseline. The latter suggests that 
AM can serve as a grouping cue, not only modulating but even enhancing the fusion 
of the lag with the lead, as it is visible through changes of the echo threshold. 
The echo threshold was partly predicted by the coherence between lead and lag, a 
measure of fine structure similarity. This is because the AM attenuates parts of 
the signal, thus affecting coherence between lead and lag at particular delays. 

Acknowledgements
This work was supported by the MRC.