Sieveke, Ida; Leibold, Christian; Kaiser, Katharina; Grothe, Benedikt; Wiegrebe, Lutz
Adjustment of interaural-time-difference analysis to sound level.
15th International Symposium on Hearing (ISH), 1. - 5. Juni 2009, Salamanca, Spanien.
To localize low-frequency sound sources in azimuth, the binaural system
compares the timing of sound waves at the two ears with microsecond precision.
A similarly high precision is also seen in the binaural processing of the envelopes
of high-frequency complex sounds. Both for low- and high-frequency sounds,
interaural time difference (ITD) acuity is to a large extent independent of sound
level. The mechanisms underlying this level-invariant extraction of ITDs by the
binaural system are, however, only poorly understood. We use high-frequency pip
trains with asymmetric and dichotic pip envelopes in a combined psychophysical,
electrophysiological, and modeling approach. Although the dichotic envelopes
cannot be physically matched in terms of ITD, the match produced perceptually by
humans is very reliable, and it depends systematically on the overall sound level.
These data are reflected in neural responses from the gerbil lateral superior olive
and lateral lemniscus. The results are predicted in an existing temporal-integration
model extended with a level-dependent threshold criterion. These data provide a
very sensitive quantification of how the peripheral temporal code is conditioned
for binaural analysis.