Factors affecting envelope ITD discrimination at high frequencies

Monaghan, Jessica J.M., Krumbholz, Katrin and Seeber, Bernhard U.

MRC Institute of Hearing Research, University Park, Nottingham, UK

In acoustic hearing at low frequencies interaural time differences are
extracted from the fine structure of sounds, but at high frequencies loss
of phase locking means that only envelope ITDs can be used. Cochlear
implant patients, who can only obtain interaural time differences from
envelopes, do not appear to be able to use them for localisation, perhaps
because envelope cues are more easily degraded by the addition of
reflections in real environments.
Using participants with normal hearing we investigated the factors
affecting discrimination of ITDs in the envelopes of high frequency
stimuli. Stimuli with low pass noise envelopes were used to investigate
the effect of a reduction in binaural coherence, but these stimuli led to
very high thresholds for all conditions.
A novel stimulus was developed to investigate the effect of binaural
coherence on envelope ITD discrimination while reducing confounding
factors such as changes in modulation depth or co-variation of onset
slope with modulation rate and the number of presentations per time
interval. Envelope coherence was varied without reducing modulation
depth by temporally jittering the position of raised cosine pulses while
ensuring that they did not temporally overlap. The envelope was used to
modulate a 4 kHz carrier tone. In two further experiments these novel
stimuli were modified to separately investigate the effect of onset slope
and modulation depth. Room simulations were performed to estimate the
reduction in modulation depth onset slope and binaural coherence that
occur in rooms. Results will be presented.

Acknowledgements
This work was funded by the Intramural Programme of the MRC.