Possible Implications of Interaural Mismatch
in the Place-of-Stimulation
on Spatial Release from Masking
in Cochlear Implant Listeners
Alan Kan, Corey Stoelb, Matthew Goupell,
Ruth Litovsky
Binaural Hearing and Speech Laboratory
Cochlear Implants (CI) restore hearing
Source: http://www.cochlear.com/uk/node/6858
Binaural Hearing and Speech Laboratory
Binaural cues tells us where sounds
come from
Interaural time
difference (ITD)
Bang!
Interaural level
difference (ILD)
Binaural Hearing and Speech Laboratory
Spatial Release from Masking (SRM)
The improvement in speech-in-noise
understanding gained from a separation
between target and masker sources
Masker
Target
Binaural Hearing and Speech Laboratory
Normal hearing listeners
SRM (dB)
SRM (dB)
CI users
Loizou et. al. (2009)
Binaural Hearing and Speech Laboratory
Interaural place-of-stimulation
mismatch
Base
Apex
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Left
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Right
20000 Hz
7000 Hz
4000 Hz
20 Hz
Binaural Hearing and Speech Laboratory
Mismatch affects SRM
Binaural Hearing and Speech Laboratory
What is the effect of mismatch on
sound image fusion?
Base
Apex
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Left
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Right
20000 Hz
20 Hz
Binaural Hearing and Speech Laboratory
Find a matched pair via pitch matching
Base
Apex
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Left
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Right
20000 Hz
20 Hz
Δ=0
Binaural Hearing and Speech Laboratory
Mismatch conditions
1.5 mm
Base
Apex
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Left
Electrode #
2
4
6
8
10 12 14 16 18 20 22
Right
20000 Hz
20 Hz
Δ=8
Δ=4 Δ=2
Right higher in frequency
Δ=-2 Δ=-4 Δ=-8
Δ=0
Left higher in frequency
Binaural Hearing and Speech Laboratory
What do you hear?
Binaural Hearing and Speech Laboratory
Mismatch affects where a sound is
perceived
Left higher in
frequency
Right higher
in frequency
Binaural Hearing and Speech Laboratory
Mismatch affects people differently
Left higher in
frequency
Right higher
in frequency
Left higher in
frequency
Right higher
in frequency
Binaural Hearing and Speech Laboratory
Increasing mismatch leads to more
split sound images
Binaural Hearing and Speech Laboratory
Most people lateralize towards high
frequencies
Binaural Hearing and Speech Laboratory
Possible Implications
Target
Lateralized sound image
Target
Split sound image
Binaural Hearing and Speech Laboratory
How does mismatch affect sounds off
the mid-line?
Typical values:
•ILD = 0; ±2; ±5; ±10 CU
•ITD = 0; ±100; ±200;
±400; ±800 µs
•Negative values mean
sound is to the left.
Binaural Hearing and Speech Laboratory
Good lateralization at no mismatch
Binaural Hearing and Speech Laboratory
Mismatch affect ILDs
Binaural Hearing and Speech Laboratory
Mismatch affect ILDs
Binaural Hearing and Speech Laboratory
Mismatch affect ITDs even more
Binaural Hearing and Speech Laboratory
Non-midline crossings increase with
mismatch
L
C
R
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Possible Implications
Masker
Lateralized sound image
Masker
Masker
Split sound image
Binaural Hearing and Speech Laboratory
Possible implications on SRM
Target
Maskers
Lateralized Image
Target
Masker
Binaural Hearing and Speech Laboratory
Possible implications on SRM
Target
Maskers
Lateralized Image
Target
Maskers
Lateralized Image
Target
Masker
Binaural Hearing and Speech Laboratory
Possible implications on SRM
Target
Maskers
Lateralized Image
Target
Maskers
Lateralized Image
Target
Target
Maskers
Maskers
Target
Split Image
Masker
Binaural Hearing and Speech Laboratory
Summary
• Interaural place-of-stimulation mismatch leads
to lateralized or split sound images which
affects perceived location of sounds
• Depending on location of target and masker,
interaural mismatch might increase or hinder
spatial release from masking
Binaural Hearing and Speech Laboratory
Thank you
• We’d like to thank our research participants
for participating in our experiments, and
Cochlear Ltd. for supplying the equipment and
technical support.
• This work is supported by NIH-NIDCD
R01003083 (Litovsky) and in part by a core
grant to the Waisman Center from the NICHD
(P30 HD03352).
Binaural Hearing and Speech Laboratory