Wk6a-Bilateral CIs (I) Flashcards
What are binaural cues?
Phase (interaural time difference) and amplitude (ILD) differences between ears, which help us localize sound
Low frequencies diffract ____ (more/less) than high frequencies, so shadow is less
More
Each frequency has a max ILD not at 90 degrees. This maximum is closer to 90 degrees for ___ (higher/lower) frequencies
Higher
NH listeners can detect ILD differences of about ___ dB, which corresponds to about ____ degrees of change in the azimuthal angle
1 dB
2-3 degrees of change
ITD is creased by path-length difference to the two ears. For NH listeners, ITD JND can be as low as ___ ms, or about ___ degree(s) displacement from the midline
10 ms
1 degree
Lower frequencies have ____ (lower/higher) ITDs compared to higher frequencies.
Higher
Which peak is always at 90 degrees azimuth, regardless of frequency: ILDs or ITDs?
ITDs
According to the research of Brughera et al regarding ITD JND, listener performance increased with frequency up to ___ Hz, at which point the threshold ITD was about ___ microseconds or 1 degree.
800
10 microseconds
According to the research by Brughera et al, at what frequency did ITD JNDs become useless?
1400 Hz
According to Brughera et al, in wideband sounds, ITDs in the ___ - ____ Hz bands are the dominant cue for NH listeners
500-1000 Hz
____ frequencies best carry the ILD info.
High
___ frequencies best carry the ITD info
Low
Anatomically, most of the acoustic processing is ____ (ipsi/contra)-lateral
Contralateral
Sound pathway: Ipsilateral cochlea Acoustic Nerve \_\_\_\_\_\_\_ cochlear nucleus \_\_\_\_\_\_\_\_ LSO Ipsilateral \_\_ & \_\_\_
Ipsilateral CN
Contralateral LSO (vis medial nucleus of trapezoidal body)
IC & DNLL
A left side stimulus will trigger a high spike rate in the contralateral ____. When the sound is from the right, we have high inhibition in the right MSO (from ipsi cochlea) and some excitation from left => low spike rate on the ipsi____
MSO
MSO
Describe how research on owls (by Jefferies) was disproven by Brand et al?
There is a lot of variability below and above 0 microseconds, but there was a spike rate increase across all frequencies for contralateral stimuli, thereby disproving the coincidence theory
Besides ITDs and ILDs, how else do we segregate talkers?
Location
In the test by Williges et al, they used one monaural listener and one binaural listener to measure the SRT when noise was presented with the target at the front, and from either side. Describe their findings
They found that the SRT of the was best for the monaural listener when the noise was opposite his good ear, and best for the binaural listener when the noise was separate from the speech signal.
Does binaural summation exist with CIs? Why?
Yes (usually about 1-2 dB in CI users);
- internal noise is reduced
- information may be complementary (good apical in one, good basal in the other)
Describe the head shadow effect, and how it affects binaural hearing.
For a spatially separated signal, one ear may have a better SNR due to head shadow, therefore this will be the ear that gives us a better SRT
- for unilateral listeners, the functioning ear is not always the better SNR ear
How many dB is head shadow (usually)?
Up to 10 dB
What is binaural interaction (or “squelch)?
All benefits beyond head shadow and summation
True/False: When a signal is presented in opposite phase at one ear, compared to the other (NoS(pi)), we get a binaural masking level difference (BMLD) of 15 dB. How do we know this is not due to a better SNR or summation?
There is no better SNR in left or right, and there is no summation b/c the signal is in opposite phase and would = 0 when added.
- when we only add noise in one ear, we only get a BMLD of 9 dB (binaural interaction)
What does squelch rely on?
Interaural temporal disparities
- 2-5 dB in NH
- 0-2 dB in CI users
Why is large BMLD/Squelch not found in CI users?
- CIS processing removes waveform fine structure (Interaural phase diffs no longer detectable)
- uncoordinated pulse trains from two independent processors cannot convey detailed ITD info
How was CIS circumvented to test if CI users can exploit ITD if preserved?
Using a 2-interval left-right discrimination with biphasic pulse trains
- both ears were stimulated with biphasic pulse trains for 500 ms, but the right was stimulated a few ms earlier than the left
- > should be perceived from right
- next, 300 ms rest, then another 500 ms burst, but left leading
- should be perceived as travelling from right to left
- *In most cases, bilateral CI users have poor ITD sensitivity (~100 microseconds, compared to 10 microseconds used by NH listeners)
Best JNDs for ITD (CI users) are found b/w 100 and 200 micro seconds, at ____ pps with worse ITDs as rate ____ (increases/decreases)
100 pps
Increases
According to Baumgaertel et al, as rate increases, lateralization ____ (decreases/increases) for most CI users
Decreases
*Also, most users would need unnaturally large ITDs, and even lower rate limit than for ITD detection
In acoustic hearing cochleas, the tonotopic organization is preserved between cochleas. However, bilateral CIs may not be identical (e.g. regarding insertion depth). How does this mismatch affect localization?
Mismatch of frequencies causes different frequency information from either ear, which impedes ITD processing.
To use ITDs in bilateral CIs, we need to ___ electrodes in each cochlea
Match
Hu and Dietz used pitch-matching to show…
That over time, the cortex can “learn” to pitch-match differently inserted electrodes (so cortex can “learn” but brainstem cannot)
Laback looked into ILD sensitivity in bilateral CI users. What did he find regarding stimulation of different electrode pairs?
Best JNDs were for closely corresponding electrodes
- concluded that, in bilateral CIs, have near normal ILD sensitivity (1.3-4 dB) compared to 1 dB NH, but only for matching electrode pairs
Summarize what we know about bilateral CI localization?
- good spatial release from masking but no binaural squelch
- near normal ILD sensitivity
- CIS and most other coding strategies do not provide ITD info in pulse timing
- ITD sensitive, but much worse and limited compared to NH
- ITD based lateralization is even more limited
- best ITD and ILD performance in electrode pairs are matched
- plasticity for pitch, but not ITD and ILD
