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Re: Influence of amplitude on place pitch in cochlear implants (CIs)

Hello Matthias,

It depends on your question: if you want to know what the speech processor does with your stimuli (roving and all) then you'll have to live with that (and of course your results would depend on which speech processing strategy you used). I would always suggest that you try to achieve full stimulus control (to the extent possible) so that you are free to ask whatever question you want. It sounds like you want the roving as a way to control what the listener attends to: if so, then don't worry about whether it's realistic or not, do the roving at the level of the electrical currents.



M Chatterjee, Ph.D.
Director, Cochlear Implants and Psychophysics Laboratory
Asst Professor, Hearing and Speech Sciences
0100 LeFrak Hall
University of Maryland, College Park
College Park, MD 20742
(301) 405 7716

>>> Matthias Milczynski <Matthias.Milczynski@xxxxxxxxxxxxxxx> 12/7/2007 6:46 AM >>>
Dear list,

I am testing pitch ranking in CI subjects. In general, I work with a  
research processor and I stream pre-processed stimuli directly to a  
subject's implant. To get rid of loudness cues I want to apply  
amplitude roving. However, when applying the roving at the front-end  
(i.e. manipulating the amplitude of wav-files to be processed by a  
particular strategy) I observe the following problem:

Let's assume a complex tone (e.g. piano) at a frequency of 164.8 Hz  
(E3), with a duration of 500 ms. Thereby the rms of the sound is  
relative small (e.g. -30 dB re full scale). Then, let's assume a tone  
at 130.8 Hz (C3) from the same instrument and at the same duration  
but at a higher rms (e.g. 10 dB higher in rms than the first tone).  
Due to the non-linear loudness growth function implemented in these  
days' CIs (assuming e.g. the ACE strategy with default adjustments)  
the stimulation pattern corresponding to the first tone will show  
activity on a few apical electrodes (up to about 900 Hz). However the  
stimulation pattern corresponding to the second tone (which is higher  
in amplitude but lower in pitch) will also show activity at  
electrodes corresponding to much higher frequencies (up to approx.  
1700 Hz). As a consequence a subject that mainly relies on place  
pitch cues could falsely rank the second tone as the higher one in  
pitch. That means that amplitude roving at the front-end can  
introduce a misleading place pitch cue. Of course the stimulation  
patterns for E3 and C3 will differ in temporal pitch cues (i.e. the  
frequency of the envelope fluctuations in the E3-pattern will be  
higher than for the C3-pattern), however the effectivity of this cue  
will be subject-dependent.

Another possibility to amplitude-rove the stimuli would be to apply  
the roving at the back-end, i.e. manipulating the electrode current  
(i.e. multiplication by a scaling factor) but this type of roving  
does not necessarily correspond to a real-life situation and is  
difficult to implement when working with the subject's own device.

I would very much appreciate any comments and/or suggestions.

Many thanks!!


Matthias Milczynski
PhD Student
ExpORL, Dept. Neurosciences, K.U.Leuven
O.& N2, Herestraat 49 bus 721
B-3000 Leuven

+32 16 330476