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
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Matthias
Milczynski
PhD
Student
ExpORL,
Dept. Neurosciences, K.U.Leuven
O.&
N2, Herestraat 49 bus 721
B-3000
Leuven
Belgium
Tel:
+32 16 330476
E-mail:
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