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.