During the pitch height scaling in experiment 2 (page 24) subjects often reported difficulties in judging the pitch height of stimuli with a low pulse rate. Therefore a scaling of sound quality depending on pulse rate was conducted. Due to the experiences in experiment 2, an effect on sound quality was expected for pulse rates below 300 pps. Furthermore, the experiment was conducted at different places of stimulation similar to experiment 2. The expectation was that low pulse rates would have less influence on sound quality at more apical electrodes where the neurons of the spiral ganglion cells are tuned to low frequencies in normal hearing according to the frequency-place transformation (Zwicker & Fastl, 1999).
a) Participants
Ten subjects took part in the pitch scaling experiment (S1, S2, S3, S5, S8, S6, S7, S8, S13 and S14). The electrode was not fully inserted into the cochlear for S3 where there were E11 and E12 external to the scala tympani (controlled by Stenvers’ view x-ray scans).
b) Procedure
A single interval line length scaling procedure was used to judge the sound quality of stimuli with varying pulse rate and electrode position. All ten subjects of experiment 2 participated also in experiment 3. The subjects were instructed to assign the sound quality of
the stimulus by touching on a scale between the endpoints ‘extremely buzzy’ and ‘extremely clear’ on a touch screen (see Fig. 10). The line was internally scaled from 0 (extremely buzzy) to 27 cm (extremely clear). Poor sound quality was assigned by touching on a position located towards the left end, better sound quality by touching towards the right end of the scale. Eight different pulse rates were presented within one block. Stimuli were presented at 100, 119, 141, 168, 200, 238, 566 and 800 pps in random order. The sound quality was tested at four different electrodes according to experiment 1 (page 16), at E1, E3, E7 and E10. Nine estimates for each pulse rate were recorded within one block. The blocks were ordered according to stimulated electrodes, E1, E7, E3 and E10. Prior to the experimental runs, a training session was conducted whereby all electrodes were stimulated once at all pulse rates applied in the experiment. The final score was calculated as the arithmetic mean of nine estimates. All conditions were tested within one session.
FIGURE 10. Screen copy of the TFT touch screen used for the scaling of sound quality (experiment 3, line length method). The task of the subject was to indicate the sound quality of the stimulus between extremely buzzy (left side) and extremely clear (right sight) by pointing at a position on the grey bar. After the scaling the ‘OK’ button was pressed to confirm the input.
c) Results
Sound quality (mm)
0 5 10 15 20 25
E1 E3 E7
E10 S5 S2
100 200 800 100 200 800 100 200 800
S8
Sound quality (mm)
0 5 10 15 20 25
S3
100 200 800
100 200 800 100 200 800
S14 S10
100 200 800 100 200 800 100 200 800
Sound quality (mm)
0 5 10 15 20 25
S6 S13
Pulse rate (pps)
S1
100 200 800
Pulse rate (pps)
Sound quality (mm)
0 5 10 15 20 25
S7
FIGURE 11. Individual results for the sound scaling experiment. The sound quality in mm line length is plotted as a function of pulse rate; the parameter is electrode number.
The averaged estimated sound quality in line length units (0 cm: extremely buzzy;
27 cm: extremely clear) is plotted as a function of pulse rate in Fig. 11 for the individual
subjects; the parameter is electrode number. The inter-individual results vary considerably.
There are three listeners with significant (t-test with 95% confidence interval) influence of pulse rate on sound quality at all electrodes (S2, S5, S8). Four listeners show significant influence of pulse rate on sound quality at least at one single electrode (S3, S6, S10, S14) and three listeners show no significant influence (S1, S7, S13). The majority of the subjects judge the sound of the lowest pulse rate as lowest perceived sound quality. At most of the electrodes an increasing sound quality with increasing pulse rate can be observed. The individual sound quality functions exhibit a split into two regions for seven out of ten subjects: one region below 200 pps with sound quality depending on pulse rate and another region above 200 pps with hardly changing sound quality estimates. Regarding the individual results, for example the estimates of listener S5, sound quality reaches a maximum at 168 pps at E1 and E3, and at 200 and 566 pps at E7 and E10, respectively. The estimates of listener S10 show a dependency of sound quality on pulse rate at E1, E3 and E7. Sound quality increases up to a pulse rate of 566 pps. At the more basal electrode E10 however, the estimates of S10 are independent of pulse rate and much lower as for the other electrodes. For listener S7 the estimates at all electrodes are independent of pulse rate and equally high for all pulse rates and electrodes. The averaged estimates of listener S13 as well as of listener S1 show no significant influence on pulse rate, partly due to the large intra-individual variation.
Figure 12 shows the averaged results over all listeners and pulse rates for the four test electrodes. The averaged sound quality estimates are increasing with increasing pulse rate at all electrodes. There are significant increases in sound quality judgments between 119 and 168 pps and between 238 and 566 pps at E1. Sound quality increases significantly between 100 and 119 pps, between 141 and 200 pps and between 238 and 566 pps at E3. At E7 and E10, sound quality increases significantly between 141 and 566 pps, respectively. There is no influence on the averaged sound quality judgments for the highest pulse rates (566 and 800 pps) applied in the experiment for all test electrodes. That means that sound quality
estimates saturate at 566 pps independent of electrode location. The averaged sound quality estimates for the apical electrodes E1 and E3 are significantly higher than the estimates for the more basal electrodes E7 and E10 at pulse rates up to 238 pps. Significant sound quality differences between E1 - E3 and E7 - E10 can only be observed at single pulse rates.
Pulse rate (pps)
Sound quality (mm)
0 5 10 15 20
25 E1
E3 E7 E10
FIGURE 12. Average sound scaling results for ten subjects in the same format as Fig. 11.
d) Discussion
Sound quality is increasing with increasing pulse rate up to about 566 pps. This means that changes in pulse rate are always resulting in changes in sound quality. This effect is hardly described in the literature. In a recent study, Fearn & Wolfe (2000) did a quality rating for stimuli with changing pulse rate in six subjects implanted with the CI22M. Each stimulus was presented twice and should be rated on a line between two bipolar quality words like
‘like-dislike’, ‘mechanical-natural’, ‘clear-fuzzy’ etc. The mean of eight positions was taken to give a quality rating of the sound. The results show that sound quality is increasing with increasing pulse rate between 100 and 400 pps. Fearn & Wolfe (2000) also observed that more basal electrodes were judged lower in sound quality than more apical electrodes. This effect occurred up to 1000 pps. For an electrode distance between the most apical and most basal electrode of 11.25 mm the difference in sound quality was 40 cu on a scale between 0
and 100. In the present sound quality experiment, a difference in sound quality between more apical and more basal electrodes can also be observed. However, the difference is much smaller: For an electrode distance between the most apical (E1) and most basal electrode (E10) of 24 mm, the sound quality difference is 3 to 4 cm on a scale between 0 and 27 cm.
This would correspond to only 14.8 cu on a scale between 0 and 100. In the present study the stimuli were only judged between extremely buzzy and extremely clear. The data of Fearn &
Wolfe (2000) also include sensations like ‘pleasant’, ‘mechanical’, ‘natural’, and ‘musical’.
Most cochlear implant patients have been deaf or have had a profound hearing loss before implantation. Therefore especially the stimulation of more basal electrodes often evokes unpleasant pitch sensations and most cochlear implant patients prefer the sound of more apical electrodes. This effect might influence the data of Fearn & Wolfe (2000) and cause the difference in the sound quality rating compared to the results of experiment 3.
The effect of poorer sound quality at low pulse rates at more basal electrodes might be due to the mechanism of tonotopic allocation at the spiral ganglion. In the more apical region, more neurons of the auditory nerve tuned to low frequencies might exist than in the more basal region. However, current studies do not report a decrease in sound quality with high pulse rates at more apical electrodes and the sound quality of very high pulse rates with varying electrode position was not examined yet.
In normal hearing a distinct change of sound quality depending on the modulation frequency of the stimulus is described as roughness (Zwicker & Fastl, 1999). For a 100%-amplitude modulated stimulus with a carrier frequency of 1 kHz maximal roughness is perceived for a modulation frequency of 70 Hz. For modulation frequencies higher than 70 Hz, roughness decreases up to about 400 Hz. In the psychoacoustic literature it is described that the sensation of pitch strength is related with stimulus frequency. The pitch strength of a pure tone is increasing with increasing frequency up to about 750 Hz (Zwicker &
Fastl, 1999). This effect might also generally contribute to a change in sound quality with
increasing pulse rate. The sensation in this experiment might be a mixture of pitch strength and roughness.