Methods

The general methods have been described in chapter 2. However, a number of more detailed considerations are outlined in this section. First, the choice of CI users and the associated choice of processing parameters is considered. The clinical population available to the author at the South of England Cochlear Implant Centre were users of the Nucleus implant, and work reported in chapter 3 used an AM of the Nucleus 24 implant with the CIS processing strategy. However, the great majority of Nucleus 24 users use the ACE or SPEAK processing strategy. Therefore, to continue to use a fixed channel (CIS) AM would introduce an additional confounding variable when

comparing AM and CI user results. Therefore, it was decided it to undertake the AM experiment using the Nucleus 24 ACE speech processing strategy. In this way, all processing variables could be matched between the AM and CI user experiments.

A further consideration was the choice of channel number and channel stimulation rate values. It was decided to reduce channel number and stimulation rate selectively from "typically used" values. In order to match the two experiments as closely as possible, the default parameters were those most commonly used by the clinical population accessed for the CI user experiment. The most common parameter settings for adult users of the Nucleus 24 implant in the available clinical population (adult Nucleus 24 users in the South of England Cochlear Implant Centre) were: 900 pps/ch stimulation rate, 12 maxima out of 20 channels with the Advanced Combination Encoder (ACE) speech processing strategy. This was therefore chosen as the default "high rate high channel number" condition for both experiments. The low stimulation rate condition was chosen as 250 pps/ch as this was the minimum permissible

stimulation rate allowed by clinical software and also corresponds to the stimulation rate used with the SPEAK speech processing strategy. However, the TMTF

measurements shown in 1.4.3 showed that differences in temporal envelope coding between these two rates are modest at low modulation rates and non-existent at higher modulation rates. Moreover, the differences in envelope coding, e.g. effective

envelope bandwidth, varied in the noise band AM to the same extent as in the CI processor. Consequently, if the temporal information provided by the implant is the key factor in determining perceptual abilities (as opposed to some physiological mechanism associated with higher pulse rates), little if any change between rates would be anticipated. Nevertheless, the design of the consonant recognition task, e.g. using the /iCi/ vowel environment which (suggested by Loizou et al. (2000b)) should be more sensitive to rate changes and the inclusion of a background noise condition should be such that any perceptual effects would be evident. The majority of the CI users had ACE maps with a channel stimulation rate of 900 pps/ch. For all conditions which did not use the CI subject’s standard stimulation rate (e.g. all 250 pps/ch conditions), re-mapping was undertaken by globally adjusting T-levels and C-levels along all 20 active electrode channels. Re-mapping was undertaken in order to account for the change in loudness (which is a function of stimulation rate, so long as pulse duration remains unchanged). Only small changes in overall electrical dynamic

range were observed in the altered low-rate MAPs, of the order of 2-3% reduction in dynamic range overall compared to the 900 pps/ch MAPs.

The decision about reducing channel number was less straightforward, given the choice of the peak-picking strategy ACE. The question was raised previously as to whether channel number is perceptually equivalent to the number of spectral peaks selected, or equivalent to the number of channels available, or to something in between the two. Dorman et al. (2002) found that performance was equivalent between fixed channel and peak-picking models where peak number in a peak- picking strategy was around the same as channel number in a fixed-channel strategy. In the present study the decision was taken to reduce both of these correspondingly to a level where channel number effects have been determined in previous work- thus the normal 12/20 condition was changed to 4/7, e.g. both channel number and peak (maxima) number were altered threefold. The 900*4/7 condition used channels 3,6,9,12,15,18 and 21. Figure 5.1 shows the frequency weighting and boundaries for the 20-channel MAPs while figure 5.2 shows frequency boundaries for the 7-channel MAP. Table 5.1 summarises the parameter values for the three MAP conditions. (The term “MAP”, coined by Cochlear Corporation, is used here to describe the particular set of parameter configurations, and their implementation, used for a particular CI user in a particular listening condition.)

0 1000 2000 3000 4000 5000 6000 7000 8000 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 Electrode number Fr e qu e n c y Lower frequency Higher frequency

Figure 5.1. Upper and lower frequency boundaries for 12*20 channel MAPs.

0 1000 2000 3000 4000 5000 6000 7000 8000 21 18 15 12 9 6 3 Electrode number Fr e que nc y Lower frequency Higher frequency

Figure 5.2. Upper and lower frequency boundaries for 4*7 channel MAP.

Table 5.1. Summary of three MAP conditions.

Map Stimulation rate Channel/maxima number

Default 900 pps/ch 12 maxima/20 channels

Reduced stimulation rate 250 pps/ch 12 maxima/20 channels

An additional advantage of this design was that equally “unfamiliar” MAP conditions could be compared in order to reduce the effect of familiarity. Where the default MAP was the one normally used, deterioration in performance in the other MAPs could be taken as a results of inadequate acclimatisation to the MAP, given that exposure was relatively limited. However, the possibility of comparing two equally unfamiliar MAPs, one with a reduced rate and one with a reduced channel number, would allow at least one comparison that was unaffected by the familiarity/acclimatisation issue and would also allow a direct comparison between lowering rate and lowering

channel number from the normally used MAP condition. Finally, it was the possibility that there was a linear equivalence in performance when trading off channel number and stimulation rate was raised in 2.3.4. It should be noted that there is no theoretical basis as such for assuming a direct comparison between linear reductions in channel number and stimulation rate for the Nucleus 24 device. Nevertheless, it was of interest to explore the possibility of “trade-off” between stimulation rate and channel number and therefore that the two “reduced” MAP conditions were roughly equivalent e.g. stimulation rate was reduced to slightly less than a third, as was channel resolution.

It was considered that, for the purposes of sample size calculation, the most important effect was the effect of noise on feature transmission values. Friesen et al. (2001) found that, for Nucleus users, consonant recognition was reduced by 10% with the addition of stationary background noise at +10 dB SNR, the same noise type and SNR used in this experiment. In the Friesen study standard deviation was also around 10. A sample size calculation based on these values (even assuming a highly conservative value for correlation in scores of over 50%) yielded a desired sample size of 11. Eleven NH subjects were recruited to experiment 4. In the event recruitment problems in the study meant that data were collected on 9 CI users. It should be noted that the sample size calculation was appropriate for within-subject comparisons but not for between-group comparisons; it was considered that the more important objective of the research was to determine differences in feature transmission as a function of processing and other variables within subjects, while the comparison between better and worse CI users, which is presented in 5.3, was of secondary importance to the overall design of the study.

The aim of the corresponding CI user experiment was to duplicate the AM processing and stimulus parameters just described with a group of adult users of the Nucleus 24 CI. Subjects were recruited from the South of England Cochlear Implant Centre and were all experienced users of the Nucleus CI24M or Nucleus CI24R Contour. Subject criteria were:

• Post-lingually deafened adult cochlear implant users aged 18 or over.

• Users of the Nucleus 24 device.

• Normally users of the ACE strategy

• Implant users for at least nine months.

• Score of at least 60% in the BKB sentences test in quiet at last review session.

• English as their first language.

No formal attempt was made to choose “better” and “worse” performers a priori. Due

to difficulties in recruiting an adequate sample, the inclusion criteria were expanded to include two subjects who normally used the SPEAK processing strategy in the bilateral condition. Both of these subjects had had experience using the ACE strategy since receiving their implants. Subject 5 had achieved a score of only 57% in the Bamford-Kowal-Bench, known as BKB sentence test (Bench et al.,1979) at his most recent review but had achieved scores above 60% on all previous occasions. Three of the subjects had bilateral implants but performed the tests using only the implant which they had had the longest. Subjects who used a hearing aid on their non-

implanted ear used their implant on its own for these tests. The subjects’ ages ranged from 25 to 85 with a mean age of 61. There were six males and three females. Subject details are given in table 5.2. It should be noted that the post hoc separation into

“worse” and “better” CI users, described in 5.3, did not co-vary with distinctions between those who normally used the ACE 900 pps/ch strategy vs. those who did not, nor did it co-vary with those who normally used bilateral CIs vs. those who did not. Consequently, it was thought that the relaxation of the inclusion criteria did not adversely affect results.

Table 5.2. Subject details for CI user experiment. Subject

number Sex Age BKB score Duration of implant use Implant type Normal strategy Other info

1 M 25 81 1yr 5m CI24R ACE 900

pps 12 of 20 2 M 70 92 2 ½ yr CI24R ACE 900 pps 12 of 20 3 M 65 90 1 yr CI24R ACE 900 pps 12 of 20 4 F 73 94 L – 6yr R – 4 yr CI24M SPEAK 250 pps 8 of 20 Bilateral implants 5 M 85 57 R – 7 yr L – 3 ½ yr CI24M ACE 720 pps 8 of 20 Bilateral implants 6 F 62 80 2 yr CI24R ACE 900 pps 12 of 20 7 M 49 98 2 yr CI24R ACE 900 pps 12 of 19 8 M 72 94 L – 6 yr R – 3 yr CI24M SPEAK 250 pps 8 of 20 Bilateral implants 9 F 48 100 1 yr CI24R ACE 900 pps 12 of 20

Each subject was tested using three different MAPs, e.g. 900*12/20 ACE, 250*12/20 ACE and 900*4/7 ACE. Mapping was undertaken using the Cochlear Custom Sound programming software by the researcher. Order of MAP condition was randomised and testing was conducted first in quiet then in noise for each MAP condition. A spare Esprit 3G processor was used to provide alternative MAPs. For most listeners, they could use their normal MAP as this was already 900*12/20 ACE. For the reduced channel condition a new MAP was created using the same seven channels as in experiment 3 (see figure 5.2). For the lower stimulation rate MAP, it was necessary to adjust T-levels and C-levels (minimal audible and maximum comfortable current levels) because of the change in loudness associated with changes in stimulation rate. Subjects were given as much time as needed to acclimatise to the new MAPs; in practice, this was not more than 15 minutes.

In summary, two parallel experiments were undertaken, one with 11 normal hearing listeners listening to an AM and the other with 9 users of the Nucleus 24 CI device. In

conditions (with and without stationary background noise at +10 dB SNR) * 3 MAP conditions. Additionally, for the AM experiment, all testing was undertaken with three different AMs, varying by the degree of the term λ from equation 3.2 from 0 to 1 to 3.3. This meant that there were a total of 2*3 listening conditions for CI users and 2*3*3 listening conditions for AM subjects. The results of the CI user experiment are reported in 5.3 while results of the AM experiment are reported in 5.4. In section 5.5 the two sets of data are considered together. All results are reported separately by transmission of six consonant features and also by total correct scores.