Abstract
Objective
This study was performed to compare cochlear implant (CI) users’ performance in Mandarin speech and tone perception between 2 types of speech-processing strategies—advanced combination encoder (ACE) and continuous interleaved sampling (CIS)—under quiet and noisy conditions.
Methods
This study involved 10 congenitally deaf children (age range, 5.7–15.3 years; mean, 9.2 years) who received the Nucleus 24-channel CI system cochlear device (CI24R; Cochlear Ltd, Lane Cove NSW, Australia). The subjects used ACE since switching on their CI devices. Speech and tone perception tests were administered under quiet and noisy (+5 dB signal-to-noise ratio) conditions with ACE and CIS strategies 20 minutes and 2 weeks apart.
Results
Regardless of the strategy used, subjects showed significantly higher scores in speech perception than in tone recognition. Under noisy conditions, subjects had significantly higher tone identification scores with the CIS than the ACE strategy ( P = .038). There was no significant difference in speech identification score between the strategies. Subjects showed significant higher tone identification and speech perception scores under quiet than noisy (+5 dB signal-to-noise ratio) conditions. Subjectively, 6 subjects preferred the ACE strategy, and the remaining 4 preferred the CIS strategy. The strategy preference of the subjects was related to speech perception performance rather than tone identification. A significant correlation was observed between tone identification and speech recognition, regardless of whether speech was evaluated by consonants ( r = 0.669, P < .001), vowels ( r = 0.426, P = .001), or sentences ( r = 0.294, P = .023).
Conclusion
There are only 4 patterns of tone in Mandarin, which is far fewer than the number of speech sounds. However, tone identification is poorer than speech perception. The CIS speech-processing strategy may improve tone identification under noisy conditions. Before improved speech strategies to code acoustic characteristics of tone can be developed, it would be worthwhile to try both CIS and ACE for CI users and to select the most suitable speech-processing strategy according to the subjective preference and objective performance.
1
Introduction
Speech-processing strategies for cochlear implant (CI) devices are core technologies for helping hearing-impaired patients understand verbal language. Modern speech processors (hardware) are very flexible and can provide 2 or more different speech-processing strategies (software) for use in listening. It has been suggested that clinical audiologists should allow CI patients to experience the differences in their sense of hearing and esthesia when different speech-processing strategies are used . Based on individual differences, the users are able to select the most suitable speech-processing strategies for themselves.
The Nucleus 24-channel CI system (CI24R; Cochlear Ltd, Lane Cove NSW, Australia) is capable of implementing 3 different speech coding strategies: spectral peak coding (SPEAK), continuous interleaved sampling (CIS), and advanced combination encoder (ACE). The CIS strategy presents high fixed rates of stimulation (720-2400 pulses per second [pps]) to a small number of channels . The ACE strategy uses high rates of stimulation (600-1800 pps) with dynamic electrode selection and a large number of available electrodes . The number of channels is limited by a stimulation rate of 14 400 pps across channels. For example, not more than 8 channels can be chosen for a rate of 1800 pps per channel. Continuous interleaved sampling is based on temporal envelope representation, whereas ACE is based on the spectral peak picking method.
Many studies have explored the effects of various speech-processing strategies on CI listeners’ speech recognition performance . However, little is known about the effects of the different strategies on the recognition of suprasegmental information, such as tone identification in tonal languages. There are 4 patterns of tone in Mandarin, and their perceptual cues are carried mostly in the fundamental frequency of vocal folds. The acoustic cues are not appropriately coded by current speech strategies . With this small number of tones, perception scores in tone were not better than those in speech . Cochlear implant users have problems in tone perception even after many years of frequent use of the device . Electric stimulation in CIs is limited to a fixed number of electrode locations, which are insufficient to convey pitch information very well. Cochlear implant users achieve only limited recognition performance for tone perception.
To the best of our knowledge, only 1 study thus far has assessed post–speech-processing strategies: tone identification was used as the testing variable under a quiet condition, and the adjustment time for subjects was only 10 to 15 minutes . The present study was performed to evaluate tone identification and speech recognition performances under both quiet and noisy conditions with 2 different speech coding strategies (ACE and CIS) in children implanted with the Nucleus CI24R. In contrast to the previous study, the subjects were allowed to adopt a new strategy for 2 weeks.
2
Patients and methods
2.1
Subjects
This study involved 10 congenitally deaf children (age range, 5.7–15.3 years; mean, 9.2 years) who received the Nucleus CI24R cochlear device ( Table 1 ). Complete insertion of the 22 active electrodes was accomplished in all cases. They had been using ACE since their CI devices were first switched on. The CI was worn all day except during bathing and sleeping. All patients had at least 12 months of experience with the ACE strategy (range, 13-65 months; mean 39.9 months) before they switched to the CIS strategy. Baseline performance evaluation with the ACE strategy was conducted before the change of strategy. Children then switched to the CIS strategy. Continuous interleaved sampling was used by each child for 20 minutes and then for 2 weeks, after which their performance was reevaluated and compared with their baseline performance with ACE. At the time of testing, each child had both stable ACE and CIS speech process programs (Mappings). Mapping parameters for the 2 strategies are shown in Table 2 .
| Subjects | Sex | Age at implantation (y) | Age at test time (y) | Duration of use (mo) | Etiology | Implant ear | Speech process |
|---|---|---|---|---|---|---|---|
| 1 | F | 1.92 | 5.67 | 45 | Congenital | R | Sprint |
| 2 | F | 1.83 | 6.33 | 54 | Congenital | R | SPrint |
| 3 | F | 1.92 | 6.5 | 55 | Congenital | R | SPrint |
| 4 | F | 4.75 | 7.67 | 35 | Congenital | L | SPrint |
| 5 | F | 2.42 | 7.83 | 65 | Congenital | R | SPrint |
| 6 | M | 4.33 | 8 | 44 | LVAS | R | SPrint |
| 7 | M | 4.83 | 9.08 | 51 | Congenital | R | SPrint |
| 8 | F | 9 | 11 | 24 | Progressive | R | SPrint |
| 9 | M | 13 | 14.17 | 13 | Progressive | L | Freedom SP |
| 10 | M | 14.08 | 15.25 | 13 | Progressive | L | Freedom SP |
| Parameter | ACE | CIS |
|---|---|---|
| Stimulation mode | Monopolar 1 + 2 | Monopolar 1 + 2 |
| Channels | 8 maximum | 8 fixed |
| Volume | 9 | 9 |
| Sensitivity | 12 | 12 |
| Stimulation rate | 900 pps | 1800 pps |
| Pulse duration | 25 μ s | 25 μ s |
| Bandwidth (frequency range) | 188-7938 Hz | 188-7938 Hz |
| Loudness growth function | 20 | 20 |
2.2
Experimental design
Speech and tone perception tests were administered under quiet and noisy (+5 dB signal-to-noise ratio [SNR]) conditions, with ACE and CIS strategies, after 20 minutes and again after 2 weeks. Advanced combination encoder was the original speech-processing strategy used by all subjects. After the speech and tone identification tests, the subjects switched to the CIS strategy. The tests were repeated after the subjects had been using the new strategy for 20 minutes and then again after 2 weeks. The sequence of testing was randomized for noisy (+5 dB SNR) and quiet conditions. All tests were conducted in a sound-treated booth equipped with a two-channel clinical audiometer (GSI 61; Grason-Stadler Inc, Eden Prairie, Minnesota) and a CD player. Stimulus levels were calibrated at 65 dB hearing level (HL) and were delivered via loudspeaker. Audiological assessments were conducted at the end of each step to confirm the hearing status. After the 2-week trial period, each subject’s parents filled out a questionnaire on a 10-point scale, based on the children’s subjective speech strategy preference in the different listening situations ( Table 3 ) .
| I. Which strategy do you prefer? (ACE or CIS or Neutral) |
| II. How strong is your preference? (scale 1–10: 1 = weak preference, 10 = strong preference) |
| III. Parents’ rating scales basing on children’s subjective preference: (rating scale: 1 = strongly disagree to 5 = strongly agree) |
| 1. The overall quality of sound is better |
| 2. Speech sounds more natural |
| 3. Speech is easier to understand in the midst of background noise |
| 4. I am able to distinguish more environmental sounds (ie, birds, water, etc) |
| 5. Music sounds better |
| 6. I am able to distinguish different speakers |
| 7. It is easier to answer phone calls |
| 8. I am able to recognize lyrics in songs with a musical background |
| 9. I am able to distinguish individual instruments when listening to music |
The study was reviewed and approved by the institutional review board of our hospital (no. 97-1217B). Informed consent was obtained from all subjects/parents before participation in the study.
2.3
Instruments
Tone identification: 4 lists of monosyllabic words, each including 25 words of different tones, were used for tone identification . Scores were presented as the percentage correct. Subjects were instructed to indicate their perception of the 4 tones in the stimuli by either using hand shapes or pointing to graphic representations of the tones. Before the experiment, the subjects were trained to express their perception of these 4 tones with both modes of tone indication. For example, stretching the 2 arms out horizontally indicated the first tone; pointing the left arm downward with right arm upward, the second tone; stretching the 2 arms upward in a V shape, the third tone; and pointing the left arm upward with right arm downward, the fourth tone. Among the 10 subjects, 2 children responded with hand shapes.
Speech identification: We used 3 lists of picture identification, including vowels (16 items), consonants (21 items), and sentences (10 items) . The responses across the 3 lists were summed, and the score was represented as the percentage correct.
Student t test and Pearson correlation coefficient test were used to assess the significance of the differences in outcome measures among the different conditions. In all analyses, P < .05 was deemed to indicate statistical significance.
2
Patients and methods
2.1
Subjects
This study involved 10 congenitally deaf children (age range, 5.7–15.3 years; mean, 9.2 years) who received the Nucleus CI24R cochlear device ( Table 1 ). Complete insertion of the 22 active electrodes was accomplished in all cases. They had been using ACE since their CI devices were first switched on. The CI was worn all day except during bathing and sleeping. All patients had at least 12 months of experience with the ACE strategy (range, 13-65 months; mean 39.9 months) before they switched to the CIS strategy. Baseline performance evaluation with the ACE strategy was conducted before the change of strategy. Children then switched to the CIS strategy. Continuous interleaved sampling was used by each child for 20 minutes and then for 2 weeks, after which their performance was reevaluated and compared with their baseline performance with ACE. At the time of testing, each child had both stable ACE and CIS speech process programs (Mappings). Mapping parameters for the 2 strategies are shown in Table 2 .
| Subjects | Sex | Age at implantation (y) | Age at test time (y) | Duration of use (mo) | Etiology | Implant ear | Speech process |
|---|---|---|---|---|---|---|---|
| 1 | F | 1.92 | 5.67 | 45 | Congenital | R | Sprint |
| 2 | F | 1.83 | 6.33 | 54 | Congenital | R | SPrint |
| 3 | F | 1.92 | 6.5 | 55 | Congenital | R | SPrint |
| 4 | F | 4.75 | 7.67 | 35 | Congenital | L | SPrint |
| 5 | F | 2.42 | 7.83 | 65 | Congenital | R | SPrint |
| 6 | M | 4.33 | 8 | 44 | LVAS | R | SPrint |
| 7 | M | 4.83 | 9.08 | 51 | Congenital | R | SPrint |
| 8 | F | 9 | 11 | 24 | Progressive | R | SPrint |
| 9 | M | 13 | 14.17 | 13 | Progressive | L | Freedom SP |
| 10 | M | 14.08 | 15.25 | 13 | Progressive | L | Freedom SP |
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