Bimodal Stimulation

As adverse as the effects of speech perception with background noise are for individuals with HI, there are perhaps more adverse for CI users, due to the reduced frequency resolution and lack of fine-structure cues provided by the devices [24, 35-37]. CIs work by filtering the incoming signal into a number of frequency bands, and extracting the amplitude envelope in each of the bands [7]. The electrical pulses that are modulated in amplitude with each extracted envelope, emit trains to the electrodes which positioned at various points along the basilar membrane. Because of this process, fine-structure cues are largely discarded.

Previous studies have demonstrated that the addition of low frequency sound provide benefits in terms of speech recognition over the unilateral CI alone in both simulated [38-41] and actual Bimodal stimulation [33-34, 42-56], in noise condition. This may be the low frequency acoustic information contains some cues that help listener to perception phonemic well. As shown by [25], better frequency resolution aids the recognition of speech in backgrounds. Turner et al.

demonstrated that high frequency electrical stimulation along with acoustic low frequency hearing in simulated EAS subject had the potential to provide a significant advantage for understanding speech in background noise, particularly when the competing signal was other talkers [34]. Ching showed the low frequency hearing from HA increases consonant voicing and manner of articulation information in Bimodal stimulation [49]. Similarly, Mok et al. demonstrated that increased low frequency phonemes information (e.g. nasals, semivowels, diphthongs and the first formant frequency) were transmitted with Bimodal stimulation relative to the CI alone [52]. Several studies have been suggested that segregation as a possible mechanism for the benefits of Bimodal hearing

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in noise [39-40, 51]. Chang et al. have suggested that listeners combine the relatively weak pitch information conveyed by the electric stimulation with the stronger pitch cue from the target in the low frequency acoustic to segregate target and background noise. However, the question remains of what low frequency cues are responsible for the Bimodal effect.

Von Ilberg et al. reported the speech recognition in quiet results for one patient implanted with a long electrode inserted 20 mm into the cochlea who had some preserved residual hearing 2 months following implantation, this patient‟s sentence recognition scores were 7% with the acoustic hearing alone, and increased to 56% with the addition of electric stimulation. Interestingly, sentence recognition with the CI alone was only 2%, suggesting a strong synergistic effect of the two modes of stimulation [33]. Gantz et al. reported results from a group of six Bimodal users, three implanted with a 6 mm electrode and the remainder with a 10 mm electrode [45]. The 6 mm-electrode patients demonstrated approximately 10% points improvement in the recognition of consonants with the addition of electric stimulation, whereas the patients implanted with the 10mm device improved nearly 40% points. The greatest improvement was noted for the perception of the speech feature place of articulation, which is in agreement with the high-frequency range of speech information assigned to the short-electrode. These authors also noted a strong effect of experience following implantation, with some patients continuing to improve as long as 10 months following implantation. Skarzynski et al. described the speech recognition results for one patient listening under Bimodal stimulation after being implanted with a long electrode of 20 mm insertion depth.

Monosyllabic word scores increased from 25% (acoustic-alone) to 90% (EAS) [46]. The electric-only score was 23%, again suggesting a strong synergistic effect for this patient. Gstoettner et al. reported speech results for one patient implanted with a 22 mm electrode who improved from 38% recognition of monosyllables in the acoustic-only mode to 90% with the addition of electric stimulation [48]. Kiefer et al. reported monosyllabic word understanding a group of 11 patients with

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preserved residual hearing [50]. Mean acoustic-alone scores were 7%-correct, and the addition of electric stimulation increased this to over 60%, with several patients showing scores increasing to over 75%. Of these 11 patients, only four obtained Bimodal scores higher than the electric alone score, suggesting that for many patients, the electric stimulation was providing the primary contribution to speech understanding. James et al. reported word recognition scores for seven subjects whose residual hearing was preserved 6 months postoperatively to 60 dB HL or better at the low frequencies [56]. The mean preoperative score was 22%, CI-alone score 6 months after surgery was 56%, and under Bimodal stimulation scores averaged 68%. For two of these subjects, the implant-alone score was equal to the Bimodal score, suggesting that the bulk of speech recognition was provided by the electric stimulation. At the other end of the spectrum, in two other patients, the implant provided little or no speech recognition alone, nor did it provide an increase over the acoustic-alone score in the Bimodal stimulation. These two patients had long durations of deafness prior to implantation.

Chang et al and Qin et al. both have shown that F0 is seem to play an important role in low frequency region then any first formant may play [39-40]. In their studies, for example, the 300 Hz low-pass speech sound, which should not yield any intelligibility, can significant improved speech recognition in noisy environment, when addition to vocoder stimulation. This seems to suggest that F0 is important, since it is very unlikely that the 300 Hz low-pass speech sound has contain any first formant information [57].

Another report has even showed that CI patients may achieve Bimodal benefit with residual hearing only up to about 125 Hz. For example, Zhang et al., examined Bimodal patients combined the 750, 500, 250 or 125 Hz low-pass speech sound and listened to both monosyllables in quiet and sentences with background noise [58]. Results showed that the majority of the EAS benefit was observed from acoustic information present below 125 Hz. For example, the recognition rate of

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monosyllables in the vocoder only, EAS-125, and EAS-broadband conditions were 56, 78 and 88 percent correct respectively. Given that the mean F0 was 123 Hz of the male talker, they concluded that perception of F0 plays a major role in Bimodal benefit, due to this low-pass acoustic signal contained only F0 as the major speech cue. Cullington et al. also observed Bimodal benefit with very low frequency acoustic information (150 Hz low-pass) and suggested that much of this benefit provided by F0 information in their study [55].

Plomp had described a model that contained two independent factors that describing the hearing loss [59]. First, the attenuation factor, it represents the threshold shift of a listener with HI.

The attenuation factor would show differences in speech intelligibility only at lower noise levels, and a convergence in performance once the level of the noise increased. Second, the distortion factor, it represents supra-threshold deficits, and specifies that performance will always be worse for listeners with HI, once the level of the noise rises to such a level that it becomes the limiting factor in the audibility of the target speech. Figure 2.1 shows the difference between these two factors in speech recognition threshold with conductive hearing loss and sensorineural hearing loss.

The distortion factor exhibited by listeners with HI can be observed when fluctuating (amplitude-modulated) maskers are used, such as competing speech. It has been known that normal hearing (NH) listeners can take advantage of the masking release which occurs in the temporal valleys of fluctuating maskers to improve speech intelligibility performance while listeners with HI are less able to [60-65]. This ability has been termed “listening in the dips” or “glimpsing.”