Electric-acoustic Stimulation Benefit Research Articles

Interactions Between Slopes of Residual Hearing and Frequency Maps in Simulated Bimodal and Electric-Acoustic Stimulation Hearing.

The aim of this study was to determine the effects of residual hearing slopes and cochlear implant frequency map settings on bimodal and electric-acoustic stimulation (EAS) benefits in speech perception. Adults with normal hearing were recruited for simulated bimodal and EAS hearing. Sentence perception was measured unilaterally and bilaterally. For the acoustic stimulation, three slopes of high-frequency hearing loss were created using low-pass filters with a cutoff frequency of 500 Hz: steep (96 dB/octave), medium (48 dB/octave), and shallow (24 dB/octave). For the electric stimulation, an eight-channel sinewave vocoder was used with an output frequency range (1000-7938 Hz) with three input frequency ranges to create frequency maps, overlap (188-7938 Hz), meet (500-7938 Hz), and gap (750-7938 Hz), relative to the cutoff frequency in the acoustic stimulation. The largest bimodal/EAS benefit occurred with the shallow slope, and the smallest occurred with the steep slope. The effects of the slopes on bimodal/EAS benefit were greatest with the meet or gap map and the least with the overlap map. EAS benefit was greater than bimodal benefit at higher signal-to-noise ratios regardless of frequency map. The results indicate that correlation between bimodal/EAS benefit and residual hearing could potentially improve if slopes were considered. The optimal frequency map differed with different slopes, suggesting that the slopes of residual hearing should be carefully considered in fitting bimodal and EAS hearing. EAS hearing provided greater benefit over bimodal hearing, suggesting that spectrotemporal integration was better within one ear (i.e., EAS) than across ears (i.e., bimodal).

A State-of-the-Art Method for Preserving Residual Hearing During Cochlear Implant Surgery.

The developments in surgical techniques and cochlear implant (CI) electrode design have expanded the indications for CI treatment. Currently, patients with high-frequency hearing loss may benefit from CIs when low-frequency residual hearing can be preserved, as this enables combined electric-acoustic stimulation (EAS). The possible benefits of EAS include, for example, improved sound quality, music perception, and speech intelligibility in noise. The risks of inner ear trauma and a deterioration or even complete loss of residual hearing vary according to the surgical technique and the type of electrode array used. Short, lateral-wall electrodes with shallower angular insertion depths have demonstrated higher rates of hearing preservation than longer electrodes. The very slow insertion of the electrode array through the round window of the cochlea contributes to insertion atraumaticity and, thus, may lead to favorable hearing preservation results. However, residual hearing can be lost even after an atraumatic insertion. Electrocochleography (ECochG) can be used to monitor inner ear hair cell function during the insertion of the electrode. Several investigators have demonstrated that the ECochG responses during surgery may predict postoperative hearing preservation results. In a recent study, we correlated the patients' subjective hearing perception with simultaneously recorded intracochlear ECochG responses during the insertion. This is the first report evaluating the association between intraoperative ECochG responses and hearing perception in a subject undergoing cochlear implantation under local anesthesia without sedation. The combination of intraoperative ECochG responses with the patient's real-time feedback to sound stimuli has excellent sensitivity for the intraoperative monitoring of cochlear function. This paper presents a state-of-the-art method for the preservation of residual hearing during CI surgery. We describe this treatment procedure with the special consideration of performing the surgery under local anesthesia, which makes it feasible for monitoring the patient's hearing during the insertion of the electrode array.

Neural correlates of speech in noise perception differences between combined electric-acoustic stimulation and standard cochlear implants

Cochlear implants have evolved to utilize residual acoustic hearing that combines to electric stimulation, known as electric-acoustic stimulation (EAS). However, there are mixed expectations about the benefits of EAS. A positive perspective expects that contributions from residual hearing provide better access to acoustic cues that is helpful for speech-in-noise perception. An opposing view concerns potentially poorer spectral resolution of EAS electrodes’ stimulation as those electrodes are often inserted close to lateral wall, which may cause poorer speech-in-noise perception. This study aimed to directly compare neural processes of speech-in-noise perception between EAS and standard CI users to provide an answer to the above alternative expectations. We used 64-channel EEG to measure cortical evoked responses to (1) background noise and (2) target word while listeners perform a word-in-noise task. Then, we compared the amplitude ratio of evoked responses to the target word and background noise, referred to as “internal SNR,” which reflects how well target sound is unmasked from the mixture of speech and noise. Based on the comparison of 55 EAS and 22 standard CI users, internal SNR was significantly larger in EAS CI users. This result indicates that EAS provides enhanced neural processes for speech unmasking.

Shifting Fundamental Frequency in Simulated Electric-Acoustic Listening

Shifting the mean fundamental frequency (F0) of target speech down in frequency may be a way to provide the benefits of electric-acoustic stimulation (EAS) to cochlear implant (CI) users whose limited residual hearing precludes a benefit typically, even with amplification. However, previous study showed a decline in the amount of benefit at the greatest downward frequency shifts, and the authors hypothesized that this might be related to F0 variation. Thus, in the present study, the authors sought to determine the relationship between mean F0, F0 variation, and the benefits of combining electric stimulation from a CI with low-frequency residual acoustic hearing. The authors measured speech intelligibility in normal-hearing listeners using an EAS simulation consisting of a sine vocoder combined either with speech low-pass filtered at 500 Hz, or with a pure tone representing target F0. The authors used extracted target voice pitch information to modulate the tone, and manipulated both the frequency of the carrier (mean F0), as well as the standard deviation of the voice pitch information (F0 variation). A decline in EAS benefit was observed at the lowest mean F0 tested, but this decline disappeared when F0 variation was reduced to be proportional to the amount of the shift in frequency (i.e., when F0 was shifted logarithmically instead of linearly). Lowering mean F0 by shifting the frequency of a pure tone carrying target voice pitch information can provide as much EAS benefit as an unshifted tone, at least in the current simulation of EAS. These results may have implications for CI users with extremely limited residual acoustic hearing.

Effects of contextual cues on speech recognition in simulated electric-acoustic stimulation.

Low-frequency acoustic cues have shown to improve speech perception in cochlear-implant listeners. However, the mechanisms underlying this benefit are still not well understood. This study investigated the extent to which low-frequency cues can facilitate listeners' use of linguistic knowledge in simulated electric-acoustic stimulation (EAS). Experiment 1 examined differences in the magnitude of EAS benefit at the phoneme, word, and sentence levels. Speech materials were processed via noise-channel vocoding and lowpass (LP) filtering. The amount of spectral degradation in the vocoder speech was varied by applying different numbers of vocoder channels. Normal-hearing listeners were tested on vocoder-alone, LP-alone, and vocoder + LP conditions. Experiment 2 further examined factors that underlie the context effect on EAS benefit at the sentence level by limiting the low-frequency cues to temporal envelope and periodicity (AM + FM). Results showed that EAS benefit was greater for higher-context than for lower-context speech materials even when the LP ear received only low-frequency AM + FM cues. Possible explanations for the greater EAS benefit observed with higher-context materials may lie in the interplay between perceptual and expectation-driven processes for EAS speech recognition, and/or the band-importance functions for different types of speech materials.

Effects of spectral degradation on attentional modulation of auditory responses to continuous speech

Auditory attention enhances phase-locked responses to the temporal envelope of the attended speech stream in a competing background. This study investigates how cortical responses are affected by spectral degradation and the extent to which low-frequency fine-structure cues facilitate sound segregation. Two competing speech streams were presented diotically to normal-hearing subjects. Each subject was tested with unprocessed speech, vocoder speech (8 to 64 channels), and six-channel high-frequency vocoder + lowpass filtered speech simulating electric-acoustic stimulation (EAS). Ongoing EEG responses were measured in each condition. Cross-correlation between speech envelope and EEG responses was calculated at different time lags. For unprocessed speech, the cross-correlation function showed opposite signs of correlations between the attended and unattended speech, supporting the neural mechanism of suppression of the competing speech stream. This suppression mechanism, however, was only evident for the 32- and 64-channel vocoder conditions, but not for the 8- and 16-channel conditions. This indicates that greater frequency resolution is required for sound segregation. For EAS, the pattern of cortical responses was similar to that of the eight-channel vocoder condition, although speech intelligibility was higher in EAS than vocoder speech. Consistent with previous psychoacoustic evidence, these neural results further argue against the segregation mechanism for EAS benefit.

Comparing the effects of reverberation and of noise on speech recognition in simulated electric-acoustic listening

Cochlear implant users report difficulty understanding speech in both noisy and reverberant environments. Electric-acoustic stimulation (EAS) is known to improve speech intelligibility in noise. However, little is known about the potential benefits of EAS in reverberation, or about how such benefits relate to those observed in noise. The present study used EAS simulations to examine these questions. Sentences were convolved with impulse responses from a model of a room whose estimated reverberation times were varied from 0 to 1 sec. These reverberated stimuli were then vocoded to simulate electric stimulation, or presented as a combination of vocoder plus low-pass filtered speech to simulate EAS. Monaural sentence recognition scores were measured in two conditions: reverberated speech and speech in a reverberated noise. The long-term spectrum and amplitude modulations of the noise were equated to the reverberant energy, allowing a comparison of the effects of the interferer (speech vs noise). Results indicate that, at least in simulation, (1) EAS provides significant benefit in reverberation; (2) the benefits of EAS in reverberation may be underestimated by those in a comparable noise; and (3) the EAS benefit in reverberation likely arises from partially preserved cues in this background accessible via the low-frequency acoustic component.

Assessment of the Subjective Benefit of Electric Acoustic Stimulation with the Abbreviated Profile of Hearing Aid Benefit

Conclusion: This study demonstrates that electric-acoustic stimulation (EAS) significantly decreases the subjective impairment in speech perception. Objectives: To assess the subjective benefit of EAS over the first 12 months after EAS fitting using the Abbreviated Profile of Hearing Aid Benefit (APHAB). Method: Twenty-three EAS users, implanted with either the PULSAR_CI¹⁰⁰ FLEX^EAS provided with the DUET EAS processor or the COMBI40+ Medium provided with the TEMPO+ speech processor, were included. Electric stimulation was activated about 1 month postoperatively; ipsilateral acoustic stimulation was added 2 months thereafter. EAS benefit was measured preoperatively with only a hearing aid and postoperatively at EAS fitting and then 3, 6 and 12 months after EAS fitting using the APHAB. Results: Subjects reported significant improvements in the global score with a mean decrease in impairment from 74% preoperatively to 45% after 3 months of EAS use. Furthermore, clinical relevance was demonstrated in multiple subscales between preoperative and first fitting reflecting a true benefit of EAS with a probability of 95%.

Influence of a single reflection on speech intelligibility in simulated electric‐acoustic stimulation.

In reverberant environments, reflections occurring within 30–50 ms of the direct speech assist intelligibility by perceptually fusing with the source, effectively increasing its level. Fusion occurs at similar delays in monaurally deaf and normal‐hearing listeners, suggesting an independence from binaural processes [Litovsky et al., J. Acoust. Soc. Am. 106, 1633–1654 (1999)]. Its effects can thus be examined in unilateral cochlear implant (CI) users. Simulated CI listening shows little benefit from early reflections, and a detriment when delays exceed 20 ms [Whitmal and Poissant, Conference on Implantable Auditory Prostheses (2009)]. Data from our laboratory show that CI users with residual acoustic hearing benefit from electric‐acoustic stimulation (EAS) in reverberation. The role of early reflections in this EAS benefit remains unclear. The present study examined the effect of a single unattenuated reflection at ten delay times on intelligibility in simulated EAS. Target stimuli consisted of sentences combined with unattenuated copies delayed by 0–66 ms. Four‐talker babble was added to increase difficulty. A four‐channel vocoder simulated electric stimulation; low‐pass filtered speech represented residual acoustic hearing. Monaural intelligibility scores from normal‐hearing listeners under anechoic and reflected conditions with electric and electric‐acoustic processing suggest that EAS may facilitate a benefit from early reflections. [Work supported by the NIDCD.]

Combined electric acoustic stimulation with the PULSARCI100 implant system using the FLEXEAS electrode array

Conclusion: This study demonstrates that electric acoustic stimulation (EAS) using the FLEXEAS electrode is a successful treatment method for preservation and stability of low frequency hearing, and results in significant improvements in speech perception. Objectives: Low frequency hearing preservation and stability following EAS surgery with the MED-EL PULSARCI100 implant using the FLEXEAS electrode and a combined processor in a multicentre setting, and the efficacy and benefits of EAS over time were evaluated. Methods: Eighteen subjects with normal to moderate hearing loss in the low frequencies and severe to profound hearing loss in the high frequencies were implanted with the FLEXEAS electrode. Implantation was performed by different surgeons at three participating centres using either the cochleostomy or the round window approach. Preoperatively, hearing was measured; a battery of speech perception tests and the subjective benefit questionnaire were administered. These tests were subsequently repeated: first at EAS fitting, which was due 3 months after surgery, and then again 3, 6 and 12 months after EAS fitting. Results: Hearing could be preserved in all subjects. Speech understanding showed significant improvement in all tests over time with the EAS condition outperforming the cochlear implant-only condition at all intervals. These considerable effects were also reflected in the subjective benefit outcome.

Lowering mean fundamental frequency to improve speech intelligibility in noise under simulated electric‐acoustic stimulation.

We have previously demonstrated that much or all of the benefits of electric‐acoustic stimulation (EAS) can be achieved when the low‐frequency speech is replaced with a tone that is modulated in both frequency and amplitude with F0 and amplitude envelope cues derived from the target speech. One advantage of this approach is that the frequency of the carrier tone can be lowered with little decline in benefit. Lowering mean F0 has the potential to provide EAS benefit to CI users who have very limited residual hearing. One drawback to this approach is that it relies on the efficacy of the pitch extraction algorithm. This is problematic because pitch extractors have trouble in background noise, an environment in which F0 is particularly useful. Here, an alternative way of lowering mean F0 that is unaffected by the presence of noise is examined under simulated EAS conditions. Speech intelligibility was measured using an algorithm based on resampling and compared to performance with the pitch‐based method we have used previously, at frequency shifts of 0, 0.5, and 1 octave. At the 0.5‐octave shift, the resampling‐based approach provided more benefit than the tone. However, at the 1‐octave shift, resampling was less beneficial. [Work supported by NIDCD.]

Signal-processing strategies for electric-acoustic stimulation.

Electric-acoustic stimulation (EAS) occurs when a cochlear implant recipient receives some amount of acoustic stimulation, typically as a consequence of residual low-frequency hearing. This combined stimulation often leads to significant improvements in speech intelligibility, especially in the presence of a competing background. We have demonstrated EAS benefit when target speech in the low-frequency region is replaced with a tone modulated in frequency with the target talker’s F0, and in amplitude with the amplitude envelope of the target speech. We have hypothesized that in some instances, the modulated tone may be more beneficial than speech. For example, a patient possessing extremely limited residual hearing (e.g., only up to 100 Hz) may not show EAS benefit because the F0 of many talkers is too high in frequency to be audible. In support of this hypothesis, we demonstrated that the tone can be shifted down considerably in frequency without affecting its contribution to speech intelligibility. In the current experiment, while generating the tone, we manipulated various signal-processing parameters to examine their effects on the benefits of EAS. One goal of this work is to develop a real-time processor that provides EAS benefit to implant users with limited residual hearing. [Work supported by NIDCD.]