Electric-Acoustic Stimulation of Hearing in Adults and Children

The purpose of this study was to test a possible way of improving speech perception through a cochlear implant, especially for children. Although implants provide a signal that is speech-like, the quality of sound is much different from typical speech. Most deaf listeners have a little low-frequency residual hearing. The idea was explored that perhaps speech perception would be improved by amplifying any residual hearing, even if a small amount, and combining it with the electric signal received through an implant. Three hypotheses were tested. H1: Recognition would be better for a simulated implant signal combined with a low-pass acoustic signal than for the implant simulation alone. H2: Greater benefits would be observed for sentence materials than for isolated words because low-frequency signals provide information primarily about sentence structure. H3: Children would show greater effects than adults because they rely more on the kinds of structure available in the low frequencies. Participants were 20 7years-olds and 20 adults, all with normal hearing. They identified individual words and sentences with two kinds of processed speech: (1) speech band-pass filtered between 250 and 8000 Hz, and then noise vocoded in four channels, which replicates the signal available through a cochlear implant; and (2) that signal combined with the original signal low-pass filtered below 250 Hz. The results revealed that both children and adults benefitted from the addition of low-frequency information to the vocoded speech for words and sentences. The effect of adding the lowfrequency signal was larger for sentences than for individual words. Adults performed better than children in all conditions, but listeners in both groups showed equivalent benefit from the addition of the low-frequency signal. Thus, H1 and H2 were supported, but not H3. Combining acoustic hearing through a high-powered hearing aid with electric hearing through a cochlear implant might enhance speech perception for deaf listeners. Electric-Acoustic Stimulation of Hearing in Adults and Children 3 I. Introduction History of Cochlear Implants In 1961 Dr. William House’s cochlear implant design was first implanted in two patients with hearing loss (Wilson & Dorman 2008). Although many were skeptical about the potential effectiveness of the device, it was an unprecedented achievement in the aspiration to restore hearing to deaf individuals. Ultimately the first archetype of the device failed, but Dr. House continued to research and improve upon his invention (Mahalakshmi & Reddy 2012). Since then cochlear implants have evolved in both sophistication and efficiency in aiding speech understanding. The first cochlear implant Dr. House developed used a single-channel processor, which provided broad electric stimulation across the cochlea. The single-channel processors were comprised of one cochlear electrode and processed frequencies through a 340-2700 Hz bandpass filter. The strategy used to process the signal was one that relied on replicating the amplitude modulations that occur in the original speech signal. While these preliminary devices were beneficial for aiding lip reading, they were not successful in supporting speech understanding because the processed signal neglected frequency information (Mahalakshmi & Reddy 2012). It was not until the 1980s that multichannel processor cochlear implants were produced and utilized. The multichannel processors, unlike the single-channel implants, have several electrodes which provide stimulation at multiple locations in the cochlea through the electrode array. The numerous electrodes allow the auditory nerve to be stimulated at different places along the basilar membrane. Additionally, the multichannel processors were receptive to the temporal details and replicated amplitude structure over several frequency bands when Electric-Acoustic Stimulation of Hearing in Adults and Children 4 processing the speech signal, which resulted in slightly better spectral resolution (Mahalakshmi & Reddy 2012). The multichannel implants model tonotopic organization to a limited extent in order to code frequencies. The electrodes located at the base of the cochlea respond to highfrequency signal components, whereas the electrodes near the apex of the cochlea are stimulated by low-frequency components (Wilson & Dorman 2008). The electrode array is organized using the filter bank model in which each electrode responds to a specific range of frequencies in the signal and stimulates a specific location along the basilar membrane accordingly. The number of filter bands is dependent on the number of stimulation channels in the device itself (Mahalakshmi & Reddy 2012). There are three primary strategies for cochlear implant signal processing: waveform, feature-extraction, and hybrid. The waveform strategy for processing relies on the filter bank model to divide the speech signal into specific frequency bands. The amplitude waveform is extracted from each frequency band and each is presented to a different electrode. Originally this approach was vulnerable to interference caused by overlapping of electric fields within the cochlea, which distorted the quality of the signal and reduced speech intelligibility. These technical problems were addressed with the advent of the continuous interleaved sampling (CIS) strategy (Mahalakshmi & Reddy 2012). The CIS strategy is a sequential pulsing in which the electrodes are stimulated intermittently (Wilson & Dorman 2004). Alternatively, the feature-extraction processing strategy relies on dissecting spectral information, such as fundamental and formant frequencies, from the speech signal. This information is then converted into signals for the electrodes. The detected fundamental frequency is converted into the stimulation rate of the implant. In the first prototypes of the featureextraction devices, only the fundamental frequency and second formant frequency were extracted Electric-Acoustic Stimulation of Hearing in Adults and Children 5 from the speech signal. However, in later models the first formant frequency was added for additional spectral resolution. The formant frequencies are used to determine which electrode is to be stimulated. In models with both F1 and F2, F1 information is used to stimulate apical electrodes and F2 information is used to stimulate basal electrodes. This strategy was highly beneficial in terms of aiding speech recognition because it emphasized features that were common to vowel sounds (Mahalakshmi & Reddy 2012). The feature-extraction method is no longer widely used. Currently, the channel vocoder-type processors are most common in cochlear implant devices (Møller 2006). Even with the progress the device has made over the last 5 decades, word recognition for implant users remains low. In one study, adult implant users averaged 42% correct for CNC word scores and 72% correct for HINT sentences presented at 70 dB SPL in quiet (Firszt et al. 2004). Although cochlear implants are able to provide a signal that is speech-like in nature to listeners, the quality of sound is much different than the original signal. Currently, cochlear implants typically only deliver high-frequency (above about 300 Hz) electric signals to the user, who in turn loses the benefits of signals displayed at lower frequencies. While these devices have a promising future, it is necessary to explore options that improve speech perception and electricacoustic stimulation may be one solution. That’s where the third processing method comes in: hybrid stimulation. That is the topic of this research project. Electric-Acoustic Stimulation There are many ways to combine electric and acoustic stimulation. Both the electric and the acoustic components can be combined into one device, a cochlear implant with a hearing aid on the same ear. This configuration is known as a hybrid cochlear implant. Alternatively, the Electric-Acoustic Stimulation of Hearing in Adults and Children 6 electric and acoustic components can be separated into two devices, with a cochlear implant on one ear and a hearing aid in the contralateral ear. That configuration is known as traditional bimodal stimulation. Given that hybrid devices were only recently developed, most of the research available today involves bimodal stimulation (Dorman et al. 2009). As many studies have suggested, the bimodal and hybrid methods of stimulation offer many possible benefits for implant users. Before the advent of electric-acoustic stimulation, cochlear implant candidacy was limited to patients with slight to no residual hearing. As a result there was little need to explore options of preserving residual hearing, especially since older surgery techniques destroyed any natural hearing with the insertion of the electrode array. However, as technological advances occurred and candidacy requirements changed, so did the ideology surrounding the need to preserve residual hearing. With the current “soft surgery” technique for cochlear implantation and the design of smaller electrode arrays there is new hope for preserving residual hearing. The gentle insertion of the smaller electrode array into the scala tympani allows for limited damage to the inner ear and the hair cells at the apex of the cochlea. Accordingly, frequencies below roughly 500 to 700 Hz can be utilized with the support of a hearing aid (Dorman et al. 2005). This arrangement is necessary for the hybrid devices. In 2005 Gantz et al. found that around 6 months after implantation with a hybrid device participants scored 69% correct on monosyllabic word recognition. The participants’ scores continued to improve significantly to 79% correct within 6 months to 2 years of implantation (Gantz et al. 2005). This is better than mean scores of patients with only implants, which are closer to 42% correct (Firszt et al. 2004). Although frequencies below 300 Hz are unintelligible when presented alone, they hold c