Electric Hearing Research Articles

Combined Electric and Contralateral Acoustic Hearing: Word and Sentence Recognition With Bimodal Hearing

The authors assessed whether (a) a full-insertion cochlear implant would provide a higher level of speech understanding than bilateral low-frequency acoustic hearing, (b) contralateral acoustic hearing would add to the speech understanding provided by the implant, and (c) the level of performance achieved with electric stimulation plus contralateral acoustic hearing would be similar to performance reported in the literature for patients with a partial insertion cochlear implant. Monosyllabic word recognition as well as sentence recognition in quiet and at +10 and +5 dB was assessed. Before implantation, scores were obtained in monaural and binaural conditions. Following implantation, scores were obtained in electric-only and electric-plus-contralateral acoustic conditions. Postoperatively, all individuals achieved higher scores in the electric-only test conditions than they did in the best pre-implant test conditions. All individuals benefited from the addition of low-frequency information to the electric hearing. A full-insertion cochlear implant provides better speech understanding than bilateral, low-frequency residual hearing. The combination of an implant and contralateral acoustic hearing yields comparable performance to that of patients with a partially inserted implant and bilateral, low-frequency acoustic hearing. These data suggest that a full-insertion cochlear implant is a viable treatment option for patients with low-frequency residual hearing.

Effect of filter spacing on melody recognition: Acoustic and electric hearing

This paper assesses the effect of filter spacing on melody recognition by normal-hearing (NH) and cochlear implant (CI) subjects. A new semitone filter spacing is proposed for music. The quality of melodies processed by the various filter spacings is also evaluated. Results from NH listeners showed nearly perfect melody recognition with only four channels of stimulation, and results from CI users indicated significantly higher scores with a 12-channel semitone spacing compared to the spacing used in their daily processor. The quality of melodies processed by the semitone filter spacing was preferred over melodies processed by the conventional logarithmic filter spacing.

The effect of fundamental frequency in simulated electric-acoustic hearing

Individuals with residual hearing restricted to low frequencies are candidates for electric-acoustic stimulation (EAS). When low-frequency acoustic information is added to either real or simulated high-frequency electric stimulation, speech recognition often improves dramatically. This may reflect the availability of fundamental frequency (f0) information in the acoustic region. We tested this directly. In particular, we examined the relative contributions of f0 and the amplitude envelope of speech by replacing the low-frequency speech with a tone that was modulated either in frequency to track the f0 of the speech, in amplitude with the extracted envelope of the low-frequency speech, or both. A female talker was combined with various backgrounds and processed with a four-channel vocoder to simulate electric hearing. Across all backgrounds, intelligibility improved significantly when a tone tracking f0 was added to vocoder stimulation and further still when both f0 and amplitude envelope cues were applied. These results confirm the importance of f0 information (at least under simulated EAS) and indicate that significant information can be provided by a tone that tracks f0 or f0 and amplitude envelope cues. [Work supported in part by NIH.]

Psychophysical Assessment of Spatial Spread of Excitation in Electrical Hearing with Single and Dual Electrode Contact Maskers

To evaluate psychophysically the spatial spread of excitation in electrical hearing with a new dual contact masker and to investigate under which conditions it is possible to stimulate fibers in the immediate neighborhood of an electrode contact, which were not excited by neighboring electrode contacts. In this study a psychophysical forward masking paradigm with a dual contact masker was used to avoid off-site listening, the electrical analogue of off-frequency listening. The masker stimulus (300 msec) is presented nonsimultaneously on two electrode contacts, one on the apical side and another on the basal side of the probe contact, followed by a probe stimulus of 20 msec.Unmasked probe thresholds were compared with masked ones at a number of masker-probe distances, whereas growth of masking curves were measured for a fixed masker contact pair. Standard selectivity measurements (single contact masking) and the recovery of forward masking with one masker contact were included for comparison with existing methods. All experiments were carried out with six participants who use the Clarion CII device with a HiFocus I electrode array. For dual contact masking the amount of masking was significantly greater than for single contact masking and the width of the masking patterns was on average 1.1 mm broader than for single contact masking, resulting in a broad region of excitation, with masker-probe overlap for distances greater than 3 mm. Masking widths for dual and single contact masking were highly correlated. Growth of masking curves were highly nonlinear. They showed a strong elevation of the slope that starts for most subjects around the middle of the dynamic range or above. For 4 out of 6 subjects, no probe threshold was found above a masker amplitude of about 400-500 microA. The ratio of the maximum measurable masked probe thresholds and unmasked probe threshold ranged from 1.7 to 2.6 (S4 excluded). Recovery of masking functions follow an exponential decay. Time constants tau for the recovery process ranged from 21.6 msec to 114.9 msec. With a dual contact masker (1) off-site listening can be avoided, leading to larger estimates of the width of excitation patterns than in single contact masking, (2) it can be estimated for which stimulation level there is complete overlap of excitation patterns of adjacent electrode contacts, (3) it can be shown that stimulation of nerve fibers in the immediate neighborhood of an electrode contact which were not excited by neighboring electrode contacts is only possible if the probe stimulation amplitude is sufficiently high in comparison with amplitudes on neighboring contacts.

Electric-stimulus rate encoding algorithm of Mandarin tonal information in cochelear implant

Abstract The modern multi-channel cochlear implants usually extract and encode the temporal envelope cues. This kind of algorithms can hardly satisfy the users who speak tonal languages, such as Mandarin. Many studies have tried to enhance the recognition ability of Mandarin through encoding tonal information and made suggestions to frequency-modulate the fixed pulse-rate carrier with the extracted tonal information based on the rate-pitch theory in electric hearing. However, how to encode and deliver the tonal information such as F 0 by changing electric-stimulus rate has not got further study in clinical electric-stimulus mode. The present paper focuses on this matter by piecewise calculating the appreciable electric-stimulus rates. Then an electric-stimulus encoding algorithm of Mandarin tone is brought forward and testified through acoustic simulations. Such conclusion can be drawn that this algorithm of electric-stimulus rate could convey tonal information well and help to enhance the Mandarin speech...

The Effect of Intensity on Pitch in Electric Hearing and Its Relationship to the Speech Perception Performance of Cochlear Implantees

This study investigated the effect of intensity on pitch in electric hearing and its relationship to the speech perception ability of cochlear implantees. Subjects were 13 adult users of the Nucleus 22 cochlear implant system, using either the Spectra22 or ESPrit22 speech processor and the SPEAK speech processing strategy. A multidimensional scaling technique was employed. Speech perception was measured using sentences and vowels. All measurements were performed in a soundfield condition, and subjects wore their own speech processors with their normally used settings. Results showed a significant correlation between the degree of deviation of the subjects' stimulus spaces from the "ideal" space and subjects' performance with the sentences, but not with the vowels. A significant correlation was found between subjects' response variability in performing the multidimensional scaling task and their speech perception measures, suggesting that spectral smearing or underlying cognitive abilities might affect implantees' speech perception performance.

Utilization of acoustic cues in identifying speech intonation contrasts in cochlear implant recipients and listeners with normal hearing

This study investigated the utilization of acoustic cues in identifying speech intonation contrasts in adult cochlear implant (CI) recipients and normal‐hearing (NH) individuals listening to spectrally and temporally degraded stimuli. Fundamental frequency, intensity, and duration patterns of a disyllabic word, popcorn, were manipulated orthogonally, resulting in 360 resynthesized stimuli. In a two‐alternative forced‐choice task, CI and NH participants identified whether each stimulus was question‐like or statement‐like. Each NH listener also identified stimuli that were noise vocoded. Preliminary results from seven CI and four NH listeners indicated that (a) weighting of fundamental frequency in the identification of CI listeners was less pronounced than that of NH listeners, (b) unlike NH listeners who showed little reliance upon intensity or duration patterns in identifying unprocessed stimuli, CI users demonstrated systematic dependence upon intensity and duration cues in identifying the same set of stimuli, and (c) with noise‐vocoded (spectrally and temporally degraded) stimuli, NH listeners’ identification exhibited weighting patterns for intensity and duration cues in a way similar to that of CI listeners. Implications for the processing of multidimensional acoustic cues for speech intonation in normal and electrical hearing will be discussed. [Work supported by NIDCD‐R01DC04786.]

Neural correlates of psychometric functions for auditory detection

The psychometric function in a 2IFC task, which describes the relation between the percentage of correct responses (pc) and the signal strength (x), can be expressed using the Gaussian probability function (φ) with a threshold (α) and a slope (β) parameter, pc=φ[(x/α)β/21/2]. For auditory detection, the psychometric functions exhibited a wide range of slopes in different conditions, such as tone in noise [β≊1, D. M. Green, M. J. McKey, and J. C. R. Licklider, J. Acoust. Soc. Am. 31, 1446–1452 (1959)] and tone in quiet [β≊2, C. S. Watson, J. R. Franks, and D. C. Hood, J. Acoust. Soc. Am. 52, 633–643 (1972)] by listeners with acoustic hearing, and detection by listeners with electrical hearing [βmax≊10, G. S. Donaldson, N. F. Viemeister, and D. A. Nelson, J. Acoust. Soc. Am. 101, 3706–3721 (1997)]. To understand the mechanisms underlying the change in the slope of the psychometric function, the performance of a neural detector engaged in the same tasks as the listeners was analyzed, and the neural correlates of the psychometric function, the neurometric function, was derived. Under the assumption that the detection was based on spike counts of the neuron(s) tuned to the signal frequency, the derived neurometric function showed slope values in good agreement with those obtained from the listeners.

Effects of interaural time differences in fine structure and envelope on lateral discrimination in electric hearing

Bilateral cochlear implant (CI) listeners currently use stimulation strategies which encode interaural time differences (ITD) in the temporal envelope but which do not transmit ITD in the fine structure, due to the constant phase in the electric pulse train. To determine the utility of encoding ITD in the fine structure, ITD-based lateralization was investigated with four CI listeners and four normal hearing (NH) subjects listening to a simulation of electric stimulation. Lateralization discrimination was tested at different pulse rates for various combinations of independently controlled fine structure ITD and envelope ITD. Results for electric hearing show that the fine structure ITD had the strongest impact on lateralization at lower pulse rates, with significant effects for pulse rates up to 800 pulses per second. At higher pulse rates, lateralization discrimination depended solely on the envelope ITD. The data suggest that bilateral CI listeners benefit from transmitting fine structure ITD at lower pulse rates. However, there were strong interindividual differences: the better performing CI listeners performed comparably to the NH listeners.

Stability of Low-Frequency Residual Hearing in Patients Who Are Candidates for Combined Acoustic Plus Electric Hearing

The purpose of this study was to investigate the stability over time of low-frequency auditory thresholds to better determine if the new technique of using a short-electrode cochlear implant that preserves residual low-frequency acoustic hearing can be a long-term solution for those with severe-to-profound hearing loss at high frequencies. The present study determined the long-term rate of decline in acoustic hearing in patients who have a preexisting hearing loss yet have not been implanted with a cochlear implant. A retrospective analysis of patients' audiograms that fit into the range for candidacy for the short-electrode device was performed to calculate the rate of change of threshold over time. The analysis of adult patients' data indicated that there was an average of only 1.05 dB hearing deterioration per year in the low frequencies and that presbycusis accounted for approximately one third to one half of this decline. The average deterioration of hearing threshold for pediatric patients was 1.2 dB per year; however, the rates of change in pediatric patients were considerably more variable (across individuals and across frequencies) than in adults. These data provide support for the idea that the short-electrode cochlear implant may be a practical solution for most adults in the long run, but this may not be the case for all pediatric patients.

Effects of pulse rate on thresholds and loudness of biphasic and alternating monophasic pulse trains in electrical hearing

Detection thresholds and most comfortable loudnesses (MCLs) were determined as a function of pulse rate for standard biphasic pulse trains (BP) and for anodic and cathodic monophasic phases alternating at fixed intervals (ALT-m). Three different phase durations were examined. With a 100-μs phase duration, thresholds for the ALT-m stimulus were substantially (up to 12 dB) lower than for the BP stimuli at relatively low rates (200 pps), but were similar to the BP thresholds at high rates (⩾1000 pps). Thresholds for BP pulse trains decreased monotonically with increasing rate, whereas the function for ALT-m waveforms was non-monotonic with a maximum between 400 and 1000 pps. These trends occurred for three different cochlear implant devices, different electrode configurations, and, generally, for different phase durations (10.8, 25, and 100 μs/phase). However, at the shorter phase durations, thresholds remained lower for the ALT-m stimulus, even at 5000 pps, the highest rate studied. Dynamic ranges of the BP pulse trains increased with increasing rate, irrespective of the phase duration under test, but for the ALT-m stimuli this was only true at the shorter phase durations tested. At a 100-μs phase duration, dynamic ranges for the ALT-m waveforms did not differ significantly as a function of rate. The results confirm previous reports that delaying charge recovery, in this case by switching from a BP to an ALT-m wave shape, can substantially reduce thresholds [Van Wieringen, A., Carlyon, R.P., Laneau, J., Wouters, J., 2005. Effects of waveform shape on human sensitivity to electrical stimulation of the inner ear. Hear. Res. 200, 73–86; Carlyon, R.P., van Wieringen, A., Deeks, J.M., Long, C.J., Lyzenga, J, Wouters, J., 2005. Effect of inter-phase gap on the sensitivity of cochlear implant users to electrical stimulation. Hear. Res. 205, 210–224]. However, at high pulse rates, this advantage only occurs at short phase durations. In addition, we show that the complex interaction between the effects of pulse shape, rate, and phase duration on thresholds can be captured by the simple linear model described by Carlyon et al.

Acoustic cues for improved speech recognition in combined acoustic and electric hearing

Speech recognition in noise improves with combined acoustic and electric hearing compared to electric hearing alone. Here, we investigate the contribution of four low-frequency hearing cues (fundamental frequency (F0), first formant (F1), voicing, and glimpsing) to speech recognition in combined hearing. Normal-hearing listeners heard vocoded speech in one ear and low-pass (LP) filtered speech in the other. Three conditions (vocode alone, LP alone, and combined) were tested. Target speech with an average F0 of 120 Hz was mixed with a time-reversed masker sentence (average F0=172 Hz) at three SNRs (5,10,15 dB). The LP speech aided performance at all three SNRs. F1 cues were then removed by replacing the LP speech with an LP equal-amplitude harmonic complex with the same F0 contour as the target speech, and an amplitude that was modulated by that of the target’s voiced portions. The benefits of combined hearing disappeared at 10- and 15-dB SNR, but persisted at SNR=5 dB. The same happened when, additionally, F0 cues were removed by fixing the F0 of the harmonic complex at 150 Hz. The results are consistent with a role for the F1 cue and of voicing and/or glimpsing cues, but not with a combination of F0 information between the two ears.

A theoretical model of cochlear processing improves spectrally degraded speech perception

Smith and Lewicki, Neural Comp. 17, 19–45, 2005a; Adv. Neural. Inf. Process. Syst. 17, 1289–1296, 2005b; Nature, 439, 7079, 2006, demonstrated that mammalian hearing follows an efficient coding principle (Barlow, Sensory Communications, 217–234, 1961; Atick, Network, 3(2), 213–251, 1992; Simoncelli and Olshausen, Ann. Rev. Neurosci., 24, 1193–1216, 2001; Laughlin and Sejnowski, ‘‘Communications in Neuronal Networks,’’ Science, 301, 1870–1874 2003). Auditory neurons efficiently code for natural sounds in the environment, maximizing information rate while minimizing coding cost (Shannon, Science, 270, 303–304, 1948). Applying the same analysis to speech coding suggests that speech acoustics are optimally adapted to the mammalian auditory code (Smith and Lewicki, Neural Comp. 17, 19–45, 2005a; Adv. Neural. Inf. Process. Syst. 17, 1289–1296, 2005b; Nature, 439, 7079, 2006). The present work applies this efficient coding theory to the problem of speech perception in individuals using cochlear implants (CI), for which there exist vast individual differences in speech perception and spectral resolution (Zeng et al., Auditory Prostheses and Electric Hearing, 20, 1–14, 2004). A machine-learning method for CI filterbank design based on the efficient-coding hypothesis is presented. Further, a pair of experiments to evaluate this approach using noise-excited vocoder speech (Shannon et al., Bell Systems Technical Journal 27, 379–423, 623–656, 1995) is described. Participants’ recognition of continuous speech and isolated syllables is significantly more accurate for speech filtered through the theoretically-motivated efficient-coding filterbank relative to the standard cochleotopic filterbank, particularly for speech transients. These findings offer insight in CI design and provide behavioral evidence for efficient coding in human perception.

Loudness Adaptation in Acoustic and Electric Hearing

The present study is aimed to evaluate and compare loudness adaptation between normal hearing and cochlear-implant subjects. Loudness adaptation for 367-s pure tones was measured in five normal-hearing subjects at three frequencies (125, 1,000, and 8,000 Hz) and three levels (30, 60, and 90 dB SPL). In addition, loudness adaptation for 367-s pulse trains was measured in five Clarion cochlear-implant subjects at three stimulation rates (100, 991, and 4,296 Hz), three levels (10, 50, and 90% of the electric dynamic range), three stimulation positions (apical, middle and basal), and two stimulation modes (monopolar and bipolar). The method of successive magnitude estimation was used to quantify loudness adaptation. Similar to the previous results, we found that loudness adaptation in normal-hearing subjects increases with decreasing level and increasing frequency. However, we also found a small but significant loudness enhancement at 90 dB SPL in acoustic hearing. Despite large individual variability, we found that loudness adaptation in cochlear-implant subjects increases with decreasing levels, but is not significantly affected by the rate, place and mode of stimulation. A phenomenological model was proposed to predict loudness adaptation as a function of stimulus frequency and level in acoustic hearing. The present results were not fully compatible with either the restricted excitation hypothesis or the neural adaptation hypothesis. Loudness adaptation may have a central component that is dependent on the peripheral excitation pattern.

The Benefits of Combined Acoustic plus Electric Hearing in the Treatment of Sensorineural Hearing Loss

The Benefits of Combined Acoustic plus Electric Hearing in the Treatment of Sensorineural Hearing Loss

Efficacy of a Cochlear Implant Simultaneous Analog Stimulation Strategy Coupled with a Monopolar Electrode Configuration

The present study was performed to evaluate the efficacy and clinical feasibility of using monopolar stimulation with the Clarion Simultaneous Analog Stimulation (SAS) strategy in patients with cochlear implants. Speech recognition by 10 Clarion cochlear implant users was evaluated by means of 4 different speech processing strategy/electrode configuration combinations; ie, SAS and Continuous Interleaved Sampling (CIS) strategies were each used with monopolar (MP) and bipolar (BP) electrode configurations. The test measures included consonants, vowels, consonant-nucleus-consonant words, and Hearing in Noise Test sentences with a +10 dB signal-to-noise ratio. Additionally, subjective judgments of sound quality were obtained for each strategy/configuration combination. All subjects but 1 demonstrated open-set speech recognition with the SAS/MP combination. The group mean Hearing in Noise Test sentence score for the SAS/MP combination was 31.6% (range, 0% to 92%) correct, as compared to 25.0%, 46.7%, and 37.8% correct for the CIS/BP, CIS/MP, and SAS/BP combinations, respectively. Intersubject variability was high, and there were no significant differences in mean speech recognition scores or mean preference ratings among the 4 strategy/configuration combinations tested. Individually, the best speech recognition performance was with the subject's everyday strategy/configuration combination in 72% of the applicable cases. If the everyday strategy was excluded from the analysis, the subjects performed best with the SAS/MP combination in 37.5% of the remaining cases. The SAS processing strategy with an MP electrode configuration gave reasonable speech recognition in most subjects, even though subjects had minimal previous experience with this strategy/configuration combination. The SAS/MP combination might be particularly appropriate for patients for whom a full dynamic range of electrical hearing could not be achieved with a BP configuration.

Temporal Masking in Electric Hearing

Temporal masking can be defined as the detection threshold of a brief signal as a function of the signal delay in a relatively long masker. The temporal masking pattern in normal acoustic hearing reveals temporal edge enhancement in which the signal detection threshold is greater near the masker onset than in the steady-state portion. Both peripheral and central mechanisms appear to underlie temporal edge enhancement, but their relative contributions remain elusive. Cochlear implants bypass cochlear mechanical processing and stimulate the auditory nerve directly, thereby providing a unique opportunity to separate the peripheral mechanisms from the central mechanisms. Here, we systematically measured temporal masking in electric hearing by examining whether a brief signal was harder to detect at the onset than in the steady-state portion of a long masker (the "overshoot" effect). The signal and the masker were presented (1) either to the same electrode or to different electrodes, (2) at the same stimulation or different rates, and (3) in a simultaneous or an interleaved fashion. A consistent pattern of results was observed, depending on the stimulus configuration between the signal and the masker. Simultaneous stimulation at the same rate and with the same electrode produced no difference in sensitivity between the onset and the steady-state conditions, but interleaved stimulation at different rates or with different electrodes produced a significant difference. Unlike acoustic hearing, high masker levels produced an overshoot effect, and low masker levels produced an undershoot effect. Although the present results are consistent with the "on-frequency vs. off-frequency" hypothesis for the overshoot effect, results also suggest a central "same vs. different" mechanism underlying temporal masking. These results have practical implications for improving cochlear implant design.

Speaker recognition with temporal cues in acoustic and electric hearing

Natural spoken language processing includes not only speech recognition but also identification of the speaker's gender, age, emotional, and social status. Our purpose in this study is to evaluate whether temporal cues are sufficient to support both speech and speaker recognition. Ten cochlear-implant and six normal-hearing subjects were presented with vowel tokens spoken by three men, three women, two boys, and two girls. In one condition, the subject was asked to recognize the vowel. In the other condition, the subject was asked to identify the speaker. Extensive training was provided for the speaker recognition task. Normal-hearing subjects achieved nearly perfect performance in both tasks. Cochlear-implant subjects achieved good performance in vowel recognition but poor performance in speaker recognition. The level of the cochlear implant performance was functionally equivalent to normal performance with eight spectral bands for vowel recognition but only to one band for speaker recognition. These results show a disassociation between speech and speaker recognition with primarily temporal cues, highlighting the limitation of current speech processing strategies in cochlear implants. Several methods, including explicit encoding of fundamental frequency and frequency modulation, are proposed to improve speaker recognition for current cochlear implant users.

Acoustic Simulations of Combined Electric and Acoustic Hearing (EAS)

Our aim was to explore the consequences for speech understanding of leaving a gap in frequency between a region of acoustic hearing and a region stimulated electrically. Our studies were conducted with normal-hearing listeners, using an acoustic simulation of combined electric and acoustic (EAS) stimulation. Simulations of EAS were created by low-pass filtering speech at 0.5 kHz (90 dB octave roll-off) and adding amplitude-modulated sine waves at higher frequencies. The gap in frequency between acoustic and simulated electric hearing was varied over the range 0.5 kHz to 3.2 kHz. Stimuli included sentences in quiet, sentences in noise, and consonants and vowels. Three experiments were conducted with sample sizes of 12 listeners. Scores were highest in conditions that minimized the frequency gap between acoustic and electric stimulation. In quiet, vowels and consonant place of articulation showed the most sensitivity to the frequency gap. In noise, scores in the simulated EAS condition were higher than the sum of the scores from the acoustic-only and simulated electric-only conditions. Our results suggest that both deep and shallow insertions of electrodes could improve the speech understanding abilities of patients with residual hearing to 500 Hz. However, performance levels will be maximized if the gap between acoustic and electric stimulation is minimized.

Pitch discrimination of patterned electric stimulation

One reason for the poor pitch performance in current cochlear-implant users may be the highly synchronized neural firing in electric hearing that lacks stochastic properties of neural firing in normal acoustic hearing. This study used three different electric stimulation patterns, jittered, probabilistic, and auditory-model-generated pulses, to mimic some aspects of the normal neural firing pattern in acoustic hearing. Pitch discrimination was measured at standard frequencies of 100, 250, 500, and 1000 Hz on three Nucleus-24 cochlear-implant users. To test the utility of the autocorrelation pitch perception model in electric hearing, one, two, and four electrodes were stimulated independently with the same patterned electric stimulation. Results showed no improvement in performance with any experimental pattern compared to the fixed-rate control. Pitch discrimination was actually worsened with the jittered pattern at low frequencies (125 and 250 Hz) than that of the control, suggesting that externally introduced stochastic properties do not improve pitch perception in electric stimulation. The multiple-electrode stimulation did not improve performance but did not degrade performance either. The present results suggest that both "the right time and the right place" may be needed to restore normal pitch perception in cochlear-implant users.