Indication for the need of flexible and frequency specific mapping functions in cochlear implant speech processors

Categorical loudness scaling of electric and acoustic stimuli was performed in cochlear implant (CI) recipients equipped with Nucleus™ systems in order to achieve a normal loudness perception in the whole dynamic range of acoustic input. For each electrode, the lower and upper limits of electric stimulus were defined by the values corresponding to “very soft” and “too loud”. Within this dynamic range, the stimulus strength intervals associated to the verbal categories “soft”, “medium”, “loud” and “very loud” were determined. The same loudness categories were used for the scaling of acoustic stimuli. From both scaling experiments, the transduction of the CI system can be assessed and the parameters of the individual mapping function yielding a normal loudness growth can be derived. Deviations from optimum mapping can be corrected at least partially by manipulating the parameters of the mapping function. In many cases, however, one mapping function is not sufficient for all channels. The results argue in favour of the development of flexible and channel-specific mapping function parameters in future CI systems. Parts of this paper were presented at the 76th annual meeting of the Deutsche Gesellschaft für HNO-Heilkunde, Kopf- und Hals-Chirurgie in Erfurt on 8 May 2005.     

Loudness growth in cochlear implants: effect of stimulation rate and electrode configuration

In cochlear implant speech processor design, acoustic amplitudes are mapped to electric currents with the intention of preserving loudness relationships across electrodes. Many parameters may affect the growth of loudness with electrical stimulation. The present study measured the effects of stimulation rate and electrode configuration on loudness growth in six Nucleus-22 cochlear implant users. Loudness balance functions were measured for stimuli that differed in terms of stimulation rate, electrode configuration and electrode location; a 2-alternative, forced-choice adaptive procedure (double-staircase) was used. First, subjects adaptively adjusted the amplitude of a 100-pulse-per-second (pps) pulse train to match the loudness of a 1000-pps standard pulse train. For a range of reference stimulation levels, the loudness of the 100-pps stimulus was matched to that of the 1000-pps standard stimulus; loudness balancing was performed for three electrode pairs [(20,22), (1,3), (1,22)]. The results showed that the loudness balance functions between the 100- and 1000-pps stimulation rates were highly subject-dependent. Some subjects' loudness balance functions were logarithmic, while others' were nearly linear. Loudness balance functions were also measured across electrode locations [(20,22) vs. (1,3)] for two stimulation rates (100, 1000 pps). Results showed that the loudness balance functions between the apical and basal electrode pairs highly depended on the stimulation rate. For all subjects, at the 1000-pps rate, the loudness balance functions between the two electrode locations were nearly linear; however, at the 100-pps rate, the loudness balance function was highly nonlinear in two out of six subjects. These results suggest that, for some cochlear implant patients, low-frequency stimulation may be processed differently at different electrode locations; for these patients, acoustic-to-electric amplitude mapping may need to be sensitive to this place-dependent processing when relatively low stimulation rates are used.     

Influence of stimulation rate and loudness growth on modulation detection and intensity discrimination in cochlear implant users

In cochlear implants (CIs), increasing the stimulation rate typically increases the electric dynamic range (DR), mostly by reducing audibility thresholds. While CI users’ intensity resolution has been shown to be fairly constant across stimulation rates, high rates have been shown to weaken modulation sensitivity, especially at low listening levels. In this study, modulation detection thresholds (MDTs) were measured in five CI users for a range of stimulation rates (250–2000 pulses per second) and modulation frequencies (5–100 Hz) at 8 stimulation levels that spanned the DR (loudness-balanced across stimulation rates). Intensity difference limens (IDLs) were measured for the same stimulation rates and levels used for modulation detection. For all modulation frequencies, modulation sensitivity was generally poorer at low levels and at higher stimulation rates. CI users were sensitive to modulation frequency only at relatively high levels. Similarly, IDLs were poorer at low levels and at high stimulation rates. When compared directly in terms of relative amplitude, IDLs were generally better than MDTs at low levels. Differences in loudness growth between dynamic and steady stimuli might explain level-dependent differences between MDTs and IDLs. The slower loudness growth associated with high stimulation rates might explain the poorer MDTs and IDLs with high rates. In general, high stimulation rates provided no advantage in intensity resolution and a disadvantage in modulation sensitivity.     

Practical model description of peripheral neural excitation in cochlear implant recipients: 1. Growth of loudness and ECAP amplitude with current

This is the first in a series of five papers, presenting the development of a practical mathematical model that describes excitation of the auditory nerve by electrical stimulation from a cochlear implant. Here are presented methods and basic data for the subjects, who were implanted with the Nucleus^® 24 cochlear implant system (three with straight and three with Contour™ electrode arrays), required as background for all papers. The growth of subjective loudness with stimulus current was studied, for low-rate pulse bursts and for single pulses. The growth of the amplitude of the compound action potential (ECAP) was recorded using the Neural Response Telemetry™ (NRT™) system. An approximately linear relationship was demonstrated between ECAP amplitude and burst loudness, although this failed at the lower end of the dynamic range, to an extent that varied with subject and stimulated electrode. Single-pulse stimuli were audible below ECAP threshold, demonstrating that the audibility of burst stimuli at such low currents was not due solely to temporal loudness summation. An approximate function was established relating the curvature of the burst loudness growth function to the maximum comfortable level (MCL). Loudness at threshold was quantified, as a percentage of loudness at MCL. The relationship between loudness and ECAP growth functions, the curvature versus MCL function and the loudness associated with threshold are relevant to the development of a mathematical model of electrically evoked auditory nerve excitation.     

Sensitivity to interaural level difference and loudness growth with bilateral bimodal stimulation

The interaural level difference (ILD) is an important cue for the localization of sound sources. The sensitivity to ILD was measured in 10 users of a cochlear implant (CI) in one ear and a hearing aid (HA) in the other severely impaired ear. For simultaneous presentation of a pulse train on the CI side and a sinusoid on the HA side the just noticeable difference (JND) in ILD and loudness growth functions were measured. The mean JND for pitch-matched electric and acoustic stimulation was 1.7 dB. A linear fit of the loudness growth functions on a decibel-versus-microampere scale shows that the slope depends on the subject's dynamic ranges.     

Loudness and auditory steady-state responses in normal-hearing subjects

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the growth of loudness in listeners with normal hearing. Individual intensity functions were obtained from measures of loudness growth using the contour test and from the electrophysiological amplitude measures of multiple amplitude-modulated (77–105Hz) tones (500, 1000, 2000, and 4000Hz) simultaneously presented to both ears and recorded over the scalp. Slope analyses for the behavioural and electrophysiological intensity functions were separately performed. Response amplitudes of the ASSRs and loudness sensation judgements increase as the stimulus intensity increases for the four frequencies studied. A significant relationship was obtained between loudness and the ASSRs. The results of this study suggest that the amplitude of the ASSRs may be used to estimate loudness growth at least for individuals with normal hearing.     

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.     

Categorical loudness scaling in cochlear implant recipients

Objective: This study investigated categorical loudness scaling in a large group of cochlear implant (CI) recipients. Design: Categorical loudness was measured for individually determined sets of current amplitudes on apical, mid and basal electrodes of the Nucleus array. Study sample: Thirty adult subjects implanted with the Nucleus CI. Results: Subjects were generally reliable in categorical loudness scaling. As expected, current levels eliciting the same loudness categories differed across subjects and electrodes in many cases. After scaling the electric levels to remove differences in dynamic ranges across subjects and electrodes, the across-subject loudness functions for the three electrodes were very similar. Conclusions: Scaled electric current to remove differences in dynamic range, as implemented in the Nucleus processor, ensures uniform loudness across the array and CI recipients. The results also showed that categorical loudness scaling for electric stimulation was similar to that for acoustic stimulation in normal hearing subjects. These findings could be used as a guide for aligning electric and acoustic loudness in CI recipients with contralateral hearing.     

Post-operative Stapedius Reflex Tests with Simultaneous Loudness Scaling in Patients Supplied with Cochlear Implants

The estimation of the maximum comfort loudness levels (MCL) by measurements of the electrically elicited stapedius reflex was examined in six experienced cochlear implant users supplied with the COMBI 40 implant system. The stapedius reflex was tested and loudness scaling was performed simultaneously using an up/down stimulation protocol close to the reflex threshold with automated recording of both test procedures. The electrical stapedius reflex threshold (ESRT) and loudness scaling were evaluated separately. Scaling at the reflex threshold ranged between normal and loud. The range of stimulus intensities corresponding to ESRT is much smaller than that at a particular loudness category. The overall correlation between ESRT and MCL was high (r=0.92), with a similar dependence of ESRT and MCL on the channel stimulated. Thus, when the stapedius reflex can be detected post-op-eratively, the ESRT can be applied successfully for the fitting procedure of the speech processor. Simultaneous loudness scaling during the entire reflex test showed that overstimulation via the implant can be avoided effectively.     

响度梯度训练法矫治听障儿童响度低下障碍的个案研究

目的本研究探讨响度梯度训练法在临床上对响度过低患者的康复效果。方法以1名存在响度低下问题的听力障碍儿童为对象，运用响度梯度训练法对其进行提高响度的康复训练，比较分析训练前后该儿童的平均言语强度、强度标准差、最高强度和最低强度。结果响度训练前后患儿平均言语强度存在显著性差异，最低强度无显著差异，最高强度有显著变化。结论响度梯度训练法对听障儿童响度低下问题有康复效果。     

Cochlear-implant spatial selectivity with monopolar, bipolar and tripolar stimulation

Sharp spatial selectivity is critical to auditory performance, particularly in pitch-related tasks. Most contemporary cochlear implants have employed monopolar stimulation that produces broad electric fields, which presumably contribute to poor pitch and pitch-related performance by implant users. Bipolar or tripolar stimulation can generate focused electric fields but requires higher current to reach threshold and, more interestingly, has not produced any apparent improvement in cochlear-implant performance. The present study addressed this dilemma by measuring psychophysical and physiological spatial selectivity with both broad and focused stimulations in the same cohort of subjects. Different current levels were adjusted by systematically measuring loudness growth for each stimulus, each stimulation mode, and in each subject. Both psychophysical and physiological measures showed that, although focused stimulation produced significantly sharper spatial tuning than monopolar stimulation, it could shift the tuning position or even split the tuning tips. The altered tuning with focused stimulation is interpreted as a result of poor electrode-to-neuron interface in the cochlea, and is suggested to be mainly responsible for the lack of consistent improvement in implant performance. A linear model could satisfactorily quantify the psychophysical and physiological data and derive the tuning width. Significant correlation was found between the individual physiological and psychophysical tuning widths, and the correlation was improved by log-linearly transforming the physiological data to predict the psychophysical data. Because the physiological measure took only one-tenth of the time of the psychophysical measure, the present model is of high clinical significance in terms of predicting and improving cochlear-implant performance.     

Electrocochleography and experimentally induced loudness recruitment

Summary The relationship between changes in loudness and the cochlear whole-nerve potential following experimentally produced deafness was studied in an animal model. Reaction time of a subject's response to an auditory stimulus has been shown to be an index of loudness in human experiments and has been adapted to nonhuman primates. In a series of experiments, four macaque monkeys were operantly conditioned to respond to 8-kHz tones over a range of 0–80 dB SPL, and their reaction times to pure tone stimuli were measured. Whole-nerve cochlear action potentials were recorded from chronic inner-ear electrodes. The relationship between behavioral and electrical measures of loudness recruitment were examined in animals with both temporary and permanent noise-induced hearing loss.Loudness recruitment was demonstrated experimentally after a 1-h exposure to a high-intensity 8-kHz octave band of noise. Excellent agreement was observed between the reaction time function and the action potential input-output function at intervals of 0.5, 12, 24, 48, and 84 h after exposure.Permanent hearing loss was produced in some of these animals by a much longer duration of exposure to the 8-kHz octave band of noise. Recruitment was observed in both the behavioral and the electrical measures. Histological studies of these damaged cochleas revealed primarily outer hair cell destruction, with a relative sparing of inner hair cells and nerve supply. The findings of this study are interpreted as strong support for the clinical electrocochleogram as an objective indicator of the presence of loudness recruitment.This investigation was supported by research grants NS-05077, NS-05065 and NS-10854, Program Project grant NS-05786, training grant NS-05679, and Postdoctoral fellowship SF 11 NS-2423-03 to J.E.P. from the National Institutes of Health     

Acoustic reflex threshold and loudness in patients with unilateral hearing losses

The relationship between the acoustic reflex threshold (ART) and loudness was examined in patients with unilateral hearing losses and subjects with simulated hearing losses using a masking method. Significant differences in the ART between the two ears of patients with unilateral hearing losses were correlated with differences in loudness at the level of the ART with differences in loudness determined by the alternate binaural loudness balance test. A similar relationship of ART and the sensation of loudness was also observed in ears with simulated hearing losses. The results obtained in the present study suggest a positive relationship between the ART and loudness, and provide some support for the assumption that a common neuronal information pathway plays an important role both in producing the loudness and eliciting the acoustic reflex. Received: 4 April 1997 / Accepted: 4 August 1997     

Comparison of linear gain and wide dynamic range compression hearing aid circuits II: aided loudness measures

OBJECTIVES: The goal of this study was to test the theoretical advantages of a single-channel wide dynamic range compression (WDRC) circuit fitted using the DSL method for increased dynamic range and normalized loudness growth. DESIGN: Ten adolescents and young adults with moderate to severe sensorineural hearing loss were fitted monaurally with the Siemens Viva 2 Pro behind-the-ear instrument set to DSL 4.0 targets for both linear gain and WDRC processing. Threshold, upper limit of comfort and loudness growth were measured in the unaided, linear gain and WDRC conditions for warble tones, environmental sounds and speech. Twelve adult listeners with normal hearing also were tested monaurally in the unaided condition to provide normative data for comparison purposes. RESULTS: The WDRC hearing aid provided a greater input dynamic range than the linear circuit for all stimuli. The dynamic range was normalized for more subjects with the WDRC than the linear hearing aid. In addition, exponential loudness growth functions fitted to the loudness growth data showed that, on average, loudness growth was more normalized with the WDRC hearing aid fitted to DSL[i/o] targets than the linear hearing aid fitted to DSL[i/o] targets. CONCLUSIONS: WDRC processing, fitted using the DSL[i/o] method, has potential applications in hearing aid fittings for listeners with moderate to severe hearing loss because it provides an audible, comfortable and tolerable amplified signal across a wider range of inputs than linear gain processing, without the need for volume control adjustments.     

Critical Bandwidth in Menière's Disease

The critical bandwidth in loudness summation was estimated in 20 patients with typical Menière's disease using noise bands centered around 1 kHz. A reduction of the normal loudness difference between broad-band noise and narrow-band noise was present at all except the highest levels. Judged individually, 7 of the 20 patients appeared to have a widened critical band, but in the pooled data the size of the critical band was normal. This was the case in patients with a hearing loss <50 dB HL as well as in patients with a hearing loss ≥50 dB HL. Expressed in terms of the critical band mechanism as an internal filter system, the single filter appears to have normal bandwidth but the interaction between adjacent filters is defective. The anatomical localisation of this interaction is discussed.     

Loudness adaptation measured by the simultaneous dichotic loudness balance technique differs between genders

Loudness adaptation was measured using the classic simultaneous, dichotic loudness balance technique. A 6-min continuous tone was introduced using headphones to a participant’s adaptingear. Immediately upon presentation of the tone and at 1-min intervals, participants adjusted the sound level of a tone of the same frequency in the contralateral controlear until both tones sounded equally loud. The control ear, which was otherwise retained in silence, measured adaptation in the adapting ear. As the constant-sound level stimulus to the adapting ear continued, the sound level that a participant selected to produce equal loudness between ears decreased, oscillating towards an apparent asymptotic value. This value was used to calculate total decibels of adaptation. The magnitude of female adaptation exceeded that of males at all time points measured following stimulus onset. The ratio total dB of adaptation to dB SL of the test tone may provide an empirical estimate for the loudness exponent, n, seen in Stevens’ power law, L = kφⁿ_, which relates the intensity of a pure tone, φ, to the loudness of the tone, L. Since dB of adaptation for females was greater than that of males, female n-values exceeded those of males, in accordance with previous research.     

Electrical field imaging as a means to predict the loudness of monopolar and tripolar stimuli in cochlear implant patients

Tripolar and other electrode configurations that use simultaneous stimulation inside the cochlea have been tested to reduce channel interactions compared to the monopolar stimulation conventionally used in cochlear implant systems. However, these "focused" configurations require increased current levels to achieve sufficient loudness. In this study, we investigate whether highly accurate recordings of the intracochlear electrical field set up by monopolar and tripolar configurations correlate to their effect on loudness. We related the intra-scalar potential distribution to behavioral loudness, by introducing a free parameter (α) which parameterizes the degree to which the potential field peak set up inside the scala tympani is still present at the location of the targeted neural tissue. Loudness balancing was performed on four levels between behavioral threshold and the most comfortable loudness level in a group of 10 experienced Advanced Bionics cochlear implant users. The effect of the amount of focusing on loudness was well explained by α per subject location along the basilar membrane. We found that α was unaffected by presentation level. Moreover, the ratios between the monopolar and tripolar currents, balanced for equal loudness, were approximately the same for all presentation levels. This suggests a linear loudness growth with increasing current level and that the equal peak hypothesis may predict the loudness of threshold as well as at supra-threshold levels. These results suggest that advanced electrical field imaging, complemented with limited psychophysical testing, more specifically at only one presentation level, enables estimation of the loudness growth of complex electrode configurations.     

A Computational Model of a Single Auditory Nerve Fiber for Electric-Acoustic Stimulation

Cochlear implant (CI) recipients with preserved acoustic low-frequency hearing in the implanted ear are a growing group among traditional CI users who benefit from hybrid electric-acoustic stimulation (EAS). However, combined ipsilateral electric and acoustic stimulation also introduces interactions between the two modalities that can affect the performance of EAS users. A computational model of a single auditory nerve fiber that is excited by EAS was developed to study the interaction between electric and acoustic stimulation. Two existing models of sole electric or acoustic stimulation were coupled to simulate responses to combined EAS. Different methods of combining both models were implemented. In the coupled model variant, the refractoriness of the simulated fiber leads to suppressive interaction between electrically evoked and acoustically evoked spikes as well as spontaneous activity. The second model variant is an uncoupled EAS model without electric-acoustic interaction. By comparing predictions between the coupled and the noninteracting EAS model, it was possible to infer electric-acoustic interaction at the level of the auditory nerve. The EAS model was used to simulate fiber populations with realistic inter-unit variability, where each unit was represented by the single-fiber model. Predicted thresholds and dynamic ranges, spike rates, latencies, jitter, and vector strengths were compared to empirical data. The presented EAS model provides a framework for future studies of peripheral electric-acoustic interaction.

     

Effects of dynamic range and amplitude mapping on phoneme recognition in Nucleus-22 cochlear implant users

OBJECTIVE: To determine the consequences for phoneme recognition of errors in setting threshold and loudness levels in cochlear implant listeners using a 4-channel continuous interleaved sampling (CIS) speech processor. DESIGN: Three Nucleus-22 cochlear implant listeners, who normally used the SPEAK speech processing strategy participated in this study. An experimental 4-channel CIS speech processor was implemented in each listener as follows. Speech signals were band-pass filtered into four broad frequency bands and the temporal envelope of the signal in each band was extracted by half-wave rectification and low-pass filtering. A power function was used to convert the extracted acoustic amplitudes to electric currents. The electric currents were dependent on the exponent of the mapping power function and the electrode dynamic range, which was determined by the minimum and maximum stimulation levels. In the baseline condition, the minimum and maximum stimulation levels were defined as the psychophysically measured threshold level (T-level) and maximum comfortable level (C-level). In the experimental conditions, the maximum stimulation levels were fixed at the C-level and the dynamic range (in dB) was changed by varying the minimum stimulation levels on all electrodes. This manipulation simulates the effect of an erroneous measurement of the T-level. Phoneme recognition was obtained as the dynamic range of electrodes was changed from 1 dB to 20 dB and as the exponent of the power-law amplitude mapping function was changed from 0.1 to 0.4. RESULTS: For each mapping condition, the electric dynamic range had a significant, but weak effect on vowel and consonant recognition. For a strong compression (p = 0.1), best vowel and consonant scores were obtained with a large dynamic range (12 dB). When the exponent of the mapping function was changed to 0.2 and 0.4, the dynamic range producing the highest scores decreased to 6 dB and 3 dB, respectively. CONCLUSIONS: Phoneme recognition with a 4-channel CIS strategy was only mildly affected by large changes in both electric threshold and loudness mapping. Errors in threshold by a factor of 2 (6 dB) and in the loudness mapping exponent by a factor of 2 were required to produce a significant decrease in performance. In these extreme conditions, the effect of the electric dynamic range on phoneme recognition could be due to two independent factors: abnormal loudness growth and a reduction in the number of discriminable intensity steps. The decrease in performance caused by a reduced electric dynamic range can be compensated by a more expansive power-law mapping function, as long as the number of discriminable intensity steps is moderately large (e.g., >8).