Electrical Excitation of the Acoustically Sensitive Auditory Nerve: Single-Fiber Responses to Electric Pulse Trains

Nearly all studies on auditory-nerve responses to electric stimuli have been conducted using chemically deafened animals so as to more realistically model the implanted human ear that has typically been profoundly deaf. However, clinical criteria for implantation have recently been relaxed. Ears with “residual” acoustic sensitivity are now being implanted, calling for the systematic evaluation of auditory-nerve responses to electric stimuli as well as combined electric and acoustic stimuli in acoustically sensitive ears. This article presents a systematic investigation of single-fiber responses to electric stimuli in acoustically sensitive ears. Responses to 250 pulse/s electric pulse trains were collected from 18 cats. Properties such as threshold, dynamic range, and jitter were found to differ from those of deaf ears. Other types of fiber activity observed in acoustically sensitive ears (i.e., spontaneous activity and electrophonic responses) were found to alter the temporal coding of electric stimuli. The electrophonic response, which was shown to greatly change the information encoded by spike intervals, also exhibited fast adaptation relative to that observed in the “direct” response to electric stimuli. More complex responses, such as “buildup” (increased responsiveness to successive pulses) and “bursting” (alternating periods of responsiveness and unresponsiveness) were observed. Our findings suggest that bursting is a response unique to sustained electric stimulation in ears with functional hair cells.     

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.     

Asymmetric Pulses in Cochlear Implants: Effects of Pulse Shape, Polarity, and Rate

Existing cochlear implants stimulate the auditory nerve with trains of symmetric biphasic (BP) pulses. Recent data have shown that modifying the pulse shape, while maintaining charge balance, may be beneficial in terms of reducing power consumption, increasing dynamic range, and limiting channel interactions. We measured thresholds and most comfortable levels (MCLs) for various 99-pulses-per-second (pps) stimuli. “Pseudomonophasic (PS)” pulses consist of a brief phase of one polarity followed immediately by a longer and lower-amplitude phase of the opposite polarity. We focused on a novel variant of PS pulses, termed the “delayed pseudomonophasic (DPS)” stimulus, in which the longer phase is presented midway between the short phases of two consecutive pulses. DPS pulse trains produced thresholds that were more than 10 dB lower than those obtained with BP pulses. This reduction was much greater than the 0- to 3-dB drop obtained with PS pulses and was still more than 6 dB when a pulse rate of 892 pps was used. A study of the relative contributions of the two phases of DPS suggested that the short, high-amplitude phase dominated the perceived loudness. This study showed major threshold and MCL reductions using a DPS stimulus compared to the widely used BP stimulus. These reductions, which were predicted by a simple linear filter model, might lead to considerable power savings if implemented in a cochlear implant speech processor.     

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.     

Auditory Nerve Fiber Responses to Combined Acoustic and Electric Stimulation

Persons with a prosthesis implanted in a cochlea with residual acoustic sensitivity can, in some cases, achieve better speech perception with “hybrid” stimulation than with either acoustic or electric stimulation presented alone. Such improvements may involve “across auditory-nerve fiber” processes within central nuclei of the auditory system and within-fiber interactions at the level of the auditory nerve. Our study explored acoustic–electric interactions within feline auditory nerve fibers (ANFs) so as to address two goals. First, we sought to better understand recent results that showed non-monotonic recovery of the electrically evoked compound action potential (ECAP) following acoustic masking (Nourski et al. 2007, Hear. Res. 232:87–103). We hypothesized that post-masking changes in ANF temporal properties and responsiveness (spike rate) accounted for the ECAP results. We also sought to describe, more broadly, the changes in ANF responses that result from prior acoustic stimulation. Five response properties—spike rate, latency, jitter, spike amplitude, and spontaneous activity—were examined. Post-masking reductions in spike rate, within-fiber jitter and across-fiber variance in latency were found, with the changes in temporal response properties limited to ANFs with high spontaneous rates. Thus, our results suggest how non-monotonic ECAP recovery occurs for ears with spontaneous activity, but cannot account for that pattern of recovery when there is no spontaneous activity, including the results from the presumably deafened ears used in the Nourski et al. (2007) study. Finally, during simultaneous (electric+acoustic) stimulation, the degree of electrically driven spike activity had a strong influence on spike rate, but did not affect spike jitter, which apparently was determined by the acoustic noise stimulus or spontaneous activity.     

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).     

Perceptual Adaptation to Spectrally Shifted Vowels: Training with Nonlexical Labels

Although normal-hearing (NH) and cochlear implant (CI) listeners are able to adapt to spectrally shifted speech to some degree, auditory training has been shown to provide more complete and/or accelerated adaptation. However, it is unclear whether listeners use auditory and visual feedback to improve discrimination of speech stimuli, or to learn the identity of speech stimuli. The present study investigated the effects of training with lexical and nonlexical labels on NH listeners’ perceptual adaptation to spectrally degraded and spectrally shifted vowels. An eight-channel sine wave vocoder was used to simulate CI speech processing. Two degrees of spectral shift (moderate and severe shift) were studied with three training paradigms, including training with lexical labels (i.e., “hayed,” “had,” “who’d,” etc.), training with nonlexical labels (i.e., randomly assigned letters “f,” “b,” “g,” etc.), and repeated testing with lexical labels (i.e., “test-only” paradigm without feedback). All training and testing was conducted over 5 consecutive days, with two to four training exercises per day. Results showed that with the test-only paradigm, lexically labeled vowel recognition significantly improved for moderately shifted vowels; however, there was no significant improvement for severely shifted vowels. Training with nonlexical labels significantly improved the recognition of nonlexically labeled vowels for both shift conditions; however, this improvement failed to generalize to lexically labeled vowel recognition with severely shifted vowels. Training with lexical labels significantly improved lexically labeled vowel recognition with severely shifted vowels. These results suggest that storage and retrieval of speech patterns in the central nervous system is somewhat robust to tonotopic distortion and spectral degradation. Although training with nonlexical labels may improve discrimination of spectrally distorted peripheral patterns, lexically meaningful feedback is needed to identify these peripheral patterns. The results also suggest that training with lexically meaningful feedback may be beneficial to CI users, especially patients with shallow electrode insertion depths.     

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.     

Perceptual dissimilarities among acoustic stimuli and ipsilateral electric stimuli

Five users of cochlear implants who had residual acoustic hearing in the implanted ear postoperatively participated in a study comparing the percepts elicited by acoustic and electric stimuli. The stimuli comprised pulse trains delivered to single electrodes and pure tones presented ipsilaterally. In the experiments, 12 equally loud stimuli with differing frequencies, electrode positions, and pulse rates were generated. Subjects listened to all of the possible pairs of stimuli in each set, and provided a relative dissimilarity rating for the members of each stimulus pair. The data were analyzed using non-metric multi-dimensional scaling techniques. Stimulus spaces were plotted in two dimensions to represent the results for each subject with each stimulus set. The results suggested that one dimension was associated with a pitch-like percept, related to the acoustic tone frequency and the active electrode position. The second dimension separated the acoustic stimuli from the electric stimuli. Generally, the electric pulse rate seemed to have a relatively small perceptual effect in this experimental context. Overall, the results show that acoustic pure tones are perceived as very different from electric pulse trains delivered to single electrode positions with constant rate, even when both the acoustic and the electric stimuli are presented to the same ear.     

Effect of Stimulus Duration on Stapedius Reflex Threshold in Electrical Stimulation via Cochlear Implant

The effect of stimulus duration on the threshold of the contralateral stapedius reflex was investigated in patients supplied with a Vienna cochlear implant (analog stimulation via CI) and compared to results of a normal-hearing reference group in case of acoustic stimulation. Changes in reflex threshold were determined at four frequencies (125, 500,1000 and 2000 Hz in case of electrostimulation and at 500, 1000, 2000 and 4000 Hz for acoustic stimulation) and for five durations (30,50, 100, 300 and 500 ms). The comparison of the two stimulation modes was accomplished by using the same instrumentation and procedure. Reflex threshold was evaluated according to an objective criterion based on the individual noise distribution of recordings without reflex and by subjective judgement. For both stimulation modes a strong effect of stimulus duration on reflex threshold was observed (p < 0.001). The amount of temporal integration reflected by the threshold difference between 500 and 50 ms was approximately 2-AdB for electrical stimulation via CI and 6 dB for acoustic stimulation in normal-hearing individuals. In case of electrostimulation, the reflex threshold for stimuli of 30 ms was most often above the limit of uncomfortable loudness sensation; the increase in reflex threshold for acoustic stimulation between 500 and 30 ms was approximately 14 dB. There was no evidence of frequency effect on reflex threshold nor an interaction between frequency and stimulus duration for either stimulation mode.     

Objective assessment of autophony in patients with patulous Eustachian tube

The objective of the present study is to evaluate the Eustachian tube (ET) acoustic patency during phonation. The sound level in the EAC during phonation of the “A” and ”N” sounds was measured by microphones in the bilateral EACs of nine normal subjects and 31 patients with patulous ET. The measured sound pressure differences between the right and left ears were correlated with the differences in severity of autophony between the bilateral ears assessed by a visual analogue scale (VAS). The patulous condition was often remarkable when the “N” sound was phonated. In some patients with patulous ET, the patulous condition was indicated only by the present method, and not by conventional ET function tests such as tubo-tympano-aerodynamic-graphy or sonotubometry.     

Loudness balance between electric and acoustic stimulation

Binaural loudness balance between electric and acoustic stimulation is obtained in auditory brainstem implant listeners who had substantial acoustic hearing in one ear. The data are well described by a linear relationship between acoustic decibels and electric microamps. Based upon this linear relationship, we propose an exponential model of loudness growth in electric stimulation. The exponential model predicts that the loudness growth function can be determined solely by the threshold and the uncomfortable loudness level in electric stimulation. This prediction is consistent with previous psychophysical data on loudness functions. Implications of this finding for speech processor designs are discussed.     

Acoustic Basis of Directional Acuity in Laboratory Mice

The acoustic basis of auditory spatial acuity was investigated in CBA/129 mice by relating patterns of behavioral errors to directional features of the head-related transfer function (HRTF). Behavioral performance was assessed by training the mice to lick a water spout during sound presentations from a “safe” location and to suppress the response during presentations from “warning” locations. Minimum audible angles (MAAs) were determined by delivering the safe and warning sounds from different locations in the inter-aural horizontal and median vertical planes. HRTFs were measured at the same locations by implanting a miniature microphone and recording the gain of sound energy near the ear drum relative to free field. Mice produced an average MAA of 31° when sound sources were located in the horizontal plane. Acoustic measures indicated that binaural inter-aural level differences (ILDs) and monaural spectral features of the HRTF change systematically with horizontal location and therefore may have contributed to the accuracy of behavioral performance. Subsequent manipulations of the auditory stimuli and the directional properties of the ear produced errors that suggest the mice primarily relied on ILD cues when discriminating changes in azimuth. The MAA increased beyond 80° when the importance of ILD cues was minimized by testing in the median vertical plane. Although acoustic measures demonstrated a less robust effect of vertical location on spectral features of the HRTF, this poor performance provides further evidence for the insensitivity to spectral cues that was noted during behavioral testing in the horizontal plane.     

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.     

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.     

Spectral loudness summation for electrical stimulation in cochlear implant users

Objective: This study investigates the effect of spectral loudness summation (SLS) in the electrical domain as perceived by cochlear implant (CI) users. Analogous to SLS in the acoustical domain, SLS was defined as the effect of electrode separation at a fixed overall stimulation rate. Design: Categorical loudness scaling (CLS) was conducted at three overall stimulation rates using single-electrode stimuli and multi-electrode stimuli presented interleaved on two or four electrodes. The specific loudness of the pulses in the multi-electrode stimuli were equalized based on single-electrode measurements at the same overall stimulation rate. At a fixed overall stimulation rate and a fixed loudness perception, SLS was calculated as the difference in mean current between single-electrode and multi-electrode stimuli. Study sample: Ten postlingually deafened adult CI users. Results: The amount of SLS varied between subjects and between the number and location of the stimulated electrodes in the multi-electrode configuration. SLS was significantly higher than 0 for a subset of the subjects. Conclusions: For a subpopulation of CI users, loudness models should account for nonlinear interactions between electrodes (in the perceptual domain). Similarly, SLS should be accounted for when using CLS outcomes for fitting purposes, at least in a subpopulation of CI users.     

Detection of loudness recruitment in patients with retrocochlear lesions using the "Würzburger Hörfeld" loudness scaling

BACKGROUND: The pathogenesis of hearing loss caused by cerebellopontine angle tumors such as acoustic neuromas is unknown. The lack of loudness recruitment is thought to be one of the features of retrocochlear hearing impairment. In contrast to conventional suprathreshold tests, the categorial loudness scaling using the "Würzburger H?rfeld" is a valuable tool to describe the individual perception of sound. The aim of the present study was to analyze the loudness growth rate in patients with acoustic neuroma. PATIENTS AND METHOD: Pure tone and speech audiometry as well as auditory brainstem response and bilateral categorial loudness scaling were performed preoperatively in 54 patients with acoustic neuroma. Loudness scaling was done in free field switching off the contralateral ear by using an ear-plug. RESULTS: An abnormal rapid loudness growth function was found in 38 of the 54 patients (70.4%) at least at one frequency on the tumor side. The contralateral side was effected only in 57.4% of the patients. The incidence of a recruitment depended on the frequency with a maximum at 4 kHz. The slope of the loudness function showed a tendency to increase with increasing hearing loss. CONCLUSIONS: Loudness recruitment is not a rare phenomenon in patients with acoustic neuroma. The underlying cause (a preexisting hair cell damage, hair cell changes resulting from an obstruction of the cochlear blood supply or a disruption of the cochlear efferents) still remains unclear.     

Automatic evaluation of prosodic features of tracheoesophageal substitute voice

In comparison with laryngeal voice, substitute voice after laryngectomy is characterized by restricted aero-acoustic properties. Until now, an objective means of prosodic differences between substitute and normal voices does not exist. In a pilot study, we applied an automatic prosody analysis module to 18 speech samples of laryngectomees (age: 64.2 ± 8.3 years) and 18 recordings of normal speakers of the same age (65.4 ± 7.6 years). Ninety-five different features per word based upon the speech energy, fundamental frequency F₀ and duration measures on words, pauses and voiced/voiceless sections were measured. These reflect aspects of loudness, pitch and articulation rate. Subjective evaluation of the 18 patients’ voices was performed by a panel of five experts on the criteria “noise”, “speech effort”, “roughness”, “intelligibility”, “match of breath and sense units” and “overall quality”. These ratings were compared to the automatically computed features. Several of them could be identified being twice as high for the laryngectomees compared to the normal speakers, and vice versa. Comparing the evaluation data of the human experts and the automatic rating, correlation coefficients of up to 0.84 were measured. The automatic analysis serves as a good means to objectify and quantify the global speech outcome of laryngectomees. Even better results are expected when both the computation of the features and the comparison method to the human ratings will have been revised and adapted to the special properties of the substitute voices.