Spinal Evoked Compound Action Potentials in Rats With Clinically Relevant Stimulation Modalities.

OBJECTIVES: Rats are commonly used for translational pain and spinal cord stimulation (SCS) research. Although many SCS parameters are configured identically between rats and humans, stimulation amplitudes in rats are often programmed relative to visual motor threshold (vMT). Alternatively, amplitudes may be programmed relative to evoked compound action potential (ECAP) thresholds (ECAPTs), a sensed measure of neural activation. The objective of this study was to characterize ECAPTs, evoked compound muscle action potential thresholds (ECMAPTs), and vMTs with clinically relevant SCS modalities. MATERIALS AND METHODS: We implanted ten anesthetized rats with two quadripolar epidural SCS leads: one for stimulating in the lumbar spine, and another for sensing ECAPs in the thoracic spine. We then delivered two SCS paradigms to the rats. The first used 50-Hz SCS with 50-, 100-, 150-, and 200-µs pulse widths (PWs), whereas the second used a 50-Hz, 150-µs PW low-rate program (LRP) multiplexed to a 1200-Hz, 50-µs PW high-rate program (HRP). We increased SCS amplitudes up to the vMT in the first paradigm, and in the second, we increased HRP amplitudes up to the HRP ECAPT with a fixed amplitude (70% of the vMT) LRP. For each test case, we captured ECAPTs, ECMAPTs, and vMTs from each rat. RESULTS: vMTs were 3.0 ± 0.7 times greater than ECAPTs, with vMTs marginally (3.0 ± 3.6%) greater than ECMAPTs (mean ± SD) across all PWs with the first paradigm. With the second paradigm, we noted a negligible increase (3.6 ± 6.2%) on the LRP ECAP as HRP amplitudes were increased. CONCLUSIONS: Our results demonstrate reasonable levels of neural activation in anesthetized rats with SCS amplitudes appropriately programmed relative to vMT or ECMAPT when using clinically relevant SCS modalities. Furthermore, we demonstrate the feasibility of ECAP recording in rats with multiplexed HRP SCS.

Using evoked compound action potentials to quantify differential neural activation with burst and conventional, 40 Hz spinal cord stimulation in ovines.

Unlike conventional dorsal spinal cord stimulation (SCS)-which uses single pulses at a fixed rate-burst SCS uses a fixed-rate, five-pulse stimuli cluster as a treatment for chronic pain; mechanistic explanations suggest burst SCS differentially modulate the medial and lateral pain pathways vs conventional SCS. Neural activation differences between burst and conventional SCS are quantifiable with the spinal-evoked compound action potential (ECAP), an electrical measure of synchronous neural activation. Methods: We implanted 7 sheep with a dorsal stimulation lead at T9/T10, a dorsal ECAP sensing lead at T6/T7, and a lead also at T9/T10 but adjacent to the anterolateral system (ALS). Both burst and conventional SCS with stimulation amplitudes up to the visual motor threshold (vMT) were delivered to 3 different dorsal spinal locations, and ECAP thresholds (ECAPTs) were calculated for all combinations. Then, changes in ALS activation were assessed with both types of SCS. Results: Evoked compound action potential thresholds and vMTs were significantly higher (P < 0.05) with conventional vs burst SCS, with no statistical difference (P > 0.05) among stimulation sites. However, the vMT-ECAPT window (a proxy for the useable therapeutic dosing range) was significantly wider (P < 0.05) with conventional vs burst SCS. No significant difference (P > 0.05) in ALS activation was noted between conventional and burst SCS. Conclusion: When dosed equivalently, no differentially unique change in ALS activation results with burst SCS vs conventional SCS; in addition, sub-ECAPT burst SCS results in no discernable excitability changes in the neural pathways feeding pain relevant supraspinal sites.

AzBio Sentence test in Hebrew (HeBio): development, preliminary validation, and the effect of noise.

The AzBio sentence test is widely used to assess speech perception pre- and post-cochlear implantation. This study created and validated a Hebrew version of AzBio (HeBio) and tested its intelligibility amidst background noise.In Experiment 1, 1,000 recorded Hebrew sentences were presented via five-channel vocoder to 10 normal hearing (NH) listeners for intelligibility testing. In Experiment 2, HeBio lists were presented to 25 post-lingual cochlear implant (CI) users amidst four-talker babble noise (4TBN) or in quiet, along with one-syllable word test. In Experiment 3, 20 NH listeners were presented with eight HeBio lists in two noise conditions [4TBN, speech shaped noise (SSN)] and four SNRs (+3, 0 dB, -3 dB, -6 dB).HeBio lists (33) produced 82% average understanding, no inter-list intelligibility differences among NH, and equal intelligibility for CI users. One-syllable words predicted 67% of the variance in HeBio among CI users. Higher intelligibility was found for SSN than for 4TBN, and the mean speech receptive threshold (SRT) was more negative for SSN than for 4TBN.HeBio results were similar to AzBio. Results obtained with two noise types were as expected. HeBio is recommended for evaluation of different populations in quiet and noise.

Cortical Auditory Evoked Potentials Recorded Directly Through the Cochlear Implant in Cochlear Implant Recipients: a Feasibility Study.

OBJECTIVES: The use of objective measures in cochlear implant (CI) mapping, has greatly contributed to the refinement of the setting of audible and comfortable stimulation levels, which serve as the basis of the mapping process, especially in cases of infants and young children. In addition, objective measures can also confirm the integrity of the CI system. Current CI objective measures mainly reflect neural activity from the auditory nerve and brainstem site. An objective cortical CI measure that reflects directly central auditory activity is greatly needed, especially since it is closely related to CI outcomes in both children and adults. Recording the brain activity currently requires an external evoked potential (EP) system including scalp electrodes, rendering it impractical for widespread clinical use. This study aimed to assess the feasibility of recording cortical auditory evoked potentials (CAEPs) directly and solely through the cochlear implant in response to external acoustic stimulation in the non-implanted ear. DESIGN: A total of nine CI users (four females and five males) participated, including seven post-lingual adults (23 to 72 years), and two pediatric cases, one teenager (15 years), and one child (8 years)-both pre-lingual. All participants had a residual hearing in the ear contralateral to the ear with CI. CAEPs were recorded in the implanted ear in response to acoustic stimulation of the non-implanted ear, consisting of a brief tonal stimulus at comfortable listening levels. Recordings used an intracranial montage consisting of an intracochlear apical electrode (active) and one of the two ( case and ring ) extra-cochlear implanted electrodes serving as reference electrodes. The CI CAEPs were compared with a single-channel conventional CAEP recording obtained simultaneously via scalp electrodes (Fz-mastoid) using a standard EP system and an external trigger from the CI system. Statistical comparisons were made between the CI and the scalp recorded CAEPs and for differences between the CI CAEP measures acquired using the ring and the case as the reference electrode. RESULTS: CAEPs recorded directly and solely through the CI were equivalent to the standard scalp recorded CAEP responses. CAEP responses acquired using the case electrode as the reference were highly correlated in terms of morphology, latencies, and amplitudes of the CAEP components. The CI CAEP latencies of the two pediatric cases were consistent with their normal developed age group and delayed relative to adult CAEP latencies, as expected. CONCLUSIONS: This study demonstrated the feasibility of recording long latency CAEPs directly and solely through CI in adults with residual hearing, in response to acoustic stimulation of the non-implanted ear. The CI CAEPs closely resembled the CAEPs recorded simultaneously by an external EP system and via scalp electrodes. The ability to record directly from the implant, without the need of an external recording system, presents an innovative method with many clinical and research implications.

A Clinical Feasibility Study of Spinal Evoked Compound Action Potential Estimation Methods.

OBJECTIVES: Spinal cord stimulation (SCS) is a treatment for chronic neuropathic pain. Recently, SCS has been enhanced further with evoked compound action potential (ECAP) sensing. Characteristics of the ECAP, if appropriately isolated from concurrent stimulation artifact (SA), may be used to control, and aid in the programming of, SCS systems. Here, we characterize the sensitivity of the ECAP growth curve slope (S) to both neural response (|Sresp|) and SA contamination (|Sart|) for four spinal ECAP estimation methods with a novel performance measure (|Sresp/Sart|). MATERIALS AND METHODS: We collected a library of 112 ECAP and associated artifact recordings with swept stimulation amplitudes from 14 human subjects. We processed the signals to reduce SA from these recordings by applying one of three schemes: a simple high-pass (HP) filter, subtracting an artifact model (AM) consisting of decaying exponential and linear components, or applying a template correlation method consisting of a triangularly weighted sinusoid. We compared these against each other and to P2-N1, a standard method of measuring ECAP amplitude. We then fit the ECAP estimates from each method with a function representing the growth curve and calculated the Sresp and Sart parameters following the fit. RESULTS: Any SA reduction scheme selected may result in under- or overestimation of neural activation or misclassification of SA as ECAP. In these experiments, the ratio of neural signal preservation to SA misclassification (|Sresp/Sart|) on the ECAP estimate was superior (p < 0.05) with the HP and AM schemes relative to the others. CONCLUSIONS: This work represents the first comprehensive assessment of spinal ECAP estimation schemes. Understanding the clinically relevant sensitivities of these schemes is increasingly important, particularly with closed-loop SCS systems using ECAP as a feedback control variable where misclassification of artifact as neural signal may lead to suboptimal therapy adjustments.

The Evoked Compound Action Potential as a Predictor for Perception in Chronic Pain Patients: Tools for Automatic Spinal Cord Stimulator Programming and Control.

OBJECTIVES: Spinal cord stimulation (SCS) is a drug free treatment for chronic pain. Recent technological advances have enabled sensing of the evoked compound action potential (ECAP), a biopotential that represents neural activity elicited from SCS. The amplitudes of many SCS paradigms - both sub- and supra-threshold - are programmed relative to the patient's perception of SCS. The objective of this study, then, is to elucidate relationships between the ECAP and perception thresholds across posture and SCS pulse width. These relationships may be used for the automatic control and perceptually referenced programming of SCS systems. METHODS: ECAPs were acquired from 14 subjects across a range of postures and pulse widths with swept amplitude stimulation. Perception (PT) and discomfort (DT) thresholds were recorded. A stimulation artifact reduction scheme was employed, and growth curves were constructed from the sweeps. An estimate of the ECAP threshold (ET), was calculated from the growth curves using a novel approach. Relationships between ET, PT, and DT were assessed. RESULTS: ETs were estimated from 112 separate growth curves. For the postures and pulse widths assessed, the ET tightly correlated with both PT (r = 0.93; p < 0.0001) and DT (r = 0.93; p < 0.0001). The median accuracy of ET as a predictor for PT across both posture and pulse width was 0.5 dB. Intra-subject, ECAP amplitudes at DT varied up to threefold across posture. CONCLUSION: We provide evidence that the ET varies across both different positions and varying pulse widths and suggest that this variance may be the result of postural dependence of the recording electrode-tissue spacing. ET-informed SCS holds promise as a tool for SCS parameter configuration and may offer more accuracy over alternative approaches for neural and perceptual control in closed loop SCS systems.

Testing Speech Perception with Cochlear Implants Through Digital Audio Streaming in a Virtual Sound Booth: A Feasibility Study.

OBJECTIVE: For patients who have received cochlear implants (CIs), speech-perception testing requires specialized equipment. This limits locations where these services can be provided, which can introduce barriers for provision of care. Providing speech test stimuli directly to the CI via wireless digital audio streaming (DAS) or wired direct audio input (DAI) allows for testing without the need for a sound booth (SB). A few studies have investigated the use of DAI for testing speech perception in CIs, but none have evaluated DAS. The goal of this study was to compare speech perception testing in CI users via DAS versus a traditional SB to determine if differences exist between the two presentation modes. We also sought to determine whether pre-processing the DAS signal with room acoustics (reverberation and noise floor) to emulate the SB environment would affect performance differences between the SB and DAS. DESIGN: In Experiment 1, speech perception was measured for monosyllabic words in quiet and sentences in quiet and in noise. Scores were obtained in a SB and compared to those obtained via DAS with unprocessed speech (DAS-U) for 11 adult CI users (12 ears). In Experiment 2, speech perception was measured for sentences in noise, where both the speech and noise stimuli were pre-processed to emulate the SB environment. Scores were obtained for 11 adult CI users (12 ears) in the SB, via DAS-U, and via DAS with the processed speech (DAS-P). RESULTS: For Experiment 1, there was no significant difference between SB and DAS-U conditions for words or sentences in quiet. However, DAS-U scores were significantly better than SB scores for sentences in noise. For Experiment 2, there was no significant difference between the SB and DAS-P conditions. Similar to Experiment 1, DAS-U scores were significantly better than SB or DAS-P scores. CONCLUSIONS: By pre-processing the test materials to emulate the noise and reverberation characteristics of a traditional SB, we can account for differences in speech-perception scores between those obtained via DAS and in a SB.

Simulating intracochlear electrocochleography with a combined model of acoustic hearing and electric current spread in the cochlea.

Intracochlear electrocochleography (ECochG) is a potential tool for the assessment of residual hearing in cochlear implant users during implantation and acoustical tuning postoperatively. It is, however, unclear how these ECochG recordings from different locations in the cochlea depend on the stimulus parameters, cochlear morphology, implant design, or hair cell degeneration. In this paper, a model is presented that simulates intracochlear ECochG recordings by combining two existing models, namely a peripheral one that simulates hair cell activation and a three-dimensional (3D) volume-conduction model of the current spread in the cochlea. The outcomes were compared to actual ECochG recordings from subjects with a cochlear implant (CI). The 3D volume conduction simulations showed that the intracochlear ECochG is a local measure of activation. Simulations showed that increasing stimulus frequency resulted in a basal shift of the peak cochlear microphonic (CM) amplitude. Increasing the stimulus level resulted in wider tuning curves as recorded along the array. Simulations with hair cell degeneration resulted in ECochG responses that resembled the recordings from the two subjects in terms of CM onset responses, higher harmonics, and the width of the tuning curve. It was concluded that the model reproduced the patterns seen in intracochlear hair cell responses recorded from CI-subjects.

Intracochlear Electrocochleography: Influence of Scalar Position of the Cochlear Implant Electrode on Postinsertion Results.

HYPOTHESIS: Electrocochleography (ECochG) recorded during cochlear implant (CI) insertion from the apical electrode in conjunction with postinsertion ECochG can identify electrophysiologic differences that exist between groups with and without a translocation of the array from the scala tympani (ST) into the scala vestibuli (SV). BACKGROUND: Translocation of the CI electrode from ST into SV can limit performance postoperatively. ECochG markers of trauma may be able to aid in the ability to detect electrode array-induced trauma/scalar translocation intraoperatively. METHODS: Twenty-one adult CI patients were included. Subjects were postoperatively parsed into two groups based on analysis of postoperative imaging: 1) ST (n = 14) insertion; 2) SV (n = 7) insertion, indicating translocation of the electrode. The ECochG response elicited from a 500 Hz acoustic stimulus was recorded from the lead electrode during insertion when the distal electrode marker was at the round window, and was compared to the response recorded from a basal electrode (e13) after complete insertion. RESULTS: No statistically significant change in mean ECochG magnitude was found in either group between recording intervals. There was a mean loss of preoperative pure-tone average of 52% for the nontranslocation group and 94% for the translocation group. CONCLUSIONS: Intraoperative intracochlear ECochG through the CI array provides a unique opportunity to explore the impact of the CI electrode on the inner ear. Specifically, a translocation of the array from ST to SV does not seem to change the biomechanics of the cochlear region that lies basal to the area of translocation in the acute period.

Intra-Cochlear Electrocochleography During Cochear Implant Electrode Insertion Is Predictive of Final Scalar Location.

HYPOTHESIS: Electrocochleography (ECochG) patterns observed during cochlear implant (CI) electrode insertion may provide information about scalar location of the electrode array. BACKGROUND: Conventional CI surgery is performed without actively monitoring auditory function and potential damage to intracochlear structures. The central hypothesis of this study was that ECochG obtained directly through the CI may be used to estimate intracochlear electrode position and, ultimately, residual hearing preservation. METHODS: Intracochlear ECochG was performed on 32 patients across 3 different implant centers. During electrode insertion, a 50-ms tone burst stimulus (500 Hz) was delivered at 110 dB SPL. The ECochG response was monitored from the apical-most electrode. The amplitude and phase changes of the first harmonic were imported into an algorithm in an attempt to predict the intracochlear electrode location (scala tympani [ST], translocation from ST to scala vestibuli [SV], or interaction with basilar membrane). Anatomic electrode position was verified using postoperative computed tomography (CT) with image processing. RESULTS: CT analysis confirmed 25 electrodes with ST position and 7 electrode arrays translocating from ST into SV. The ECochG algorithm correctly estimated electrode position in 26 (82%) of 32 subjects while 6 (18%) electrodes were wrongly identified as translocated (sensitivity = 100%, specificity = 77%, positive predictive value = 54%, and a negative predictive value = 100%). Greater hearing loss was observed postoperatively in participants with translocated electrode arrays (36 ±â€Š15 dB) when compared with isolated ST insertions (28 ±â€Š20 dB HL). This result, however, was not significant (p = 0.789). CONCLUSION: Intracochlear ECochG may provide information about CI electrode location and hearing preservation.

A Dynamically Focusing Cochlear Implant Strategy Can Improve Vowel Identification in Noise.

OBJECTIVES: The standard, monopolar (MP) electrode configuration used in commercially available cochlear implants (CI) creates a broad electrical field, which can lead to unwanted channel interactions. Use of more focused configurations, such as tripolar and phased array, has led to mixed results for improving speech understanding. The purpose of the present study was to assess the efficacy of a physiologically inspired configuration called dynamic focusing, using focused tripolar stimulation at low levels and less focused stimulation at high levels. Dynamic focusing may better mimic cochlear excitation patterns in normal acoustic hearing, while reducing the current levels necessary to achieve sufficient loudness at high levels. DESIGN: Twenty postlingually deafened adult CI users participated in the study. Speech perception was assessed in quiet and in a four-talker babble background noise. Speech stimuli were closed-set spondees in noise, and medial vowels at 50 and 60 dB SPL in quiet and in noise. The signal to noise ratio was adjusted individually such that performance was between 40 and 60% correct with the MP strategy. Subjects were fitted with three experimental strategies matched for pulse duration, pulse rate, filter settings, and loudness on a channel-by-channel basis. The strategies included 14 channels programmed in MP, fixed partial tripolar (σ = 0.8), and dynamic partial tripolar (σ at 0.8 at threshold and 0.5 at the most comfortable level). Fifteen minutes of listening experience was provided with each strategy before testing. Sound quality ratings were also obtained. RESULTS: Speech perception performance for vowel identification in quiet at 50 and 60 dB SPL and for spondees in noise was similar for the three tested strategies. However, performance on vowel identification in noise was significantly better for listeners using the dynamic focusing strategy. Sound quality ratings were similar for the three strategies. Some subjects obtained more benefit than others, with some individual differences explained by the relation between loudness growth and the rate of change from focused to broader stimulation. CONCLUSIONS: These initial results suggest that further exploration of dynamic focusing is warranted. Specifically, optimizing such strategies on an individual basis may lead to improvements in speech perception for more adult listeners and improve how CIs are tailored. Some listeners may also need a longer period of time to acclimate to a new program.

Pure-Tone Masking Patterns for Monopolar and Phantom Electrical Stimulation in Cochlear Implants.

OBJECTIVES: Monopolar stimulation of the most apical electrode produces the lowest pitch sensation in cochlear implants clinically. A phantom electrode that uses out-of-phase electrical stimulation between the most apical and the neighboring basal electrode can produce a lower pitch sensation than that associated with the most apical electrode. However, because of the absence of contacts beyond the apical tip of the array, the ability to assess the spread of electrical excitation associated with phantom stimulation is limited in the typical cochlear implant subject with no residual hearing. In the present study, the spread of electrical excitation associated with monopolar and phantom stimulation of the most apical electrode was assessed using electrical masking of acoustic thresholds in cochlear implant subjects with residual, low-frequency, acoustic hearing. DESIGN: Eight subjects with an Advanced Bionics cochlear implant and residual hearing in the implanted ear participated in this study (nine ears in total). Unmasked and masked thresholds for acoustic pure tones were measured at 125, 250, 500, 750, 1000, and 2000 Hz in the presence of monopolar and phantom electrode stimulation presented at the apical-most end of the array. The current compensation for phantom electrode stimulation was fixed at 50%. The two electrical maskers were loudness balanced. Differences between the unmasked and masked acoustic thresholds can be attributed to (1) the electrical stimulus-induced interference in the transduction/conduction of the acoustic signal through cochlear periphery and the auditory nerve and/or (2) masking at the level of the central auditory system. RESULTS: The results show a significant elevation in pure-tone thresholds in the presence of the monopolar and phantom electrical maskers. The unmasked thresholds were subtracted from the masked thresholds to derive masking patterns as a function of the acoustic probe frequency. The masking patterns show that phantom stimulation was able to produce more masking than that associated with the monopolar stimulation of the most apical electrode. CONCLUSION: These results suggest that for some cochlear implant subjects, phantom electrode stimulation can shift the neural stimulation pattern more apically in the cochlea, which is consistent with reports that phantom electrode stimulation produces lower pitch sensations than those associated with monopolar stimulation of the most apical electrode alone.

Feasibility of Using Electrocochleography for Objective Estimation of Electro-Acoustic Interactions in Cochlear Implant Recipients with Residual Hearing.

Although cochlear implants (CI) traditionally have been used to treat individuals with bilateral profound sensorineural hearing loss, a recent trend is to implant individuals with residual low-frequency hearing. Patients who retain some residual acoustic hearing after surgery often can benefit from electro-acoustic stimulation (EAS) technologies, which combine conventional acoustic amplification with electrical stimulation. However, interactions between acoustic and electrical stimulation may affect outcomes adversely and are time-consuming and difficult to assess behaviorally. This study demonstrated the feasibility of using the Advanced Bionics HiRes90K Advantage implant electronics and HiFocus Mid Scala/1j electrode to measure electrocochleography (ECochG) responses in the presence of electrical stimulation to provide an objective estimate of peripheral physiologic EAS interactions. In general, electrical stimulation reduced ECochG response amplitudes to acoustic stimulation. The degree of peripheral EAS interaction varied as a function of acoustic pure tone frequency and the intra-cochlear location of the electrically stimulated electrode. Further development of this technique may serve to guide and optimize clinical EAS system fittings in the future.

Real-Time Intracochlear Electrocochleography Obtained Directly Through a Cochlear Implant.

HYPOTHESIS: Utilizing the cochlear implant to record electrophysiologic responses during device placement is a feasible and efficacious technique for monitoring near real-time cochlear physiology during and following electrode insertion. BACKGROUND: Minimizing intracochlear trauma during cochlear implantation has emerged as a highly researched area to help improve patient performance. Currently, conventional cochlear implant technology allows for the recording of electrically evoked compound action potentials (eCAPs). Acoustically evoked potentials may be more sensitive in detecting physiologic changes occurring as a result of electrode insertion. Electrocochleography obtained from within the cochlea allows hair cell and neural response monitoring along the cochlear spiral at locations where changes most likely would occur. METHODS: Intracochlear electrocochleography (ECochG) was recorded from the cochlear implant during surgery in 14 subjects. A long acquisition time (54.5 ms), capable of measuring potentials from the low frequency-serving apical region of the cochlea (125 and 500 Hz) was employed. Two distinct intracochlear processing methods were used and compared in obtaining electrophysiologic data. RESULTS: Measureable intracochlear ECochG responses were obtained from all 14 participants. The 1st harmonic distortions (cochlear microphonic and auditory nerve neurophonic) generally increased steadily with electrode insertion. Electrode and frequency scan following insertion revealed that response amplitude varied based on location of recording electrode and frequency of stimulation. Exquisite sensitivity to manipulation during round window muscle packing was demonstrated. CONCLUSION: Intracochlear ECochG recorded from the electrode array of the cochlear implant is a highly feasible technique that sheds light on cochlear micromechanics during cochlear implant electrode placement.

An Objective Estimation of Air-Bone-Gap in Cochlear Implant Recipients with Residual Hearing Using Electrocochleography.

Although, cochlear implants (CI) traditionally have been used to treat individuals with bilateral profound sensorineural hearing loss, a recent trend is to implant individuals with residual low-frequency hearing. Notably, many of these individuals demonstrate an air-bone gap (ABG) in low-frequency, pure-tone thresholds following implantation. An ABG is the difference between audiometric thresholds measured using air conduction (AC) and bone conduction (BC) stimulation. Although, behavioral AC thresholds are straightforward to assess, BC thresholds can be difficult to measure in individuals with severe-to-profound hearing loss because of vibrotactile responses to high-level, low-frequency stimulation and the potential contribution of hearing in the contralateral ear. Because of these technical barriers to measuring behavioral BC thresholds in implanted patients with residual hearing, it would be helpful to have an objective method for determining ABG. This study evaluated an innovative technique for measuring electrocochleographic (ECochG) responses using the cochlear microphonic (CM) response to assess AC and BC thresholds in implanted patients with residual hearing. Results showed high correlations between CM thresholds and behavioral audiograms for AC and BC conditions, thereby demonstrating the feasibility of using ECochG as an objective tool for quantifying ABG in CI recipients.

Optimizations for the Electrically-Evoked Stapedial Reflex Threshold Measurement in Cochlear Implant Recipients.

OBJECTIVE: The electrically-evoked stapedial reflex threshold (eSRT) has proven to be useful in setting upper stimulation levels of cochlear implant recipients. However, the literature suggests that the reflex can be difficult to observe in a significant percentage of the population. The primary goal of this investigation was to assess the difference in eSRT levels obtained with alternative acoustic admittance probe tone frequencies. DESIGN: A repeated-measures design was used to examine the effect of 3 probe tone frequencies (226, 678, and 1000 Hz) on eSRT in 23 adults with cochlear implants. RESULTS: The mean eSRT measured using the conventional probe tone of 226 Hz was significantly higher than the mean eSRT measured with use of 678 and 1000 Hz probe tones. The mean eSRT were 174, 167, and 165 charge units with use of 226, 678, and 1000 Hz probe tones, respectively. There was not a statistically significant difference between the average eSRTs for probe tones 678 and 1000 Hz. Twenty of 23 participants had eSRT at lower charge unit levels with use of either a 678 or 1000 Hz probe tone when compared with the 226 Hz probe tone. Two participants had eSRT measured with 678 or 1000 Hz probe tones that were equal in level to the eSRT measured with a 226 Hz probe tone. Only 1 participant had an eSRT that was obtained at a lower charge unit level with a 226 Hz probe tone relative to the eSRT obtained with a 678 and 1000 Hz probe tone. CONCLUSIONS: The results of this investigation demonstrate that the use of a standard 226 Hz probe tone is not ideal for measurement of the eSRT. The use of higher probe tone frequencies (i.e., 678 or 1000 Hz) resulted in lower eSRT levels when compared with the eSRT levels obtained with use of a 226 probe tone. In addition, 4 of the 23 participants included in this study did not have a measureable eSRT with use of a 226 Hz probe tone, but all of the participants had measureable eSRT with use of both the 678 and 1000 Hz probe tones. Additional work is required to understand the clinical implication of these changes in the context of cochlear implant programming.

Loudness and pitch perception using Dynamically Compensated Virtual Channels.

Reducing power consumption is important for the development of smaller cochlear implant (CI) speech processors. Simultaneous electrode stimulation may improve power efficiency by minimizing the required current applied to a given electrode. Simultaneous in-phase stimulation on adjacent electrodes (i.e. virtual channels) can be used to elicit pitch percepts intermediate to the ones provided by each of the physical electrodes in isolation. Virtual channels are typically implemented in monopolar stimulation mode, producing broad excitation patterns. Focused stimulation may reduce the excitation patterns, but is inefficient in terms of power consumption. To create a more power efficient virtual channel, we developed the Dynamically Compensated Virtual Channel (DC-VC) using four adjacent electrodes. The two central electrodes are current steered using the coefficient α (0<α<1 ) whereas the two flanking electrodes are used to focus/unfocus the stimulation with the coefficient σ (-1<σ<1). With increasing values of σ, power can be saved at the potential expense of generating broader electric fields. Additionally, reshaping the electric fields might also alter place pitch coding. The goal of the present study is to investigate the tradeoff between place pitch encoding and power savings using simultaneous electrode stimulation in the DC-VC configuration. A computational model and psychophysical experiments in CI users have been used for that purpose. Results from 10 adult Advanced Bionics CI users have been collected. Results show that the required current to produce comfortable levels is significantly reduced with increasing σ as predicted by the computational model. Moreover, no significant differences in the estimated number of discriminable steps were detected for the different values of σ. From these results, we conclude that DC-VCs can reduce power consumption without decreasing the number of discriminable place pitch steps.

Electrocochleography in Cochlear Implant Recipients With Residual Hearing: Comparison With Audiometric Thresholds.

OBJECTIVES: To determine whether electrocochleography (ECoG) thresholds, especially cochlear microphonic and auditory nerve neurophonic thresholds, measured using an intracochlear electrode, can be used to predict pure-tone audiometric thresholds following cochlear implantation in ears with residual hearing. DESIGN: Pure-tone audiometric thresholds and ECoG waveforms were measured at test frequencies from 125 to 4000 Hz in 21 Advanced Bionics cochlear implant recipients with residual hearing in the implanted ear. The "difference" and "summation" responses were computed from the ECoG waveforms measured from two alternating phases of stimulation. The interpretation is that difference responses are largely from the cochlear microphonic while summating responses are largely from the auditory nerve neurophonic. The pure-tone audiometric thresholds were also measured with same equipment used for ECoG measurements. RESULTS: Difference responses were observed in all 21 implanted ears, whereas summation response waveforms were observed in only 18 ears. The ECoG thresholds strongly correlated (r = 0.87, n = 150 for difference response; r = 0.82, n = 72 for summation response) with audiometric thresholds. The mean difference between the difference response and audiometric thresholds was -3.2 (±9.0) dB, while the mean difference between summation response and audiometric thresholds was -14 (±11) dB. In four out of 37 measurements, difference responses were measured to frequencies where no behavioral thresholds were present. CONCLUSIONS: ECoG thresholds may provide a useful metric for the assessment of residual hearing in cochlear implant subjects for whom it is not possible to perform behavioral audiometric testing.

Reducing Channel Interaction Through Cochlear Implant Programming May Improve Speech Perception: Current Focusing and Channel Deactivation.

Speech perception among cochlear implant (CI) listeners is highly variable. High degrees of channel interaction are associated with poorer speech understanding. Two methods for reducing channel interaction, focusing electrical fields, and deactivating subsets of channels were assessed by the change in vowel and consonant identification scores with different program settings. The main hypotheses were that (a) focused stimulation will improve phoneme recognition and (b) speech perception will improve when channels with high thresholds are deactivated. To select high-threshold channels for deactivation, subjects' threshold profiles were processed to enhance the peaks and troughs, and then an exclusion or inclusion criterion based on the mean and standard deviation was used. Low-threshold channels were selected manually and matched in number and apex-to-base distribution. Nine ears in eight adult CI listeners with Advanced Bionics HiRes90k devices were tested with six experimental programs. Two, all-channel programs, (a) 14-channel partial tripolar (pTP) and (b) 14-channel monopolar (MP), and four variable-channel programs, derived from these two base programs, (c) pTP with high- and (d) low-threshold channels deactivated, and (e) MP with high- and (f) low-threshold channels deactivated, were created. Across subjects, performance was similar with pTP and MP programs. However, poorer performing subjects (scoring < 62% correct on vowel identification) tended to perform better with the all-channel pTP than with the MP program (1 > 2). These same subjects showed slightly more benefit with the reduced channel MP programs (5 and 6). Subjective ratings were consistent with performance. These finding suggest that reducing channel interaction may benefit poorer performing CI listeners.

Design and evaluation of a cochlear implant strategy based on a "Phantom" channel.

Unbalanced bipolar stimulation, delivered using charge balanced pulses, was used to produce "Phantom stimulation", stimulation beyond the most apical contact of a cochlear implant's electrode array. The Phantom channel was allocated audio frequencies below 300 Hz in a speech coding strategy, conveying energy some two octaves lower than the clinical strategy and hence delivering the fundamental frequency of speech and of many musical tones. A group of 12 Advanced Bionics cochlear implant recipients took part in a chronic study investigating the fitting of the Phantom strategy and speech and music perception when using Phantom. The evaluation of speech in noise was performed immediately after fitting Phantom for the first time (Session 1) and after one month of take-home experience (Session 2). A repeated measures of analysis of variance (ANOVA) within factors strategy (Clinical, Phantom) and interaction time (Session 1, Session 2) revealed a significant effect for the interaction time and strategy. Phantom obtained a significant improvement in speech intelligibility after one month of use. Furthermore, a trend towards a better performance with Phantom (48%) with respect to F120 (37%) after 1 month of use failed to reach significance after type 1 error correction. Questionnaire results show a preference for Phantom when listening to music, likely driven by an improved balance between high and low frequencies.