Electric-acoustic interactions in the hearing cochlea: Single fiber recordings

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.

Sensitivity to interaural time differences in the inferior colliculus of cochlear implanted rats with or without hearing experience

For deaf patients cochlear implants (CIs) can restore substantial amounts of functional hearing. However, binaural hearing, and in particular, the perception of interaural time differences (ITDs) with current CIs has been found to be notoriously poor, especially in the event of early hearing loss. One popular hypothesis for these deficits posits that a lack of early binaural experience may be a principal cause of poor ITD perception in pre-lingually deaf CI patients. This is supported by previous electrophysiological studies done in neonatally deafened, bilateral CI-stimulated animals showing reduced ITD sensitivity. However, we have recently demonstrated that neonatally deafened CI rats can quickly learn to discriminate microsecond ITDs under optimized stimulation conditions which suggests that the inability of human CI users to make use of ITDs is not due to lack of binaural hearing experience during development. In the study presented here, we characterized ITD sensitivity and tuning of inferior colliculus neurons under bilateral CI stimulation of neonatally deafened and hearing experienced rats. The hearing experienced rats were not deafened prior to implantation. Both cohorts were implanted bilaterally between postnatal days 64-77 and recorded immediately following surgery. Both groups showed comparably large proportions of ITD sensitive multi-units in the inferior colliculus (Deaf: 84.8%, Hearing: 82.5%), and the strength of ITD tuning, quantified as mutual information between response and stimulus ITD, was independent of hearing experience. However, the shapes of tuning curves differed substantially between both groups. We observed four main clusters of tuning curves - trough, contralateral, central, and ipsilateral tuning. Interestingly, over 90% of multi-units for hearing experienced rats showed predominantly contralateral tuning, whereas as many as 50% of multi-units in neonatally deafened rats were centrally tuned. However, when we computed neural d' scores to predict likely limits on performance in sound lateralization tasks, we did not find that these differences in tuning shapes predicted worse psychoacoustic performance for the neonatally deafened animals. We conclude that, at least in rats, substantial amounts of highly precise, "innate" ITD sensitivity can be found even after profound hearing loss throughout infancy. However, ITD tuning curve shapes appear to be strongly influenced by auditory experience although substantial lateralization encoding is present even in its absence.

The role of electroneural versus electrophonic stimulation on psychoacoustic electric-acoustic masking in cochlear implant users with residual hearing

Cochlear implant (CI) candidates with residual low-frequency hearing are nowadays often implanted with CI electrode arrays that allow preserving their acoustic hearing in the implanted ear. These subjects receiving combined electric-acoustic stimulation (EAS) show enhanced speech perception scores when compared to traditional CI users without acoustic component. However, these benefits are limited by interaction effects such as masking between electric and acoustic stimulation. This study evaluates ipsilateral electric-acoustic masking in a psychophysical experiment conducted in 5 EAS subjects. The elevation of acoustic pure tone thresholds through simultaneous presentation of electric pulse trains and vice versa is measured for different acoustic frequencies and different settings of the electric stimuli. Electric-acoustic interaction could originate either from electroneural stimulation of auditory nerve fibers or from electrophonic stimulation of hair cells. The two fundamental goals of this study are to investigate the effects of stimulation rate and phase duration of the electric stimulus on electric-acoustic masking and to investigate the origin of electric-acoustic masking by assessing the contributions of electroneural versus electrophonic stimulation. The amount of electric-acoustic masking in the present study was independent of pulse rate and phase duration of the electric stimuli. Moreover, the results demonstrate that electric-acoustic masking depends on the spatial distance between the locations of electric or acoustic excitation in the cochlea, but not on the spectral content of the electric stimulus. We thereby conclude that psychoacoustic electric-acoustic masking in EAS users is dominated by electroneural-acoustic interaction, whereas the contribution of electrophonic stimulation is negligible.

Electrode estimation in the acoustic region of the human Cochlea

Background: In this study, a method to estimate number of electrodes in the acoustic region of Electric Acoustic Stimulation (EAS) subjects was proposed. Aims/Objectives: To develop and validate an anatomy-based method for EAS subjects to estimate the number of electrodes within the acoustic region.Material and methods: The postoperative CTs of adults with various degree of hearing implanted with lateral wall electrodes with mean insertion depth of 23.9 mm (18.0-28.2 mm) and mean insertion angle of 505° (355-695°) were evaluated.Results: The difference between the estimated and measured angle varied between -18 and 25°, with a mean of 0.9°. For the insertion angle of 230° and higher, the maximum difference was 24°. Taking this uncertainty into account, all electrodes in the acoustic region were predicted correctly.Conclusions and significance: The method decides on non-overlapping acoustic and electric stimulation in terms of place in the cochlea. With the accuracy of 0.84 mm for the electrode arrays inserted for more than 230°, the method was sufficient to estimate the exact number of electrodes in the acoustic region of cochlear implantees. The benefit of this method may be in fitting of EAS subjects with some portion of the electrode array in the acoustic region.

Effects of Low Frequency Residual Hearing on Music Perception and Psychoacoustic Abilities in Pediatric Cochlear Implant Recipients

Studies have demonstrated the benefits of low frequency residual hearing in music perception and for psychoacoustic abilities of adult cochlear implant (CI) users, but less is known about these effects in the pediatric group. Understanding the contribution of combined electric and acoustic stimulation in this group can help to gain a better perspective on decisions regarding bilateral implantation. We evaluated the performance of six unilaterally implanted children between 9 and 13 years of age with contralateral residual hearing using the Clinical Assessment of Music Perception (CAMP), spectral ripple discrimination (SRD), and temporal modulation transfer function (TMTF) tests and compared findings with previous research. Our study sample performed similarly to normal hearing subjects in pitch direction discrimination (0.81 semitones) and performed well above typical CI users in melody recognition (43.37%). The performance difference was less in timbre recognition (48.61%), SRD (1.47 ripple/octave), and TMTF for four modulation frequencies. These findings suggest that the combination of low frequency acoustic hearing with the broader frequency range of electric hearing can help to increase clinical CI benefit in pediatric users and decisions regarding second-side implantation should consider these factors.

The Summating Potential Is a Reliable Marker of Electrode Position in Electrocochleography: Cochlear Implant as a Theragnostic Probe

OBJECTIVE

For the increasing number of cochlear implantations in subjects with residual hearing, hearing preservation, and thus the prevention of implantation trauma, is crucial. A method for monitoring the intracochlear position of a cochlear implant (CI) and early indication of imminent cochlear trauma would help to assist the surgeon to achieve this goal. The aim of this study was to evaluate the reliability of the different electric components recorded by an intracochlear electrocochleography (ECochG) as markers for the cochleotopic position of a CI. The measurements were made directly from the CI, combining intrasurgical diagnostics with the therapeutical use of the CI, thus, turning the CI into a "theragnostic probe."

DESIGN

Intracochlear ECochGs were measured in 10 Dunkin Hartley guinea pigs of either sex, with normal auditory brainstem response thresholds. All subjects were fully implanted (4 to 5 mm) with a custom six contact CI. The ECochG was recorded simultaneously from all six contacts with monopolar configuration (retroauricular reference electrode). The gross ECochG signal was filtered off-line to separate three of its main components: compound action potential, cochlear microphonic, and summating potential (SP). Additionally, five cochleae were harvested and histologically processed to access the spatial position of the CI contacts. Both ECochG data and histological reconstructions of the electrode position were fitted with the Greenwood function to verify the reliability of the deduced cochleotopic position of the CI.

RESULTS

SPs could be used as suitable markers for the frequency position of the recording electrode with an accuracy of ±1/4 octave in the functioning cochlea, verified by histology. Cochlear microphonics showed a dependency on electrode position but were less reliable as positional markers. Compound action potentials were not suitable for CI position information but were sensitive to "cochlear health" (e.g., insertion trauma).

CONCLUSIONS

SPs directly recorded from the contacts of a CI during surgery can be used to access the intracochlear frequency position of the CI. Using SP monitoring, implantation may be stopped before penetrating functioning cochlear regions. If the technique was similarly effective in humans, it could prevent implantation trauma and increase hearing preservation during CI surgery. Diagnostic hardware and software for recording biological signals with a CI without filter limitations might be a valuable add-on to the portfolios of CI manufacturers.

Physiological Mechanisms in Combined Electric-Acoustic Stimulation

OBJECTIVE

Electrical stimulation is normally performed on ears that have no hearing function, i.e., lack functional hair cells. The properties of electrically-evoked responses in these cochleae were investigated in several previous studies. Recent clinical developments have introduced cochlear implantation (CI) in residually-hearing ears to improve speech understanding in noise. The present study documents the known physiological differences between electrical stimulation of hair cells and of spiral ganglion cells, respectively, and reviews the mechanisms of combined electric and acoustic stimulation in the hearing ears.

DATA SOURCES

Literature review from 1971 to 2016.

CONCLUSIONS

Compared with pure electrical stimulation the combined electroacoustic stimulation provides additional low-frequency information and expands the dynamic range of the input. Physiological studies document a weaker synchronization of the evoked activity in electrically stimulated hearing ears compared with deaf ears that reduces the hypersynchronization of electrically-evoked activity. The findings suggest the possibility of balancing the information provided by acoustic and electric input using stimulus intensity. Absence of distorting acoustic-electric interactions allows exploiting these clinical benefits of electroacoustic stimulation.

Electric-acoustic forward masking in cochlear implant users with ipsilateral residual hearing

In order to investigate the temporal mechanisms of the auditory system, psychophysical forward masking experiments were conducted in cochlear implant users who had preserved acoustic hearing in the ipsilateral ear. This unique electric-acoustic stimulation (EAS) population allowed the measurement of threshold recovery functions for acoustic or electric probes in the presence of electric or acoustic maskers, respectively. In the electric masking experiment, the forward masked threshold elevation of acoustic probes was measured as a function of the time interval after the offset of the electric masker, i.e. the masker-to-probe interval (MPI). In the acoustic masking experiment, the forward masked threshold elevation of electric probe stimuli was investigated under the influence of a preceding acoustic masker. Since electric pulse trains directly stimulate the auditory nerve, this novel experimental setup allowed the acoustic adaptation properties (attributed to the physiology of the hair cells) to be differentiated from the subsequent processing by more central mechanisms along the auditory pathway. For instance, forward electric masking patterns should result more from the auditory-nerve response to electrical stimulation, while forward acoustic masking patterns should primarily be the result of the recovery from adaptation at the hair-cell neuron interface. Electric masking showed prolonged threshold elevation of acoustic probes, which depended significantly on the masker-to-probe interval. Additionally, threshold elevation was significantly dependent on the similarity between acoustic stimulus frequency and electric place frequency, the electric-acoustic frequency difference (EAFD). Acoustic masking showed a reduced, but statistically significant effect of electric threshold elevation, which did not significantly depend on MPI. Lastly, acoustic masking showed longer decay times than electric masking and a reduced dependency on EAFD. In conclusion, the forward masking patterns observed for combined electric-acoustic stimulation provide further insights into the temporal mechanisms of the auditory system. For instance, the asymmetry in the amount of threshold elevation, the dependency on EAFD and the time constants for the recovery functions of acoustic and electric masking all indicate that there must be several processes with different latencies (e.g. neural adaptation, depression of spontaneous activity, efferent systems) that are involved in forward masking recovery functions.

Cochlear Implant Stimulation of a Hearing Ear Generates Separate Electrophonic and Electroneural Responses

Electroacoustic stimulation in subjects with residual hearing is becoming more widely used in clinical practice. However, little is known about the properties of electrically induced responses in the hearing cochlea. In the present study, normal-hearing guinea pig cochleae underwent cochlear implantation through a cochleostomy without significant loss of hearing. Using recordings of unit activity in the midbrain, we were able to investigate the excitation patterns throughout the tonotopic field determined by acoustic stimulation. With the cochlear implant and the midbrain multielectrode arrays left in place, the ears were pharmacologically deafened and electrical stimulation was repeated in the deafened condition. The results demonstrate that, in addition to direct neuronal (electroneuronal) stimulation, in the hearing cochlea excitation of the hair cells occurs ("electrophonic responses") at the cochlear site corresponding to the dominant temporal frequency components of the electrical stimulus, provided these are < 12 kHz. The slope of the rate-level functions of the neurons in the deafened condition was steeper and the firing rate was higher than in the hearing condition at those sites that were activated in the two conditions. Finally, in a monopolar stimulation configuration, the differences between hearing status conditions were smaller than in the narrower (bipolar) configurations.

SIGNIFICANCE STATEMENT

Stimulation with cochlear implants and hearing aids is becoming more widely clinically used in subjects with residual hearing. The neurophysiological characteristics underlying electroacoustic stimulation and the mechanism of its benefit remain unclear. The present study directly demonstrates that cochlear implantation does not interfere with the normal mechanical and physiological function of the cochlea. For the first time, it double-dissociates the electrical responses of hair cells (electrophonic responses) from responses of the auditory nerve fibers (electroneural responses), with separate excited cochlear locations in the same animals. We describe the condition in which these two responses spatially overlap. Finally, the study implicates that using the clinical characteristics of stimulation makes electrophonic responses unlikely in implanted subjects.

Neurocognitive factors in sensory restoration of early deafness: a connectome model

Progress in biomedical technology (cochlear, vestibular, and retinal implants) has led to remarkable success in neurosensory restoration, particularly in the auditory system. However, outcomes vary considerably, even after accounting for comorbidity-for example, after cochlear implantation, some deaf children develop spoken language skills approaching those of their hearing peers, whereas other children fail to do so. Here, we review evidence that auditory deprivation has widespread effects on brain development, affecting the capacity to process information beyond the auditory system. After sensory loss and deafness, the brain's effective connectivity is altered within the auditory system, between sensory systems, and between the auditory system and centres serving higher order neurocognitive functions. As a result, congenital sensory loss could be thought of as a connectome disease, with interindividual variability in the brain's adaptation to sensory loss underpinning much of the observed variation in outcome of cochlear implantation. Different executive functions, sequential processing, and concept formation are at particular risk in deaf children. A battery of clinical tests can allow early identification of neurocognitive risk factors. Intervention strategies that address these impairments with a personalised approach, taking interindividual variations into account, will further improve outcomes.