Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis

Responses of Cat Auditory Nerve Fibers to Biphasic Electrical Current Pulses

Discharge patterns of single auditory nerve fibers were recorded from normal-hearing cats implanted with a 12-band intracochlear electrode array. Stimuli were biphasic current pulses of specifiable width, amplitude, and rate. Acoustic tuning curves were obtained to determine the cochlear positions of the fibers. Response latencies to electrical stimuli formed two groups. Short latency (0.3 to 0.7 ms) responses were attributed to direct activation of spiral ganglion neurons. At high stimulus intensities, these often exhibited abrupt shifts toward even shorter latencies. Long latency (> 1.5 ms) responses were probably caused by electrophonic activation of functional hair cells. Response thresholds to electrical stimuli depended on a fiber's proximity to the stimulating electrodes, and they did not depend on a fiber's acoustic response threshold or spontaneous discharge rate. High intensity (> 1.5 mA) stimuli could excite fibers over a wide range of characteristic frequencies, even for the narrowest (0.45 mm) electrode separations. Response threshold was an exponentially decreasing function of pulse width for widths up to 300 μs/phase. Fiber discharges were highly phase-locked at all suprathreshold intensities, and saturation discharge rates usually equaled stimulus pulse rates for rates up to at least 800 pulses/s. Dynamic ranges were small (1 to 6 dB), increased with pulse rate, and were uncorrelated with electrical response threshold. Within the dynamic range, shapes of poststimulus time and interspike interval histograms resembled those obtained in response to acoustic stimuli. Depolarization block caused fiber activity to cease in 2 to 5 seconds for sustained stimuli presented at high (> 600 pulses/s) pulse rates and intensities.

Cochlear implantation in adults with prelingual deafness. Part II. Underlying constraints that affect audiological outcomes

OBJECTIVES/HYPOTHESIS

To discuss the underlying physiological and anatomical constraints on audiological performance of late-implanted prelingually deafened adult cochlear implant patients.

STUDY DESIGN

Retrospective review.

METHODS

Published literature on the topic of auditory pathway responses to prolonged congenital deafness was reviewed. In particular, the authors sought to identify the anatomical and physiological changes that take place in both the peripheral and central auditory pathways in response to prolonged deafness, as well as how they are altered by chronic electrical stimulation.

RESULTS

The currently available evidence suggests that the colonization of the auditory cortex by other sensory modalities is the main limiting factor in postimplantation performance, not the pathological degenerative changes of the auditory nerve, cochlear nucleus, or auditory midbrain.

CONCLUSION

The reviewed evidence, although circumstantial, suggests that emphasizing aurally based educational programs before (with hearing aids) and after cochlear implantation could reduce the cortical colonization phenomenon and potentially improve postimplantation audiological performance of patients with long-term prelingual deafness.

Matching the neural adaptation in the rat ventral cochlear nucleus produced by artificial (electric) and acoustic stimulation of the cochlea

To investigate neural adaptive properties, near-field evoked potentials were recorded from a chronically implanted electrode in the ventral cochlear nucleus in awake Long-Evans rats exposed to acoustic stimuli or receiving intracochlear electric stimulation. Stimuli were 250-ms trains of repetitive acoustic clicks (10, 30 and 50 dB SPL) or biphasic electric pulses (30, 50 and 70 microA) with intratrain pulse rates ranging from 100 to 1000 pulses per second (pps). The amplitude of the first negative (N(1)) to positive (P(1)) component of the average evoked potentials was measured for each consecutive individual pulse in the train. While a progressive exponential decrease in N(1)-P(1) amplitude was observed as a function of the position of the pulse within the train for both types of stimulation, the decrement of electric responses (adaptive pattern) was substantially less prominent than that observed for acoustic stimuli. Based on this difference, the present work was extended by modifying electric stimuli in order to try to restore normal adaptation phenomena. The results suggest the feasibility of mimicking acoustic adaptation by stimulation with exponentially decreasing electric pulse trains, which may be clinically applicable in the auditory implant field.

Discharge properties of identified cochlear nucleus neurons and auditory nerve fibers in response to repetitive electrical stimulation of the auditory nerve

Using the in vitro isolated whole brain preparation of the guinea pig maintained at 29 degrees C, we intracellularly recorded and stained cochlear nucleus (CN) neurons and auditory nerve (AN) fibers. Discharge properties of CN cells and AN axons were tested in response to 50-ms trains of electrical pulses delivered to the AN at rates ranging from 100 to 1000 pulses per second (pps). At low stimulation rates (200-300 pps), the discharges of AN fibers and a large proportion of principal cells (bushy, octopus, stellate) in the ventral cochlear nucleus (VCN) followed with high probability each pulse in the train, resulting in synchronization of discharges within large populations of AN fibers and CN cells. In contrast, at high stimulation rates (500 pps and higher), AN fibers and many VCN cells exhibited "primary-like", "onset" and some other discharge patterns resembling those produced by natural sound stimuli. Unlike cells in the VCN, principal cells (pyramidal, giant) of the dorsal CN did not follow the stimulating pulses even at low rates. Instead, they often showed "pauser" and "build-up" patterns of activity, characteristic for these cells in conditions of normal hearing. We hypothesize that, at low stimulation rates, the response behavior of AN fibers and VCN cells is different from the patterns of neuronal activity related to normal auditory processing, whereas high stimulation rates produce more physiologically meaningful discharge patterns. The observed differences in discharge properties of AN fibers and CN cells at different stimulation rates can contribute to significant advantages of high- versus low-rate electrical stimulation of the AN used for coding sounds in modern cochlear implants.

Deafness-induced changes in the auditory pathway: implications for cochlear implants

A profound sensorineural hearing loss induces significant pathological and atrophic changes within the cochlea and central auditory pathway. We describe these deafness-induced morphological and functional changes following controlled lesions of the cochlea in experimental animals. Such changes are generally consistent with the limited number of reports describing deafness-induced changes observed in human material. The implications of these pathophysiological changes within the auditory pathway on cochlear implant function are discussed. Finally, the plastic response of the deafened auditory system to electrical stimulation of the auditory nerve is reviewed in light of the clinical implications for cochlear implant recipients.

Additive noise can enhance temporal coding in a computational model of analogue cochlear implant stimulation

Conventional analogue multichannel cochlear implants are unlikely to convey formant information by the fine time structure of evoked discharges. Theoretically, however, the addition of noise to the channel outputs could enhance the representation of formants by time coding. In this study, the potential benefit of noise in analogue coding schemes was investigated using a computer model of cochlear implant stimulation. The cochlear nerve was modelled by the Frankenhauser-Huxley equations. For all five vowels investigated, the optimal addition of noise to the first channel of the simulated implant (200-671 Hz) caused enhancement of the first formant representation (as seen in amplitude spectra of the simulated discharges). For vowels with a low-frequency second formant, clear enhancement of the second formant resulted from the optimal addition of noise to the third channel (1200-2116 Hz). On the basis of the present computational study, additive noise would be expected to enhance the coding of temporal information by the discharges of a single nerve fiber.

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (approximately 1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.

Single unit activity in the inferior colliculus of the rat elicited by electrical stimulation of the cochlea

The activity of single neurons (n = 182) of the central nucleus of the inferior colliculus (CIC) of the rat was recorded in response to unilateral electrical stimulation of the left cochlea and/or acoustical stimulation of the right ear. The probability of response to both modes of stimulation was comparable (90 per cent for contralateral and 60 per cent for ipsilateral presentation). Response patterns consisted predominantly of onset excitations. Response latencies to electrical stimuli ranged from 3 to 21 ms, with an average value of 9.7 ms (SD = 3.5 ms) in the ipsilateral CIC and 6.6 ms (SD = 3.4 ms) in the contralateral CIC. With respect to binaural inputs, the majority of units were excited by stimulation of either ear (EE; about 60 per cent) while about one third were influenced by one ear only (EO). Units excited by one ear and inhibited by the other (EI) were rare. The main difference between the present implanted rats and normal animals was the virtual absence here of inhibitory effects for both types of stimuli when they were delivered to the ipsilateral ear (very few EI units).

Electrical stimulation of the auditory nerve: the coding of frequency, the perception of pitch and the development of cochlear implant speech processing strategies for profoundly deaf people

1. The development of speech processing strategies for multiple-channel cochlear implants has depended on encoding sound frequencies and intensities as temporal and spatial patterns of electrical stimulation of the auditory nerve fibres so that speech information of most importance of intelligibility could be transmitted. 2. Initial physiological studies showed that rate encoding of electrical stimulation above 200 pulses/s could not reproduce the normal response patterns in auditory neurons for acoustic stimulation in the speech frequency range above 200 Hz and suggested that place coding was appropriate for the higher frequencies. 3. Rate difference limens in the experimental animal were only similar to those for sound up to 200 Hz. 4. Rate difference limens in implant patients were similar to those obtained in the experimental animal. 5. Satisfactory rate discrimination could be made for durations of 50 and 100 ms, but not 25 ms. This made rate suitable for encoding longer duration suprasegmental speech information, but not segmental information, such as consonants. The rate of stimulation could also be perceived as pitch, discriminated at different electrode sites along the cochlea and discriminated for stimuli across electrodes. 6. Place pitch could be scaled according to the site of stimulation in the cochlea so that a frequency scale was preserved and it also had a different quality from rate pitch and was described as tonality. Place pitch could also be discriminated for the shorter durations (25 ms) required for identifying consonants. 7. The inaugural speech processing strategy encoded the second formant frequencies (concentrations of frequency energy in the mid frequency range of most importance for speech intelligibility) as place of stimulation, the voicing frequency as rate of stimulation and the intensity as current level. Our further speech processing strategies have extracted additional frequency information and coded this as place of stimulation. The most recent development, however, presents temporal frequency information as amplitude variations at a constant rate of stimulation. 8. As additional speech frequencies have been encoded as place of stimulation, the mean speech perception scores have continued to increase and are now better than the average scores that severely-profoundly deaf adults and children with some residual hearing obtain with a hearing aid.

Enhancement of vowel coding for cochlear implants by addition of noise

Profoundly deaf people, who gain no benefit from conventional hearing aids, can receive speech cues by direct electrical stimulation of the cochlear nerve. This is achieved by an electronic device, a cochlear implant, which is surgically inserted into the ear. Here we show physiological results from the isolated sciatic nerve of the toad Xenopus laevis, used to predict the response of the human cochlear nerve to vowels coded by a cochlear implant. These results suggest that standard analogue cochlear implants do not evoke the patterns of neural excitation that are normally associated with acoustic stimulation. Adding noise to the stimulus, however, enhanced distinguishing features of the vowel encoded by the fine time structure of neural discharges. On the basis of these results, and those concerning stochastic resonance, we advocate a cochlear implant coding strategy in which noise is deliberately added to cochlear implant signals.

The use of long-duration current pulses to assess nerve survival

This study investigated the usefulness of long-duration current pulses in assessing the status of the auditory nerve in ears with various degrees of retrograde neural degeneration. Guinea pigs were deafened with aminoglycosides prior to acute implantation of the cochlea and collection of electrically evoked auditory brainstem responses (EABRs). Analysis of wave I evoked with long-duration current pulses suggests that this evoked response is sensitive to degeneration of the peripheral processes of the auditory nerve. Correlations with spiral ganglion cell density show that EABR measures obtained with long-duration pulses are comparable to those previously established for estimating nerve survival. Further analysis indicates that this measure may provide unique information about the degenerative state of the nerve. Threshold EABR measures using long-duration pulses are evidently more place-specific than other measures. Also, results suggest that long-duration pulses may be sensitive to two phases of the degenerative process: degradation of the peripheral processes and subsequent degeneration of neural processes central to the spiral ganglion.

Middle latency responses to electrical stimulation of the auditory nerve in unanaesthetized guinea pigs

Middle latency responses (MLR) to sinusoidal and pulsatile electrical stimulation (ES) of the cochlea and to acoustical stimulation (AS) were evaluated in awake guinea pigs with chronically implanted electrodes. The ear, which was later electrically stimulated, was deafened by local intracochlear application of gentamicin, the opposite ear was left intact. Waveforms and P1-P2 interpeak intervals of the electrically evoked MLR (ES-MLR) were similar to those evoked by acoustical stimulation of the intact ear (AS-MLR) and the latencies of the ES-MLR were shorter by about 1-3 ms. Thresholds of ES-MLR in the frequency range 0.5-32 kHz increased with increasing ES frequency (slope 3.2 dB/octave), thresholds were 3.5-9.5 dB lower for intracochlear than for extracochlear ES. Dynamic ranges for ES-MLR varied between 6-20 dB. MLR amplitude-intensity functions for ES were steeper (slope 2-12 microV/dB) than those for AS (slope 0.2-2 microV/dB). Maximal ES-MLR amplitudes exceeded usually 1.5-3 times the amplitudes of the acoustically evoked MLR. Both types of stimulations evoked larger MLR amplitudes to contralateral stimulation than to ipsilateral stimulation (average ratio = 4.1 +/- 2.2 for AS and 3.3 +/- 2.2 for ES). Because of the relatively long latency and therefore insensitivity to electrical artifact, the ES-MLR can be used for the evaluation of different strategies of the electrical stimulation of the cochlea in awake guinea pig.

Chronic intracochlear electrical stimulation in the neonatally deafened cat. II. Temporal properties of neurons in the inferior colliculus

The major focus of this study was to define the effects of chronic intracochlear electrical stimulation (ICES) on single unit responses in the inferior colliculus from three experimental groups: 1) normal adults 2) neonatally-deafened/unstimulated adults; and 3) neonatally-deafened/chronically stimulated adults. The major findings include: 1) IC neurons in normal adults showed a diversity of perstimulus responses to ICES which were qualitatively similar to those evoked by acoustic stimuli. They responded with: an onset burst, a sustained discharge, a decrease in their spontaneous activity, or a strong post-stimulus response. The excitatory responses showed either a monotonic or a nonmonotonic increase in activity with increasing stimulus intensity. Response latencies ranged from 5 to over 40 ms. 2) Responses to ICES in normal and deafened/unstimulated animals were virtually indistinguishable from one another. 3) In contrast, responses to ICES in neonatally deafened stimulated animals were different from normal and from deafened, unstimulated animals. Their perstimulus response latencies were significantly shorter, their late response latencies were significantly longer, and the frequency of occurrence of inhibitory and late responses were significantly higher. From these results we conclude that the responses to intracochlear electrical stimulation are directly comparable to those observed following normal acoustic stimulation; that development of cochleotopic organization of the inferior colliculus is not affected by the almost complete lack of normal acoustic input experienced by neonatally deafened animals; and that the basic response properties of IC units are likewise unaffected by neonatal deafening. Moreover, the results suggest that, although the limited regime of electrical stimulation employed in these studies produced no major qualitative distortions in the perstimulus response patterns of IC neurons, it did result in some quantitative changes in those responses.

Discrimination of complex electrical stimulation through a multichannel intracochlear implant

A model has been developed to describe the electric fields generated in the inner ear when electrical stimuli are presented through a multichannel implant in the scala tympani of the cochlea. The model relies on the hypothesis that stimuli which excite the largest number of neural elements provide the greatest probability of successful discrimination by the implanted subject. It suggests that the effective stimulus is determined by the linear combination of electrical fields produced by the individual channels, and that excitation takes place in a spatially restricted area of the auditory nerve in the vicinity of the stimulating electrodes. The model was tested by biophysical measurements of the potential developed in the stimulated cochlea, and by a psychophysical study of the ability of a monkey to discriminate complex electrical signals using dual channel stimulation. The experimental findings are in agreement with the computer simulations.

Inner ear implants for experimental electrical stimulation of auditory nerve arrays

Electrode arrays chronically implanted in the inner ear are gaining increased use for experimental studies of the auditory nervous system, as well as for studies related to development of improved auditory prostheses. Commercially available electrode arrays are designed for human use and thus may be unsuitable for experimental studies, particularly in small animals. This paper describes a simple, inexpensive method for making custom electrode arrays in a variety of configurations, suitable for animals ranging from small rodents to non-human primates.

Effect of electrical pulse shape on AVCN unit responses to cochlear stimulation

Electrical stimulation of the cochlea with a multiple-electrode array is best accomplished using pulsatile instead of continuous stimulation. The optimum shapes of electrical pulses for this purpose are still uncertain due to a lack of knowledge about their stimulation efficiency and requirements of the encoding strategy. We presented an extensive set of charge-balanced, rectangular pulse shapes to the guinea pig cochlea. Durations per phase for these constant-current pulses ranged from 20 microseconds to 900 microseconds with initially positive and initially negative polarities. Spike counts from single units in the anteroventral cochlear nucleus differed significantly for different pulse shapes, as did their initial latencies. Implications for stimulation efficiency and encoding strategies are discussed.

Comparisons of psychophysical and neurophysiological studies of cochlear implants

This paper compares psychophysical and neural studies of electrical stimulation of the auditory nerve with the goal of evaluating the relevance of single-unit animal models for the development of cochlear prostheses for profoundly deaf humans. Comparative psychophysical studies with implanted deaf subjects indicate that animal models, at least nonhuman primates, provide a close match to humans, though this is not always true for acoustic stimulation of normal-hearing subjects. However, the human-animal comparisons, especially those involving electrical stimuli, need further study using more carefully matched conditions. Comparisons of psychophysical and neurophysiological thresholds for electrical stimulation in animals reveal consistently higher thresholds in the neural studies. A number of factors which may account for these differences are discussed. A partial resolution of the problem could result from conducting neurophysiological and behavioral studies in the same animal. Finally, comparison of psychophysical and neurophysiological studies of temporal encoding suggest that there may be more information encoded in the auditory nerve than is used by the system, at least for nonspectral frequency discrimination. This points to a need for further analysis of the processing of this information at higher levels in the auditory pathway.

Temporal response patterns of single auditory nerve fibers elicited by periodic electrical stimuli

Single auditory nerve fibers exhibit firing synchronized to one or both phases of periodic AC stimulus currents. Responses to biphasic pulses depend on order and excitation sites of the two phases. Sine and triangle stimuli between 100 Hz and 500 Hz elicit similar response patterns. Responses to square waves are sometimes more synchronized and generally shifted in phase with respect to sine wave responses. Preferred firing phase(s): (1) are largely independent of stimulus intensity; (2) vary among fibers; (3) may shift continuously or discontinuously over several seconds before steady state is achieved. Responses to an unprocessed synthetic vowel stimulus were dominated by pitch period, first formant, and 'spurious' components.

Responses of cochlear nucleus units to electrical stimulation through a cochlear prosthesis: channel interaction

The responses of 39 single units in the ventral cochlear nucleus of acute anesthetized guinea-pigs were studied with continuous electrical stimuli presented through a dual-channel implant in the scala tympani. Implants had four electrodes placed along the axis of the cochlea with 1 mm separations. With a specific pair (either apical or basal) of stimulating electrodes, about half of the units responded when current was flowing apically, while the rest responded to current in the opposite direction. No obvious relation existed between the effective polarity of the basal and apical pairs of electrodes. Two response types were observed while stimulating through both pairs simultaneously. Seventy-nine percent of the units responded to the sum of the current waveforms presented through the two pairs. Twenty-one percent responded to the difference between the waveforms. Both types of responses were observed for suprathreshold as well as for some intensities of stimulation that alone were subthreshold. The type of response was not dependent on the absolute threshold or threshold difference between the two pairs. For equal peak-intensities of stimuli, two-channel stimulation evoked larger responses than single-channel stimulation, provided the two channels were in their effective polarities. Responses to dual-channel stimulation were consistently larger than the summed responses to the two individual single-channel stimuli. The observed responses to the dual-channel stimulation indicate that the adequate stimulus was determined by the linear combination of fields produced by the individual channels in the vicinity of the stimulating electrodes before the auditory nerve is stimulated, and that excitation takes place in a spatially restricted area of the auditory nerve.