Comparison of electrode sites in electrical stimulation of the cochlea

There is considerable controversy about the “best” location for single channel cochlear implant electrodes. We measured the electrically induced auditory brain stem response (EABR) in a series of normal to totally denervated cat ears in response to promontory (P), round window (RW) and scala tympani (ST) stimulation. The status of the ganglion cell population was then assessed by light microscopy. In ears with light to medium ganglion cell loss the ST EABR yielded the most definitive input-output functions. RW responses were present at increased thresholds and smaller peak amplitudes. P responses were worse or even missing completely. In severely damaged ears, including some with no detectable ganglion cells, ST and RW EABRs were both markedly reduced with considerable overlap between the two sites. P responses, when present, were almost buried in the electrical noise near threshold. Extrapolating these results to humans suggests that when ganglion cell loss is very severe the RW or ST is an acceptable stimulation site. When ganglion cell loss is moderate or better, ST electrodes are superior.     

Tuning characteristics of cochlear nucleus units in response to electrical stimulation of the cochlea

The responses of single units in the ventral cochlear nucleus of acute anesthetized guinea pigs were studied with continuous sinusoidal electrical stimuli presented through a multi-electrode implant in the scala tympani. Implants had two or four electrodes along the axis of the scala with 1 mm separations. Best frequencies were consistently in the 100 Hz range (50-250 Hz) with thresholds of about 0.063 mA peak-to-peak. Tuning curves were usually symmetrical with slopes of 3-4 dB/octave, both below and above the best frequency. The relative sharpness of the tuning curves, as measured by Q10dB, averaged 0.2. Dynamic ranges as determined by the intensity-rate functions for the various frequencies were 2-15 dB. No significant difference was found between tuning characteristics of units in response to stimulation via the apical or basal pair of implant electrodes. The findings suggest some limitations on the applicability of independent stimulating channels in multi-electrode implants.     

An experimental study on the auditory brainstem response to electrical stimulation of the cochlea

This animal research was undertaken to study some important issues for cochlear implantation. This included investigations of 1) the difference in the auditory brainstem responses (ABRs) to extra- and intracochlear electrical stimulations. 2) stimulus parameters and ABRs to a bipolar intracochlear stimulation, and 3) electrode placements and ABRs. There were no distinct differences in waveforms and input-output curves of ABRs between extracochlear electrical stimulation and intracochlear bipolar stimulation of the cochlea. Changes in amplitude and latency of ABRs were investigated at changes of the pulse amplitude or the pulse width of stimulating electric rectangular waves. Increase in the pulse amplitude or the pulse width resulted in increase of the amplitude of ABRs and no increase in the ABR latency. This results showed the amplitude of ABR was dependent on the amount of electricity; coulomb. Electrically evoked ABRs were studied for stimulations at different sites of the cochlea. The ABR elicited with a bipolar electrode placed at the third turn of the cochlea had a longer latency of each peak of ABR waves than the one at the basal turn. This indicates that electrical stimulation with a bipolar electrode can give rise to a localized excitation in the cochlea and a latency change along the longitudinal axis of the cochlea, so that it suggests a multichannel cochlea implant system can transmit more information as a spacious pattern than a single channel system.     

Changes over time in thresholds for electrical stimulation of the cochlea

The purpose of this paper is to better characterize changes over time that occurred in psychophysical detection thresholds for electrical stimulation of the cochlea. Threshold changes observed in nonhuman primates implanted with cochlear electrode arrays can be divided into at least three types based on the patterns of change over time. Short-term increases and subsequent decreases in threshold were commonly observed during the first months after implantation and were often followed by periods of long-term threshold stability. Long-term slow increases in thresholds and more rapid increases after a period of threshold stability have also been observed. The threshold changes may be divided into at least two classes based on their dependence on the waveforms used for the threshold measurements. Some changes occurred primarily in thresholds for long phase-duration signals while other changes were equal in magnitude (in decibels) for all tested stimuli. This suggests that at least two mechanisms underlay these threshold changes. The observed changes in thresholds have implications for experimental studies of electrical stimulation and for clinical application of auditory prostheses.     

Protocol for determining the auditory percepts of electrical stimulation of the cochlea

The percepts of electrical stimulation of the auditory nerve can best be determined in man. An eight channel bipolar electrode array has been developed which allows discreet stimulation of segments of auditory nerve via the scala tympani. In addition, a programmable stimulator has been developed which allows the delivery of different waveforms and stimulus patterns. Hopefully this will begin to allow the generation of the elements of speech.     

Brainstem location of efferent neurones projecting to the guinea pig cochlea

Retrograde transport of horseradish peroxidase using tetramethylbenzidine as a chromagen was used to map the cell bodies of origin of the efferent projection to the cochlea in the guinea pig. Large numbers of small labelled neurones were found within the body of the lateral superior olivary nucleus (LSO) ipsi-lateral to the injected cochlea. Labelled cells within the contra-lateral LSO were extremely rare (1% or less). Large labelled neurones were found both ipsi-laterally and contra-laterally in the dorsomedial periolivary region, including the medial nucleus of trapezoid body and in the ventral and lateral nuclei of the trapezoid body and the ventral nucleus of lateral lemniscus. It was concluded that some aspects of the distribution of these efferent neurones in the guinea pig are similar to the situation in the rat, whilst others resemble more closely the arrangement found in cat.     

Spatial distribution of neural activity evoked by electrical stimulation of the cochlea

Activity in the central auditory system was mapped with 2-deoxyglucose (2-DG) autoradiography, using either pure tones or electrical stimulation of the normal cochlea. Electrical stimulation with both monopolar (distant reference electrode) and bipolar prostheses near threshold increased 2-DG uptake in auditory nuclei in a manner similar to that seen with a pure tone: increased 2-DG uptake was restricted to a small frequency region of brainstem and mid-brain auditory nuclei. The position of this area was related to the cochlear location of the prosthesis. At higher current amplitudes only the bipolar prosthesis retained spatial restriction of evoked neural activity, while stimulation through a monopolar prosthesis produced evoked activity in all frequency regions of auditory nuclei and in non-auditory nuclei. Activation of non-auditory structures was consistent with spread of current through the brainstem, rather than activation of peripheral nerves. At all current amplitudes, a monopolar prosthesis evoked higher levels of 2-DG uptake than a bipolar prosthesis.

The results suggest that while a bipolar prosthesis provides greater spatial restriction of evoked neural activity and a greater dynamic range, a monopolar prosthesis produces higher levels of evoked activity.     

Discharge patterns of cat primary auditory fibers with electrical stimulation of the cochlea

Intact and destroyed cat cochleae were electrically stimulated through round window electrodes. Intact cochleae provided information about fiber properties with acoustic stimuli. With sinusoidal currents thresholds for synchronization were 4–68 μA rms. Thresholds were independent of the fiber's characteristic frequencies and thus of their places of origin in the intact cochleae. This shows large current spread. Phase-locking of the responses to electric stimulation was much stronger than it was to acoustic stimulation. Destroyed cochleae had no spontaneous activity and showed even stronger phase-locking. Thresholds obtained using 0.2 ms per phase biphasic pulse stimuli were 60–350 μA_pp. Action potentials were found to be released with as little as 0.3 ms latency. The neuronal responses to any electric stimulus differed considerably from the responses to corresponding acoustic stimuli. Vestibular fibers were easily activated by electric stimulation.     

Some experimental observations of responses evoked from the cochlea during two-tone stimulation

When the gerbil cochlea is stimulated with two closely spaced tones, a broad frequency spread of distortion is generated which can be detected in both the meatal sound field and in the round window cochlear microphonic (CM) response. When the frequency ratio f2/f1 is 1.025 or less and f2 = 6 kHz, there is evidence that more than one group of distortion components combines to give the pattern seen in the two cochlear responses. One group is apparently limited to frequencies below 4.5 kHz. This group may be an expression at close stimulus frequency ratios of the 'pulse' described for more widely spaced, lower frequency stimulus conditions, but shown here to be low-pass filtered. Acoustic and CM components within this group show comparable behaviour. A second distortion group consists of a symmetrical distribution of components above and below the stimuli, with no evidence of low-pass filtering. Acoustic and CM manifestations of this symmetrical grouping show little comparable behaviour. It is suggested that interaction could occur between these two groups of distortion to cause some of the complex behaviour reported for 2f1-f2.     

电刺激耳蜗记录下丘的空间调谐曲线

目的探索耳蜗电刺激的听觉中枢电活动机理，为多道人工耳蜗电刺激的部位代码提供生理依据。方法利用记录单单位和多单位电位反应的方法，分别描记１７只猫听觉下丘核团对耳蜗内单极电刺激，耳蜗内双极电刺激和纯音刺激反应的空间调谐曲线。结果耳蜗内双极电刺激能兴奋下丘中的特定区域，类似于对纯音刺激的反应；而耳蜗内单极电刺激使下丘细胞广泛地被兴奋，不能提供部位代码。结论使用双极电极时，刺激电流的局限范围是提高部位代码的关键；耳蜗内单极电刺激部位代码的电生理结果与心理物理学的结论以及人工耳蜗植入患者的临床效果相矛盾，对此的解释有待进一步研究。     

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.     

电刺激耳蜗记录下丘的空间调谐曲线

探索耳蜗电刺激的听觉中枢电活动机理，为多道人工耳蜗电刺激的部位代码提供依据。方法 利用记录单单位和多单位电位反应的方法，分别描记１７只猫听觉下丘核团对耳蜗内单极电刺激，耳蜗内双极电刺激和纯音刺激反应的空间调谐曲线。结果 耳蜗内双极电刺激能兴奋下丘中的特定区域，类似于纯音刺激的反应；而耳蜗内单极电刺激使下丘细胞广泛地被兴奋，不能提供部位代码。     

Electrical and physiological changes during short-term and chronic electrical stimulation of the normal cochlea

In the electrical stimulation (ES) of auditory pathways, the type of stimulus and the electrode/tissue interface are critical parameters for the safety and efficacy of the protocol. In this study the influence of alternate pulses, applied between round window and vertex electrodes in chronically implanted guinea pigs, and maintained during 1 and 25 daily periods of 2 h (short-term and long-term experiments, respectively), was investigated. ES consisted of monophasic current pulses of ±70 μA and 300 μs in duration at a rate of 167/s, with alternate polarity. Compound Action Potential (CAP) audiograms, amplitudes and latencies of click-evoked CAPs, amplitudes and latencies of electrically-evoked auditory responses (EARs), and electrode impedances, were measured periodically outside or during the ES periods. Short-term ES induced no change in CAP thresholds, amplitude and latency in response to clicks at 80 dB above normal threshold, but induced a slight latency increase and amplitude decrease of the EAR, correlated with an exponential decay of the electrode impedance. On a long-term basis, CAP audiograms and latencies did not change significantly, whereas CAP amplitudes and electrode impedances increased, in correlation with each other. In control guinea pigs receiving no ES, the same CAP amplitude and impedance changes were observed over the same long-term period. The EAR and CAP changes can be explained by a variation of the electrical impedance of the electrode/tissue interface. This is possibly due to a change in electrotytes around the electrode under the influence of the ES for the short-term variation, and to an electrode encapsulation by fibrous tissue independent of the ES for the long-term change. In itself, and under the conditions of this experiment, the ES demonstrated no adverse effects on the auditory function and can be safely used for inner-ear exploration.     

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

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.     

Cochleotopic fos immunoreactivity in cochlea and cochlear nuclei evoked by bipolar cochlear electrical stimulation

Fos-like immunoreactivity evoked by basal, second or apical turn bipolar intracochlear electrical stimulation was evaluated in the spiral ganglion and cochlear nuclei. At stimulation levels of six times the electrically evoked auditory brain stem response thresholds, immunoreactive neurons were observed at appropriate discrete cochleotopic regions relative to stimulation site. The number of neurons increased with stimulus level and closely correlated to wave I amplitude. At 10 times thresholds, some spread in fos-like immunoreactivity to adjacent cochlear turns was found. However, fos-like immunoreactivity at this high level of stimulation still clearly showed a differential distribution in density of expression. These results indicated that the restricted topographic distribution of activity evoked by high levels of electrical stimulation is initiated at first order primary neurons of the system. For the profoundly deaf with cochlear implants, this indicates that place (channel) information can be maintained in the spiral ganglion and central nervous system even at very high levels of electrical stimulation. Together with our previous studies, these results indicate that fos provides a marker which can be used for evaluation of extent and pattern of cellular activation throughout the central auditory pathways, including activation of auditory nerve cells.     

Vestibular effects of electrical stimulation of the cochlea. Monitored in the awake primate.

The effect of electrical stimulation of the cochlea on vestibular response was monitored in six rhesus monkeys. Eye movements and single-unit activity from the vestibular portion of the eighth nerve, vestibular nuclei, reticular formation, and abducens nucleus were observed while electrical stimulation was delivered through an implanted cochlear prosthesis. In one animal of this series, neural activity from the inferior colliculus and cochlear nuclear complex was also recorded. Electrical stimulation elicited eye-movement responses in only one animal. In the animals from which single-unit activity was recorded, no positive vestibular effects were noted. In one animal of this study, responses were elicited from auditory structures by relatively low intensities of electrical stimulation.     

Histopathologic study of the cochlear nuclei after 10 years of electrical stimulation of the human cochlea

This report provides histologic data on the cochlear nuclei of a patient who used a cochlear implant in the right ear for about 10 years and in the left ear for about 1 year. The most pronounced abnormality in the cochlear nuclei is sites of gliosis. However, since areas of necrosis with similar size, shape, and content of cells were observed outside the nuclei, the gliosis was not specific to the nuclei. There were no other unusual changes in the cochlear nuclei, such as excessive chromatolysis, excessive accumulation of lipofuscin, swollen neurons, or accumulation of glial cells. Comparison showed no statistically significant differences in sizes of cell bodies, size of nuclei, and total density of neurons in cochlear nuclei between the right and left sides. However, the density of normal-looking neurons (with visible nucleoli and without visible pathologic features) was noticeably less on the longer-stimulated side. Some differences between the longer- and shorter-stimulated sides may be due to differences before implantation or other factors, as well as to effects of stimulation.