Vestibular implants: the first steps in humans.

Currently there is no efficient treatment for patients with severe bilateral vestibular function impairment. Presence of oscillopsia is their main complaint. It has a significant negative impact on their quality of life. Recently it has been shown that angular vestibulo-ocular reflex can be partially restored in animals. In humans it is possible to elicit a nystagmic response by electric stimulation of ampullary parts of the vestibular nerve. Controlled eye movements can be generated by frequency and intensity modulation of the restored baseline firing rate of the vestibular nerve. During adaptation phase to the electric stimulus, patients experience nystagmus with associated inconveniences. By repetition of "on/off periods" the duration of the adaptation phase can be significantly decreased. Results show that permanent electric stimulation is necessary to maintain this "optimal" adaptation state.

[Have we made progress in the development of a vestibular implant?]. / Queues avancées cette année dans le développement d'un implant vestibulaire?

The concept of a vestibular implant to restore balance in patients suffering from bilateral loss of function is similar to that of a cochlear implant. Motion sensors will capture head movements and this information will be transmitted to the central nervous system via electrodes implanted in the vestibular system. However, several key questions must be answered before such prosthesis could be used in humans. One is to restore a baseline neural activity in the system that can be then adequately modulated by the prosthesis, without causing unbearable symptoms. We showed that this is possible in human. This is an important prerequisite for the feasibility of a vestibular implant.

Subretinal electrode implantation in the P23H rat for chronic stimulations.

BACKGROUND: In age related macular degeneration and inherited dystrophies, preservation of retinal ganglion cells has been demonstrated. This finding has led to the development of various models of subretinal or epiretinal implant in order to restore vision. This study addresses the development of a polyimide subretinal electrode platform in the dystrophic P23H rat in vivo. METHODS: A technique was developed for implanting a subretinal electrode into the subretinal space and stabilising the distal extremity of the cabling on the rat cranium in order to allow future electrical stimulations of the retina. RESULTS: In vivo imaging of the retina with the scanning laser ophthalmoscope demonstrated reabsorption of the surgically induced retinal detachment and the absence of major tissue reactions. These in vivo observations were confirmed by retinal histology. The extraocular fixation system on the rat cranium was effective in stabilising the distal connector for in vivo stimulation. CONCLUSION: This study demonstrates that a retinal implant can be introduced into the subretinal space of a dystrophic rat with a stable external connection for repeatable electrical measurements and stimulation. This in vivo model should therefore allow us to evaluate the safety and efficacy of electrical stimulations on dystrophic retina.

Ganglion cells from chick retina display multiple functional nAChR subtypes.

We have examined the properties of nicotinic acetylcholine receptors in embryonic chick retinal ganglion cells. Ganglion cells, identified according to morphological and physiological criteria, displayed spontaneous or induced action potentials. In 94/99 cells acetylcholine pulses evoked responses. In current clamp mode, acetylcholine provoked membrane depolarization and triggered action potentials. Under voltage clamp conditions, acetylcholine evoked inward currents that were readily blocked by d-tubocurarine. Antagonists specific for homomeric (alpha-bungarotoxin) and heteromeric (dihydro-beta-erythroidine) receptors revealed that ganglion cells express multiple functional receptor subtypes. These findings demonstrate that ACh modulates the electrical activity of these cells and is likely to mediate synaptic transmission. The presence of multiple receptor subtypes may contribute to processing and transmission of information in the retina.

Channel interactions with high-rate biphasic electrical stimulation in cochlear implant subjects.

Channel interactions were assessed using high-rate stimulation in cochlear implant subjects using the Ineraid electrode array. Stimulation currents were applied on one intracochlear electrode and their effects on psychophysical detection thresholds on an adjacent electrode were measured. Stimuli were trains of brief, biphasic, 50-micros/phase pulses presented at a rate of 2000 pulses per second per channel. In experiment I, we studied how the detection of a probe signal was influenced by a sub-threshold perturbation signal presented either simultaneously or non-simultaneously (with no overlap) on an adjacent electrode. Results showed that simultaneous activation led to strong channel interactions, producing threshold changes consistent with instantaneous electric field summation. Non-simultaneous activation revealed much weaker interactions, producing threshold changes of opposite sign. In experiment II, we studied how the temporal delay between perturbation and probe pulses, as well as how the level of the perturbation signal influenced non-simultaneous channel interactions. First, threshold changes when reversing the polarity of the perturbation did progressively vanish when increasing the delay between pulses. This suggested that non-overlapping stimulation of adjacent electrodes produced channel interactions that were in part due to residual polarization of the nerve membrane. Second, increasing the perturbation to supra-threshold levels produced threshold elevations that were independent of the interpulse interval. This suggested channel interactions due to neural masking. These results provide insights into the different concurrently active mechanisms of channel interactions in cochlear implant systems using this type of stimuli.

Effects of the acoustical dynamic range on speech recognition with cochlear implants.

The amplitude compression function in a speech processor for cochlear implants maps the wide acoustical dynamic range of sounds into the smaller electrical dynamic range available on the implanted electrodes. In this study, we examined the effects of systematic variations of the acoustical dynamic range of the compression function on speech recognition with cochlear implants. Statistical measures of the amplitude distribution of speech sounds were made in each channel of a research speech processor providing more than 50 dB of input signal-to-noise ratio. Several systematic variations of the dynamic range of the compression function were implemented on this basis, and speech recognition was determined using vowel and consonant identification tests in three experienced cochlear implant users. Results demonstrated that the acoustical dynamic range of the compression function does have a significant effect on speech recognition with cochlear implants. They suggest that a dynamic range of about 45 dB is necessary for optimal speech recognition.

[Necessary visual information for restoring reading with a retinal implant in a blind patients with massive retinal degeneration of photoreceptors]. / Information visuelle nécessaire à la restauration d'une lecture au moyen d'un implant rétinien chez un aveugle par dégénérescence massive des photorécepteurs.

DEFINITION OF THE PROBLEM: Our goal is to determine the minimum of information necessary for elementary reading, using a retinal implant. This concerns particularly the fragmentation (pixellisation) of the presented image and its position in the visual field. Fragmentation corresponds to the number of electrodes available, the position of the image in the visual field is equivalent to the site of the implant on the retina. MATERIAL AND METHODS: 10 degrees x 10 degrees windows, containing isolated words or letters, were presented to six healthy subjects on a computer screen. A coupling between the computer and an eye tracker stabilizes these images in an area of the visual field. This coupling constantly corrects the position of the image on the screen according to the direction of gaze. RESULTS: 1) A rapid decrease of the performance is observed at a certain threshold of pixellisation, dependent on the eccentricity of presentation of the images. 2) In central vision, about 400 pixels are sufficient to recognize 80% of the four-letters words. At 10 degrees of eccentricity, about 1225 pixels are needed. 3) An acceptable comprehension of a text (identification of four words out of five), is impossible at eccentricities higher than 10 degrees 4) About 50 pixels are sufficient for a satisfactory recognition of isolated letters, independently of their eccentricity. CONCLUSION: These data validate the method of investigation and provide valuable indications regarding minimal visual requirements in prosthetic vision.

Within-patient longitudinal speech reception measures with continuous interleaved sampling processors for ineraid implanted subjects.

OBJECTIVE: To assess within-subject changes in speech reception over time in a group of Ineraid subjects fitted with continuous interleaved sampling (CIS) wearable processors fabricated in Geneva. To compare asymptotic performance between CIS and Ineraid processors for the same subjects. DESIGN: Twelve patients, all users of the 4-channel Ineraid cochlear implant system for several years and with no previous experience of CIS processors in daily life, were equipped with Geneva Wearable Processors programmed to implement a high-rate CIS sound processing strategy using four to six channels. Their speech reception performance with CIS processors was monitored over a period of 1 yr with consonant and vowel identification tests. For comparison, speech reception performance also was measured with Ineraid processors before switching to CIS and after 6 mo of non-use of Ineraid processors. RESULTS: At fitting, CIS processors produced significantly better consonant identification but no better vowel identification. Subsequently, consonant and vowel scores with CIS processors improved progressively to asymptote after 6 mo of daily use. At 6 mo and beyond, performance with CIS processors was significantly superior to that obtained with Ineraid processors on both consonant and vowel identification tests. Control tests made with Ineraid processors after 6 mo of non-use of the device yielded results that were indistinguishable from those obtained before the study. CONCLUSIONS: The full potential of the CIS strategy is not revealed at fitting. Accumulation of daily experience provides significant improvements, asymptotic performance being reached after about 4 to 6 mo of use. All Ineraid users might greatly benefit from CIS processors.

Electrically evoked compound action potential (ECAP) of the cochlear nerve in response to pulsatile electrical stimulation of the cochlea in the rat: effects of stimulation at high rates.

Some cochlear implant patients achieve better speech recognition with pulsatile electrical stimulation presented at high rates. The present study aimed to explore, in an animal model of cochlear implants, how the excitability of the cochlear nerve is affected by pulsatile electrical stimulation delivered at high rates, of up to 1,000-2,000 pulses per second (pps). Adult rats (n=23) were implanted with two or three stimulating electrodes in the left cochlea. In four of these rats, the left cochlea was deafened by local perfusion with 1 per cent or 4 per cent neomycin solutions prior to implantation. Pulsatile stimuli consisted of 20 micros electrical pulses, delivered in trains of 200 ms duration, separated by a pause of 200 ms. The pulse rates ranged from 100 to 2,000 pps (intra-train pulse rate). Electrically evoked compound action potentials (ECAPs) of the cochlear nerve were recorded either intracochlearly or from epidural electrodes (extra-cochlearly). With increasing pulse rates, the average ECAP amplitude decreased, whereas the average ECAP latency and its variability (SD) increased. For rates above 300 pps, the amplitude of the ECAP to the individual successive pulses delivered in the train progressively decreased during the initial part of the train, corresponding to a short-term adaptation of the cochlear nerve. This effect progressively increased for pulse rates ranging from 300 to 2,000 pps. In addition, there was a phenomenon of long-term adaptation, as indicated by a decrease in the amplitude of the ECAP to the first pulse of the train, indicating that the pause of 200 ms between each train was not long enough for full recovery of the cochlear nerve. This long-term adaptation was progressively more pronounced for increasing pulse rates. To characterize further the recovery in excitability of the cochlear nerve, forward masking experiments were conducted, showing a decrease of the ECAP amplitude when the interval between the first pulse (masker) and the second pulse (probe) was shorter than 2 ms. This ECAP decrease was slow for intervals between 2 and 1 ms and then abrupt for shorter intervals. The observations described above were similar for extra- and intra-cochlear recordings and were little, if at all, affected by treatment of the cochlea with neomycin.

Colour vision in AIDS patients without HIV retinopathy.

Patients suffering from AIDS develop ocular complications, the most frequent being HIV retinopathy. It is however not clear, if functional visual impairments can be observed as early indicators of ocular complications, before clinical diagnosis of HIV retinopathy is made at fundus examination. To address this issue, we measured colour vision in a group of 49 AIDS subjects with normal clinical fundi using the 'two equation method'. This method, combining red-green Rayleigh and the blue-green Moreland metameric matches, enables more complete and quantitative assessments of colour vision than those based on pigmentary tests. Data were collected on our computer controlled colorimeter and compared to those of normal subjects. While most AIDS subjects without HIV retinopathy demonstrated normal colour vision, a significant portion of them had wider matches than normal subjects (11% for the Rayleigh equation and 16% for the Moreland equation). Furthermore, matching ranges of the Moreland equation were significantly correlated with CD4 lymphocyte counts. Patients with low CD4 values tended to produce larger matching ranges than the patients with high CD4 values. A within subject study on 17 patients confirmed this trend and showed that the patients who increased/decreased their CD4 blood counts generally improved/impaired their colour discrimination in the Moreland match. No such correlation was found between the matching ranges of the Rayleigh equation and the CD4 counts. These results show that colour discrimination is slightly reduced in some AIDS subjects, although there are no detectable ocular complications. They also suggest two different types of colour vision impairments in AIDS patients without retinopathy: one reversible process affecting colour discrimination in the blue-green range; and another irreversible process affecting colour discrimination in the red-green range.

Effect of high-frequency electrical stimulation of the auditory nerve in an animal model of cochlear implants.

HYPOTHESIS: Electrical stimulation of the cochlea at high rates induces significant adaptation of the auditory nerve. BACKGROUND: A new development of cochlear implants is the use of speech processors delivering electrical pulses on the implanted electrodes at high rates, such as 1,000 pulses per second (pps) and above. Such a stimulation mode allows subjects with cochlear implants to reach excellent understanding of speech. METHODS: Long Evans-rats received implantation of stimulating electrodes in the left cochlea. Two hundred-millisecond trains of short (20 microns) monophasic pulses were delivered in 50% duty cycle at 500 microA above threshold. The pulse rate in the train was increased from 100 pps to 1,500 pps. Electrically evoked auditory brainstem responses (EABR) were recorded. The amplitude of the compound action potential of the auditory nerve to each single pulse in the train was measured as the first vertex positive wave (WAVE I) of the EABR. RESULTS: At 100 and 200 pps, WAVE I amplitudes to each pulse were large and remained stable throughout the pulse train. For increasing pulse rates, WAVE I amplitudes progressively decreased during the first 40 to 50 ms of the train and reached 80% at 300 pps to 15% at 1,500 pps of the maximal amplitude observed for the first pulse in the train. CONCLUSIONS: The decrease of the WAVE I amplitude in response to high-rate pulsatile stimulation reflects an adaptation of the auditory nerve due, at least in part, to the refractory period of auditory nerve fibers.

[Stimulation parameters for automatic examination of color vision]. / Paramètres de stimulation pour l'examen automatisé de la vision colorée.

PURPOSE: We have developed a polyvalent computer controlled instrument, which uses the "two equation method" (Rayleigh and Moreland equations) to measure human colour vision. This "colorimeter" (or anomaloscope) was used to determine the influence of some important stimulation parameters. METHODS: The influence of stimulus exposure time, observation field size, absolute stimulus luminance, saturation and luminance mismatches between mixture and reference stimuli were measured on our computer controlled colorimeter. All subjects were normal observers. RESULTS: 1) An exposure time of 2s was found to be optimal for clinical work. 2) The Moreland equation on a 7 degrees observation field yields results in which population variability is comparable to a Rayleigh equation on a 2 degrees field. 3) A retinal illuminance between 40 and 1000 trolands can be used for automated Moreland matches. 4) The saturation of the reference field for the Moreland match can be preset to a fixed value. 5) It is important to vary automatically the radiance of the reference field to provide an approximate luminance match as the ratio of primaries in the mixture field is changed. CONCLUSION: These measurements allows us to determine optimal conditions for automated colour vision examinations and to make recommendations for an international standard.

Speech recognition with a CIS strategy for the ineraid multichannel cochlear implant.

Wilson et al. proposed a new sound-processing strategy for multichannel cochlear implants, the Continuous Interleaved Sampling (CIS) strategy. Their study was performed on seven American patients, selected for their excellent performance with the Ineraid multichannel cochlear implant, and involved refined testing of several parameter modifications of the CIS strategy during a 1-week period. At the end of the week, the CIS strategy produced large improvements in speech-recognition tests for all subjects. To evaluate the generality of this promising result, the goal of this study was to assess whether similar improvements of performance could be observed in a typical population of Ineraid users and implemented as a clinical protocol. Therefore we designed one unique, predetermined CIS processor that could be temporarily fitted to the patients in < 2 h, and we evaluated speech recognition with consonant-and vowel-identification tests in a group of patients with performances ranging from star to almost chance levels and speaking six different native languages. Scores of vowel and consonant identifications obtained with this predetermined CIS processor and with the standard processor of the Ineraid system were compared in 15 Ineraid users. Fourteen of 15 patients had significantly better scores of consonant identification with the new CIS strategy. The group mean scores of vowel identification with either strategy were not statistically different. In agreement with these observations, most patients immediately reported that the CIS strategy sounded subjectively "more clear" for real-time speech recognition. It is now possible to implement a CIS speech-coding strategy as a standard clinical procedure to improve speech-recognition performances of all Ineraid users.

First field trials with a portable CIS processor for the Ineraid multichannel cochlear implant.

The continuous interleaved sampling (CIS) strategy is a promising sound processing strategy for multichannel cochlear implants which provides immediate improvements in speech recognition when tested on Ineraid users: patients with only a few hours of experience (in laboratory testing) with the CIS strategy score better than with the Ineraid prosthesis they used since they, were implanted. The goal of this study was to evaluate the benefits that can be gained by the use of the new strategy in every day life. Two patients, implanted with the Ineraid multichannel cochlear implant, were equipped with a portable numerical processor programmed to implement a high rate CIS strategy. Their speech recognition was evaluated periodically with consonant and vowel identification tests for more than 6 months of use. Tests were also made with the Ineraid processor during the same experimental sessions and patients were regularly interviewed about their experience. Performance with the portable CIS processor was superior or equal to that obtained previously in the laboratory with the same strategy. Both patients achieved the best scores in 6 years of cochlear implant use. Qualitative reports from the patients suggest that the CIS strategy can improve "hearing" performance of cochlear implant users in many important situations of every day life. Altogether, these results hold great promises for all users of the Ineraid multichannel cochlear implant.

Ineraid cochlear implant in the ossified cochlea: surgical techniques and results.

Extensive ossification of the cochlea is a common finding in patients with total deafness caused by meningitis, labyrinthitis, or otosclerosis. When the cochlea is totally ossified, the prognosis for achieving free-running speech without lip reading is poor. However, subtotal ossification with residual cochlear patency in the upper basal turn, the middle turn, or the apical turn can be maximally exploited by selective insertion of electrodes. Results can be surprisingly adequate in spite of the poor anatomic conditions. Computed tomography and exploratory cochleotomy, in which electrically evoked auditory brainstem responses are elicited and recorded by intracochlear stimulation are good diagnostic indicators of ossification and residual auditory nerve excitability. "Apical cochleostomy" is an adequate procedure to help penetrate the basal turn in retrograde insertion of the cochlear implant electrode array. Results of this study indicate that the surgical technique and placement of the Ineraid electrode array play a major role in the ability of the patient to achieve understanding of free-running speech.

Low-pass filtering in amplitude modulation detection associated with vowel and consonant identification in subjects with cochlear implants.

Temporal auditory analysis of acoustic events in various frequency channels is influenced by the ability to detect amplitude modulations which for normal hearing involves low-pass filtering with a cutoff frequency around 100 Hz and a rejection slope of about 10 dB per decade. These characteristics were established in previous studies measuring modulation transfer functions. For cochlear implant subjects, the delivery of detailed amplitude modulation information has been recently shown to result in very significant improvements in speech understanding. Several previous studies on cochlear implant subjects have reported capacities for temporal resolution rather equivalent to those of normally hearing subjects but with some notable individual differences. Recently two studies on some cochlear implant subjects indicated modulation transfer functions often quite similar to those of normal hearing but exhibiting marked individual differences in shape and absolute sensitivity. The present study compared amplitude modulation detection and phonetic recognition in a group of cochlear implant subjects to determine the extent to which the two tasks are correlated. Nine individuals who had been implanted with an Ineraid device and who demonstrated open speech understanding ranging from excellent to poor were chosen and tested in the present study. For each subject modulation transfer functions were measured at the most apical electrode and phonetic recognition of isolated vowels and intervocalic consonants was assessed. Results showed a strong correlation between the depth of high-frequency rejection in modulation transfer functions and success in vowel and consonant intelligibility. These results emphasize the importance of temporal speech features and offer perspectives for customizing signal processing in cochlear implants.

Vowel and consonant identification tests can be used to compare performances in a multilingual group of cochlear implant patients.

Vowel and consonant identification tests were conducted in the sound-only condition in a multilingual group of 13 totally deaf patients who are users of the Ineraid multichannel cochlear implant. Native languages ranged across French, German, Italian, Spanish, Albanian and Swahili. We found high correlations (r > -0.83) among vowel or consonant identification scores and 'subjective ranking' scores established on the basis of a subjective evaluation of the patient's speech reception abilities in the sound-only condition. Detailed analysis demonstrates that the identification of vowel and consonant is dominated by the perception of acoustic cues characteristic of the set of stimuli used as well as by the strengths and weaknesses of the speech processing of the cochlear implant system. We did not find any systematic pattern in the results that could be related to the native language of the patients. These results suggest that vowel as well as consonant identification tests are effective means to compare the performance of cochlear implant patients even across different native languages. They also indicate that, in the future, one can conduct a fewer number of the many different (e.g. nonsense-syllable, word, sentence, speech-tracking) tests when evaluating the speech recognition abilities of patients with the implant.

Auditory magnetic fields in cochlear-implant patients.

Neuromagnetic recordings were used to check if electrical stimulation of the auditory nerve in cochlear-implant patients activates cortical auditory areas in a similar manner as acoustic stimulation in normal hearing subjects or whether the excitation processes are different. The waveform of the evoked magnetic field complex elicited by electrical stimulation of the auditory nerve as well as the distribution of the magnetic responses over the head resemble that of auditory-evoked fields elicited by acoustic stimulation of normal subjects. The location and direction of the equivalent dipole are consistent with activation of the auditory cortex. Neuromagnetic recordings could be used in the future (i) to check activation of the auditory cortices in cochlear-implant patients; (ii) to gather information about the functional properties of this activity, and (iii) to study the integrity of central auditory pathways in totally deaf patients.