fMRI Evidence for Activation of Multiple Cortical Regions in the Primary Auditory Cortex of Deaf Subjects Users of Multichannel Cochlear Implants

To investigate the activation of the auditory cortex by fMRI, three deaf subjects users of the Ineraid cochlear implant participated in our study. Possible interference between fMRI acquisition and the implanted electrodes was controlled and safe experimental conditions were obtained. For each subject, electrical stimuli were applied on different intracochlear electrodes, in monopolar mode. Stimulation of each electrode was actually producing auditory sensations of different pitches, as demonstrated by psychophysical pitch-ranking measurements in the same subjects. Because deaf subjects did not hear scanner noise, the data were collected in ‘silent background' conditions, i.e. as a result of pure auditory sensations. Functional maps showed activation of the primary auditory cortex, predominantly in the left hemisphere. Stimulation of each different intracochlear electrode revealed different clusters of activation. After cluster grouping, at least three regions have been identified in the auditory cortex of each subject, and comparisons with previous architectonic and functional studies are proposed. However, a tonotopic organization could not be clearly identified within each region. These arguments, obtained without interference with unwanted scanner noise, plead in favor of a functional subdivision of the primary auditory cortex into multiple cortical regions in cochlear implant user

Partial withdrawal of deeply inserted cochlear electrodes: observations of two patients

Three patients implanted in our department received the preformed Clarion S-Series cochlear implant with the electrode Positioning System (EPS). The EPS is a device designed to bring the electrode array closer to the modiolus and deeper into the cochlea. Two of these patients still complained because they were perceiving too low pitch sounds, and because of the presence of echoes and poor discrimination after 3years of implant use and many tuning sessions. We hypothesized that the electrode array was too deeply inserted and could be stimulating overlapping populations of neurons in the low frequency range. The EPS was removed through a transcanal tympanotomy under local anesthesia and the array was pulled 2-3mm out of the cochlea. The angle of electrode insertion into the cochlea and the patients' performances on consonant identification tests were evaluated before and after the removal surgery and over the long term, 3years after the surgery. Immediately after the removal surgery the angle of insertion of the electrode array decreased from 720° to 485° in one case and from 675° to 485° in the other. Both patients reported subjective improvements after the removal which were confirmed by tests of performance at the long term by one of the patients. These observations show that (1) the electrode array can be moved without deterioration of performances even several years after being implanted; revision surgery may be beneficial in some cases, (2) neighboring electrodes might stimulate overlapping populations of neurons, inducing a deterioration of performances; for anatomical reasons, this is most likely to occur in the apex of the cochlea and (3) tuning of the external processor should be a customized procedur

A two-microphone noise reduction system for cochlear implant users with nearby microphones. Part II: Performance Evaluation

Users of cochlear implants (auditory aids, which stimulate the auditory nerve electrically at the inner ear) often suffer from poor speech understanding in noise. We evaluate a small (intermicrophone distance 7 mm) and computationally inexpensive adaptive noise reduction system suitable for behind-the-ear cochlear implant speech processors. The system is evaluated in simulated and real, anechoic and reverberant environments. Results from simulations show improvements of 3.4 to 9.3 dB in signal to noise ratio for rooms with realistic reverberation and more than 18 dB under anechoic conditions. Speech understanding in noise is measured in 6 adult cochlear implant users in a reverberant room, showing average improvements of 7.9–9.6 dB, when compared to a single omnidirectional microphone or 1.3–5.6 dB, when compared to a simple directional two-microphone device. Subjective evaluation in a cafeteria at lunchtime shows a preference of the cochlear implant users for the evaluated device in terms of speech understanding and sound quality

Acoustic to Electric Pitch Comparisons in Cochlear Implant Subjects with Residual Hearing

The aim of this study was to assess the frequency-position function resulting from electric stimulation of electrodes in cochlear implant subjects with significant residual hearing in their nonimplanted ear. Six cochlear implant users compared the pitch of the auditory sensation produced by stimulation of an intracochlear electrode to the pitch of acoustic pure tones presented to their contralateral nonimplanted ear. Subjects were implanted with different Clarion® electrode arrays, designed to lie close to the inner wall of the cochlea. High-resolution radiographs were used to determine the electrode positions in the cochlea. Four out of six subjects presented electrode insertions deeper than 450°. We used a two-interval (one acoustic, one electric), two-alternative forced choice protocol (2I-2AFC), asking the subject to indicate which stimulus sounded the highest in pitch. Pure tones were used as acoustic stimuli. Electric stimuli consisted of trains of biphasic pulses presented at relatively high rates [higher than 700 pulses per second (pps)]. First, all electric stimuli were balanced in loudness across electrodes. Second, acoustic pure tones, chosen to approximate roughly the pitch sensation produced by each electrode, were balanced in loudness to electric stimuli. When electrode insertion lengths were used to describe electrode positions, the pitch sensations produced by electric stimulation were found to be more than two octaves lower than predicted by Greenwood's frequency-position function. When insertion angles were used to describe electrode positions, the pitch sensations were found about one octave lower than the frequency-position function of a normal ear. The difference found between both descriptions is because of the fact that these electrode arrays were designed to lie close to the modiolus. As a consequence, the site of excitation produced at the level of the organ of Corti corresponds to a longer length than the electrode insertion length, which is used in Greenwood's function. Although exact measurements of the round window position as well as the length of the cochlea could explain the remaining one octave difference found when insertion angles were used, physiological phenomena (e.g., stimulation of the spiral ganglion cells) could also create this difference. From these data, analysis filters could be determined in sound coding strategies to match the pitch percepts elicited by electrode stimulation. This step might be of main importance for music perception and for the fitting of bilateral cochlear implant

Reading with a Simulated 60-Channel Implant

First generation retinal prostheses containing 50–60 electrodes are currently in clinical trials. The purpose of this study was to evaluate the theoretical upper limit (best possible) reading performance attainable with a state-of-the-art 60-channel retinal implant and to find the optimum viewing conditions for the task. Four normal volunteers performed full-page text reading tasks with a low-resolution, 60-pixel viewing window that was stabilized in the central visual field. Two parameters were systematically varied: (1) spatial resolution (image magnification) and (2) the orientation of the rectangular viewing window. Performance was measured in terms of reading accuracy (% of correctly read words) and reading rates (words/min). Maximum reading performances were reached at spatial resolutions between 3.6 and 6 pixels/char. Performance declined outside this range for all subjects. In optimum viewing conditions (4.5 pixels/char), subjects achieved almost perfect reading accuracy and mean reading rates of 26 words/min for the vertical viewing window and of 34 words/min for the horizontal viewing window. These results suggest that, theoretically, some reading abilities can be restored with actual state-of-the-art retinal implant prototypes if “image magnification” is within an “optimum range.” Future retinal implants providing higher pixel resolutions, thus allowing for a wider visual span might allow faster reading rates

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 µA) with intratrain pulse rates ranging from 100 to 1000 pulses per second (pps). The amplitude of the first negative (N1) to positive (P1) component of the average evoked potentials was measured for each consecutive individual pulse in the train. While a progressive exponential decrease in N1-P1 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

Forward masking in different cochlear implant systems

The goal of this study was to evaluate, from a psychophysical standpoint, the neural spread of excitation produced by the stimulation of different types of intracochlear electrode arrays: the Ineraid, the Clarion S-Series on its own or with the Electrode Positioning System (EPS), and the Clarion HiFocus-I with the EPS. The EPS is an independent silicone part designed to bring the electrode array close to the modiolus. Forward masking was evaluated in 12 adult subjects (3 Ineraid, 4 Clarion S-Series, 3 Clarion S-Series+EPS, 3 HiFocus-I+EPS) by psychophysical experiments conducted using trains of biphasic stimuli (813 pulses per second, 307.6 micros/phase). Masker signals (+8 dB re: threshold, 300 ms) were applied to the most apical electrode. Probe signals (30 ms, 10-ms postmasker) were delivered to more basal electrodes. Masked and unmasked detection thresholds of probe signals were measured. For both Clarion HiFocus-I subjects, measurements were conducted in both monopolar and bipolar stimulus configurations. No major differences were found in forward masking between the different intracochlear electrode arrays tested in the monopolar configuration at suprathreshold levels equivalent to those used in speech-coding strategies, but significant differences were found between subjects. A significant negative correlation also was found between the level of forward masking and the consonant identification performance. These measurements showed that the neural spread of excitation was more restricted in the bipolar configuration than in the monopolar configuration for HiFocus-I subjects. It was found that CIS strategies implemented without using apical electrodes, which showed high levels of masking, could improve consonant identification