Biomedical subjects

Alain Sigrist

Publications and source records attributed to Alain Sigrist.

5 recordsLinked to original sources

PubMed2007

Implications of deep electrode insertion on cochlear implant fitting.

Using long Med-El Combi40+ electrode arrays, it is now possible to cover the whole range of the cochlea, up to about two turns. Such insertion depths have received little attention. To evaluate the contribution of deeply inserted electrodes, five Med-El cochlear implant users were tested on vowel and consonant identification tests with fittings with first one, two, and up to five apical electrodes being deactivated. In addition, subjects performed pitch-ranking experiments, using loudness-balanced stimuli, to identify electrodes creating pitch confusions. Radiographs were taken to measure each electrode insertion depth. All subjects used each modified fitting for two periods of about 3 weeks. During the experiment, the same stimulation rate and frequency range were maintained across all the fittings used for each individual subject. After each trial period the subject had to perform three consonant and three vowel identification tests. All subjects showed deep electrode insertions ranging from 605 degrees to 720 degrees. The two subjects with the deepest electrode insertions showed significantly increased vowel- and consonant-identification performances with fittings with the two or three most apical electrodes deactivated compared to their standard fitting with all available electrodes activated. The other three subjects did not show significant improvements in performance when one or two of their most apical electrodes were deactivated. Four out of five subjects preferred to continue use of a fitting with one or more apical electrodes deactivated. The two subjects with the deepest insertions also showed pitch confusions between their most apical electrodes. Two possible reasons for these results are discussed. One is to reduce neural interactions related to electrodes producing pitch confusions. Another is to improve the alignment of the frequency components of sounds coded by the electrical signals delivered to each electrode to the overall pitch of the auditory perception produced by the electrical stimulation of auditory nerve fibers.

Adult↗

PubMed2006

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 degrees . 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 implants.

Acoustic Stimulation↗

PubMed2005

Measurements of electrode position inside the cochlea for different cochlear implant systems.

CONCLUSIONS: This study demonstrates that the exact location of an electrode inside the cochlea needs to be assessed using two complementary measures, namely the length and angle of insertion, both of which are mandatory if one wants to prevent erroneous outcomes. Knowledge of the contact position may become very useful when tuning a cochlear implant processor in a patient with contralateral residual hearing, or in cases of binaural implants. OBJECTIVE: Multichannel cochlear implants restore useful hearing to deaf patients. However, several types of intracochlear electrodes are presently available, each featuring a specific technology or design. The aim of this study was to determine precisely the intracochlear position of the contacts for different electrode arrays. MATERIAL AND METHODS: Electrode array insertions were estimated using special radiographs. A total of 26 cochlear implantations were included in the study: 6 Ineraid; 5 Clarion HiFocus I; 11 Clarion HiFocus II; and 4 Med-El Combi40+. In each case, a measurable reference or marker ring placed close to the round window (within 2 mm) could be identified. Insertion lengths and angles were measured and then plotted on a graphl based on 3D reconstructions. RESULTS: Both Clarion HiFocus I and II electrode arrays were found to be placed close to the inner wall of the cochlea. Ineraid and Med-El Combi40+ electrode arrays were both placed close to the organ of Corti, the Med-El Combi40+ arrays demonstrating the deepest insertions overall. In spite of marked differences in the positions of the contacts, we did not find any correlation with speech perception performance for the different types of implants studied.

Adult↗

PubMed2004

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 users.

Adult↗

PubMed2002

Functional MRI of auditory cortex activated by multisite electrical stimulation of the cochlea.

Electrical stimulation of the ear of deaf patients via cochlear implants offers a unique occasion to study activity of central auditory pathways with fMRI, without bias due to scanner noise. Such measurements, however, require one to control the possible interference between fMRI acquisition and the implanted electrodes. A series of measurements on a customized phantom designed to characterize the level of induced currents during MRI acquisition is presented. These experiments demonstrate that the major artifactual contribution is due to radiofrequency interaction and that safe experimental conditions can be obtained with proper shielding of the stimulation cables. The induced currents could be reduced to low levels (<50 microA for a duration <2 ms), below the acoustic perceptual threshold of cochlear implant subjects. Subsequent fMRI experiments on a patient using an Ineraid cochlear implant were conducted. Results revealed bilateral localized activation of the primary auditory cortex. Stimulation of two different intracochlear electrodes elicited activity in two neighboring, but different, regions, in agreement with the known tonotopical organization of the auditory cortex. This work paves the way for fMRI studies of a broad selection of auditory paradigms without interference from unwanted noise.

Artifacts↗