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

Abstract

The aim of this study was to assess the frequencyposition 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 implants.

An Electric Frequency-to-place Map for a Cochlear Implant Patient with Hearing in the Nonimplanted Ear

“…The pitch match for an electrode only 3.4 mm into the cochlea was 3,447 Hz. These data are consistent with other reports, e.g., Boëx et al (2006), of a frequency-to-place map for the electrically stimulated cochlea in which perceived pitches for stimulation on individual electrodes are significantly lower than those predicted by the Greenwood function for stimulation at the level of the hair cell. …”

“…A similar outcome was obtained by James et al (2001). Boëx et al (2006) tested six patients fit with Clarion implant systems who, most generally, had better hearing thresholds in the nonimplanted ear than the patients in Blamey et al (1996) and James et al (2001). For example, patient H70 had a 0 dB HL threshold at 250 Hz and a 45-dB HL threshold at 4 kHz.…”

“…A summed voxel projection of highresolution CT scan data was used to create a radiograph-type image (see Fig. 2) from which electrode insertion angle was estimated in the manner described by Boëx et al (2006). 3D image volumes.…”

Assessment and improvement of sound quality in cochlear implant users

“…CIs are designed to imitate this by transmitting pitch information to the electrode at the cochlear location biologically optimized to transmit the assigned frequency. Most often, however, the programmed characteristic frequency and the theoretical characteristic frequency (based on the location along the basilar membrane) do not match, leading to an inaccurate pitch percept 23, 24. This physical mismatch is due to a combination of variation in size of the cochlea, the length of the electrode array, proximity to nerve fibers, and insertion depth, among other factors.…”

“…Furthermore, anatomic variations in cochlea lengths58 and intraoperative events affect individual electrode contact placement after array insertion. Consequentially, electrodes programmed to carry low‐frequency information (apically located on the electrode array) and electrodes programmed to carry high‐frequency information (basally located on the electrode array) commonly stimulate areas of the basilar membrane that contain spiral ganglion cells associated with a lower frequency and higher frequency, respectively 23, 24. This place‐pitch mismatch exists between the frequencies transmitted by the individual channels and the corresponding characteristic frequency given the final electrode position (Fig.…”