Acoustic to electric pitch comparisons in cochlear implant subjects with residual hearing

Abstract

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.

Fig. 1

Hearing thresholds (pure tones) of the nonimplanted ear of each subject.

Fig. 2

Radiographs obtained for each subject following the modified Stenver's view. The angle of the midsagittal plane of the skull made with the film plane at the time of the radiograph is indicated in parentheses. The line going through thetop of the semi-circular- canal middle of the vestibule round windowtop of the superior semicircular canal and the center of the vestibule crosses the electrode array at the estimated site of the round window. The line going through the estimated site of the round window to the center of the first turn of the spiral made by the electrode array is used as the 0° reference line. In cases of deep electrode insertions, the line going through the estimated site of the round window to the center of the second turn of the spiral made by the electrode array is determined as the 720° line.

Fig. 3

Estimation of electrode positions (insertion lengths and angles) for each subject in comparison to the positions of the outer (solid line) and inner (second dashed line) walls of the cochlea, as well as the organ of Corti (first dashed line) and the spiral ganglion (dotted line), as described in the reconstruction work of Kawano et al. (1996).

Fig. 4

Pitch-comparison data for electrode 7 in subject Cp18. The proportions of pitch judgments in which the acoustic stimulus were judged higher in pitch are plotted as a function of the frequency of the acoustic stimuli.

Fig. 5

Equivalent acoustic frequency versus electrode insertion length (mm), for all six subjects. The solid lines represent Greenwood's (1990) frequency–position function for a normal ear, using proportion of basilar length (a = 0.06; k = 1; total length: 35 mm). Both dashed lines represent the same function shifted down by one or two octaves.

Fig. 6

Equivalent acoustic frequency versus electrode insertion angle for all six subjects. The solid lines represent the frequency–position function of a normal ear, adapted for angles. The dashed line represents the same function shifted down by one octave. The curved line represents the frequency–position function estimated for the stimulation of the spiral ganglion cells (see text for details).