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Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation.

ABSTRACT: OBJECTIVES:In this study, localization accuracy and sensitivity to acoustic interaural time differences (ITDs) in subjects using cochlear implants with combined electric-acoustic stimulation (EAS) were assessed and compared with the results of a normal hearing control group. METHODS:Eight CI users with EAS (2 bilaterally implanted, 6 unilaterally implanted) and symmetric binaural acoustic hearing and 24 normal hearing subjects participated in the study. The first experiment determined mean localization error (MLE) for different angles of sound incidence between ± 60° (frontal and dorsal presentation). The stimuli were either low-pass, high-pass or broadband noise bursts. In a second experiment, just noticeable differences (JND) of ITDs were measured for pure tones of 125 Hz, 250 Hz and 500 Hz (headphone presentation). RESULTS:Experiment 1: MLE of EAS subjects was 8.5°, 14.3° and 14.7°, (low-, high-pass and broadband stimuli respectively). In the control group, MLE was 1.8° (broadband stimuli). In the differentiation between sound incidence from front and back, EAS subjects performed on chance level. Experiment 2: The JND-ITDs were 88.7 ?s for 125 Hz, 48.8 ?s for 250 Hz and 52.9 ?s for 500 Hz (EAS subjects). Compared to the control group, JND-ITD for 125 Hz was on the same level of performance. No statistically significant correlation was found between MLE and JND-ITD in the EAS cohort. CONCLUSIONS:Near to normal ITD sensitivity in the lower frequency acoustic hearing was demonstrated in a cohort of EAS users. However, in an acoustic localization task, the majority of the subjects did not reached the level of accuracy of normal hearing. Presumably, signal processing time delay differences between devices used on both sides are deteriorating the transfer of precise binaural timing cues.

SUBMITTER: Kortje M

PROVIDER: S-EPMC7571672 | biostudies-literature | 2020

REPOSITORIES: biostudies-literature

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Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation.

Körtje Monika M Baumann Uwe U Stöver Timo T Weissgerber Tobias T

PloS one 20201019 10

<h4>Objectives</h4>In this study, localization accuracy and sensitivity to acoustic interaural time differences (ITDs) in subjects using cochlear implants with combined electric-acoustic stimulation (EAS) were assessed and compared with the results of a normal hearing control group.<h4>Methods</h4>Eight CI users with EAS (2 bilaterally implanted, 6 unilaterally implanted) and symmetric binaural acoustic hearing and 24 normal hearing subjects participated in the study. The first experiment determ ...[more]

PMID: 33075114

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Project description:Electrical interaural time delay (ITD) discrimination was measured using 300-ms bursts applied to binaural pitch matched electrodes at basal, mid, and apical locations in each ear. Six bilateral implant users, who had previously shown good ITD sensitivity at a pulse rate of 100 pulses per second (pps), were assessed. Thresholds were measured as a function of pulse rate between 100 and 1,000 Hz, as well as modulation rate over that same range for high-rate pulse trains at 6,000 pps. Results were similar for all three places of stimulation and showed decreasing ITD sensitivity as either pulse rate or modulation rate increased, although the extent of that effect varied across subjects. The results support a model comprising a common ITD mechanism for high- and low-frequency places of stimulation, which, for electrical stimulation, is rate-limited in the same way across electrodes because peripheral temporal responses are largely place invariant. Overall, ITD sensitivity was somewhat better with unmodulated pulse trains than with high-rate pulse trains modulated at matched rates, although comparisons at individual rates showed that difference to be significant only at 300 Hz. Electrodes presenting with the lowest thresholds at 600 Hz were further assessed using bursts with a ramped onset of 10 ms. The slower rise time resulted in decreased performance in four of the listeners, but not in the two best performers, indicating that those two could use ongoing cues at 600 Hz. Performance at each place was also measured using single-pulse stimuli. Comparison of those data with the unmodulated 300-ms burst thresholds showed that on average, the addition of ongoing cues beyond the onset enhanced overall ITD sensitivity at 100 and 300 Hz, but not at 600 Hz. At 1,000 Hz, the added ongoing cues actually decreased performance. That result is attributed to the introduction of ambiguous cues within the physiologically relevant range and increased dichotic firing.

Project description:OBJECTIVE:The primary objective of this study was to assess the effect of electric and acoustic overlap for speech understanding in typical listening conditions using semidiffuse noise. DESIGN:This study used a within-subjects, repeated measures design including 11 experienced adult implant recipients (13 ears) with functional residual hearing in the implanted and nonimplanted ear. The aided acoustic bandwidth was fixed and the low-frequency cutoff for the cochlear implant (CI) was varied systematically. Assessments were completed in the R-SPACE sound-simulation system which includes a semidiffuse restaurant noise originating from eight loudspeakers placed circumferentially about the subject's head. AzBio sentences were presented at 67 dBA with signal to noise ratio varying between +10 and 0 dB determined individually to yield approximately 50 to 60% correct for the CI-alone condition with full CI bandwidth. Listening conditions for all subjects included CI alone, bimodal (CI + contralateral hearing aid), and bilateral-aided electric and acoustic stimulation (EAS; CI + bilateral hearing aid). Low-frequency cutoffs both below and above the original "clinical software recommendation" frequency were tested for all patients, in all conditions. Subjects estimated listening difficulty for all conditions using listener ratings based on a visual analog scale. RESULTS:Three primary findings were that (1) there was statistically significant benefit of preserved acoustic hearing in the implanted ear for most overlap conditions, (2) the default clinical software recommendation rarely yielded the highest level of speech recognition (1 of 13 ears), and (3) greater EAS overlap than that provided by the clinical recommendation yielded significant improvements in speech understanding. CONCLUSIONS:For standard-electrode CI recipients with preserved hearing, spectral overlap of acoustic and electric stimuli yielded significantly better speech understanding and less listening effort in a laboratory-based, restaurant-noise simulation. In conclusion, EAS patients may derive more benefit from greater acoustic and electric overlap than given in current software fitting recommendations, which are based solely on audiometric threshold. These data have larger scientific implications, as previous studies may not have assessed outcomes with optimized EAS parameters, thereby underestimating the benefit afforded by hearing preservation.

Project description:BackgroundHarmony is an important part of tonal music that conveys context, form and emotion. Two notes sounded simultaneously form a harmonic interval. In normal-hearing (NH) listeners, some harmonic intervals (e.g., minor 2nd, tritone, major 7th) typically sound more dissonant than others (e.g., octave, major 3rd, 4th). Because of the limited spectro-temporal resolution afforded by cochlear implants (CIs), music perception is generally poor. However, CI users may still be sensitive to relative dissonance across intervals. In this study, dissonance ratings for harmonic intervals were measured in 11 unilaterally deaf CI patients, in whom ratings from the CI could be compared to those from the normal ear.MethodsStimuli consisted of pairs of equal amplitude MIDI piano tones. Intervals spanned a range of two octaves relative to two root notes (F3 or C4). Dissonance was assessed in terms of subjective pleasantness ratings for intervals presented to the NH ear alone, the CI ear alone, and both ears together (NH + CI). Ratings were collected for both root notes for within- and across-octave intervals (1-12 and 13-24 semitones). Participants rated the pleasantness of each interval by clicking on a line anchored with "least pleasant" and "most pleasant." A follow-up experiment repeated the task with a smaller stimulus set.ResultsWith NH-only listening, within-octave intervals minor 2nd, major 2nd, and major 7th were rated least pleasant; major 3rd, 5th, and octave were rated most pleasant. Across-octave counterparts were similarly rated. With CI-only listening, ratings were consistently lower and showed a reduced range. Mean ratings were highly correlated between NH-only and CI-only listening (r = 0.845, p < 0.001). Ratings were similar between NH-only and NH + CI listening, with no significant binaural enhancement/interference. The follow-up tests showed that ratings were reliable for the least and most pleasant intervals.DiscussionAlthough pleasantness ratings were less differentiated for the CI ear than the NH ear, there were similarities between the two listening modes. Given the lack of spectro-temporal detail needed for harmonicity-based distinctions, temporal envelope interactions (within and across channels) associated with a perception of roughness may contribute to dissonance perception for harmonic intervals with CI-only listening.

Project description: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.

Dataset Information

Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation.

Publications

Sensitivity to interaural time differences and localization accuracy in cochlear implant users with combined electric-acoustic stimulation.

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