The effect of different cochlear implant microphones on acoustic hearing individuals' binaural benefits for speech perception in noise

Abstract

Objectives: Cochlear implant microphones differ in placement, frequency response, and other characteristics such as whether they are directional. Although normal-hearing (NH) individuals are often used as controls in studies examining cochlear implant users' binaural benefits, the considerable differences across cochlear implant microphones make such comparisons potentially misleading. The goal of this study was to examine binaural benefits for speech perception in noise for NH individuals using stimuli processed by head-related transfer functions (HRTFs) based on the different cochlear implant microphones.

Design: HRTFs were created for different cochlear implant microphones and used to test participants on the Hearing in Noise Test. Experiment 1 tested cochlear implant users and NH individuals with HRTF-processed stimuli and with sound field (SF) testing to determine whether the HRTFs adequately simulated SF testing. Experiment 2 determined the measurement error and performance-intensity function for the Hearing in Noise Test with NH individuals listening to stimuli processed with the various HRTFs. Experiment 3 compared NH listeners' performance across HRTFs to determine how the HRTFs affected performance. Experiment 4 evaluated binaural benefits for NH listeners using the various HRTFs, including ones that were modified to investigate the contributions of interaural time and level cues.

Results: The results indicated that the HRTFs adequately simulated SF testing for the Hearing in Noise Test. They also demonstrated that the test-retest reliability and performance-intensity function were consistent across HRTFs, and that the measurement error for the test was 1.3 dB, with a change in signal-to-noise ratio of 1 dB reflecting a 10% change in intelligibility. There were significant differences in performance when using the various HRTFs, with particularly good thresholds for the HRTF based on the directional microphone when the speech and masker were spatially separated, emphasizing the importance of measuring binaural benefits separately for each HRTF. Evaluation of binaural benefits indicated that binaural squelch and spatial release from masking were found for all HRTFs, and binaural summation was found for all but one HRTF, although binaural summation was less robust than the other types of binaural benefits. In addition, the results indicated that neither interaural time nor level cues dominated binaural benefits for the NH participants.

Conclusions: This study provides a means to measure the degree to which cochlear implant microphones affect acoustic hearing with respect to speech perception in noise. It also provides measures that can be used to evaluate the independent contributions of interaural time and level cues. These measures provide tools that can aid researchers in understanding and improving binaural benefits in acoustic hearing individuals listening via cochlear implant microphones.

Figure 1

Spectral characteristics of the HRTFs.

Figure 2

Relationship between sound field testing and testing with the HRTF-processed stimuli for the Noise Side condition. The symbols labeled AH represent data from normal hearing individuals, who used the AH HRTF; all other symbols represent data from cochlear implant users. Each data point represents one participant and condition. Because the CI users (Freedom, Tempo+, and Opus 2 HRTFs) were tested with both Noise Left and Noise Right conditions, there are two data points for each of those participants, one for each condition. The acoustic hearing individuals were only tested in the Noise Right condition. The dashed line indicates equivalence between the two testing modalities.

Figure 3

Test-retest scores for Noise Front (NF) and Noise Side (NS) testing. Each data point represents one participant. The dotted line indicates test-retest equivalence. The parallel dashed lines indicate the measurement error.

Figure 4

Percent intelligibility as a function of signal-to-noise ratio along with the regression line indicating the relationship between the two. Each data point indicates the trimmed mean and winsorized standard error.

Figure 5

Differences in performance on the Noise Front and Noise Right conditions for acoustic hearing individuals listening to stimuli processed by the various HRTFs. Bars represent trimmed means with winsorized standard errors.

Figure 6

Binaural summation for acoustic hearing individuals listening to stimuli processed by the various HRTFs. Bars represent trimmed means with winsorized standard errors. The arrow indicates that negative scores represent binaural summation, with a greater magnitude reflecting increased binaural summation.

Figure 7

Spatial release from masking for acoustic hearing individuals listening to stimuli processed by the various HRTFs, including modified HRTFs with either ITD or ILD cues preserved. Bars represent trimmed means with winsorized standard errors. Asterisks indicate a significant difference between performance using the ITD and ILD HRTFs. The arrow indicates that negative scores represent spatial release from masking, with a greater magnitude reflecting increased spatial release from masking.

Figure 8

Binaural squelch for acoustic hearing individuals listening to stimuli processed by the various HRTFs, including modified HRTFs with either ITD or ILD cues preserved. Bars represent trimmed means with winsorized standard errors. Asterisks indicate a significant difference between performance using the ITD and ILD HRTFs. The arrow indicates that negative scores represent binaural squelch, with a greater magnitude reflecting increased binaural squelch.

Figure 9

Histograms of three types of distributions: A normal distribution with a mean of 0 and a S.D. of 2 (A); The same normal distribution with 10% of the data replaced by data sampled from a normal distribution with a mean of 0 and a S.D. of 10, resulting in a heavy-tailed distribution (B); The original normal distribution with 10% of the data replaced with a log normal distribution, resulting in a skewed distribution (C).