Advantages of Pulse Rate Compared to Modulation Frequency for Temporal Pitch Perception in Cochlear Implant Users.

Most cochlear implants encode the fundamental frequency of periodic sounds by amplitude modulation of constant-rate pulsatile stimulation. Pitch perception provided by such stimulation strategies is markedly poor. Two experiments are reported here that consider potential advantages of pulse rate compared to modulation frequency for providing stimulation timing cues for pitch. The first experiment examines beat frequency distortion that occurs when modulating constant-rate pulsatile stimulation. This distortion has been reported on previously, but the results presented here indicate that distortion occurs for higher stimulation rates than previously reported. The second experiment examines pitch resolution as provided by pulse rate compared to modulation frequency. The results indicate that pitch discrimination is better with pulse rate than with modulation frequency. The advantage was large for rates near what has been suggested as the upper limit of temporal pitch perception conveyed by cochlear implants. The results are relevant to sound processing design for cochlear implants particularly for algorithms that encode fundamental frequency into deep envelope modulations or into precisely timed pulsatile stimulation.

Pitch perception is more robust to interference and better resolved when provided by pulse rate than by modulation frequency of cochlear implant stimulation.

Cochlear implants are medical devices that have been used to restore hearing to more than half a million people worldwide. Most recipients achieve high levels of speech comprehension through these devices, but speech comprehension in background noise and music appreciation in general are markedly poor compared to normal hearing. A key aspect of hearing that is notably diminished in cochlear implant outcomes is the sense of pitch provided by these devices. Pitch perception is an important factor affecting speech comprehension in background noise and is critical for music perception. The present article summarizes two experiments that examine the robustness and resolution of pitch perception as provided by cochlear implant stimulation timing. The driving hypothesis is that pitch conveyed by stimulation timing cues is more robust and better resolved when provided by variable pulse rates than by modulation frequency of constant-rate stimulation. Experiment 1 examines the robustness for hearing a large, one-octave, pitch difference in the presence of interfering electrical stimulation. With robustness to interference characterized for an otherwise easily discernible pitch difference, Experiment 2 examines the resolution of discrimination thresholds in the presence of interference as conveyed by modulation frequency or by pulse rate. These experiments test for an advantage of stimulation with precise temporal cues. The results indicate that pitch provided by pulse rate is both more robust to interference and is better resolved compared to when provided by modulation frequency. These results should inform the development of new sound processing strategies for cochlear implants designed to encode fundamental frequency of sounds into precise temporal stimulation.

Two-microphone spatial filtering improves speech reception for cochlear-implant users in reverberant conditions with multiple noise sources.

This study evaluates a spatial-filtering algorithm as a method to improve speech reception for cochlear-implant (CI) users in reverberant environments with multiple noise sources. The algorithm was designed to filter sounds using phase differences between two microphones situated 1 cm apart in a behind-the-ear hearing-aid capsule. Speech reception thresholds (SRTs) were measured using a Coordinate Response Measure for six CI users in 27 listening conditions including each combination of reverberation level (T60=0, 270, and 540 ms), number of noise sources (1, 4, and 11), and signal-processing algorithm (omnidirectional response, dipole-directional response, and spatial-filtering algorithm). Noise sources were time-reversed speech segments randomly drawn from the Institute of Electrical and Electronics Engineers sentence recordings. Target speech and noise sources were processed using a room simulation method allowing precise control over reverberation times and sound-source locations. The spatial-filtering algorithm was found to provide improvements in SRTs on the order of 6.5 to 11.0 dB across listening conditions compared with the omnidirectional response. This result indicates that such phase-based spatial filtering can improve speech reception for CI users even in highly reverberant conditions with multiple noise sources.

Two-microphone spatial filtering provides speech reception benefits for cochlear implant users in difficult acoustic environments.

This article introduces and provides an assessment of a spatial-filtering algorithm based on two closely-spaced (∼1 cm) microphones in a behind-the-ear shell. The evaluated spatial-filtering algorithm used fast (∼10 ms) temporal-spectral analysis to determine the location of incoming sounds and to enhance sounds arriving from straight ahead of the listener. Speech reception thresholds (SRTs) were measured for eight cochlear implant (CI) users using consonant and vowel materials under three processing conditions: An omni-directional response, a dipole-directional response, and the spatial-filtering algorithm. The background noise condition used three simultaneous time-reversed speech signals as interferers located at 90°, 180°, and 270°. Results indicated that the spatial-filtering algorithm can provide speech reception benefits of 5.8 to 10.7 dB SRT compared to an omni-directional response in a reverberant room with multiple noise sources. Given the observed SRT benefits, coupled with an efficient design, the proposed algorithm is promising as a CI noise-reduction solution.

Training improves cochlear implant rate discrimination on a psychophysical task.

The purpose of this study was to determine the extent to which cochlear implant (CI) rate discrimination can be improved through training. Six adult CI users took part in a study that included 32 h of training and assessment on rate discrimination measures. Rate difference limens (DLs) were measured from 110 to 3520 Hz in octave steps using 500 ms biphasic pulse trains; the target and standard stimuli were loudness-balanced with the target always at an adaptively lower rate. DLs were measured at four electrode positions corresponding to basal, mid-basal, mid-apical, and apical locations. Procedural variations were implemented to determine if rate discrimination was impacted by random variations in stimulus amplitude or by amplitude modulation. DLs improved by more than a factor of 2 across subjects, electrodes, and standard rates. Factor analysis indicated that the effect of training was comparable for all electrodes and standard rates tested. Neither level roving nor amplitude modulation had a significant effect on rate DLs. In conclusion, the results demonstrate that training can significantly improve CI rate discrimination on a psychophysical task.