The sensitivity of normal hearing (NH) listeners to interaural time differences (ITD) in the envelope of high-frequency carriers is limited with respect to the envelope modulation rate. Increasing the envelope rate reduces the sensitivity, an effect that has been termed binaural adaptation (Hafter and Dye, 1983). In another study (Laback and Majdak, 2008), it was hypothesized that introducing binaural jitter may improve ITD sensitivity in bilateral cochlear implant (CI) listeners by avoiding periodicity. Indeed, the results showed large improvements at high rates (≥ 800 pps). This was interpreted as an indication for a recovery from binaural adaptation.
In this study, we further investigated this effect using NH subjects. We attempted to understand the underlying mechanisms by applying a well-established model of peripheral auditory processing.
Bandpass-filtered clicks (4 kHz) with a pulse rate of 600 pps were used at a nominal pulse rate of 600 pulses per second (pps). It was found that randomly jittering the timing of the pulses significantly increases detectability of the ITD. A second experiment was performed to observe the effect of place and rate for pulse trains. It was shown that ITD sensitivity for jittered pulse trains at 1200 pps were significantly higher than periodic pulse trains at 600 pps. Therefore, with the addition of jitter, listeners were not solely benefiting from the longest interpulse intervals and instances of reduced rate. A third experiment, using a 900 pps pulse train, confirmed the improvement in ITD sensitivity. This occurred even when random amplitude modulation, a side-effect in the case of large amounts of jitter, is ruled out. A model of peripheral auditory processing up to the brain stem (Nucleus Cochlearis) has been applied to study the mechanisms underlying the improvements in ITD sensitivity. It was found that the irregular timing of the jittered pulses increases the synchrony of firing of the cochlear nucleus. These results suggest that a recovery from binaural adaptation activated by a temporal irregularity is possibly occurring at the level of the cochlear nucleus.
Together with the results of Laback and Majdak (2008) on the effect of binaural jitter in CI listeners, these results suggest that the binaural adaptation effect first observed by Hafter and Dye (1983) is related to the synchrony of neural firings across auditory nerve fibers. The nerve fibers, in turn, innervate cochlear nucleus cells. At higher rates, periodic pulse trains result in little synchrony of the response to the ongoing signal. Jittering the pulse timing increases the probability of synchronous firing across AN fibers at certain instances of time. Further studies are required to determine if other aspects of binaural adaptation can also be attributed to this explanation.
Normal hearing (NH) listener sensitivity to interaural time differences (ITD) in the envelope of high-frequency carriers is limited with respect to the envelope modulation rate. Increasing the envelope rate reduces the sensitivity, an effect that has been termed binaural adaptation (Hafter and Dye, 1983). In other studies (Laback and Majdak, 2008; Goupell et al., 2008), it has been shown that introducing binaural jitter improves ITD sensitivity at higher rates in bilateral cochlear implant (CI) listeners as well as in NH listeners. The results were interpreted in terms of a recovery from binaural adaptation. Sensorineural hearing impairment often results in reduced ITD sensitivity (e.g. Hawkins and Wightman, 1980). The present study investigates if a similar recovery from binaural adaptation, and thus an improvement in ITD sensitivity, can be achieved in hearing impaired listeners.
Bandpass-filtered clicks (4 kHz) with pulse rates of 400 and 600 pulses per second (pps) are used. Different amounts of jitter (the minimum representing the periodic condition) and different ITDs are tested. Listeners with a moderate cochlear hearing loss are selected. Additional stimuli tested are bandpass-filtered noise bands at 4 kHz and low-frequency stimuli at 500 Hz (sinusoids, SAMs, noise bands and jittered pulse trains). The levels of the stimuli are adjusted in pretests to achieve a centered auditory image at a comfortable loudness.
Data collected so far show improvements in ITD sensitivity in some individuals but not in others.
The results may lead to the design of a new hearing aid processing algorithm that attempts to improve ITD sensitivity.
The sensitivity of normal hearing listeners to interaural time differences (ITD) in the envelope of high-frequency carriers is limited with respect to the envelope modulation rate. Increasing the envelope rate reduces the sensitivity, an effect that has been termed binaural adaptation (Hafter and Dye, 1983). Cochlear implant (CI) listeners show a similar limitation in ITD sensitivity with respect to the rate of unmodulated pulse trains containing ITD. Unfortunately, such high rates are needed to appropriately sample the modulation information of the acoustic signal. This study tests the ideas that (1) similar "binaural adaptation" mechanisms are limiting the performance in both subject groups, (2) the effect is related to the periodicity of pulse trains, and (3) introducing jitter (randomness) into the pulse timing causes a recovery from binaural adaptation and thus improves ITD sensitivity at higher pulse rates.
These ideas have been studied by testing the ITD sensitivity of five CI listeners. The parameters' pulse rate, amount of jitter (where the minimum represents the periodic condition), and ITD were all varied. We showed that introducing binaurally synchronized jitter in the stimulation timing causes large improvements in ITD sensitivity at higher pulse rates (? 800 pps). Our experimental results demonstrate that a purely temporal trigger can cause recovery from binaural adaptation.
Applying binaurally jittered in stimulation strategies may improve several aspects of binaural hearing in bilateral recipients of CIs, including localization of sound sources and speech segregation in noise.
This study examined the sensitivity of four cochlear implant (CI) listeners to ITD in different portions of four-pulse sequences in lateralization discrimination. ITD was present either in all the pulses (referred to as condition "Wave"), the two middle pulses (Ongoing), the first pulse (Onset), the last pulse (Offset), or both the first and last pulse (Gating). All ITD conditions were tested at different pulse rates (100, 200, 400, and 800 pulses per second, pps). Also, five normal hearing (NH) subjects were tested. The NH subjects listened to an acoustic simulation of CI stimulation.
All CI and NH listeners were sensitive in condition "Gating" at all pulse rates for which they showed sensitivity in condition "Wave". The sensitivity in condition "Onset" increased with the pulse rate for three CI listeners as well as for all NH listeners. The performance in condition "Ongoing" varied among the subjects. One CI listener showed sensitivity up to 800 pps, two up to 400 pps, and one at 100 pps only. The group of NH listeners showed sensitivity up to 200 pps.
CI listeners' ability to detect ITD from the middle pulses of short trains indicates fine timing relevance of stimulation pulses to lateralization. This is also relevant to CI stimulation strategies.
Objective and Methods:
This project cluster includes several studies on the perception of interaural time differences (ITD) in cochlear implant (CI), hearing impaired (HI), and normal hearing (NH) listeners. Studying different groups of listeners allows for identification of the factors that are most important to ITD perception. Furthermore, the comparison between the groups allows for the development of strategies to improve ITD sensitivity in CI and HI listeners.
This project investigated the perception of interaural intensity differences among cochlear implant (CI) listeners in relation to the spectral composition and the temporal structure of the signal.
The perception thresholds (just noticeable differences, JND) of CI listeners were examined using differently structured signals. The stimuli were applied directly to the clinical signal processing units, while the parameters of the ongoing stimulation were closely monitored.
JNDs of IIDs in CI listeners ranged from 1.5 - 2.5 dB for a detection level of 80 percent. The type of stimulus seems to bear little relevance on the detection performance, with the exception of one single type of signal - a pulse train with a frequency of 20 Hz. This means that JNDs of CI listeners are only irrelevantly higher than those of normal hearing listeners. CI implantees are sensitive to IIDs, and the JNDs correlate to a difference in arrival angles ranging from 5-10 degrees. Since the JNDs are within the minimal level widths of the transfer of amplitudes by the CI system, the reduction of level width in future systems seems advisable.
Humans' ability to localize sound sources in a 3-D space was tested.
The subjects listened to noises filtered with subject-specific head-related transfer functions (HRTFs). In the first experiment with new subjects, the conditions included a type of visual environment (darkness or structured virtual world) presented via head mounted display (HMD) and pointing method (head and finger/shooter pointing).
The results show that the errors in the horizontal dimension were smaller when head pointing was used. Finger/shooter pointing showed smaller errors in the vertical dimension. Generally, the different effects of the two pointing methods was significant but small. The presence of a structured, virtual visual environment significantly improved the localization accuracy in all conditions. This supports the idea that using a visual virtual environment in acoustic tasks, like sound localization, is beneficial. In Experiment II, the subjects were trained before performing acoustic tasks for data collection. The performance improved for all subjects over time, which indicates that training is necessary to obtain stable results in localization experiments.
FWF (Austrian Science Fund): Project # P18401-B15
In this project, head-related transfer functions (HRTFs) are measured and prepared for localization tests with cochlear implant listeners. The method and apparatus used for the measurement is the same as used for the general HRTF measurement (see project HRTF-System); however, the place where sound is acquired is different. In this project, the microphones built into the behind-the-ear (BtE) processors of cochlear implantees are used. The processors are located on the pinna, and the unprocessed microphone signals are used to calculate the BtE-HRTFs for different spatial positions.
The BtE-HRTFs are then used in localization tests like Loca BtE-CI.
Previous studies show that cochlear implant (CI) listeners show sensitivity to interaural time difference (ITD) in the fine structure at comparable, or sometimes even higher pulse rates than normal hearing (NH) subjects. This study investigates whether the differences between the two subject groups are due to an effect of auditory filtering that is absent in the case of electric stimulation.
The effects of center frequency and pulse rate on the sensitivity to ongoing envelope ITD were investigated using bandpass-filtered pulse trains. Three center frequencies (4.6, 6.5, and 9.2 kHz) were tested, and the bandwidth was scaled to stimulate an approximately constant range on the basilar membrane. The pulse rate was varied from 200 to 588 pulses per second (pps).
The results show a small but significant decrease in performance with an increase in center frequency. Furthermore, performance decreases with an increase in pulse rate, yielding a rate limit of approximately 500 pps. The lack of an interaction between pulse rate and center frequency indicates that auditory filtering was not the limiting factor in ITD perception. This suggests the existence of other limiting mechanisms, such as phase locking or more central binaural processes. The comparison of the ITD rate limits in CI subjects with those in NH subjects was considered unaffected by the auditory filtering in NH listeners.
FWF (Austrian Science Fund): Project # P18401-B15
Bilateral use of current cochlear implant (CI) systems allows for the localization of sound sources in the left-right dimension. However, localization in the front-back and up-down dimensions (within the so-called sagittal planes) is restricted as a result of insufficient transmission of the relevant information.