Psychoakustik und Experimentelle Audiologie - Abgeschlossene Projekte

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.

Subprojects:

  • FsGd: Effects of ITD in Ongoing, Onset, and Offset in Cochlear Implant Listeners
  • ITD Sync: Effects of interaural time difference in fine structure and envelope on lateral discrimination in electric hearing
  • ITD Jitter CI: Recovery from binaural adaptation with cochlear implants
  • ITD Jitter NH: Recovery from binaural adaptation in normal hearing
  • ITD Jitter HI: Recovery from binaural adaptation with sensorineural hearing impairment
  • ITD CF: Effect of center frequency and rate on the sensitivity to interaural delay in high-frequency click trains
  • IID-CI: Perception of Interaural Intensity Differences by Cochlear Implant Listeners

   

Objective:

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.

Method:

In normal hearing listeners, localization within the sagittal planes is mediated when the pinna (outer ear) evaluates the spectral coloring of incoming waveforms at higher frequencies. Current CI systems do not provide these so-called pinna cues (or spectral cues), because of behind-the-ear microphone placement and the processor's limited analysis-frequency range.

While these technical limitations are relatively manageable, some fundamental questions arise:

  • What is the minimum number of channels required to encode the pinna cues relevant to vertical plane localization?
  • To what extent can CI listeners learn to localize sound sources using pinna cues that are mapped to tonotopic regions associated with lower characteristic frequencies (according to the position of typically implanted electrodes)?
  • Which modifications of stimulation strategies are required to facilitate the localization of sound sources for CI listeners?

Application:

The improvement of sound source localization in the front-back dimension is regarded as an important aspect in daily traffic safety.

Funding:

FWF (Austrian Science Fund): Project #P18401-B15

Status:

Finished in Sept. 2010

Subprojects:

  • ElecRang: Effects of upper-frequency boundary and spectral warping on speech intelligibility in electrical stimulation
  • SpecSens: Sensitivity to spectral peaks and notches
  • Loca-BtE-CI: Localization with behind-the-ear microphones
  • Loca Methods: Pointer method for localizing sound sources
  • Loca#Channels: Number of channels required for median place localization
  • SpatStrat: Development and evaluation of a spatialization strategy for cochlear implants
  • HRTF-Sim: Numerical simulation of HRTFs

Objective:

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.

Objective:

This project investigates the effect on cochlear implant (CI) speech understanding caused by spectral peaks and notches, such as those resulting from the head-related transfer function filtering of a sound source. This is required to determine how spectral localization cues are best encoded with CIs, without destroying speech information.

Application:

Results from this project are required for the development of a 3-D localization strategy for CIs. Furthermore, the results give insight into the robustness of speech cues against spectral disruption in electric hearing.

Funding:

FWF (Austrian Science Fund): Project #P18401-B15

Objective:

Humans' ability to localize sound sources in a 3-D space was tested.

Method:

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).

Results:

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.

Funding:

FWF (Austrian Science Fund): Project # P18401-B15

Publications:

  • Majdak, P., Goupell, M., and Laback, B. (2010). 3-D localization of virtual sound sources: effects of visual environment, pointing method, and training, Attention, Perception, & Psychophysics 72, 454-469.
  • Majdak, P., Laback, B., Goupell, M., and Mihocic M. (2008). "The Accuracy of Localizing Virtual Sound Sources: Effects of Pointing Method and Visual Environment", presented at AES convention, Amsterdam.

Objective and Methods:

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.

Results:

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.

Application:

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.

Funding:

Internal

Publications:

  • Laback, B., Majdak, P., Baumgartner, W. D. (2007). Lateralization discrimination of interaural time delays in four-pulse sequences in electric and acoustic hearing, J. Acoust. Soc. Am. 121, 2182-2191.
  • Laback, B., Majdak, P., Baumgartner., W.D. (2005). Interaural time differences in temporal fine structure, onset, and offset in bilateral electrical hearing, presented at the 28th Meeting of the Association for Research in Otolaryngology, New Orleans. 
  • Laback, B., Majdak, P., Baumgartner, W. D. (2006). Interaural Time Differences in Ongoing and Gating Signal Portions in Acoustic and Electric Hearing: Model Results, Proceedings of DAGA 2006, Braunschweig. 
  • Laback, B., Majdak, P., Baumgartner, W. D. (2005). Fine structure and gating interaural time differences in electrical and acoustical hearing: effects of stimulus duration, presented at the Conference on Implantable Auditory Prostheses (CIAP), Asilomar.
  • Laback, B., Majdak, P., Baumgartner., W.D. (2004). Sensitivity to interaural time differences in temporal fine-structure, onset, and offset in bilateral electrical hearing, presented at 5th Wullstein Symposium on Bilateral Cochlear Implants and Binaural Signal Processing, Würzburg.

Objective:

A recently developed stimulation strategy for cochlear implants attempts to encode temporal fine structure information, which is known to be important in perceiving pitch and interaural time differences (ITD). So-called "sequences" of pulses are triggered with each zero-crossing of the acoustic input waveform. It is expected that adaptation effects at the auditory nerve level limit the information flow. The goal of this project is to find optimum parameter values for this new stimulation strategy, which is intended to be applied in clinical applications.

Method:

The effects of a parameter's pulse rate within each sequence, the number of sequences per second, and the temporal shape of the sequence on ITD perception are studied systematically.

Application:

The optimum parameter values determined in the experiments are intended to be used in the clinical application of the new stimulation strategy.

Objective:

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.

Method and Results:

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.

Application:

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.

Funding:

Internal

Publications:

  • Laback, B., and Majdak, P. (2007). Binaural jitter improves interaural time-difference sensitivity of cochlear implantees at high pulse rates, Proc Natl Acad Sci USA (PNAS) 105, 2, 814-817.
  • Laback, B., and Majdak, P. (2008). Reply to van Hoesel: Binaural jitter with cochlear implants, improved interaural time-delay sensitivity, and normal hearing, letter to Proc Natl Acad Sci USA 12, 105, 32.
  • Laback, B., and Majdak, P. (2007). Binaural stimulation in neural auditory prostheses or hearing aids, provisional US und EP patent application (submitted 20.06.07).

Objective:

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. 

Method and Results:

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.

Application:

The results may lead to the design of a new hearing aid processing algorithm that attempts to improve ITD sensitivity.

Funding:

Internal

Objective:

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. 

Method and Results:

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.

Application:

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. 

Funding:

Internal

Publications:

  • Goupell, M. J., Laback, B., Majdak, P. (2009): Enhancing sensitivity to interaural time differences at high modulation rates by introducing temporal jitter, in: J. Acoust. Soc. Am. 126, 2511-2521.
  • Laback, B., and Majdak, P. (2007): Binaural jitter improves interaural time-difference sensitivity of cochlear implantees at high pulse rates, in: Proc. Natl. Acad. Sci. USA (PNAS) 105, 2, 814-817.
  • Laback, B., and Majdak, P. (2008): Reply to van Hoesel: Binaural jitter with cochlear implants, improved interaural time-delay sensitivity, and normal hearing, letter to Proc. Natl. Acad. Sci. USA 12, 105, 32.