Effekte von binauralem Jitter auf die Wahrnehmung von interauralen Laufzeitdifferenzen bei Hörgeschädigten

Diploma Thesis (pdf. 43000 KB)

Interaural time differences (ITDs) are relative time shifts between signals arriving at the left and right ear when the source of sound is outside the median plane. ITD provides important information for the localization of sound sources.

The maximum ITD occurs when a sound source arrives completely from the side (at 90 degree azimuth), resulting in an ITD of approximately 660μs at a frequency of 1.6 kHz. The normal hearing human’s just noticeable difference (JND) for ITD in a signal under optimum conditions can be as low as 10μs (Blauert, 1997)

There are two sources of ITD information which are processed differently by the auditory system: First, ITD in the rapidly varying fine structure of a sound for frequencies below 1.5 kHz, which is the main cue for localizing sounds and understanding speech in noise (Smith, et al., 2002; Wightman and Kistler, 1992; Mcpherson and Middlebrooks, 2002). Second, ITD in the slowly varying envelope for frequencies above 1.5 kHz, which contributes to sound localization (Mcpherson and Middlebrooks, 2002; Henning, 1974)

Several studies showed that HI listeners have reduced sensitivity to ITD compared to normal hearing (NH) listeners and that ITD sensitivity varies highly between individuals (e.g. Koehnke and Besing, 1997). One study found a positive correlation between the degree of hearing loss and ITD sensitivity (Hawkins and Wightman, 1980), whereas other studies did not find such a correlation (see Appendix).

We hypothesize that the limited ITD sensitivity is related to a phenomenon called binaural adaptation. Binaural adaptation has been demonstrated using high-frequency filtered pulse trains. While at lower pulse rates, increasing the stimulus duration improves ITD sensitivity, as predicted by a model of temporal integration of ITD information, such an improvement does not occur at higher pulse rates (Hafter and Dye, 1983). Binaural adaptation has such a strong effect on ITD perception at higher pulse rates that the onset of a sound receives maximum perceptual weight whereas the ongoing signal contributes little. (Saberi, 1996; Stecker and Hafter, 2002).

Previous studies found that introducing randomness (jitter) in the stimulation timing improves ITD sensitivity in cochlear implant (CI) listeners (Laback and Majdak, 2007) as well as NH listeners (Laback et al., 2008). In order to preserve the ITD, the randomness had to be synchronized between the two ears and was referred to as binaural jitter. The jitter followed a rectangular distribution, where the parameter k defined the width of the distribution and therefore the amount of jitter. The parameter k was defined relative to the nominal interpulse interval and ranged form 0 (periodic condition, no jitter) to 1 (maximum jitter).

The current study tests the hypothesis that HI listeners also benefit from binaural jitter in the perception of ITDs. Experiments performed with an acoustic stimulation (NH listeners) are so far tested only with high-frequency filtered pulse trains. The current study will also include frequencies below 1500, assuming that the effect of binaural jitter is effective even at frequencies for which fine-structure ITD is processed by the auditory system. This assumption is reasonable since the beneficial effect of binaural jitter shown with CI listeners has been observed at tonotopic places and pulse rates falling into this low-frequency region (Laback and Majdak, 2007).

To test the hypothesis, five to ten HI listeners with a flat moderate sensorineural hearing loss will be tested. ITD sensitivity will be measured by using a left/right discrimination task, where the subject reports the lateral position of a target stimulus relative to a centered standard stimulus (Laback and Majdak, 2007). The stimulus will be a bandpass-filtered pulse train. The following independent variables will be tested:

1. Frequency region (high-frequency bandpass-filtered pulse train (AM)); low-frequency sinusoid) 2. Amount of jitter (k) 3. ITD 4. Pulse rate (in pulses per second, pps).

All experiments will be performed using the software framework “ExpSuite”, developed at the Acoustics Research Institute of the Austrian Academy of Science. Some of the experimental programs have already been used in previous experiments, while some new routines will be programmed for the current project. For example, sensorineurally HI impaired listeners often have different absolute hearing thresholds on the two ears. Therefore it is necessary to adjust the interaural level difference of the stimulus to obtain a centralized auditory image. For that purpose, a special experimental procedure will be developed and implemented into ExpSuite.