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Effects of binaural jitter on sensitivity to interaural time differences in hearing-impaired listeners

Authors Könsgen, A.
Year 2009
Thesis Type Diploma thesis
Topic Psychoacoustics
Abstract Interaural time differences (ITDs) provide important information for the localization of sound sources and understanding of speech in noise. For pulse trains at higher rates, the sensitivity to the ongoing envelope ITD is reduced, which is known as bin- aural adaptation. The effect of binaural adaptation is that for high pulse rates the begin of the signal receives maximum perceptual weight whereas the ongoing signal contributes little to ITD perception. Introducing binaurally-synchronized randomness of the timing of individual pulses (binaural jitter) improves ITD sensitivity of normal hearing and of cochlear implant listeners. This study tests the hypothesis that for higher pulse rates, binaural jitter improves ITD sensitivity also in hearing-impaired (HI) listeners with a moderate sensorineural hearing loss. Additionally, the effect of amplitude modulation in low-frequency signals is tested. ITD sensitivity was measured in twelve HI listeners at 4000 Hz and 500 Hz, using a left/right discrimination task. For 4000 Hz, stimuli were narrow-band noise (NBN) and bandpass-filtered click trains with and without jitter using pulse rates of 400 and 600 pulses per second. ITD sensitivity improved with increasing amount of jitter, supporting the hypothesis. ITD sensitivity for pulse trains with moderate and large amount of jitter was similar to that for NBN. For 500 Hz, stimuli were pure tones with random frequency modulation (“jittered tones“), NBN, sinusoidally amplitude modulated (SAM) tones and pure tones. Jittered tones showed no improvement in ITD sensitivity compared to pure tones. SAM tones showed slightly higher ITD sensitivity than all other stimuli. NBN showed slightly lower ITD sensitivity than all other stimuli. The results indicate that at 500 Hz the fine structure is the dominant information for ITD perception, while the envelope has little effect. Overall, HI listeners show a 2-3 times lower ITD sensitivity at both 500 Hz and 4000 Hz compared to NH listeners. Consistent with the literature, ITD sensitivity does not correlate with the degree of hearing loss. The results show a practical possibility to improve ITD sensitivity of HI listeners at high frequencies by introducing binaural jitter in hearing aids.
Supervisors Höldrich, R., Laback, B.