Project number		Stand-alone Projects  P24183
Title		Binaural Hearing and the Cochlear Phase Response
Principal investigator		LABACK Bernhard
Approval date		03.10.2011
University / Research institution		Institut für Schallforschung, Österreichische Akademie der Wissenschaften
Scientific field(s)
Keywords		phase, cochlea, interaural time difference, masking, binaural , hearing
Homepage		http://www.kfs.oeaw.ac.at/

Early psychoacoustic research suggested that the human auditory system is insensitive to differences in the relative phases of spectral components of a multicomponent sound. However, research from the last two decennia provides evidence that listeners can detect phase differences between the stimulus components that interact within a single auditory filter. The most impressive demonstration of phase sensitivity is given by the masker-phase effect, i.e. the more than 20-dB variation in masking effect caused by a harmonic complex when varying the phase relations between its components. This masking paradigm is widely used to obtain a psychoacoustical measure of the phase response of the cochlea. Interaural time differences (ITD) are important information for the auditory system to determine the lateral position of a sound source. Studies on ITD perception using multicomponent stimuli set the relative phases of the components either to a constant or to random values. Because ITD perception relies on the temporal properties of the cochlear response to the signal, it can be expected that the phase relation between the components of a harmonic complex affects ITD perception, particularly at higher frequency regions where multiple components interact within an auditory filter. Therefore, the first main goal of BiPhase is to systematically study the effect of phase variations in harmonic complexes on ITD sensitivity in normal-hearing (NH) listeners. Cochlear hearing impairment is known to often cause degraded ITD sensitivity compared to NH listeners. A critical factor in case of multicomponent stimuli appears to be the physiologically- demonstrated alteration of the phase response in the impaired cochlea. Furthermore, psychoacoustic studies showed that the masker-phase effect is markedly reduced in hearing-impaired (HI) listeners. Although the origins for this reduction are still subject of research, it appears likely that phase relations that are optimal for ITD perception in normal hearing can be suboptimal in impaired hearing. Thus, the second main goal of BiPhase is to systematically study phase effects on ITD sensitivity in HI listeners and compare them to the effects found in NH listeners. The reduction or even absence of compression in impaired cochleae has been identified as an important reason for the reduction of the masker-phase effect in HI listeners. Assuming that compression is not required to observe phase effects in ITD perception (compression may actually reduce phase effects), ITD-based phase effects could be stronger than masking-based phase effects in HI listeners. Therefore, the third main goal of BiPhase is to compare phase effects measured with the ITD-based and masking-based paradigms in NH and HI listeners. BiPhase is expected to provide more insight into cochlear phase effects for multicomponent stimuli in normal and impaired hearing. The ITD-based paradigm could be a useful tool to determine the cochlear phase response in hearing impairment. The better understanding of phase effects on ITD perception may provide the basis for developing better processing schemes for hearing devices in order to restore the access to ITD cues in HI listeners. This in turn may improve their ability to localize sound sources and understand speech in noise.