Detection of simultaneous modulation of interaural time and level differences: Effects of modulation rate and relative phase (L)
Human and Information Science Laboratory, NTT Communication Science Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan.The Journal of the Acoustical Society of America (Impact Factor: 1.5). 07/2012; 132(1):1-4. DOI: 10.1121/1.4728199
The binaural system is known to be sluggish, i.e., unable to track modulations in interaural parameters even at a relatively slow rate. The present study evaluated the binaural system's sensitivity to modulation phase rather than to modulation magnitude. The detectability of simultaneous modulations in interaural time and level differences with various relative phases were measured. It was found that for modulation rates up to 10-20 Hz, the detectability varied with the relative phase. This indicates that information about higher rates is lost at or below the level of cue integration.
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ABSTRACT: The present study examined the dynamic properties of the across-frequency integration mechanism, specifically the extent to which the information about the direction of changes in the interaural-time difference (ITD) is integrated or compared across frequencies. The stimulus was a complex tone consisting of two sinusoidal carriers, one at 400 and the other at 700 Hz. A sinusoidal modulation in the ITD was imposed on one carrier alone or the two carriers simultaneously. The ITD of each carrier was centered at 0 μs, and the modulation started and ended with the zero phase. ITD modulations, when imposed on the two carriers simultaneously, were in-phase or anti-phase between them. Experiment 1 measured the threshold modulation depth for detecting the modulation with an adaptive method. The thresholds were generally lower when both carriers were modulated than when only one was, indicating across-frequency integration of the information about the presence of modulation. The threshold, however, was not significantly different between the in-phase and antiphase conditions, even when the modulation rate was as low as 1 Hz. Experiment 2 measured the discriminability between in-phase and anti-phase modulations. Modulation depth was fixed at a suprathreshold value (600 μs). The performance varied largely among the listeners, and it was near the chance level for half of listeners even for a 1-Hz rate. The study failed to present compelling evidence that the auditory system is sensitive to the relative phase of ITD modulations for the conditions tested. This suggests that the directional information of even slow (∼1 Hz) ITD modulation is not combined effectively across frequencies, at least for the conditions tested.
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