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Effect of difference in modulation depth between the ears on interaural time difference (ITD) thresholds Srividya G B (2nd year MASTERS student)*, Arivudai Nambi (Associate Professor)** Email –id : *firstname.lastname@example.org , **email@example.com Introduction and Aim Ability to locate the sound source has an important role in spatial release from masking which in turn results in better speech understanding in noise(Shamma&Micheyl, 2010). Inter-aural time difference (ITD) is one of the essential cues in localizing the sound source and the ITD information is effectively carried by the temporal envelope of the speech signal (Bernstein &Trahiotis, 2009).In real life situations, the temporal envelope of speech signal reaching both ears undergo different levels of degradation due to the presence of noise and reverberation. Effect of this difference in the envelope between the ears on ITD thresholds is unknown. Hence, In the current study, ITD threshold for amplitude modulated signal was estimated as the function of difference in temporal envelope depth between the ears. Method 10 young adults within the age range of 20-30 years participated in the perceptual experiment. The participants had hearing sensitivity within normal limits in both ears. ITD threshold for amplitude modulated high pass filtered noise (>2000Hz) was estimated as the function of difference in temporal envelope depth. 16Hz sinusoidal tone was used to amplitude modulate the noise. In one ear, the modulation depth was fixed at 90% and in the other ear the modulation depth was decreased in the step size 10% up to 50%. ITD threshold was estimated as the function of decrease in modulation depth in one ear. Two-down one-up adaptive procedure along with 3AFC task was used to estimate the ITD thresholds. Results One-way ANOVA with repeated measures were performed to investigate that main effect of the difference in modulation depth between the ears on ITD thresholds.Analysis revealed a significant main effect (F (1,9)= 371.547, p<0.001). Pair-wise comparisons with Bonferroni’s adjustments revealed that ITD thresholds were significantly deteriorated when the modulation depths were decreased to 80% (p<0.001), 70% (p<0.001), 60% (p<0.001) and 50% (p<0.001). However, no significant difference in ITD threshold was found between 60% and 50% modulation depth condition (p>0.05). This result may suggest that, ITD threshold deteriorates with decrease in modulation depth up to 60% and then remains plateau. Conclusion Results of the current study highlight the importance of inter-aural envelope coherence in ITD perception. Hence, technological advancement in future should preserve inter-aural envelope coherence for better sound localization.
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