Effect of inter-aural modulation depth difference on interaural time difference thresholds for speech: An observational study

Introduction

The ability to locate the sound source has an important role in spatial release from masking, which results in better speech understanding in noise. When target and masker signals overlap in time but are spatially dispersed, the auditory system utilizes the spatial cues to segregate the target from the masker¹. Spatial acuity in the horizontal plane is essential for speech perception in noise². Inter-aural intensity differences (IID) and inter-aural time differences (ITD) due to the head shadow effect help determine the direction of the sound source in the horizontal plane. The coding of ITD and IID cues in the auditory system relies on the signal’s temporal envelope (ENV) and temporal fine structure (TFS). ENV refers to the amplitude fluctuations, and TFS refers to the fast frequency variations in the signal. ITD coding for low frequencies relies both on the TFS and ENV. However, at higher frequencies, TFS contributes mainly to IID coding, and ENV contributes predominantly to ITD coding³. Hence, ITD for a speech signal is effectively conveyed through ENV, as well as the TFS, in the low frequency bands. Nevertheless, in the high frequency bands, the ITD is predominantly conveyed by the ENV.

ITD cues are more important than IID in perceiving a signal against background noise⁴. In individuals with auditory disorders, such as cochlear hearing loss⁵ and auditory neuropathy⁶, the perception of TFS is affected to a greater extent than the ENV, and hence they have to rely on ITD cues present in the ENV for better localization and understanding. Even with technologically advanced hearing aids and cochlear implants, the hearing impaired still face difficulties in localization and speech understanding in noise⁷. Band-pass filtering and compression in hearing aids and cochlear implants reduce the ENV modulation depth and onset gradient, both of which are necessary for the perception of ITD cues⁸. ENV fluctuations will be reduced to a different degree in both the ears depending on the source location and compression setting. Other environmental factors, such as background noise and reverberation, also distort the modulation depth in both ears⁹. However, the effect of such a difference in ENV depth between the ears on ITD thresholds is unknown. Therefore, the aim of this study was to investigate the effect of inter-aural ENV depth difference on ITD thresholds for speech stimuli.

Methods

The study was approved (Ref No: IEC KMC MLR 12-18/506) by the Institutional Ethics Committee, Kasturba Medical College, Mangalore, India. The observational study design was used to assess the ITD thresholds of participants as a function of inter-aural ENV depth difference. All the measurements were carried out in a single session. The study was conducted at the Department of Audiology & SLP, Kasturba Medical College, Mangalore, India between 19^th December 2018 and 20^th February 2019.

Data analysis

The ITD threshold of each participant was estimated as the geometric average of last eight reversals of the transformed up-down procedure. A repeated measure of ANOVA was used to investigate the main effect of inter-aural modulation depth differences on ITD thresholds. The level of significance for this analysis was 0.05. A series of paired ‘t’ tests were performed for post hoc pairwise comparisons. The level of significance considered for these analyses was 0.05. However the ‘p’ level was adjusted using Holm’s sequential Bonferroni procedure¹⁵ for each comparison. Statistical analyses were done using EZR version 1.35 software¹⁶.

Result and discussion

Repeated measures of ANOVA revealed a significant main effect of interaural modulation depth differences on ITD threshold (F(2,10)= 9.04, p= 0.006). Post hoc pairwise comparison performed with Holm’s sequential Bonferroni correction revealed that the ITD threshold increased when the modulation depth in one ear was reduced to 71%, and this reduction was marginally significant (t= -2.37, p= 0.06). Reducing the modulation depth from 100 to 50% further increased the ITD threshold significantly (t= - 3.24, p= 0.02). ITD threshold for 50% modulation depth was significantly different from ITD threshold for 71% modulation depth (t= -5.29, p= 0.003). These results suggest that as the difference in the interaural modulation depth increases, the ITD threshold increases. The mean and standard error of the mean for each interaural modulation depth condition is represented in Figure 1.

Figure 1. Mean and standard error of the mean of ITD thresholds for each inter-aural modulation depth condition.

The current study investigated the effect of inter-aural modulation depth difference on ITD thresholds. Inter-aural modulation depth differences negatively impacted ITD thresholds. ITD thresholds increased with an increase in the inter-aural modulation depth difference. Trahiotis et al.¹⁷ indicated that the depth of amplitude modulations reaching the ears strongly influences the ITD sensitivity. The difference in the modulation depth between ears may lead to the reduction in the inter-aural ENV coherence, which would have negatively influenced the binaural processing abilities of neurons in estimating the ITD. A strong relationship between the binaural coherence and ITD cues for sound source identification has been reported in the past^8,18. Results of the current study have potential implications in auditory devices such as the cochlear implant, where the ENV cues are mainly used for the perception. One limitation of the current study is the sample size. However, the small sample size could be justified as the primary aim of the current was to show the existence of an effect rather than generalizing the effect to a larger population. However, the study needs to be repeated with a large sample size to generalize the results to a large population.

Conclusion

The temporal ENV is an essential acoustic cue for conveying sound source information, which in turn helps in source segregation. The results of this study suggest that inter-aural ENV coherence in terms of ENV depth is essential for sound source perception.

Data availability