Measuring and Modeling Cue Dependent Spatial Release from Masking in the Presence of Typical Delays in the Treatment of Hearing Loss

Abstract

In asymmetric treatment of hearing loss, processing latencies of the modalities typically differ. This often alters the reference interaural time difference (ITD) (i.e., the ITD at 0° azimuth) by several milliseconds. Such changes in reference ITD have shown to influence sound source localization in bimodal listeners provided with a hearing aid (HA) in one and a cochlear implant (CI) in the contralateral ear. In this study, the effect of changes in reference ITD on speech understanding, especially spatial release from masking (SRM) in normal-hearing subjects was explored. Speech reception thresholds (SRT) were measured in ten normal-hearing subjects for reference ITDs of 0, 1.75, 3.5, 5.25 and 7 ms with spatially collocated (S0N0) and spatially separated (S0N90) sound sources. Further, the cues for separation of target and masker were manipulated to measure the effect of a reference ITD on unmasking by A) ITDs and interaural level differences (ILDs), B) ITDs only and C) ILDs only. A blind equalization-cancellation (EC) model was applied to simulate all measured conditions. SRM decreased significantly in conditions A) and B) when the reference ITD was increased: In condition A) from 8.8 dB SNR on average at 0 ms reference ITD to 4.6 dB at 7 ms, in condition B) from 5.5 dB to 1.1 dB. In condition C) no significant effect was found. These results were accurately predicted by the applied EC-model. The outcomes show that interaural processing latency differences should be considered in asymmetric treatment of hearing loss.

Clinical Feasibility and Familiarization Effects of Device Delay Mismatch Compensation in Bimodal CI/HA Users

“…This is well in line with previous studies reporting SRM in bimodal HA/CI subjects being only moderated by monaural head shadow and not by binaural unmasking based on ITD (Dieudonné & Francart, 2020;Williges et al, 2019). Further, this is well in line with a previous study investigating the effects of interaural time offsets on SRM when only ILD are present in the signal, showing no effect of a device delay mismatch (Angermeier et al, 2022). Only if sufficient ITD coding and sensitivity can be achieved in bimodal subjects in the future, a device delay mismatch might impair SRM.…”

“…Another topic that has so far only been investigated in normal-hearing listeners is speech understanding and especially the improvement in speech reception thresholds (SRTs) when the masker and the target are spatially separated compared to the situation when target and masker come from the same direction, the so-called spatial release from masking (SRM); for an overview of SRM, see Litovsky (2012) . In normal-hearing listeners, an increased device delay mismatch has been shown to have a significant influence on ITD-based SRM but not on ILD-based SRM (Angermeier et al, 2022). Other studies only investigated the influence of a device delay mismatch in hearing impaired listeners on SRTs when speech and noise were presented from the same loudspeaker in front of the subjects and could not report any effect of device delay mismatch in SSD subjects (Seebacher et al, 2019).…”

Effect of a processing delay between direct and delayed sound in simulated open fit hearing aids on speech intelligibility in noise

“…The experiments were performed in an audiometric booth using the German matrix sentence test “Oldenburger Satztest” (OLSA) ( Wagener et al, 1999 ), with the same experimental setup as used by Angermeier et al (2022) . The stimuli were presented via an external soundcard (RME Fireface 802) and circumaural closed headphones (Sennheiser HD 280 Pro), and participants entered their responses via a tablet (Samsung Tab A) displaying all possible words of the OLSA speech material as a 10 × 5 matrix.…”

“…SRM highly depends on the correct processing of ITD and ILD in the auditory system ( Litovsky, 2012 ; Glyde et al, 2013 ). Angermeier et al (2022) investigated the effect of τ on spatial release from masking (SRM) in normal-hearing subjects without direct sound reaching the eardrum(s). Five values of τ were imposed on the ITD (0, 1.75, 3.5, 5.25, and 7 ms).…”

“…It increases to −13 dB for ILD only and −12 dB for ITD only ( Bronkhorst and Plomp, 1989 ). However, the study of Angermeier et al (2022) did not investigate the effect of direct sound that reaches the eardrum in addition to the delayed sound in subjects with unilateral HL and OF. The latency offsets between 1.75 and 10 ms introduced by Angermeier et al (2022) represent the range of typical values of τ of current commercial HA ( Stone and Moore, 2003 ; Bramsløw, 2010 ; MED-EL Medical Electronics, 2023 ).…”