Further simulations of the effect of cochlear-implant pre-processing and head movement on interaural level differences

Abstract

We simulated the effect of several automatic gain control (AGC) and AGC-like systems and head movement on the output levels, and resulting interaural level differences (ILDs) produced by bilateral cochlear-implant (CI) processors. The simulated AGC systems included unlinked AGCs with a range of parameter settings, linked AGCs, and two proprietary multi-channel systems used in contemporary CIs. The results show that over the range of values used clinically, the parameters that most strongly affect dynamic ILDs are the release time and compression ratio. Linking AGCs preserves ILDs at the expense of monaural level changes and, possibly, comfortable listening level. Multichannel AGCs can whiten output spectra, and/or distort the dynamic changes in ILD that occur during and after head movement. We propose that an unlinked compressor with a ratio of approximately 3:1 and a release time of 300-500 ms can preserve the shape of dynamic ILDs, without causing large spectral distortions or sacrificing listening comfort.

Reaching to Sounds Improves Spatial Hearing in Bilateral Cochlear Implant Users

“…AGC may distort ILDs across frequencies (which are fundamental for sound localization in CI users; see Dorman et al 2014) in a more or less aggressive way as a function of the processor (and manufacturer) type and of the settings made by the hearing care professional. In the present experiment, the lack of synchronization of AGCs between bilateral CI processors (Pastore et al 2021; Dwyer et al 2021), the different duration of sound stimuli as a function of response time (Boyle et al 2009), and the possible AGC distortions as a function of head movements (Archer-Boyd & Carlyon 2019; see also Archer-Boyd & Carlyon 2021) could have substantially distorted the available ILDs. Yet, irrespective of the modulations introduced by the CI processor our spatial training was robust enough to allow generalization of training effects from the training task (in which sound duration continued until response, 9.6 sec on average), to a test task (in which the sound lasted only 3 sec).…”

“…A key factor causing such poor performance is the reduced availability of auditory cues. CIs limit input resolution in the temporal and frequency domains (Moore & Shannon 2009), alter sound levels through automatic gain control (AGC) (see, for instance, Dorman et al 2014; Archer-Boyd & Carlyon 2019, 2021), and change the auditory cues by the effect of microphone filtering strategies (both related to noise reduction and to emphasize higher frequencies) (Seeber & Fastl 2008). In addition, the two CI processors work in isolation from one another, leading to independent acoustic streams to the brain (for discussion, see Verhaert et al 2012; Rosskothen-Kuhl et al 2021).…”

Listening to Music Through Hearing Aids: Potential Lessons for Cochlear Implants

“…Also, it is not clear whether the benefits found were a result of better preservation of ILD cues because of the synchronisation, or whether they resulted from the fact that synchronisation of AGC systems generally results in less overall compression and more slowly changing compression. Finally, it should be noted that linking of AGC systems distorts the trajectory of the changes in level at each ear as the user moves their head (Archer-Boyd & Carlyon, 2021); in other words, monaural envelope cues are distorted. Clearly, more research is needed into the potential benefits of linking the AGC systems in bilaterally fitted CIs, especially in the context of music perception.…”

“…Currently, the slowly-adapting AGC systems that are used in CIs operate independently at the two ears. The use of independent AGC at the two ears distorts interaural level difference (ILD) cues for sound localisation (Wiggins & Seeber, 2011), especially when the head is moved (Archer-Boyd & Carlyon, 2019; Archer-Boyd & Carlyon, 2021). The magnitude of the effect of head movement varies in a complex way with the attack and release time of the AGC system and with the speed and type of movement of the head of the CI user.…”