Abstract
The upper limit of rate-based pitch perception and rate discrimination can differ substantially across cochlear implant (CI) users. One potential reason for this difference is the presence of a biological limitation on temporal encoding in the electrically-stimulated auditory pathway, which can be inherent to the electrical stimulation itself and/or to the degenerative processes associated with hearing loss. Electrophysiological measures, like the electrically-evoked frequency following response (eFFR) and auditory change complex (eACC), could potentially provide valuable insights in the temporal processing limitations at the level of the brainstem and cortex in the electrically-stimulated auditory pathway. Obtaining these neural responses, free from stimulation artifacts, is challenging, especially when the neural response is phase-locked to the stimulation rate, as is the case for the eFFR. In this study we investigated the feasibility of measuring eFFRs, free from stimulation artifacts, to stimulation rates ranging from 94 to 196 pulses per second (pps) and eACCs to pulse rate changes ranging from 36 to 108%, when stimulating in a monopolar configuration. A high-sampling rate EEG system was used to measure the electrophysiological responses in five CI users, and linear interpolation was applied to remove the stimulation artifacts from the EEG. With this approach, we were able to measure eFFRs for pulse rates up to 162 pps and eACCs to the different rate changes. Our results show that it is feasible to measure electrophysiological responses, free from stimulation artifacts, that could potentially be used as neural correlates for rate and pitch processing in CI users.
Highlights
Most contemporary cochlear-implant (CI) speech-processing strategies apply fixed-rate pulse trains to all electrodes, with each pulse train being modulated by the sound’s envelope on a restricted frequency region (Wouters et al, 2015)
This is the first study, to the authors’ knowledge, that demonstrates the feasibility of measuring evoked frequency following response (eFFR) to pulse trains and evoked ACC (eACC) to rate changes in adult CI users
The artifact component at the modulation frequency is lower in magnitude than at the carrier rate (Gransier et al, 2020b), with the result that shorter blanking lengths are needed to obtain artifact free evoked auditory steadystate response (eASSR) compared to artifact free eFFRs
Summary
Most contemporary cochlear-implant (CI) speech-processing strategies apply fixed-rate pulse trains to all electrodes, with each pulse train being modulated by the sound’s envelope on a restricted frequency region (Wouters et al, 2015). ACCs can be evoked by frequency changes in normal-hearing humans (Dimitrijevic et al, 2008) and animals (Presacco and Middlebrooks, 2018), and by differences in stimulation electrodes in CI users (Brown et al, 2008; He et al, 2014; Mathew et al, 2018; Mathew et al, 2017; Richardson et al, 2020), Obtaining electrophysiological responses in CI users, noninvasively by means of EEG, is extremely challenging due to the electrical stimulation artifacts that corrupt the EEG recording This is especially the case when the neural response is phase-locked to the temporal structure of the stimulus, as occurs for the electrically-evoked FFR (eFFR). In order to use electrophysiological measures to assess the neural mechanism of rate processing and rate-based pitch processing in CI users, it is essential to assess if these responses can be obtained free from stimulation artifacts
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