Independent component analysis for cochlear implant artifacts attenuation from electrically evoked auditory steady-state response measurements

OBJECTIVE

Electrically evoked auditory steady-state responses (EASSRs) are potentially useful for objective cochlear implant (CI) fitting and follow-up of the auditory maturation in infants and children with a CI. EASSRs are recorded in the electro-encephalogram (EEG) in response to electrical stimulation with continuous pulse trains, and are distorted by significant CI artifacts related to this electrical stimulation. The aim of this study is to evaluate a CI artifacts attenuation method based on independent component analysis (ICA) for three EASSR datasets.

APPROACH

ICA has often been used to remove CI artifacts from the EEG to record transient auditory responses, such as cortical evoked auditory potentials. Independent components (ICs) corresponding to CI artifacts are then often manually identified. In this study, an ICA based CI artifacts attenuation method was developed and evaluated for EASSR measurements with varying CI artifacts and EASSR characteristics. Artifactual ICs were automatically identified based on their spectrum.

MAIN RESULTS

For 40 Hz amplitude modulation (AM) stimulation at comfort level, in high SNR recordings, ICA succeeded in removing CI artifacts from all recording channels, without distorting the EASSR. For lower SNR recordings, with 40 Hz AM stimulation at lower levels, or 90 Hz AM stimulation, ICA either distorted the EASSR or could not remove all CI artifacts in most subjects, except for two of the seven subjects tested with low level 40 Hz AM stimulation. Noise levels were reduced after ICA was applied, and up to 29 ICs were rejected, suggesting poor ICA separation quality.

SIGNIFICANCE

We hypothesize that ICA is capable of separating CI artifacts and EASSR in case the contralateral hemisphere is EASSR dominated. For small EASSRs or large CI artifact amplitudes, ICA separation quality is insufficient to ensure complete CI artifacts attenuation without EASSR distortion.

Development and validation of a method to record electrophysiological responses in direct acoustic cochlear implant subjects

Acoustic hearing implants, such as direct acoustic cochlear implants (DACIs), can be used to treat profound mixed hearing loss. Electrophysiological responses in DACI subjects are of interest to confirm auditory processing intra-operatively, and to assist DACI fitting postoperatively. We present two related studies, focusing on DACI artifacts and electrophysiological measurements in DACI subjects, respectively. In the first study we aimed to characterize DACI artifacts, to study the feasibility of measuring frequency-specific electrophysiological responses in DACI subjects. Measurements of DACI artifacts were collected in a cadaveric head to disentangle possible DACI artifact sources and compared to a constructed DACI artifact template. It is shown that for moderate stimulation levels, DACI artifacts are mainly dominated by the artifact from the radio frequency (RF) communication signal, that can be modeled if the RF encoding protocol is known. In a second study, the feasibility of measuring intra-operative responses, without applying the RF artifact models, in DACI subjects is investigated. Auditory steady-state and brainstem responses were measured intra-operatively in three DACI subjects, immediately after implantation, to confirm proper DACI functioning and coupling to the inner ear. Intra-operative responses could be measured in two of the three tested subjects. Absence of intra-operative responses in the third subject can possibly be explained by the hearing loss, attenuation of intra-operative responses, the difference between electrophysiological and behavioral threshold, and a temporary threshold shift due to the DACI surgery. In conclusion, RF artifacts can be modeled, such that electrophysiological responses to frequency-specific stimuli could possibly be measured in DACI subjects, and intra-operative responses in DACI subjects can be obtained.

Template Subtraction to Remove CI Stimulation Artifacts in Auditory Steady-State Responses in CI Subjects

Cochlear implant (CI) stimulation artifacts are currently removed from electrically evoked steady-state response (EASSR) measurements based on a linear interpolation (LI) over the artifact-contaminated signal parts. LI is only successful if CI stimulation artifacts are shorter than the interpulse interval, i.e., for contralateral channels and stimulation pulse rates up to 500 pulses per second (pps). The objective of this paper is to develop and evaluate a template subtraction (TS) method to remove continuous CI stimulation artifacts in order to accurately measure EASSRs. The template construction (TC) is based on an EEG recording containing CI stimulation artifacts but no synchronous neural response. The constructed templates are subtracted from the recording of interest. Response amplitudes and latencies are compared for the TS and LI method, and for different TC durations. The response amplitudes and latencies in contralateral channels are the same after TS and LI, as expected. In ipsilateral channels, response amplitudes and latencies are within the expected range only after TS. The TC duration can be reduced from 5 min to 1 min without a significant effect on response latency. TS with a TC duration of only 1 min allows to remove all CI stimulation artifacts in individual contra- and ipsilateral EEG recording channels.

Auditory steady-state responses in cochlear implant users: Effect of modulation frequency and stimulation artifacts

Previous studies have shown that objective measures based on stimulation with low-rate pulse trains fail to predict the threshold levels of cochlear implant (CI) users for high-rate pulse trains, as used in clinical devices. Electrically evoked auditory steady-state responses (EASSRs) can be elicited by modulated high-rate pulse trains, and can potentially be used to objectively determine threshold levels of CI users. The responsiveness of the auditory pathway of profoundly hearing-impaired CI users to modulation frequencies is, however, not known. In the present study we investigated the responsiveness of the auditory pathway of CI users to a monopolar 500 pulses per second (pps) pulse train modulated between 1 and 100 Hz. EASSRs to forty-three modulation frequencies, elicited at the subject's maximum comfort level, were recorded by means of electroencephalography. Stimulation artifacts were removed by a linear interpolation between a pre- and post-stimulus sample (i.e., blanking). The phase delay across modulation frequencies was used to differentiate between the neural response and a possible residual stimulation artifact after blanking. Stimulation artifacts were longer than the inter-pulse interval of the 500pps pulse train for recording electrodes ipsilateral to the CI. As a result the stimulation artifacts could not be removed by artifact removal on the bases of linear interpolation for recording electrodes ipsilateral to the CI. However, artifact-free responses could be obtained in all subjects from recording electrodes contralateral to the CI, when subject specific reference electrodes (Cz or Fpz) were used. EASSRs to modulation frequencies within the 30-50 Hz range resulted in significant responses in all subjects. Only a small number of significant responses could be obtained, during a measurement period of 5 min, that originate from the brain stem (i.e., modulation frequencies in the 80-100 Hz range). This reduced synchronized activity of brain stem responses in long-term severely-hearing impaired CI users could be an attribute of processes associated with long-term hearing impairment and/or electrical stimulation.