Cochlear Implant Artifact Suppression in EEG Measurements
Cochlear implants (CIs) aim to restore hearing in severely to profoundly deaf adults, children and infants. Electrically evoked auditory steady-state responses (EASSRs) are neural responses to continuous modulated pulse trains, and can be objectively detected at the modulation frequency in the electro-encephalogram (EEG). EASSRs provide a number of advantages over other objective measures, because frequency-specific stimuli are used, because targeted brain areas can be studied, depending on the chosen stimulation parameters, and because they can objectively be detected using statistical methods. EASSRs can potentially be used to determine appropriate stimulation levels during CI fitting, without behavioral input from the subjects. Furthermore, speech understanding in noise varies greatly between CI subjects. EASSRs lend themselves well to study the underlying causes of this variability, such as the integrity of the electrode-neuron interface or changes in the auditory cortex following deafness and following cochlear implantation.
EASSRs are distorted by electrical artifacts, caused by the CI's radiofrequency link and by the electrical pulses used to stimulate the auditory nerve. CI artifacts may also be present at the modulation frequency, leading to inaccurate EASSR detection and unreliable EASSR amplitude and phase estimations. CI artifacts that are shorter than the interpulse interval (IPI), i.e., the inverse of the pulse rate (in pulses per second (pps)), can be removed with a linear interpolation (LI) over the EEG samples affected by CI artifacts. For clinically used monopolar (MP) mode stimulation, i.e., between an intracochlear and an extracochlear electrode, CI artifacts are longer than for bipolar (BP) mode stimulation, i.e., between two intracochlear electrodes.
In this thesis, CI artifacts are characterized based on the CI artifact duration and based on the CI artifact amplitude growth function (AGF). Furthermore, three methods for CI artifact suppression to enable reliable estimation of EASSR parameters are developed and evaluated.
The CI artifacts are larger and longer in recording channels closer to the implant.
Appropriate reference electrode selection may lead to smaller and shorter CI artifacts, that are more easily suppressed. Using LI, CI artifacts may be suppressed in contralateral recording channels for 500 pps stimulation for our recording set-up. More advanced CI artifact suppression methods are needed to measure EASSRs in ipsilateral channels (for source localization or lateralization studies) and in infants and children.
First, a CI artifact suppression method based on independent component analysis (ICA) is developed.
Independent components (ICs) associated with CI artifact are automatically identified and rejected based on the component at the pulse rate.
In some cases, CI artifacts are successfully removed, although mixed results are obtained in other cases.
Because the ICA method is not fully robust, and since multichannel recordings are needed, a second method, based on template subtraction (TS), is developed. With TS, for each stimulation pulse amplitude, the CI artifact pulse templates are constructed based on a recording containing no significant EASSR. The templates are then put in the correct order and subtracted from the recording of interest. With TS, reliable EASSR amplitudes, phases and latencies are obtained for a high signal-to-noise ratio (SNR) dataset. The template construction recording duration can be reduced to 60 s, while reliable EASSR parameter estimations are still obtained.
Because the previous method requires additional data collection, a third method for EASSR parameter estimation in the presence of CI artifacts is developed. The method is based on a Kalman filter (KF), as proposed in (Luke, 2016). The CI artifact model presented in (Luke, 2016) consists of constant triangular pulses presented at the stimulation pulse rate, and proved to work well for CI artifacts in contralateral recording channels for BP mode stimulation. In more general cases, i.e., with MP mode stimulation and in ipsilateral channels, CI artifacts are modulated and have an exponentially decaying tail. An extended state-space model is developed that contains additional components modeling these CI artifact features. With the new KF method, reliable EASSR amplitudes, phases and latencies are again obtained for a high signal-to-noise ratio (SNR) dataset, without the need for additional data collection.
The insights provided in this thesis and the developed CI artifact suppression methods may assist researchers and clinicians to record EASSRs in the presence of CI artifacts for clinical stimulation parameters. These responses may then be used to improve CI rehabilitation or CI stimulation strategies, leading to a better quality-of-life for all patients with a CI.