Electrically evoked auditory steady state responses in cochlear implant users

Theta, beta and gamma rate modulations in the developing auditory system

In the brain, the temporal analysis of many important auditory features relies on the synchronized firing of neurons to the auditory input rhythm. These so-called neural oscillations play a crucial role in sensory and cognitive processing and deviances in oscillatory activity have shown to be associated with neurodevelopmental disorders. Given the importance of neural auditory oscillations in normal and impaired sensory and cognitive functioning, there has been growing interest in their developmental trajectory from early childhood on. In the present study, neural auditory processing was investigated in typically developing young children (n = 40) and adults (n = 27). In all participants, auditory evoked theta, beta and gamma responses were recorded. The results of this study show maturational differences between children and adults in neural auditory processing at cortical as well as at brainstem level. Neural background noise at cortical level was shown to be higher in children compared to adults. In addition, higher theta response amplitudes were measured in children compared to adults. For beta and gamma rate modulations, different processing asymmetry patterns were observed between both age groups. The mean response phase was also shown to differ significantly between children and adults for all rates. Results suggest that cortical auditory processing of beta develops from a general processing pattern into a more specialized asymmetric processing preference over age. Moreover, the results indicate an enhancement of bilateral representation of monaural sound input at brainstem with age. A dissimilar efficiency of auditory signal transmission from brainstem to cortex along the auditory pathway between children and adults is suggested. These developmental differences might be due to both functional experience-dependent as well as anatomical changes. The findings of the present study offer important information about maturational differences between children and adults for responses to theta, beta and gamma rates. The current study can have important implications for the understanding of developmental disorders which are known to be associated with deviances in neural auditory processing.

Hemispheric asymmetry in auditory processing of speech envelope modulations in prereading children

The temporal envelope of speech is an important cue contributing to speech intelligibility. Theories about the neural foundations of speech perception postulate that the left and right auditory cortices are functionally specialized in analyzing speech envelope information at different time scales: the right hemisphere is thought to be specialized in processing syllable rate modulations, whereas a bilateral or left hemispheric specialization is assumed for phoneme rate modulations. Recently, it has been found that this functional hemispheric asymmetry is different in individuals with language-related disorders such as dyslexia. Most studies were, however, performed in adults and school-aged children, and only a little is known about how neural auditory processing at these specific rates manifests and develops in very young children before reading acquisition. Yet, studying hemispheric specialization for processing syllable and phoneme rate modulations in preliterate children may reveal early neural markers for dyslexia. In the present study, human cortical evoked potentials to syllable and phoneme rate modulations were measured in 5-year-old children at high and low hereditary risk for dyslexia. The results demonstrate a right hemispheric preference for processing syllable rate modulations and a symmetric pattern for phoneme rate modulations, regardless of hereditary risk for dyslexia. These results suggest that, while hemispheric specialization for processing syllable rate modulations seems to be mature in prereading children, hemispheric specialization for phoneme rate modulation processing may still be developing. These findings could have important implications for the development of phonological and reading skills.

Objective assessment of spectral ripple discrimination in cochlear implant listeners using cortical evoked responses to an oddball paradigm

Cochlear implants (CIs) can partially restore functional hearing in deaf individuals. However, multiple factors affect CI listener's speech perception, resulting in large performance differences. Non-speech based tests, such as spectral ripple discrimination, measure acoustic processing capabilities that are highly correlated with speech perception. Currently spectral ripple discrimination is measured using standard psychoacoustic methods, which require attentive listening and active response that can be difficult or even impossible in special patient populations. Here, a completely objective cortical evoked potential based method is developed and validated to assess spectral ripple discrimination in CI listeners. In 19 CI listeners, using an oddball paradigm, cortical evoked potential responses to standard and inverted spectrally rippled stimuli were measured. In the same subjects, psychoacoustic spectral ripple discrimination thresholds were also measured. A neural discrimination threshold was determined by systematically increasing the number of ripples per octave and determining the point at which there was no longer a significant difference between the evoked potential response to the standard and inverted stimuli. A correlation was found between the neural and the psychoacoustic discrimination thresholds (R² = 0.60, p<0.01). This method can objectively assess CI spectral resolution performance, providing a potential tool for the evaluation and follow-up of CI listeners who have difficulty performing psychoacoustic tests, such as pediatric or new users.

Cochlear implant artifact attenuation in late auditory evoked potentials: a single channel approach

Recent evidence suggests that late auditory evoked potentials (LAEP) provide a useful objective metric of performance in cochlear implant (CI) subjects. However, the CI produces a large electrical artifact that contaminates LAEP recordings and confounds their interpretation. Independent component analysis (ICA) has been used in combination with multi-channel recordings to effectively remove the artifact. The applicability of the ICA approach is limited when only single channel data are needed or available, as is often the case in both clinical and research settings. Here we developed a single-channel, high sample rate (125 kHz), and high bandwidth (0-100 kHz) acquisition system to reduce the CI stimulation artifact. We identified two different artifacts in the recording: 1) a high frequency artifact reflecting the stimulation pulse rate, and 2) a direct current (DC, or pedestal) artifact that showed a non-linear time varying relationship to pulse amplitude. This relationship was well described by a bivariate polynomial. The high frequency artifact was completely attenuated by a 35 Hz low-pass filter for all subjects (n = 22). The DC artifact could be caused by an impedance mismatch. For 27% of subjects tested, no DC artifact was observed when electrode impedances were balanced to within 1 kΩ. For the remaining 73% of subjects, the pulse amplitude was used to estimate and then attenuate the DC artifact. Where measurements of pulse amplitude were not available (as with standard low sample rate systems), the DC artifact could be estimated from the stimulus envelope. The present artifact removal approach allows accurate measurement of LAEPs from CI subjects from single channel recordings, increasing their feasibility and utility as an accessible objective measure of CI function.

The polarity sensitivity of the electrically stimulated human auditory nerve measured at the level of the brainstem

Recent behavioral studies have suggested that the human auditory nerve of cochlear implant (CI) users is mainly excited by the positive (anodic) polarity. Those findings were only obtained using asymmetric pseudomonophasic (PS) pulses where the effect of one phase was measured in the presence of a counteracting phase of opposite polarity, longer duration, and lower amplitude than the former phase. It was assumed that only the short high-amplitude phase was responsible for the excitation. Similarly, it has been shown that electrically evoked compound action potentials could only be obtained in response to the anodic phases of asymmetric pulses. Here, experiment 1 measured electrically evoked auditory brainstem responses to standard symmetric, PS, reversed pseudomonophasic, and reversed pseudomonophasic with inter-phase gap (6 ms) pulses presented for both polarities. Responses were time locked to the short high-amplitude phase of asymmetric pulses and were smaller, but still measurable, when that phase was cathodic than when it was anodic. This provides the first evidence that cathodic stimulation can excite the auditory system of human CI listeners and confirms that this stimulation is nevertheless less effective than for the anodic polarity. A second experiment studied the polarity sensitivity at different intensities by means of a loudness balancing task between pseudomonophasic anodic (PSA) and pseudomonophasic cathodic (PSC) stimuli. Previous studies had demonstrated greater sensitivity to anodic stimulation only for stimuli producing loud percepts. The results showed that PSC stimuli required higher amplitudes than PSA stimuli to reach the same loudness and that this held for current levels ranging from 10 to 100% of the dynamic range.

Electrically evoked amplitude modulation following response in cochlear implant candidates: comparison with auditory nerve response telemetry, subjective electrical stimulation, and speech perception

OBJECTIVE

To establish the objective, noninvasive recording of the electrically evoked amplitude modulation following response (EAMFR) for the assessment of auditory nerve function in cochlear implant candidates.

STUDY DESIGN

Prospective clinical study from 2007 to 2010.

SETTING

Cochlear Implant Programme at the Charité-Universitätsmedizin Berlin (Germany).

PATIENTS

Thirty-eight patients with severe-to-total bilateral sensorineural hearing impairment.

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURES

The EAMFR thresholds were correlated with the mean thresholds of the intraoperative electrically evoked compound action potential (ECAP) derived by auditory nerve response telemetry and with the subjective electrical stimulation test. The intraindividual left-right similarity of the EAMFR and ECAP thresholds was investigated in 9 bilaterally implanted patients. Additionally, the correlation between auditory performance and EAMFR thresholds was investigated.

RESULTS

The EAMFR auditory nerve function test produced clear responses in all patients. The EAMFR thresholds correlated significantly with the mean ECAP thresholds (r = 0.58, p < 0.01) and the subjective electrical stimulation test (r = 0.32, p < 0.05). In the bilaterally implanted patients, there was either no side difference, or the ear with the lower preoperative EAMFR threshold also was the one with the lower intraoperative mean ECAP threshold. No correlation was found between EAMFR thresholds and postoperative speech recognition scores.

CONCLUSION

These novel results support the notion that the EAMFR auditory nerve function test is a useful objective and noninvasive tool to provide information about the responsiveness of the auditory nerve to electrical stimulation in cochlear implant candidates.

Improved electrically evoked auditory steady-state response thresholds in humans

Electrically evoked auditory steady-state responses (EASSRs) are EEG potentials in response to periodic electrical stimuli presented through a cochlear implant. For low-rate pulse trains in the 40-Hz range, electrophysiological thresholds derived from response amplitude growth functions correlate well with behavioral T levels at these rates. The aims of this study were: (1) to improve the correlation between electrophysiological thresholds and behavioral T levels at 900 pps by using amplitude-modulated (AM) and pulse-width-modulated (PWM) high-rate pulse trains, (2) to develop and evaluate the performance of a new statistical method for response detection which is robust in the presence of stimulus artifacts, and (3) to assess the ability of this statistical method to determine reliable electrophysiological thresholds without any stimulus artifact removal. For six users of a Nucleus cochlear implant and a total of 12 stimulation electrode pairs, EASSRs to symmetric biphasic bipolar pulse trains were recorded with seven scalp electrodes. Responses to six different stimuli were analyzed: two low-rate pulse trains with pulse rates in the 40-Hz range as well as two AM and two PWM high-rate pulse trains with a carrier rate of 900 pps and modulation frequencies in the 40-Hz range. Responses were measured at eight different stimulus intensities for each stimulus and stimulation electrode pair. Artifacts due to the electrical stimulation were removed from the recordings. To determine the presence of a neural response, a new statistical method based on a two-sample Hotelling T (2) test was used. Measurements from different recording electrodes and adjacent stimulus intensities were combined to increase statistical power. The results show that EASSRs to modulated high-rate pulse trains account for some of the temporal effects at 900 pps and result in improved electrophysiological thresholds that correlate very well with behavioral T levels at 900 pps. The proposed statistical method for response detection based on a two-sample Hotelling T (2) test has comparable performance to previously used one-sample tests and does not require stimulus artifacts to be removed from the EEG signal for the determination of reliable electrophysiological thresholds.

Towards a closed-loop cochlear implant system: application of embedded monitoring of peripheral and central neural activity

Although the cochlear implant (CI) is widely considered the most successful neural prosthesis, it is essentially an open-loop system that requires extensive initial fitting and frequent tuning to maintain a high, but not necessarily optimal, level of performance. Two developments in neuroscience and neuroengineering now make it feasible to design a closed-loop CI. One development is the recording and interpretation of evoked potentials (EPs) from the peripheral to the central nervous system. The other is the embedded hardware and software of a modern CI that allows recording of EPs. We review EPs that are pertinent to behavioral functions from simple signal detection and loudness growth to speech discrimination and recognition. We also describe signal processing algorithms used for electric artifact reduction and cancellation, critical to the recording of electric EPs. We then present a conceptual design for a closed-loop CI that utilizes in an innovative way the embedded implant receiver and stimulators to record short latency compound action potentials ( ~1 ms), auditory brainstem responses (1-10 ms) and mid-to-late cortical potentials (20-300 ms). We compare EPs recorded using the CI to EPs obtained using standard scalp electrodes recording techniques. Future applications and capabilities are discussed in terms of the development of a new generation of closed-loop CIs and other neural prostheses.