Measuring temporal response properties of auditory nerve fibers in cochlear implant recipients.

Auditory nerve fibers' (ANFs) refractoriness and facilitation can be quantified in electrically evoked compound action potentials (ECAPs) recorded via neural response telemetry (NRT). Although facilitation has been observed in animals and human cochlear implant (CI) recipients, no study has modeled this in human CI users until now. In this study, recovery and facilitation effects at different masker and probe levels for three test electrodes (E6, E12 and E18) in 11 CI subjects were recorded. The ECAP recovery and facilitation were modeled by exponential functions and the same function used for +10 CL masker offset condition can be applied to all other masker offsets measurements. Goodness of fit was evaluated for the exponential functions. A significant effect of probe level was observed on a recovery time constant which highlights the importance of recording the recovery function at the maximum acceptable stimulus level. Facilitation time constant and amplitude showed no dependency on the probe level. However, facilitation was stronger for masker level at or around the threshold of the ECAP (T-ECAP). There was a positive correlation between facilitation magnitude and amplitude growth function (AGF) slope, which indicates that CI subjects with better peripheral neural survival have stronger facilitation.

Speech perception in tinnitus is related to individual distress level - A neurophysiological study.

Individuals suffering from tinnitus often complain about difficulties understanding speech in noisy environments even in the absence of a peripheral hearing loss. This EEG study aimed to investigate whether aspects of phonetic perception are affected by the experience of tinnitus. We examined a sample of individuals with chronic, subjective tinnitus (n = 30, age range 30-50 yrs.), who underwent behavioural screening (standard tinnitus questionnaires) and comprehensive audiometric testing that covered peripheral and central hearing abilities (pure tone audiometry, suprathreshold audiometry (frequency selectivity and temporal compression), and speech in noise performance). In addition, participants performed a phoneme discrimination task embedded in an active oddball paradigm, while auditory evoked potentials (AEPs) were recorded. In particular, we aimed to investigate if reported speech difficulties in chronic tinnitus trace back to deficits in more elementary speech processes such as phonetic processing. Furthermore, we explored whether central hearing loss and tinnitus' psychometric profile may account for deficiencies in speech perception. The analysis of behavioural and audiometric data showed indications of mild to moderate symptoms of tinnitus distress when peripheral hearing loss was not in evidence. Nevertheless, tinnitus distress was negatively related to speech in noise performance which may be indicative of a lack of inhibitory competence. We further observed an effect of tinnitus distress on phoneme discrimination. More precisely, higher tinnitus distress was associated with higher accuracy and longer reaction times, while the effect on reaction times was mediated by the individual N2 ERP peak amplitudes. Our results suggest that tinnitus clearly interacts with the central auditory system in that responsiveness to salient input changes as a function of tinnitus-related distress, irrespective of peripheral hearing loss. Akin to individuals with higher psychological stress, persons with increased tinnitus-related distress demonstrate higher sensitivity during auditory processing. Taken together, we interpret our findings in light of a Bayesian approach (Sedley et al., 2016). According to this view, tinnitus distress-related factors (e.g., attention, stress) influence the excitability of the central auditory system and this, in turn, affects the sensory precision of inflowing auditory input, including spoken language.

Neuroanatomical and resting state EEG power correlates of central hearing loss in older adults.

To gain more insight into central hearing loss, we investigated the relationship between cortical thickness and surface area, speech-relevant resting state EEG power, and above-threshold auditory measures in older adults and younger controls. Twenty-three older adults and 13 younger controls were tested with an adaptive auditory test battery to measure not only traditional pure-tone thresholds, but also above individual thresholds of temporal and spectral processing. The participants' speech recognition in noise (SiN) was evaluated, and a T1-weighted MRI image obtained for each participant. We then determined the cortical thickness (CT) and mean cortical surface area (CSA) of auditory and higher speech-relevant regions of interest (ROIs) with FreeSurfer. Further, we obtained resting state EEG from all participants as well as data on the intrinsic theta and gamma power lateralization, the latter in accordance with predictions of the Asymmetric Sampling in Time hypothesis regarding speech processing (Poeppel, Speech Commun 41:245-255, 2003). Methodological steps involved the calculation of age-related differences in behavior, anatomy and EEG power lateralization, followed by multiple regressions with anatomical ROIs as predictors for auditory performance. We then determined anatomical regressors for theta and gamma lateralization, and further constructed all regressions to investigate age as a moderator variable. Behavioral results indicated that older adults performed worse in temporal and spectral auditory tasks, and in SiN, despite having normal peripheral hearing as signaled by the audiogram. These behavioral age-related distinctions were accompanied by lower CT in all ROIs, while CSA was not different between the two age groups. Age modulated the regressions specifically in right auditory areas, where a thicker cortex was associated with better auditory performance in older adults. Moreover, a thicker right supratemporal sulcus predicted more rightward theta lateralization, indicating the functional relevance of the right auditory areas in older adults. The question how age-related cortical thinning and intrinsic EEG architecture relates to central hearing loss has so far not been addressed. Here, we provide the first neuroanatomical and neurofunctional evidence that cortical thinning and lateralization of speech-relevant frequency band power relates to the extent of age-related central hearing loss in older adults. The results are discussed within the current frameworks of speech processing and aging.

Investigating the use of a Gammatone filterbank for a cochlear implant coding strategy.

BACKGROUND: Contemporary speech processing strategies in cochlear implants (CIs) such as the Advanced Combination Encoder (ACE) use a standard Fast Fourier Transform (FFT) filterbank to extract envelopes. The assignment of the FFT bins to approximate the frequency resolution of the basilar membrane is only partly based on physiology, especially since the bins are distributed linearly below 1000Hz and logarithmically above 1000Hz. NEW METHOD: A Gammatone filterbank which provides a closer approximation to the bandwidths of filters in the human auditory system could replace the standard FFT filterbank in the ACE strategy. An infinite impulse response (IIR) all-pole design of the Gammatone filterbank was compared to the FFT filterbank with 128, 256 and 512 points resolutions and the effect of the frequency boundaries of the filters was also investigated. RESULTS: Melodic contour identification (MCI) and just noticeable difference (JND) experiments, both involving synthetic clarinet notes in octaves 3 and 4, were conducted with 6 normal hearing (NH) participants using noise vocoded stimuli; and 10 CI recipients just performed the MCI experiment. The MCI results for both NH and CI subjects, showed a significant effect of the filterbank on the percentage correct responses of the participants. COMPARISON WITH EXISTING METHODS: The Gammatone filterbank can better resolve the harmonics of tested synthetic clarinet notes which led to better performances in the MCI experiment. CONCLUSIONS: The total delay of the Gammatone filterbank can be made smaller than the delay of the FFT filterbank with the same frequency resolution at low frequencies.

A neural-based vocoder implementation for evaluating cochlear implant coding strategies.

Most simulations of cochlear implant (CI) coding strategies rely on standard vocoders that are based on purely signal processing techniques. However, these models neither account for various biophysical phenomena, such as neural stochasticity and refractoriness, nor for effects of electrical stimulation, such as spectral smearing as a function of stimulus intensity. In this paper, a neural model that accounts for stochastic firing, parasitic spread of excitation across neuron populations, and neuronal refractoriness, was developed and augmented as a preprocessing stage for a standard 22-channel noise-band vocoder. This model was used to subjectively and objectively assess consonant discrimination in commercial and experimental coding strategies. Stimuli consisting of consonant-vowel (CV) and vowel-consonant-vowel (VCV) tokens were processed by either the Advanced Combination Encoder (ACE) or the Excitability Controlled Coding (ECC) strategies, and later resynthesized to audio using the aforementioned vocoder model. Baseline performance was measured using unprocessed versions of the speech tokens. Behavioural responses were collected from seven normal hearing (NH) volunteers, while EEG data were recorded from five NH participants. Psychophysical results indicate that while there may be a difference in consonant perception between the two tested coding strategies, mismatch negativity (MMN) waveforms do not show any marked trends in CV or VCV contrast discrimination.