Methodological considerations when measuring and analyzing auditory steady-state responses with multi-channel EEG

Age dependent deficits in speech recognition in quiet and noise are reflected in MGB activity and cochlear onset coding

The slowing and reduction of auditory responses in the brain are recognized side effects of increased pure tone thresholds, impaired speech recognition, and aging. However, it remains controversial whether central slowing is primarily linked to brain processes as atrophy, or is also associated with the slowing of temporal neural processing from the periphery. Here we analyzed electroencephalogram (EEG) responses that most likely reflect medial geniculate body (MGB) responses to passive listening of phonemes in 80 subjects ranging in age from 18 to 76 years, in whom the peripheral auditory responses had been analyzed in detail (Schirmer et al., 2024). We observed that passive listening to vowels and phonemes, specifically designed to rely on either temporal fine structure (TFS) for frequencies below the phase locking limit (<1500 Hz), or on the temporal envelope (TENV) for frequencies above phase locking limit, entrained lower or higher neural EEG responses. While previous views predict speech content, particular in noise to be encoded through TENV, here a decreasing phoneme-induced EEG amplitude over age in response to phonemes relying on TENV coding could also be linked to poorer speech-recognition thresholds in quiet. In addition, increased phoneme-evoked EEG delay could be correlated with elevated extended high-frequency threshold (EHF) for phoneme changes that relied on TFS and TENV coding. This may suggest a role of pure-tone threshold averages (PTA) of EHF for TENV and TFS beyond sound localization that is reflected in likely MGB delays. When speech recognition thresholds were normalized for pure-tone thresholds, however, the EEG amplitudes remained insignificant, and thereby became independent of age. Under these conditions, poor speech recognition in quiet was found together with a delay in EEG response for phonemes that relied on TFS coding, while poor speech recognition in ipsilateral noise was observed as a trend of shortened EEG delays for phonemes that relied on TENV coding. Based on previous analyses performed in these same subjects, elevated thresholds in extended high-frequency regions were linked to cochlear synaptopathy and auditory brainstem delays. Also, independent of hearing loss, poor speech-performing groups in quiet or with ipsilateral noise during TFS or TENV coding could be linked to lower or better outer hair cell performance and delayed or steeper auditory nerve responses at stimulus onset. The amplitude and latency of MGB responses to phonemes requiring TFS or TENV coding, dependent or independent of hearing loss, may thus be a new predictor of poor speech recognition in quiet and ipsilateral noise that links deficits in synchronicity at stimulus onset to neocortical activity. Amplitudes and delays of speech EEG responses to syllables should be reconsidered for future hearing-aid studies.

Neural Adaptation at Stimulus Onset and Speed of Neural Processing as Critical Contributors to Speech Comprehension Independent of Hearing Threshold or Age

Background: It is assumed that speech comprehension deficits in background noise are caused by age-related or acquired hearing loss. Methods: We examined young, middle-aged, and older individuals with and without hearing threshold loss using pure-tone (PT) audiometry, short-pulsed distortion-product otoacoustic emissions (pDPOAEs), auditory brainstem responses (ABRs), auditory steady-state responses (ASSRs), speech comprehension (OLSA), and syllable discrimination in quiet and noise. Results: A noticeable decline of hearing sensitivity in extended high-frequency regions and its influence on low-frequency-induced ABRs was striking. When testing for differences in OLSA thresholds normalized for PT thresholds (PTTs), marked differences in speech comprehension ability exist not only in noise, but also in quiet, and they exist throughout the whole age range investigated. Listeners with poor speech comprehension in quiet exhibited a relatively lower pDPOAE and, thus, cochlear amplifier performance independent of PTT, smaller and delayed ABRs, and lower performance in vowel-phoneme discrimination below phase-locking limits (/o/-/u/). When OLSA was tested in noise, listeners with poor speech comprehension independent of PTT had larger pDPOAEs and, thus, cochlear amplifier performance, larger ASSR amplitudes, and higher uncomfortable loudness levels, all linked with lower performance of vowel-phoneme discrimination above the phase-locking limit (/i/-/y/). Conslusions: This study indicates that listening in noise in humans has a sizable disadvantage in envelope coding when basilar-membrane compression is compromised. Clearly, and in contrast to previous assumptions, both good and poor speech comprehension can exist independently of differences in PTTs and age, a phenomenon that urgently requires improved techniques to diagnose sound processing at stimulus onset in the clinical routine.

Dysfunction of specific auditory fibers impacts cortical oscillations, driving an autism phenotype despite near-normal hearing

Autism spectrum disorder is discussed in the context of altered neural oscillations and imbalanced cortical excitation-inhibition of cortical origin. We studied here whether developmental changes in peripheral auditory processing, while preserving basic hearing function, lead to altered cortical oscillations. Local field potentials (LFPs) were recorded from auditory, visual, and prefrontal cortices and the hippocampus of BdnfPax2 KO mice. These mice develop an autism-like behavioral phenotype through deletion of BDNF in Pax2+ interneuron precursors, affecting lower brainstem functions, but not frontal brain regions directly. Evoked LFP responses to behaviorally relevant auditory stimuli were weaker in the auditory cortex of BdnfPax2 KOs, connected to maturation deficits of high-spontaneous rate auditory nerve fibers. This was correlated with enhanced spontaneous and induced LFP power, excitation-inhibition imbalance, and dendritic spine immaturity, mirroring autistic phenotypes. Thus, impairments in peripheral high-spontaneous rate fibers alter spike synchrony and subsequently cortical processing relevant for normal communication and behavior.

A multi-channel EEG mini-cap can improve reliability for recording auditory brainstem responses in chinchillas

BACKGROUND

Disabling hearing loss affects nearly 466 million people worldwide (World Health Organization). The auditory brainstem response (ABR) is the most common non-invasive clinical measure of evoked potentials, e.g., as an objective measure for universal newborn hearing screening. In research, the ABR is widely used for estimating hearing thresholds and cochlear synaptopathy in animal models of hearing loss. The ABR contains multiple waves representing neural activity across different peripheral auditory pathway stages, which arise within the first 10 ms after stimulus onset. Multi-channel (e.g., 32 or higher) caps provide robust measures for a wide variety of EEG applications for the study of human hearing. However, translational studies using preclinical animal models typically rely on only a few subdermal electrodes.

NEW METHOD

We evaluated the feasibility of a 32-channel rodent EEG mini-cap for improving the reliability of ABR measures in chinchillas, a common model of human hearing.

RESULTS

After confirming initial feasibility, a systematic experimental design tested five potential sources of variability inherent to the mini-cap methodology. We found each source of variance minimally affected mini-cap ABR waveform morphology, thresholds, and wave-1 amplitudes.

COMPARISON WITH EXISTING METHOD

The mini-cap methodology was statistically more robust and less variable than the conventional subdermal-needle methodology, most notably when analyzing ABR thresholds. Additionally, fewer repetitions were required to produce a robust ABR response when using the mini-cap.

CONCLUSIONS

These results suggest the EEG mini-cap can improve translational studies of peripheral auditory evoked responses. Future work will evaluate the potential of the mini-cap to improve the reliability of more centrally evoked (e.g., cortical) EEG responses.