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.

The effects of broadband elicitor duration on a psychoacoustic measure of cochlear gain reduction.

Physiological and psychoacoustic studies of the medial olivocochlear reflex (MOCR) in humans have often relied on long duration elicitors (>100 ms). This is largely due to previous research using otoacoustic emissions (OAEs) that found multiple MOCR time constants, including time constants in the 100s of milliseconds, when elicited by broadband noise. However, the effect of the duration of a broadband noise elicitor on similar psychoacoustic tasks is currently unknown. The current study measured the effects of ipsilateral broadband noise elicitor duration on psychoacoustic gain reduction estimated from a forward-masking paradigm. Analysis showed that both masker type and elicitor duration were significant main effects, but no interaction was found. Gain reduction time constants were ∼46 ms for the masker present condition and ∼78 ms for the masker absent condition (ranging from ∼29 to 172 ms), both similar to the fast time constants reported in the OAE literature (70-100 ms). Maximum gain reduction was seen for elicitor durations of ∼200 ms. This is longer than the 50-ms duration which was found to produce maximum gain reduction with a tonal on-frequency elicitor. Future studies of gain reduction may use 150-200 ms broadband elicitors to maximally or near-maximally stimulate the MOCR.

Cortical signatures of auditory object binding in children with autism spectrum disorder are anomalous in concordance with behavior and diagnosis.

Organizing sensory information into coherent perceptual objects is fundamental to everyday perception and communication. In the visual domain, indirect evidence from cortical responses suggests that children with autism spectrum disorder (ASD) have anomalous figure-ground segregation. While auditory processing abnormalities are common in ASD, especially in environments with multiple sound sources, to date, the question of scene segregation in ASD has not been directly investigated in audition. Using magnetoencephalography, we measured cortical responses to unattended (passively experienced) auditory stimuli while parametrically manipulating the degree of temporal coherence that facilitates auditory figure-ground segregation. Results from 21 children with ASD (aged 7-17 years) and 26 age- and IQ-matched typically developing children provide evidence that children with ASD show anomalous growth of cortical neural responses with increasing temporal coherence of the auditory figure. The documented neurophysiological abnormalities did not depend on age, and were reflected both in the response evoked by changes in temporal coherence of the auditory scene and in the associated induced gamma rhythms. Furthermore, the individual neural measures were predictive of diagnosis (83% accuracy) and also correlated with behavioral measures of ASD severity and auditory processing abnormalities. These findings offer new insight into the neural mechanisms underlying auditory perceptual deficits and sensory overload in ASD, and suggest that temporal-coherence-based auditory scene analysis and suprathreshold processing of coherent auditory objects may be atypical in ASD.

Spectrally specific temporal analyses of spike-train responses to complex sounds: A unifying framework.

Significant scientific and translational questions remain in auditory neuroscience surrounding the neural correlates of perception. Relating perceptual and neural data collected from humans can be useful; however, human-based neural data are typically limited to evoked far-field responses, which lack anatomical and physiological specificity. Laboratory-controlled preclinical animal models offer the advantage of comparing single-unit and evoked responses from the same animals. This ability provides opportunities to develop invaluable insight into proper interpretations of evoked responses, which benefits both basic-science studies of neural mechanisms and translational applications, e.g., diagnostic development. However, these comparisons have been limited by a disconnect between the types of spectrotemporal analyses used with single-unit spike trains and evoked responses, which results because these response types are fundamentally different (point-process versus continuous-valued signals) even though the responses themselves are related. Here, we describe a unifying framework to study temporal coding of complex sounds that allows spike-train and evoked-response data to be analyzed and compared using the same advanced signal-processing techniques. The framework uses a set of peristimulus-time histograms computed from single-unit spike trains in response to polarity-alternating stimuli to allow advanced spectral analyses of both slow (envelope) and rapid (temporal fine structure) response components. Demonstrated benefits include: (1) novel spectrally specific temporal-coding measures that are less confounded by distortions due to hair-cell transduction, synaptic rectification, and neural stochasticity compared to previous metrics, e.g., the correlogram peak-height, (2) spectrally specific analyses of spike-train modulation coding (magnitude and phase), which can be directly compared to modern perceptually based models of speech intelligibility (e.g., that depend on modulation filter banks), and (3) superior spectral resolution in analyzing the neural representation of nonstationary sounds, such as speech and music. This unifying framework significantly expands the potential of preclinical animal models to advance our understanding of the physiological correlates of perceptual deficits in real-world listening following sensorineural hearing loss.

Auditory Brainstem Response Latency in Noise as a Marker of Cochlear Synaptopathy.

Evidence from animal and human studies suggests that moderate acoustic exposure, causing only transient threshold elevation, can nonetheless cause "hidden hearing loss" that interferes with coding of suprathreshold sound. Such noise exposure destroys synaptic connections between cochlear hair cells and auditory nerve fibers; however, there is no clinical test of this synaptopathy in humans. In animals, synaptopathy reduces the amplitude of auditory brainstem response (ABR) wave-I. Unfortunately, ABR wave-I is difficult to measure in humans, limiting its clinical use. Here, using analogous measurements in humans and mice, we show that the effect of masking noise on the latency of the more robust ABR wave-V mirrors changes in ABR wave-I amplitude. Furthermore, in our human cohort, the effect of noise on wave-V latency predicts perceptual temporal sensitivity. Our results suggest that measures of the effects of noise on ABR wave-V latency can be used to diagnose cochlear synaptopathy in humans. SIGNIFICANCE STATEMENT: Although there are suspicions that cochlear synaptopathy affects humans with normal hearing thresholds, no one has yet reported a clinical measure that is a reliable marker of such loss. By combining human and animal data, we demonstrate that the latency of auditory brainstem response wave-V in noise reflects auditory nerve loss. This is the first study of human listeners with normal hearing thresholds that links individual differences observed in behavior and auditory brainstem response timing to cochlear synaptopathy. These results can guide development of a clinical test to reveal this previously unknown form of noise-induced hearing loss in humans.

Functional modeling of the human auditory brainstem response to broadband stimulation.

Population responses such as the auditory brainstem response (ABR) are commonly used for hearing screening, but the relationship between single-unit physiology and scalp-recorded population responses are not well understood. Computational models that integrate physiologically realistic models of single-unit auditory-nerve (AN), cochlear nucleus (CN) and inferior colliculus (IC) cells with models of broadband peripheral excitation can be used to simulate ABRs and thereby link detailed knowledge of animal physiology to human applications. Existing functional ABR models fail to capture the empirically observed 1.2-2 ms ABR wave-V latency-vs-intensity decrease that is thought to arise from level-dependent changes in cochlear excitation and firing synchrony across different tonotopic sections. This paper proposes an approach where level-dependent cochlear excitation patterns, which reflect human cochlear filter tuning parameters, drive AN fibers to yield realistic level-dependent properties of the ABR wave-V. The number of free model parameters is minimal, producing a model in which various sources of hearing-impairment can easily be simulated on an individualized and frequency-dependent basis. The model fits latency-vs-intensity functions observed in human ABRs and otoacoustic emissions while maintaining rate-level and threshold characteristics of single-unit AN fibers. The simulations help to reveal which tonotopic regions dominate ABR waveform peaks at different stimulus intensities.

Evidence against attentional state modulating scalp-recorded auditory brainstem steady-state responses.

Auditory brainstem responses (ABRs) and their steady-state counterpart (subcortical steady-state responses, SSSRs) are generally thought to be insensitive to cognitive demands. However, a handful of studies report that SSSRs are modulated depending on the subject׳s focus of attention, either towards or away from an auditory stimulus. Here, we explored whether attentional focus affects the envelope-following response (EFR), which is a particular kind of SSSR, and if so, whether the effects are specific to which sound elements in a sound mixture a subject is attending (selective auditory attentional modulation), specific to attended sensory input (inter-modal attentional modulation), or insensitive to attentional focus. We compared the strength of EFR-stimulus phase locking in human listeners under various tasks: listening to a monaural stimulus, selectively attending to a particular ear during dichotic stimulus presentation, and attending to visual stimuli while ignoring dichotic auditory inputs. We observed no systematic changes in the EFR across experimental manipulations, even though cortical EEG revealed attention-related modulations of alpha activity during the task. We conclude that attentional effects, if any, on human subcortical representation of sounds cannot be observed robustly using EFRs. This article is part of a Special Issue entitled SI: Prediction and Attention.

Rapid acquisition of auditory subcortical steady state responses using multichannel recordings.

OBJECTIVE: Auditory subcortical steady state responses (SSSRs), also known as frequency following responses (FFRs), provide a non-invasive measure of phase-locked neural responses to acoustic and cochlear-induced periodicities. SSSRs have been used both clinically and in basic neurophysiological investigation of auditory function. SSSR data acquisition typically involves thousands of presentations of each stimulus type, sometimes in two polarities, with acquisition times often exceeding an hour per subject. Here, we present a novel approach to reduce the data acquisition times significantly. METHODS: Because the sources of the SSSR are deep compared to the primary noise sources, namely background spontaneous cortical activity, the SSSR varies more smoothly over the scalp than the noise. We exploit this property and extract SSSRs efficiently, using multichannel recordings and an eigendecomposition of the complex cross-channel spectral density matrix. RESULTS: Our proposed method yields SNR improvement exceeding a factor of 3 compared to traditional single-channel methods. CONCLUSIONS: It is possible to reduce data acquisition times for SSSRs significantly with our approach. SIGNIFICANCE: The proposed method allows SSSRs to be recorded for several stimulus conditions within a single session and also makes it possible to acquire both SSSRs and cortical EEG responses without increasing the session length.

A comparison of spectral magnitude and phase-locking value analyses of the frequency-following response to complex tones.

Two experiments, both presenting diotic, harmonic tone complexes (100 Hz fundamental), were conducted to explore the envelope-related component of the frequency-following response (FFRENV), a measure of synchronous, subcortical neural activity evoked by a periodic acoustic input. Experiment 1 directly compared two common analysis methods, computing the magnitude spectrum and the phase-locking value (PLV). Bootstrapping identified which FFRENV frequency components were statistically above the noise floor for each metric and quantified the statistical power of the approaches. Across listeners and conditions, the two methods produced highly correlated results. However, PLV analysis required fewer processing stages to produce readily interpretable results. Moreover, at the fundamental frequency of the input, PLVs were farther above the metric's noise floor than spectral magnitudes. Having established the advantages of PLV analysis, the efficacy of the approach was further demonstrated by investigating how different acoustic frequencies contribute to FFRENV, analyzing responses to complex tones composed of different acoustic harmonics of 100 Hz (Experiment 2). Results show that the FFRENV response is dominated by peripheral auditory channels responding to unresolved harmonics, although low-frequency channels driven by resolved harmonics also contribute. These results demonstrate the utility of the PLV for quantifying the strength of FFRENV across conditions.

Evaluation of a sustainable remediation option: beneficial reuse of petroleum-contaminated sediment as an energy source.

The characteristics of petroleum-contaminated sediment (PCS) have been evaluated to assess whether the practice of its beneficial reuse as a sole or supplemental energy source is sustainable relative to other sediment remediation options such as monitored natural recovery (MNR), capping, or off-site disposal. Some of these remediation options for PCS are energy-intensive and/or require land utilization. The energy and compositional analysis results indicate a low carbon content (15-17%(wt)) and corresponding low energy values of 5,200 kJ/kg (2,200 Btu/lb) to 5,600 kJ/kg (2,400 Btu/lb). However, given other decision-making criteria, the sediment may contain enough value to be added as a supplemental fuel given that it is normally considered a waste product and is readily available. The thermogravimetric profiles obtained under both combustion and pyrolytic conditions showed that the sulfur contents were comparable to typical low sulfur bituminous or lignite coals found in the United States, and most of the volatiles could be vaporized below 750 degrees C. The heavy metal concentrations determined before and after combustion of the PCS indicated that further engineering controls may be required for mercury, arsenic, and lead. Due to the potential for reduction of public health and environmental threats, potential economic savings, and conservation of natural resources (petroleum and land), removal of PCS by dredging and beneficial reuse as a supplemental fuel clearly has merit to be considered as a sustainable remediation option.