Stability of Auditory Steady State Responses Over Time

Automatic audiometry using auditory steady-state response and sequential test strategy applied to volunteers with normal hearing

PURPOSE

In the present study, a new procedure to perform automatic audiometry using multifrequency Auditory Steady-State Response (ASSR) is proposed.

METHODS

The automatic audiometry procedure consists of detecting the presence of multifrequency ASSR in real-time using the sequential test strategy and by adjusting the stimulus intensity independently. The ASSR audiometric thresholds of 18 adult volunteers with normal hearing were determined by automatically (four simultaneous frequencies per ear) at modulation frequencies in the 80 Hz range. The exam time and the difference between ASSR thresholds and pure-tone behavioural hearing thresholds were estimated as performance measures.

RESULTS

The results showed that automatic audiometry can reduce the number of intensity levels used to obtain the ASSR threshold by up to 58% when compared to audiometry without using the techniques applied in automatic audiometry. In addition, the average of the difference between ASSR thresholds and Pure-Tone Behavioural Hearing thresholds was around 19 dB, which is similar to the results reported in similar studies.

CONCLUSIONS

The audiometric procedure proposed in this study is fully automatic, i.e., does not require any human supervision throughout the exam, and is able to significantly reduce the conventional exam time.

Naturalistic viewing conditions can increase task engagement and aesthetic preference but have only minimal impact on EEG quality

Free gaze and moving images are typically avoided in EEG experiments due to the expected generation of artifacts and noise. Yet for a growing number of research questions, loosening these rigorous restrictions would be beneficial. Among these is research on visual aesthetic experiences, which often involve open-ended exploration of highly variable stimuli. Here we systematically compare the effect of conservative vs. more liberal experimental settings on various measures of behavior, brain activity and physiology in an aesthetic rating task. Our primary aim was to assess EEG signal quality. 43 participants either maintained fixation or were allowed to gaze freely, and viewed either static images or dynamic (video) stimuli consisting of dance performances or nature scenes. A passive auditory background task (auditory steady-state response; ASSR) was added as a proxy measure for overall EEG recording quality. We recorded EEG, ECG and eyetracking data, and participants rated their aesthetic preference and state of boredom on each trial. Whereas both behavioral ratings and gaze behavior were affected by task and stimulus manipulations, EEG SNR was barely affected and generally robust across all conditions, despite only minimal preprocessing and no trial rejection. In particular, we show that using video stimuli does not necessarily result in lower EEG quality and can, on the contrary, significantly reduce eye movements while increasing both the participants' aesthetic response and general task engagement. We see these as encouraging results indicating that - at least in the lab - more liberal experimental conditions can be adopted without significant loss of signal quality.

Free-Field Cortical Steady-State Evoked Potentials in Cochlear Implant Users

Auditory steady-state evoked potentials (SS-EPs) are phase-locked neural responses to periodic stimuli, believed to reflect specific neural generators. As an objective measure, steady-state responses have been used in different clinical settings, including measuring hearing thresholds of normal and hearing-impaired subjects. Recent studies are in favor of recording these responses as a part of the cochlear implant (CI) device-fitting procedure. Considering these potential benefits, the goals of the present study were to assess the feasibility of recording free-field SS-EPs in CI users and to compare their characteristics between CI users and controls. By taking advantage of a recently developed dual-frequency tagging method, we attempted to record subcortical and cortical SS-EPs from adult CI users and controls and measured reliable subcortical and cortical SS-EPs in the control group. Independent component analysis (ICA) was used to remove CI stimulation artifacts, yet subcortical responses of several CIs were heavily contaminated by these artifacts. Consequently, only cortical SS-EPs were compared between groups, which were found to be larger in the controls. The lower cortical SS-EPs' amplitude in CI users might indicate a reduction in neural synchrony evoked by the modulation rate of the auditory input across different neural assemblies in the auditory pathway. The brain topographies of cortical auditory SS-EPs, the time course of cortical responses, and the reconstructed cortical maps were highly similar between groups, confirming their neural origin and possibility to obtain such responses also in CI recipients. As for subcortical SS-EPs, our results highlight a need for sophisticated denoising algorithms to pinpoint and remove artifactual components from the biological response.

Evidence for enhanced neural tracking of the speech envelope underlying age-related speech-in-noise difficulties

When we grow older, understanding speech in noise becomes more challenging. Research has demonstrated the role of auditory temporal and cognitive deficits in these age-related speech-in-noise difficulties. To better understand the underlying neural mechanisms, we recruited young, middle-aged, and older normal-hearing adults and investigated the interplay between speech understanding, cognition, and neural tracking of the speech envelope using electroencephalography. The stimuli consisted of natural speech masked by speech-weighted noise or a competing talker and were presented at several subject-specific speech understanding levels. In addition to running speech, we recorded auditory steady-state responses at low modulation frequencies to assess the effect of age on nonspeech sounds. The results show that healthy aging resulted in a supralinear increase in the speech reception threshold, i.e., worse speech understanding, most pronounced for the competing talker. Similarly, advancing age was associated with a supralinear increase in envelope tracking, with a pronounced enhancement for older adults. Additionally, envelope tracking was found to increase with speech understanding, most apparent for older adults. Because we found that worse cognitive scores were associated with enhanced envelope tracking, our results support the hypothesis that enhanced envelope tracking in older adults is the result of a higher activation of brain regions for processing speech, compared with younger adults. From a cognitive perspective, this could reflect the inefficient use of cognitive resources, often observed in behavioral studies. Interestingly, the opposite effect of age was found for auditory steady-state responses, suggesting a complex interplay of different neural mechanisms with advancing age.NEW & NOTEWORTHY We measured neural tracking of the speech envelope across the adult lifespan and found a supralinear increase in envelope tracking with age. Using a more ecologically valid approach than auditory steady-state responses, we found that young and older, as well as middle-aged, normal-hearing adults showed an increase in envelope tracking with increasing speech understanding and that this association is stronger for older adults.

A Method for Tracking the Time Evolution of Steady-State Evoked Potentials

Neural entrainment refers to the synchronization of neural activity to the periodicity of sensory stimuli. This synchronization defines the generation of steady-state evoked responses (i.e., oscillations in the electroencephalogram phase-locked to the driving stimuli). The classic interpretation of the amplitude of the steady-state evoked responses assumes a stereotypical time-invariant neural response plus random background fluctuations, such that averaging over repeated presentations of the stimulus recovers the stereotypical response. This approach ignores the dynamics of the steady-state, as in the case of the adaptation elicited by prolonged exposures to the stimulus. To analyze the dynamics of steady-state responses, it can be assumed that the time evolution of the response amplitude is the same in different stimulation runs separated by sufficiently long breaks. Based on this assumption, a method to characterize the time evolution of steady-state responses is presented. A sufficiently large number of recordings are acquired in response to the same experimental condition. Experimental runs (recordings) are column-wise averaged (i.e., runs are averaged but epoch within recordings are not averaged with the preceding segments). The column-wise averaging allows analysis of steady-state responses in recordings with remarkably high signal-to-noise ratios. Therefore, the averaged signal provides an accurate representation of the time evolution of the steady-state response, which can be analyzed in both the time and frequency domains. In this study, a detailed description of the method is provided, using steady-state visually evoked potentials as an example of a response. Advantages and caveats are evaluated based on a comparison with single-trial methods designed to analyze neural entrainment.

Objective Binaural Loudness Balancing Based on 40-Hz Auditory Steady-State Responses. Part I: Normal Hearing

Psychophysical procedures are used to balance loudness across the ears. However, they can be difficult and require active cooperation. We investigated whether 40-Hz auditory steady-state response (ASSR) amplitudes can be used to objectively estimate the balanced loudness across the ears for a group of young, normal-hearing participants. The 40-Hz ASSRs were recorded using monaural stimuli with carrier frequencies of 500, 1000, or 2000 Hz over a range of levels between 40 and 80 dB SPL. Behavioral loudness balancing was performed for at least one reference level of the left ear. ASSR amplitude growth functions were listener dependent, but median across-ear ratios in ASSR amplitudes were close to 1. The differences between the ASSR-predicted balanced levels and the behaviorally found balanced levels were smaller than 5 dB in 59% of cases and smaller than 10 dB in 85% of cases. The differences between the ASSR-predicted balanced levels and the reference levels were smaller than 5 dB in 54% of cases and smaller than 10 dB in 87% of cases. No clear hemispheric lateralization was found for 40-Hz ASSRs, with the exception of responses evoked by stimulus levels of 40 to 60 dB SPL at 2000 Hz.

Estimation of auditory steady-state responses based on the averaging of independent EEG epochs

The amplitude of auditory steady-state responses (ASSRs) generated in the brainstem of rats exponentially decreases over the sequential averaging of EEG epochs. This behavior is partially due to the adaptation of the ASSR induced by the continuous and monotonous stimulation. In this study, we analyzed the potential clinical relevance of the ASSR adaptation. ASSR were elicited in eight anesthetized adult rats by 8-kHz tones, modulated in amplitude at 115 Hz. We called independent epochs to those EEG epochs acquired with sufficiently long inter-stimulus interval, so the ASSR contained in any given epoch is not affected by the previous stimulation. We tested whether the detection of ASSRs is improved when the response is computed by averaging independent EEG epochs, containing only unadapted auditory responses. The improvements in the ASSR detection obtained with standard, weighted and sorted averaging were compared. In the absence of artifacts, when the ASSR was elicited by continuous acoustic stimulation, the computation of the ASSR amplitude relied upon the averaging method. While the adaptive behavior of the ASSR was still evident after the weighting of epochs, the sorted averaging resulted in under-estimations of the ASSR amplitude. In the absence of artifacts, the ASSR amplitudes computed by averaging independent epochs did not depend on the averaging procedure. Averaging independent epochs resulted in higher ASSR amplitudes and halved the number of EEG epochs needed to be acquired to achieve the maximum detection rate of the ASSR. Acquisition protocols based on averaging independent EEG epochs, in combination with appropriate averaging methods for artifact reduction might contribute to develop more accurate hearing assessments based on ASSRs.

Objective Binaural Loudness Balancing Based on 40-Hz Auditory Steady-State Responses. Part II: Asymmetric and Bimodal Hearing

In Part I, we investigated 40-Hz auditory steady-state response (ASSR) amplitudes for the use of objective loudness balancing across the ears for normal-hearing participants and found median across-ear ratios in ASSR amplitudes close to 1. In this part, we further investigated whether the ASSR can be used to estimate binaural loudness balance for listeners with asymmetric hearing, for whom binaural loudness balancing is of particular interest. We tested participants with asymmetric hearing and participants with bimodal hearing, who hear with electrical stimulation through a cochlear implant (CI) in one ear and with acoustical stimulation in the other ear. Behavioral loudness balancing was performed at different percentages of the dynamic range. Acoustical carrier frequencies were 500, 1000, or 2000 Hz, and CI channels were stimulated in apical or middle regions in the cochlea. For both groups, the ASSR amplitudes at balanced loudness levels were similar for the two ears, with median ratios between left and right ear stimulation close to 1. However, individual variability was observed. For participants with asymmetric hearing loss, the difference between the behavioral balanced levels and the ASSR-predicted balanced levels was smaller than 10 dB in 50% and 56% of cases, for 500 Hz and 2000 Hz, respectively. For bimodal listeners, these percentages were 89% and 60%. Apical CI channels yielded significantly better results (median difference near 0 dB) than middle CI channels, which had a median difference of −7.25 dB.