Auditory Steady-state Response Amplitude Research Articles

Simultaneous acquisition of 40- and 80-Hz auditory steady-state responses for a direct comparison of response amplitude, residual noise and signal-to-noise ratio.

The purpose of this study was to establish a paradigm that allows for the simultaneous recording of auditory steady-state responses (ASSRs) to two largely different modulation rates. In 21 normal-hearing adults, ASSRs for 40- and 80-Hz modulation rates were recorded in (1) a classical monotic single-stimulus condition, (2) a monotic simultaneous condition, where 40 Hz was paired with a 1-kHz carrier and 80 Hz with a 2-kHz carrier, and (3) a dichotic simultaneous condition with the same modulation rate/carrier pairing. Response amplitudes, residual noises, and signal-to-noise ratios were compared across conditions. Whereas the multiple-stimulus paradigms reduced the 40-Hz ASSR amplitude compared to the single condition, there was hardly any change for the 80-Hz ASSR. In all conditions, the 40-Hz ASSR was considerably larger than the 80-Hz ASSR. The residual noise was only 1.4 times larger for 40 Hz than for the 80 Hz. The proposed procedure using multiple stimuli with largely different modulation rates can be used to study differences in their responses and residual noise under identical states of vigilance. The amplitude reduction caused by the interaction between multiple stimuli has to be taken into account when interpreting the results.

Visual and auditory steady-state responses in attention-deficit/hyperactivity disorder.

We designed a study to investigate the patterns of the steady-state visual evoked potential (SSVEP) and auditory steady-state response (ASSR) in adolescents with attention-deficit/hyperactivity disorder (ADHD) when performing a motor response inhibition task. Thirty 12- to 18-year-old adolescents with ADHD and 30 healthy control adolescents underwent an electroencephalogram (EEG) examination during steady-state stimuli when performing a stop-signal task. Then, we calculated the amplitude and phase of the steady-state responses in both visual and auditory modalities. Results showed that adolescents with ADHD had a significantly poorer performance in the stop-signal task during both visual and auditory stimuli. The SSVEP amplitude of the ADHD group was larger than that of the healthy control group in most regions of the brain, whereas the ASSR amplitude of the ADHD group was smaller than that of the healthy control group in some brain regions (e.g., right hemisphere). In conclusion, poorer task performance (especially inattention) and neurophysiological results in ADHD demonstrate a possible impairment in the interconnection of the association cortices in the parietal and temporal lobes and the prefrontal cortex. Also, the motor control problems in ADHD may arise from neural deficits in the frontoparietal and occipitoparietal systems and other brain structures such as cerebellum.

BS12. Confirmation of previous results of the occlusion effect through auditory steady state response in normal hearing adults

The occlusion effect (OE) is a well known phenomenon in audiology clinical practice. It is often defined as the increase in sound pressure in front of the tympanic membrane for low frequencies through 1000 Hz, and the subsequent improvement in hearing sensitivity at these frequencies when a bone conducted signal is presented to the skull and the opening of the ear canal is occluded. Hence, some authors recommend the application of a correction factor to compensate for the OE. It is also well known that behavioral audiometry is not suitable for difficult to test population (i.e., children, malingers, people with some kind of neurodevelopmental disorder), so they need to be evaluated through objective electrophysiological measures. However, only two studies have assessed the OE using auditory steady state responses (ASSRs), so there is no agreement about their use in clinical practice. The aim of this study was to confirm the findings from previous ASSRs studies of the OE for a larger sample of normal hearing adults. Thirty-two normal hearing adults (32 ears) with a mean age of 21 ± 2 years participated in this study. For each participant, one ear was selected at random, and the first measures to be obtained (occluded or unoccluded) were randomly determined. The stimulus comprised a combination of four sinusoidal carrier tones, 500, 1000, 2000, and 4000 Hz, modulated in amplitude (95% depth) at the following rates: 104.2, 107.8, 111.4, and 115 Hz, respectively. It was presented through bone conduction for each participant under two different conditions (occluded and unoccluded ear). For both conditions, bracketing technique was used for determining thresholds with a vertical montage (negative electrode: ipsilateral mastoid; positive: Cz and ground: front). The overall ambient noise was 52 dB sound pressure level. Repeated measures analysis of variances was performed to compare occluded and unoccluded ASSR thresholds and amplitudes at 30 dB hearing level for 500, 1000, 2000, and 4000 Hz. Occlusion caused a significant decrease of bone conducted ASSR thresholds at low frequencies and a significant increase at 4000 Hz. Mean ASSR amplitudes were significantly higher after occlusion at low frequencies. Some subjects; however, showed no OE at frequencies at which it is expected to be present. The electrophysiological correction factors were 12 and 8 dB HL for 500 and 1000 Hz, respectively. Despite the high ambient noise issue, results of previous studies are confirmed here in a larger sample of cases. Our findings indicate that threshold variations at high frequencies need to be considered when conducting bone conduction testing with ear canal occlusion. The fact that the occlusion effect is not present in every individual may have some clinical implications. Our findings also suggest that correction factors recommended for behavioral testing can be applicable when using ASSRs.

Stability of Auditory Steady State Responses Over Time.

Auditory steady state responses (ASSRs) are used in clinical practice for objective hearing assessments. The response is called steady state because it is assumed to be stable over time, and because it is evoked by a stimulus with a certain periodicity, which will lead to discrete frequency components that are stable in amplitude and phase over time. However, the stimuli commonly used to evoke ASSRs are also known to be able to induce loudness adaptation behaviorally. Researchers and clinicians using ASSRs assume that the response remains stable over time. This study investigates (1) the stability of ASSR amplitudes over time, within one recording, and (2) whether loudness adaptation can be reflected in ASSRs. ASSRs were measured from 14 normal-hearing participants. The ASSRs were evoked by the stimuli that caused the most loudness adaptation in a previous behavioral study, that is, mixed-modulated sinusoids with carrier frequencies of either 500 or 2000 Hz, a modulation frequency of 40 Hz, and a low sensation level of 30 dB SL. For each carrier frequency and participant, 40 repetitions of 92 sec recordings were made. Two types of analyses were used to investigate the ASSR amplitudes over time: with the more traditionally used Fast Fourier Transform and with a novel Kalman filtering approach. Robust correlations between the ASSR amplitudes and behavioral loudness adaptation ratings were also calculated. Overall, ASSR amplitudes were stable. Over all individual recordings, the median change of the amplitudes over time was -0.0001 μV/s. Based on group analysis, a significant but very weak decrease in amplitude over time was found, with the decrease in amplitude over time around -0.0002 μV/s. Correlation coefficients between ASSR amplitudes and behavioral loudness adaptation ratings were significant but low to moderate, with r = 0.27 and r = 0.39 for the 500 and 2000 Hz carrier frequency, respectively. The decrease in amplitude of ASSRs over time (92 sec) is small. Consequently, it is safe to use ASSRs in clinical practice, and additional correction factors for objective hearing assessments are not needed. Because only small decreases in amplitudes were found, loudness adaptation is probably not reflected by the ASSRs.

Background Suppression and its Relation to Foreground Processing of Speech Versus Non-speech Streams

Since sound perception takes place against a background with a certain amount of noise, both speech and non-speech processing involve extraction of target signals and suppression of background noise. Previous works on early processing of speech phonemes largely neglected how background noise is encoded and suppressed. This study aimed to fill in this gap. We adopted an oddball paradigm where speech (vowels) or non-speech stimuli (complex tones) were presented with or without a background of amplitude-modulated noise and analyzed cortical responses related to foreground stimulus processing, including mismatch negativity (MMN), N2b, and P300, as well as neural representations of the background noise, that is, auditory steady-state response (ASSR). We found that speech deviants elicited later and weaker MMN, later N2b, and later P300 than non-speech ones, but N2b and P300 had similar strength, suggesting more complex processing of certain acoustic features in speech. Only for vowels, background noise enhanced N2b strength relative to silence, suggesting an attention-related speech-specific process to improve perception of foreground targets. In addition, noise suppression in speech contexts, quantified by ASSR amplitude reduction after stimulus onset, was lateralized towards the left hemisphere. The left-lateralized suppression following N2b was associated with the N2b enhancement in noise for speech, indicating that foreground processing may interact with background suppression, particularly during speech processing. Together, our findings indicate that the differences between perception of speech and non-speech sounds involve not only the processing of target information in the foreground but also the suppression of irrelevant aspects in the background.

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.

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.

Optimized auditory transcranial alternating current stimulation improves individual auditory temporal resolution

BackgroundTemporal resolution of cortical, auditory processing mechanisms is suggested to be linked to peak frequency of neuronal gamma oscillations in auditory cortex areas (individual gamma frequency, IGF): Individuals with higher IGF tend to have better temporal resolution. HypothesisModulating ongoing gamma activity with transcranial alternating current stimulation (tACS) is expected to improve performance in gap detection (GD) tasks (shorter GD thresholds) if the frequency is higher and to decrease GD performance (longer GD thresholds) if the frequency is lower than IGF. MethodsFor 26 healthy participants the IGF and temporal resolution were identified using an auditory steady state response (ASSR) paradigm and a between-channel GD task. Finite element modelling was used to generate an optimized tACS electrode montage (one channel per hemisphere: FC5-TP7/P7 and FC6-TP8/P8). Afterwards, GD thresholds were examined during tACS (tACS frequency group A: above IGF, tACS frequency group B: below IGF). Relative changes of GD thresholds were compared between groups. Additionally, effects of tACS on oscillatory activity were investigated comparing relative changes of ASSR amplitudes before and after stimulation. ResultsPerformance of group-A-participants improved significantly during tACS in comparison to performance of group-B-participants. Significant relative changes of ASSR amplitudes were found in both groups. ConclusionThe possibility to improve gap detection with individualized stimulation protocols for tACS further supports the link between oscillatory activity and temporal resolution, whereby the improvement of temporal resolution is particularly relevant for the clinical aspect of auditory tACS.

P197 Electroencephalographic effects of transcranial random noise stimulation of the temporal cortex

Question Transcranial random noise stimulation (tRNS) is an innovative technique of non-invasive electrical stimulation with a handful of publications most of them investigating processes of the motor system. With respect to the auditory system only. Limited evidence is available for modulation of auditory processing in healthy controls and patients with chronic tinnitus. Here, we aimed to investigate the effects of tRNS over the auditory cortex on resting state and evoked activity in healthy subjects. Methods We used EEG to measure tRNS induced changes in resting state activity and in auditory steady state responses (ASSRs). Stimuli were 1000 Hz carrier frequency tones, amplitude modulated at 20 Hz and 40 Hz and applied in randomized order. Nineteen healthy subjects participated in a placebo-controlled randomized design study. Each subject received 20 min of high-frequency, low-frequency, and sham tRNS applied over auditory cortices with 2 mA, with a one week interval between the stimulation sessions. Results Data analysis is done at the moment of abstract submission. Thus, we hypothesize that we will be able to replicate previous findings that high-frequency tRNS will modulate the 40 Hz ASSR amplitude and the mean and variability of theta band power during resting state. Further analyses will aim at testing differences in the effects of high- and low-frequency tRNS. Conclusions Our findings of tRNS effects for processes of the auditory system will be discussed with respect to previous findings of tRNS effects on motor cortex excitability and to current concepts of tRNS mechanisms such as increase of stochastic resonance.

Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation.

Introduction:Present methods of measuring the attenuation of hearing protection devices (HPDs) have limitations. Objective measurements such as field microphone in real-ear do not assess bone-conducted sound. Psychophysical measurements such as real-ear attenuation at threshold (REAT) are biased due to the low frequency masking effects from test subjects’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is explored as a technique which might overcome these limitations.Subjects and Methods:Pure tone stimuli (500 and 1000 Hz), amplitude modulated at 40 Hz, are presented to 10 normal-hearing adults through headphones at three levels in 10 dB steps. Two conditions were assessed: unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level are linearized using least-square regressions. The “physiological attenuation” is then calculated as the average difference between the two measurements. The technical feasibility of measuring earplug attenuation is demonstrated for the group average attenuation across subjects.Results:No significant statistical difference is found between the average REAT attenuation and the average ASSR-based attenuation.Conclusion:Feasibility is not yet demonstrated for individual subjects since differences between the estimates occurred for some subjects.

Intraoperative auditory steady-state monitoring during surgery in the cerebellopontine angle for estimation of postoperative hearing classes.

OBJECTIVE Brainstem auditory evoked potentials (BAEPs) have been used for intraoperative monitoring of the auditory nerve for many years. However, BAEPs yield limited information about the expected postoperative hearing quality and speech perception. The auditory steady-state response (ASSR) enables objective audiograms to be obtained in patients under anesthesia. These ASSRs could be used for intraoperative estimation of hearing classes to improve the postoperative outcome and quality of life. Studies investigating the clinical use of ASSRs during total intravenous anesthesia are currently lacking. The work presented in this article therefore investigates the application of ASSRs for intraoperative estimation of hearing classes. METHODS In 43 patients undergoing surgery for vestibular schwannoma, ASSR measurements were performed at the beginning and end of the surgical procedure. ASSR stimuli consisted of 80-dB hearing level amplitude-modulated tones with 5-minute duration, 90-Hz modulation, and 3 different carrier frequencies: 500, 1000, and 2000 Hz. Stimulation was performed unilaterally with and without contralateral masking, using single and combined carriers. Evoked responses were recorded and analyzed in the frequency domain. ASSRs were compared with extraoperative hearing classes and BAEPs using ANOVA, correlation, and receiver operating characteristic statistics. RESULTS ASSRs yielded high and consistent area under the curve (AUC) values (mean 0.83) and correlation values (mean -0.63), indicating reliable prediction of hearing classes. Analysis of BAEP amplitude changes showed lower AUC (mean 0.79) and correlation values (0.63, 0.37, and 0.50 for Waves I, III, and V, respectively). Latencies showed low AUC values (mean 0.6) and no significant correlation. Combination of several carriers for simultaneous evaluation reduced ASSR amplitudes and respective AUC values. Contralateral masking did not show a significant effect. CONCLUSIONS ASSRs robustly estimate hearing class in patients under total intravenous anesthesia, even when using short measurement durations. The method provides a diagnostic performance that exceeds conventional BAEP monitoring and enables objective and automated evaluation. On the basis of these findings, continuous intraoperative auditory monitoring could become a promising alternative or adjunct to BAEPs.

Frequency characteristics of neuromagnetic auditory steady-state responses at the supra-threshold level

Auditory steady-state response (ASSR) is a neuronal electrical component recorded as a continuous sinusoidal signal phase-locked to a modulation frequency of sounds. Since ASSR amplitude shows high correlation with perceptual characteristics at hearing threshold level, the ASSR has been examined as a candidate for an objective index of the human hearing ability per frequency. However, the relationship between ASSR and perception at the supra-threshold level has not been clarified. In this study, characteristics of ASSR magnitude relative to loudness at the supra-threshold level were investigated. Neuromagnetic 40-Hz ASSR was recorded in response to sinusoidally amplitude-modulated sweep tones with carrier frequency covering the frequency range of 0.1-12.5 kHz. Sound intensity was equalized at 50-, 60-, and 70-dB SPL with an accuracy of ±0.5-dB SPL at the phasic peak of the modulation frequency. Corresponding loudness characteristics were modeled by substituting the detected individual hearing thresholds into a standard formula (ISO226:2003(E)). The strength of the ASSR component was maximum at 0.5 kHz, and it decreased linearly on logarithmic scale toward lower and higher frequencies, whereas loudness model was plateaued between 0.5 and 4 kHz. The results indicated that frequency characteristics of the ASSR were not equivalent to those of SPL and loudness model.

Short-term enhancement and suppression of dolphin auditory evoked responses following echolocation click emission.

Biosonar gain control mechanisms in a bottlenose dolphin were investigated by measuring the auditory steady-state response (ASSR) to an external tone while the animal echolocated. The dolphin performed an echo change-detection task that utilized electronically synthesized echoes with echo delays corresponding to 25- and 50-m target range. During the task, amplitude modulated tones with carrier frequencies from 25 to 125 kHz were continuously presented and the instantaneous electroencephalogram stored for later analysis. ASSRs were extracted from the electroencephalogram by synchronously averaging time epochs temporally aligned with the onset of the external tone modulation cycle nearest to each of the dolphin's echolocation clicks. Results showed an overall suppression of the ASSR amplitude for tones with frequencies near the click center frequencies. A larger, temporary suppression of the ASSR amplitude was also measured at frequencies above 40-50 kHz, while a temporary enhancement was observed at lower frequencies. Temporal patterns for ASSR enhancement or suppression were frequency-, level-, and range-dependent, with recovery to pre-click values occurring within the two-way travel time. Suppressive effects fit the patterns expected from forward masking by the emitted biosonar pulse, while the specific mechanisms responsible for the frequency-dependent enhancement are unknown.

Comparing auditory steady-state responses amplitude evoked by simultaneous air- and bone-conducted stimulation in newborns

Objective: The aim of this study was to evaluate the potential interactions of the simultaneous presentation of air- and bone-conducted stimuli on auditory steady-state responses (ASSR) amplitude in newborns. Design: Bone- and air-conducted stimuli were sinusoidal carrier tones of 500 and 2000 Hz respectively modulated in amplitude (95% depth). Air- and bone- conducted stimuli were either simultaneously recorded in the same ear using insert earphones and bone vibrator respectively, or recorded individually (single stimulation). Study sample: Sixty-nine well babies (135 ears) with ages ranging from 1 to 16 days (mean of 9.2 ± 7.9 days) were tested in this study. Results: No significant changes in ASSR amplitude by air-conducted stimuli were observed when evoked by simultaneous or single stimulation. The same trend prevailed for ASSR amplitudes evoked by bone-conducted stimuli. Conclusions: The results of this study suggest that the simultaneous stimulation of air-and bone-conducted stimuli does not alter ASSR amplitude values in well babies. Therefore, the results support the use of this technique as a potential hearing screening tool to discriminate between conductive and sensorineural hearing loss.

Can place-specific cochlear dispersion be represented by auditory steady-state responses?

The present study investigated to what extent properties of local cochlear dispersion can be objectively assessed through auditory steady-state responses (ASSR). The hypothesis was that stimuli compensating for the phase response at a particular cochlear location generate a maximally modulated basilar membrane (BM) response at that BM position, due to the large “within-channel” synchrony of activity. This would lead, in turn, to a larger ASSR amplitude than other stimuli of corresponding intensity and bandwidth. Two stimulus types were chosen: 1] Harmonic tone complexes consisting of equal-amplitude tones with a starting phase following an algorithm developed by Schroeder [IEEE Trans. Inf. Theory 16, 85–89 (1970)] that have earlier been considered in behavioral studies to estimate human auditory filter phase responses; and 2] simulations of auditory-filter impulse responses (IR). In both cases, also the temporally reversed versions of the stimuli were considered. The ASSRs obtained with the Schroeder tone complexes were found to be dominated by “across-channel” synchrony and, thus, do not reflect local place-specific information. In the case of the more frequency-specific stimuli, no significant differences were found between the responses to the IR and its temporally reversed counterpart. Thus, whereas ASSRs to narrowband stimuli have been used as an objective indicator of frequency-specific hearing sensitivity, the method does not seem to be sensitive enough to reflect local cochlear dispersion.

Confirmation of previous results of the occlusion effect through auditory steady-state responses in normal-hearing adults

Context: The occlusion effect (OE) is a well-known phenomenon in audiology clinical practice. Hence, some authors recommend the application of a correction factor to compensate for the OE. However, only two studies have assessed the OE using auditory steady-state responses (ASSRs). Aims: The aim of this study is to confirm the findings from previous ASSRs studies of the OE for a larger sample of normal-hearing adults. Settings and Design: Thirty-two normal-hearing adults (32 ears) with a mean age of 21 ± 2 years participated in this study. For each participant, one ear was selected at random, and the first measures to be obtained (occluded or unoccluded) were randomly determined. Subjects and Methods: The stimulus comprised a combination of four sinusoidal carrier tones, 500, 1000, 2000, and 4000 Hz, modulated in amplitude (95% depth) at the following rates: 104.2, 107.8, 111.4, and 115 Hz, respectively. It was presented through bone conduction for each participant under two different conditions (occluded and unoccluded ear). The overall ambient noise was 52 dB sound pressure level. Statistical Analysis Used: Repeated-measures analysis of variances was performed to compare occluded and unoccluded ASSR thresholds and amplitudes at 30 dB hearing level for 500, 1000, 2000, and 4000 Hz. Results: Occlusion caused a significant decrease of bone-conducted ASSR thresholds at low frequencies and a significant increase at 4000 Hz. Mean ASSR amplitudes were significantly higher after occlusion at low frequencies. However, some participants showed no OE at frequencies at which it is expected to be present. Conclusions: Despite the high ambient noise issue, results of previous studies are confirmed here in a larger sample of cases.

V34. Paired associative stimulation of the temporal cortex

Background Paired associative stimulation (PAS) is a specific repetitive transcranial magnetic stimulation (rTMS) protocol where every pulse is combined with an external stimulus. Based on a close timing between the TMS induced action potential and the cortical arrival of the external stimulus in the stimulated area neural plasticity is induced by mechanisms of spike-timing dependent plasticity. After a first proof-of-principle study of PAS of the auditory cortex by our group, we here extend these findings by investigating the effects of stimulus duration and of sham stimulation on primary and secondary auditory cortical activity. Methods The left temporal cortex of 18 healthy controls with normal hearing was stimulated in two experimental parts. One PAS session consisted of 200 stimulations with an interstimulus interval of 45 ms between tone onset (400 ms length, 4 kHz frequency) and the TMS pulse proven to be effective in the pilot study. Part 1 contrasted effects of a short (23 ms) and a long (400 ms) PAS-tone on secondary auditory cortex activity (short vs. long condition); part 2 contrasted effects of verum and sham stimulation (6-fold reduced intensity, not targeting the cortex) on primary and secondary auditory cortex activity (verum vs. sham condition). Primary auditory cortex activity was measured with auditory steady state responses (ASSR) as elicited by 40 Hz amplitude-modulated (AM) sine tones. Secondary auditory cortex activity was elicited with ASSRs using a 20 Hz AM tone and late auditory evoked potentials (AEPs) for the 40 Hz and 20 Hz AM tone. Evoked activity was done for the PAS tone (4 kHz carrier frequency) and a control tone (1 kHz carrier frequency) - each tone presented 70 times (60 dB SL, 800 ms length). As STDP is based on close timing of pre- and post-synaptic activity we concentrated on evoked activity by using analyses time-locked to the tone onset. Results For part 1, the short PAS condition showed an increase in AEP amplitude accompanied by a decrease for the long PAS condition specific for the PAS carrier frequency. Verum and sham stimulation showed comparable decreases in AEP amplitude independent from the PAS protocol. Part 2 showed no effects for the contrast short vs. long condition. 20 Hz ASSR amplitude showed sham-controlled decreases for the PAS-specific carrier frequency. ASSR amplitudes of the control tone or of the 40 Hz AM tone showed no significant effects. Discussion PAS of the auditory cortex seem to be restricted to the secondary auditory cortex as effects could only be found for the AEPs and the 20 Hz ASSRs which have neural generators rather in secondary than in primary auditory cortex. AEPs findings raise the importance of habituation effects and of duration of the PAS tone and challenge the validity of the transfer of PAS using sine tones to evoked potentials using AM tones.

Evidence for differential modulation of primary and nonprimary auditory cortex by forward masking in tinnitus

It has been proposed that tinnitus is generated by aberrant neural activity that develops among neurons in tonotopic of regions of primary auditory cortex (A1) affected by hearing loss, which is also the frequency region where tinnitus percepts localize (Eggermont and Roberts 2004; Roberts et al., 2010, 2013). These models suggest (1) that differences between tinnitus and control groups of similar age and audiometric function should depend on whether A1 is probed in tinnitus frequency region (TFR) or below it, and (2) that brain responses evoked from A1 should track changes in the tinnitus percept when residual inhibition (RI) is induced by forward masking. We tested these predictions by measuring (128-channel EEG) the sound-evoked 40-Hz auditory steady-state response (ASSR) known to localize tonotopically to neural sources in A1. For comparison the N1 transient response localizing to distributed neural sources in nonprimary cortex (A2) was also studied. When tested under baseline conditions where tinnitus subjects would have heard their tinnitus, ASSR responses were larger in a tinnitus group than in controls when evoked by 500 Hz probes while the reverse was true for tinnitus and control groups tested with 5 kHz probes, confirming frequency-dependent group differences in this measure. On subsequent trials where RI was induced by masking (narrow band noise centered at 5 kHz), ASSR amplitude increased in the tinnitus group probed at 5 kHz but not in the tinnitus group probed at 500 Hz. When collapsed into a single sample tinnitus subjects reporting comparatively greater RI depth and duration showed comparatively larger ASSR increases after masking regardless of probe frequency. Effects of masking on ASSR amplitude in the control groups were completely reversed from those in the tinnitus groups, with no change seen to 5 kHz probes but ASSR increases to 500 Hz probes even though the masking sound contained no energy at 500 Hz (an “off-frequency” masking effect). In contrast to these findings for the ASSR, N1 amplitude was larger in tinnitus than control groups at both probe frequencies under baseline conditions, decreased after masking in all conditions, and did not relate to RI. These results suggest that aberrant neural activity occurring in the TFR of A1 underlies tinnitus and its modulation during RI. They indicate further that while neural changes occur in A2 in tinnitus, these changes do not reflect the tinnitus percept. Models for tinnitus and forward masking are described that integrate these findings within a common framework.

Habituation of Auditory Steady State Responses Evoked by Amplitude-Modulated Acoustic Signals in Rats.

Generation of the auditory steady state responses (ASSR) is commonly explained by the linear combination of random background noise activity and the stationary response. Based on this model, the decrease of amplitude that occurs over the sequential averaging of epochs of the raw data has been exclusively linked to the cancelation of noise. Nevertheless, this behavior might also reflect the non-stationary response of the ASSR generators. We tested this hypothesis by characterizing the ASSR time course in rats with different auditory maturational stages. ASSR were evoked by 8-kHz tones of different supra-threshold intensities, modulated in amplitude at 115 Hz. Results show that the ASSR amplitude habituated to the sustained stimulation and that dishabituation occurred when deviant stimuli were presented. ASSR habituation increased as animals became adults, suggesting that the ability to filter acoustic stimuli with no-relevant temporal information increased with age. Results are discussed in terms of the current model of the ASSR generation and analysis procedures. They might have implications for audiometric tests designed to assess hearing in subjects who cannot provide reliable results in the psychophysical trials.

Characterization of cochlear implant-related artifacts during sound-field recording of the auditory steady state response using an amplitude modulated stimulus: A comparison among normal hearing adults, cochlear implant recipients, and implant-in-a-box

Recent work has investigated the use of electric stimuli to evoke auditory steady state response (ASSR) in cochlear implant (CI) users. While more control can be exerted using electric stimuli, acoustic stimuli present natural listening environment for CI users. However, ASSR using acoustic stimuli in the presence of a CI could lead to artifacts. Five experiments investigated the presence and characteristics of CI-artifacts during sound-field ASSR using amplitude modulated (AM) stimulus (carrier frequency: 2 kHz; modulation frequency: 82.031 Hz). Experiment 1 investigated differences between 10 normal hearing (NH) and 10 CI participants in terms of ASSR amplitude versus intensity and onset phase versus intensity. Experiment 2 explored similar relationships for an implant-in-a-box. Experiment 3 investigated correlations between electrophysiological ASSR thresholds (ASSRe) and behavioral thresholds to the AM stimulus (BTAM) for the NH and CI groups. Mean threshold differences (ASSRe-BTAM) were computed for each group and group differences were studied. Experiment 4 investigated the presence of transducer-related artifacts using masking. Experiment 5 investigated the effect of manipulation of intensity and external components of the CI on the ASSR. Overall, results of this study provide the first comprehensive description of the characteristics of CI-artifacts during sound-field ASSR. Implications for future research to further characterize CI-artifacts, thereby leading to strategies to minimize them are discussed.