Auditory Steady-state Response Amplitude Research Articles

Viability of intraoperative auditory steady state responses during intracranial surgery.

For intraoperative monitoring of auditory nerve function, the auditory steady-state response (ASSR) analysis may be an alternative to brain stem auditory evoked potentials, offering frequency specificity and short detection times. Clinical studies investigating the viability of ASSR under total intravenous anesthesia have not been performed. During craniotomy under total intravenous anesthesia with propofol and remifentanil in 20 patients, ASSR were recorded. An additional control patient undergoing cerebellopontine angle surgery was included, in whom the auditory nerve could not be preserved. One-minute sinus tones (500, 1,000, 2,000 Hz) were applied with 60-, 70-, and 80-decibel hearing level. Stimuli were amplitude modulated with 40, 90, or 110 Hz and applied monaurally to the left and right ears. Time to detect a significant response and response amplitudes at 40, 90, or 110 Hz in the evoked EEG spectra was evaluated. Overall, 90-Hz ASSR were successfully detected in all 20 patients, 110 Hz in 18 patients, and 40 Hz in 14 patients after a median of 10 seconds. No ASSR could be detected in the control patient at the end of the surgical procedure. Time-to-significance and ASSR amplitudes were influenced by stimulus intensity, carrier, and modulation frequency (Scheirer-Ray-Hare test, P < 0.005). Ipsilateral responses were higher than contralateral (P < 0.0001). In conclusion, 90- and 110-Hz ASSR can be reliably detected under total intravenous anesthesia. Our results are in line with those from previous studies in awake patients. Auditory steady-state response during anesthesia may enable intraoperative frequency-specific audiometry and monitoring of the auditory nerve.

Predicting Hearing Thresholds in Occupational Noise-Induced Hearing Loss by Auditory Steady State Responses

Currently available behavioral tools for the assessment of noise-induced hearing loss (NIHL) depend on the reliable cooperation of the subject. Furthermore, in workers' compensation cases, there is considerable financial gain to be had from exaggerating symptoms, such that accurate assessment of true hearing threshold levels is essential. An alternative objective physiologic tool for assessing NIHL is the auditory steady state response (ASSR) test, which combines frequency specificity with a high level of auditory stimulation, making it applicable for the evaluation of subjects with a moderate to severe deficit. The primary aim of the study was to assess the value of the multifrequency ASSR test in predicting the behavioral warble-tone audiogram in a large sample of young subjects with NIHL of varying severity or with normal hearing. The secondary goal was to assess suprathreshold ASSR growth functions in these two groups. The study group included 157 subjects regularly exposed to high levels of occupational noise, who attended a university-associated audiological clinic for evaluation of NIHL from 2009 through 2011. All underwent a behavioral audiogram, and on the basis of the findings, were divided into those with NIHL (108 subjects, 216 ears) or normal hearing (49 subjects, 98 ears). The accuracy of the ASSR threshold estimations for frequencies of 500, 1000, 2000, and 4000 Hz was compared between groups, and the specificity and sensitivity of the ASSR test in differentiating ears with or without NIHL was calculated using receiver operating characteristic analysis. Linear regression analysis was used to formulate an equation to predict the behavioral warble-tone audiogram at each test frequency using ASSR thresholds. Multifrequency ASSR amplitude growth as a function of stimulus intensity was compared between the NIHL and normal-hearing groups for 1000 Hz and 4000 Hz carrier frequencies. In the subjects with NIHL, ASSR thresholds to various frequencies were significantly and highly correlated with the behavioral warble-tone thresholds; Pearson correlation coefficients ranged from 0.6 to 0.8 over the four frequencies tested. Differences between thresholds ranged from 10 to 13 dB. The configuration of the ASSR waveforms closely approximated the behavioral audiogram. The sensitivity for screening hearing thresholds was 92%; by frequency, sensitivity ranged between 92.7 and 98.4%, but specificity was lower, especially at the low frequencies. ASSR accurately predicted moderate and severe NIHL. The mean ASSR growth amplitude to increasing stimulus level for 1000 and 4000 Hz was significantly steeper in the NIHL than in the normal-hearing group, with no significant difference between frequencies. The ASSR test has a high sensitivity to detect moderate to severe hearing loss in subjects with NIHL. Its use can facilitate the early identification of noise-exposed workers with NIHL. It may also serve an important medico-legal function in cases of workers' compensation. The ASSR test is not, by itself, an appropriate tool for hearing screening in the general population.

Association of Auditory Steady State Responses with Perception of Temporal Modulations and Speech in Noise

Amplitude modulations in the speech convey important acoustic information for speech perception. Auditory steady state response (ASSR) is thought to be physiological correlate of amplitude modulation perception. Limited research is available exploring association between ASSR and modulation detection ability as well as speech perception. Correlation of modulation detection thresholds (MDT) and speech perception in noise with ASSR was investigated in twofold experiments. 30 normal hearing individuals and 11 normal hearing individuals within age range of 18–24 years participated in experiments 1 and 2, respectively. MDTs were measured using ASSR and behavioral method at 60 Hz, 80 Hz, and 120 Hz modulation frequencies in the first experiment. ASSR threshold was obtained by estimating the minimum modulation depth required to elicit ASSR (ASSR-MDT). There was a positive correlation between behavioral MDT and ASSR-MDT at all modulation frequencies. In the second experiment, ASSR for amplitude modulation (AM) sweeps at four different frequency ranges (30–40 Hz, 40–50 Hz, 50–60 Hz, and 60–70 Hz) was recorded. Speech recognition threshold in noise (SRTn) was estimated using staircase procedure. There was a positive correlation between amplitude of ASSR for AM sweep with frequency range of 30–40 Hz and SRTn. Results of the current study suggest that ASSR provides substantial information about temporal modulation and speech perception.

Investigating the temporal dynamics of dolphin biosonar using phantom echoes and auditory evoked potentials

Phantom echo generation replaces physical targets with electronic signals that can be delayed in time, scaled in amplitude, and broadcast to an echolocating animal to simulate echoes from distant objects. Compared to physical targets, phantom echoes have the advantages of allowing for instantaneous changes in target characteristics, independent manipulation of echo delay and echo amplitude, and easy randomization of target range. At the Navy Marine Mammal Program in San Diego, California, phantom echo generation is combined with measurements of auditory evoked potentials to investigate the temporal dynamics of biosonar signal emission and reception in bottlenose dolphins. The studies are primarily focused on examining automatic gain control mechanisms by measuring changes in hearing sensitivity — assessed via the auditory steady-state response (ASSR) to an amplitude modulated tone — over time courses corresponding to single biosonar click-echo pairs. Results show the ASSR amplitude initially decreases at ...

Effective Masking Levels for 500 and 2000 Hz Bone Conduction Auditory Steady State Responses in Infants and Adults With Normal Hearing

Few studies have investigated effective masking levels (EMLs) needed to isolate the test ear for bone conduction assessments in infants. The objective of this study was to determine EMLs for 500 and 2000 Hz bone conduction auditory steady state responses (ASSRs) to amplitude (AM)/frequency-modulated (FM) stimuli for infants and adults with normal hearing. Maturational factors that contribute to infant-adult differences in EMLs will also be investigated. The present study and previously published 1000 and 4000 Hz EML data will be compared to investigate EML across four frequencies. These findings will provide a starting point for implementing clinical masking for infant bone conduction testing using physiological measures. Participants were 15 infants (7 to 35 weeks) and 15 adults (21 to 56 years) with normal hearing. Bone-conducted single ASSR stimuli (research MASTER) were 100% AM and 25% FM at 85 and 101 Hz for 500 and 2000 Hz carrier frequencies, respectively. They were presented at 25 and 35 dB HL for 500 Hz and at 35 and 45 dB HL for 2000 Hz for both infants and adults (approximately 10 and 20 dB SL at each frequency for infants). Air-conducted narrowband maskers were presented to both ears simultaneously. Real-ear to coupler differences were measured to account for differences in the sound pressure developed in infant and adult ear canals as a result of ear-canal size. Data analyses were conducted for mean EMLs across frequency (500 to 4000 Hz) and between age groups. Masked and unmasked ASSR amplitudes were compared for 500 and 2000 Hz. Both infants and adults required much more masking (25 to 33 dB) to eliminate responses at 500 compared with 2000 Hz. On average, infants required 16 dB more masking at 500 Hz and similar amounts of masking at 2000 Hz compared with adults. When adjusted for ear-canal size and bone conduction sensitivity, the pattern of results did not change. Across all four frequencies, infants showed a systematic decrease in mean EMLs with an increase in frequency; all pair-wise comparisons were significant except 2000 versus 4000 Hz. Adults showed smaller frequency-dependent changes in EML (only significantly greater for 500 versus 2000 Hz and 4000 Hz). When ear-canal size and bone conduction sensitivity were taken into account, only 500 Hz required more masking than other frequencies in infants; there were no significant frequency-dependent trends for adults, although the greater EMLs at 1000 versus 2000 Hz and 4000 Hz approached significance. Unmasked and masked amplitudes tended to be larger for 2000 Hz but not for 500 Hz when comparing infants with adults. EMLs appropriate for infants for bone conduction ASSRs elicited to AM/FM stimuli are considerably higher at 500 compared with 2000 Hz. Infants also need more masking at 500 Hz compared with adults but the same amount of masking at 2000 Hz. Comparisons across four frequencies reveal a systematic decrease in EML with an increase in frequency in infants, which is not apparent in adults. Recommended EMLs for AM/FM bone-conducted ASSR stimuli presented at 35 dB HL for 500, 1000, 2000, and 4000 Hz, respectively, are: (1) infants: 81, 68, 59, and 45 dB SPL, and (2) adults: 66, 63, 59, and 55 dB SPL.

Auditory steady-state response measurement of the temporal dynamics of hearing sensitivity in an echolocating bottlenose dolphin (Tursiops truncatus)

Studies with some echolocating odontocetes demonstrate that receiver-based automatic gain control (AGC) compensates for reductions in echo strength resulting from acoustic spreading loss. This study examined AGC in an echolocating bottlenose dolphin by measuring changes in hearing sensitivity over time courses corresponding to single click-echo pairs. The electrophysiological auditory steady-state response (ASSR) elicited by a 113-kHz sinusoidally amplitude-modulated tone was recorded while the dolphin performed a target discrimination task. Auditory electrophysiological responses were extracted from the instantaneous electroencephalogram and coherently averaged using the modulation rate of the 113-kHz tone as a reference. A Fourier transform was then performed with a 10-ms sliding window to obtain the ASSR amplitude as a function of time relative to the dolphin’s outgoing click and received echo. The ASSR amplitude initially decreased at the time of click emission and then recovered over a course of 25 to 70 ms, depending on target range. This relatively long time course of recovery appears to be consistent with forward-masking, as opposed to an AGC mechanism based on the contraction and gradual release of middle ear muscles coincident with click emission. [Work funded by SSC Pacific Naval Innovative Science and Engineering (NISE) program.]

Using the auditory steady-state response to assess temporal dynamics of hearing sensitivity during bottlenose dolphin echolocation

The auditory steady-state response (ASSR) to an external tone was measured in an echolocating dolphin to determine if hearing sensitivity changes could be tracked over time scales corresponding to single click-echo pairs. Individual epochs containing click-echo pairs were first extracted from the instantaneous electroencephalogram. Epochs were coherently averaged using the external tone modulation rate as a timing reference, then Fourier transformed using a sliding, 10-ms temporal window to obtain the ASSR amplitude as a function of time. The results revealed a decrease in the ASSR amplitude at the time of click emission, followed by a 25-70 ms recovery.

Monotic Versus Dichotic Multiple-Stimulus Auditory Steady State Responses in Young Children

In their recent study on infant multiple auditory steady state responses (ASSR), the authors found that ASSR amplitudes unexpectedly decreased when switching from 1-ear stimulation to 2-ear stimulation, a result not present in adults. In addition, residual EEG noise levels increased in the 2-ear condition. In the present study, to determine whether to use 1-ear or 2-ear multiple ASSR Protocols clinically, the authors tested a new group of 19 young children to determine whether these unexpected findings could be replicated. ASSR amplitude and EEG noise were compared for 1-ear (4 stimuli) versus 2-ear (8 stimuli) multiple stimuli presented at 60 dBHL. Results indicated a small but significant decrease in amplitudes going from 1-ear (40.1 nV) to 2-ear (37.9 nV) multiple stimuli. EEG noise was not significantly different between the 2 conditions. Despite small amplitude decreases, the 2-ear stimulus condition was more efficient for infants and young children with normal hearing.

Ipsi- and contralateral interaction in the 40 Hz auditory steady state responses (ASSRs) with two carriers at 60 dB SPL

Objective: Auditory steady state responses have been suggested for simultaneous threshold assessment using the multiple ASSR (MASSR) technique. However, at least at high stimulation levels, strong interactions reduce response amplitudes. The present study investigates ASSR interaction at a moderate stimulus level. Design: Sinusoidal carriers modulated at rates near 40 Hz were used as probe. Unmodulated and modulated interferers were presented ipsi- or contralaterally. Study sample: Twenty normal-hearing subjects participated. Results: Unmodulated interferers did not significantly change ASSR amplitudes. Modulated interferers, presented ipsilaterally or contralaterally, both significantly reduced the ASSR SNR by 13% and 8%, respectively. Conclusions: To compensate for the average SNR reduction would require a 32% and 18% longer measurement time for ipsi- and contralateral interferers, respectively, far less than the doubling of measurement time for two single measurements, emphasizing the MASSR technique advantage. However, the largest reduction for a single subject was 22% for the amplitude and 28% for the SNR, almost completely undoing the benefit in measurement time in MASSR. The individually varying interaction effects even at 60 dB SPL clearly limits the advantage of using the MASSR for modulation rates near 40 Hz over corresponding single ASSR measurements, at least for two simultaneous carriers.

Hierarchical Neural Encoding of Temporal Regularity in the Human Auditory Cortex

Temporal regularity provides an important cue for the identification of natural sounds. Here, we measured auditory evoked cortical magnetic fields to investigate the neural processing of temporal regularity that cannot be tonotopically represented in the auditory periphery. Auditory steady state responses (ASSR) and sustained fields (SF) elicited by 40Hz amplitude modulated periodic and non-periodic noises were analyzed. Periodic noises of 40-, 20-, and 5-Hz were prepared in the form of repeating frozen noises where the same noise segment appears at either each period (40Hz), every second period (20Hz), or every eighth period (5Hz) of amplitude modulation. Compared to non-periodic white noises, periodic noises with repetition rates of 5-, 20-, and 40-Hz caused significantly increased SF amplitudes in both hemispheres. ASSR amplitudes were significantly enhanced for 20- and 40-Hz periodic noises in the right hemisphere while no enhancement was observed for periodic noises in the left hemisphere. The observed variation of the regularity effect between evoked response components and hemispheres may reflect the differences in the temporal integration window lengths adopted between ASSR and SF generators and also between the right and left auditory pathways.

Intra and intersubject variability in auditory steady-state response amplitude with high modulation rates to 1000 Hz amplitude modulated and tone pip stimuli

Objective: This study set out to provide further information on how high modulation/stimulus rates affect the auditory steady-state response (ASSR) amplitude for a 1000-Hz tone, and how this effect varies between individuals. Both sinusoidal amplitude modulated tones and tone pip stimuli were investigated. Design: Modulation/stimulus rates were 70, 80, and 90 Hz and the peak to peak stimulus levels were matched for the two different types of stimuli, at 90.5 dBSPLppe. Study sample: The study was carried out on fourteen normally-hearing adults (9 males and 5 females) Results: Overall the ASSR amplitude to the two types of stimuli was similar. In general there was an increasing response amplitude between rates of 70 and 90 Hz; The relationship between the amplitude of the response and the modulation /stimulus rate varied considerably between subjects. Conclusions: Optimum stimulus rates based on group data may not give the best rate in a significant proportion of subjects. Currently tone pip ABR is the primary method used in assessing hearing in babies. Finding a way of avoiding suboptimal stimulus rates for ‘80-Hz’ ASSR in babies will improve the likelihood of ASSR being seen as an alternative.

Sorted averaging improves quality of auditory steady-state responses

Increasing signal-to-noise ratio (SNR) is essential for the recording of auditory evoked potentials with electroencephalography (EEG). Several protocols have been proposed to increase the SNR, starting with an averaging of EEG epochs which decreases noise level. Since artifacts decrease the SNR by increasing the noise level, artifact detection and reduction protocols are other important tools to reduce the noise level. The current study focuses on the sorted averaging protocol where the epochs are sort according to their estimated root-mean-square (RMS) amplitude. Calculating an estimated SNR by averaging of the sorted epochs, this process of averaging can be interrupted at the maximum SNR, i.e., at an optimal number of epochs. In contrast to the often used protocol that weighs every epoch by its inverse average root-mean-square amplitude, sorted averaging is a linear operation, i.e., it does not change signal amplitudes. In this study, the sorted averaging is, for the first time, applied to auditory steady-state responses (ASSR) which are evoked by amplitude modulated tones or trains of transient acoustic stimuli. In contrast to other evoked potentials, the ASSR is analyzed in the frequency domain, using the property of auditory system to retain the modulation frequency (or the repetition rate) of the stimulus.ASSR were recorded in 11 subjects with normal hearing. Results of four artifact processing protocols (1) fixed rejection level, (2) adaptive rejection level, (3) weighted averaging and (4) sorted averaging were compared. The results showed a higher normalized SNR with a sorted averaging protocol than with adaptive rejection level and weighted averaging protocols. An advantage of the sorted averaging protocol is that, compared to a fixed-rejection threshold, the ASSR amplitudes were unchanged when the sorted averaging protocol was used, whereas they were significantly reduced by the weighted averaging protocol. The residual noise was also significantly lower for the sorted averaging protocol than for the weighted averaging and adaptive rejection protocols. Thus, the sorted averaging may be a powerful tool to increase the quality of ASSR.

Auditory steady state response in sound field

Objective: Physiological and behavioral responses were compared in normal-hearing subjects via analyses of the auditory steady-state response (ASSR) and conventional audiometry under sound field conditions. Design: The auditory stimuli, presented through a loudspeaker, consisted of four carrier tones (500, 1000, 2000, and 4000 Hz), presented singly for behavioral testing but combined (multiple frequency technique), to estimate thresholds using the ASSR. Study sample: Twenty normal-hearing adults were examined. Results: The average differences between the physiological and behavioral thresholds were between 17 and 22 dB HL. The Spearman rank correlation between ASSR and behavioral thresholds was significant for all frequencies (p < 0.05). Significant differences were found in the ASSR amplitude among frequencies, and strong correlations between the ASSR amplitude and the stimulus level (p < 0.05). Conclusions: The ASSR in sound field testing was found to yield hearing threshold estimates deemed to be reasonably well correlated with behaviorally assessed thresholds.

Steady-state MEG responses elicited by a sequence of amplitude-modulated short tones of different carrier frequencies

The auditory steady-state response (ASSR) is a weak potential or magnetic response elicited by periodic acoustic stimuli with a maximum response at about a 40-Hz periodicity. In most previous studies using amplitude-modulated (AM) tones of stimulus sound, long lasting tones of more than 10 s in length were used. However, characteristics of the ASSR elicited by short AM tones have remained unclear. In this study, we examined magnetoencephalographic (MEG) ASSR using a sequence of sinusoidal AM tones of 0.78 s in length with various tone frequencies of 440–990 Hz in about one octave variation. It was found that the amplitude of the ASSR was invariant with tone frequencies when the level of sound pressure was adjusted along an equal-loudness curve. The amplitude also did not depend on the existence of preceding tone or difference in frequency of the preceding tone. When the sound level of AM tones was changed with tone frequencies in the same range of 440–990 Hz, the amplitude of ASSR varied in a proportional manner to the sound level. These characteristics are favorable for the use of ASSR in studying temporal processing of auditory information in the auditory cortex. The lack of adaptation in the ASSR elicited by a sequence of short tones may be ascribed to the neural activity of widely accepted generator of magnetic ASSR in the primary auditory cortex.

Auditory Steady State Responses in Normal-Hearing and Hearing-Impaired Adults

The aim of this study was to assess the between-session repeatability of auditory steady state response (ASSR) amplitudes and to examine F ratio response detection parameters. Suprathreshold ASSRs were recorded from 20 normal-hearing and 10 hearing-impaired subjects. Amplitudes and latencies were recorded in two test sessions conducted on separate days. The repeatability coefficients (limits of expected variation in repeat measurements) for amplitude and latency of ASSRs were calculated. The test time required for the responses to reach significance at 1%, 2%, and 5% F ratios was analyzed. The percentage false response detection rate was calculated to determine the suitability of current ASSR threshold estimation protocols for use in audiology clinics. The repeatability coefficients for the amplitude of ASSRs were 29 nV for the normal-hearing subjects and 57 nV for the hearing-impaired subjects. The repeatability coefficients for the latency of ASSR were 1.10 msec for the normal-hearing subjects and 1.19 msec for the hearing-impaired subjects. High false-positive detection rates were found for detection procedures that used variable test time ("stop when significance reached" methods). The results of this study showed that ASSR amplitudes are highly variable between test sessions with an average estimated variability in response amplitude of ± 40% for normal-hearing participants and ± 97% for hearing-impaired participants. This could be a possible cause of test-retest differences in ASSR threshold measurements, as it could potentially lead to thresholds that were above the EEG noise level and significant in one test session subsequently falling below the EEG noise level in the repeat test session leading to insignificant response and thus poorer ASSR threshold.

Effective Masking Levels for Bone Conduction Auditory Steady State Responses in Infants and Adults Run down Normal Hearing

To obtain ear-specific bone conduction thresholds, masking of the nontest ear is often required. Bone conduction masking has not been formally investigated for infants assessed physiologically. The objective of this study was to determine effective masking levels (EMLs) for auditory steady state responses (ASSRs) elicited by bone-conducted stimuli in a group of normal-hearing infants and adults. Participants were 15 infants younger than 6 mo and 15 adults, all with normal hearing. EML was defined as the lowest level of a binaural air-conducted masker that resulted in absent bone conduction ASSRs. Stimuli were single bone-conducted tones that were 100% amplitude modulated and 25% frequency modulated at 85 and 101 for 1000 and 4000 Hz, respectively. The stimuli were calibrated in dB HL (ANSI S3.6-1996) and expressed in dB HL or dB SL (dB relative to mean bone conduction ASSR thresholds reported in a previous study). The maskers were 1 and 4 kHz narrowband noise generated by a clinical audiometer. Unmasked and masked ASSRs were obtained for each participant. Real ear-to-coupler differences (RECDs) were also obtained for each participant and were used to convert masker dB SPL measured in the coupler to dB SPL in the individual ear canal. Infant EMLs for ASSRs elicited to bone-conducted stimuli in dB HL were 6 to 7 dB higher and 8 to 10 dB lower for 1000 and 4000 Hz, respectively, compared with adults. When masker was adjusted for RECDs, infant EMLs were 12 dB higher at 1000 Hz and similar at 4000 Hz compared with adults. When the stimulus levels were corrected for the mean differences in ASSR bone conduction thresholds between infants and adults and the masker levels adjusted for RECDs, infants had lower EMLs at 1000 Hz and equal EMLs at 4000 Hz, in comparison to adults. Frequency- and level-dependent effects on ASSR amplitude due to masking were found and differed between infants and adults. Our findings indicate that there are frequency- and level-dependent infant-adult differences in EMLs for bone conduction ASSRs and confirm that a 1000 Hz stimulus is 12 dB more effective for infants compared with adults. The following infant preliminary masking levels for bone conduction stimuli are recommended: (i) 1000 Hz: 48 and 58 dB SPL at 15 and 25 dB HL, respectively, and (ii) 4000 Hz: 40 and 45 dB SPL at 25 and 35 dB HL, respectively.

Is the effect of tinnitus on auditory steady-state response amplitude mediated by attention?

Objectives: Auditory steady-state response (ASSR) amplitude enhancement effects have been reported in tinnitus patients. As ASSR amplitude is also enhanced by attention, the effect of tinnitus on ASSR amplitude could be interpreted as an effect of attention mediated by tinnitus. As N1 attention effects are significantly larger than those on the ASSR, if the effect of tinnitus on ASSR amplitude were due to attention, there should be similar amplitude enhancement effects in tinnitus for the N1 component of the auditory-evoked response. Methods: MEG recordings which were previously examined for the ASSR (Diesch et al., 2010a) were analyzed with respect to the N1m component. Like the ASSR previously, the N1m was analyzed in the source domain (source space projection). Stimuli were amplitude-modulated (AM) tones with one of three carrier frequencies matching the tinnitus frequency or a surrogate frequency 1½ octave above the audiometric edge frequency in controls, the audiometric edge frequency, and a frequency below the audiometric edge. Single AM-tones were presented in a single condition and superpositions of three AM-tones differing in carrier and modulation frequency in a composite condition. Results: In the earlier ASSR study (Diesch et al., 2010a), the ASSR amplitude in tinnitus patients, but not in controls, was significantly larger in the (surrogate) tinnitus condition than in the edge condition. Patients showed less evidence than controls of reciprocal inhibition of component ASSR responses in the composite condition. In the present study, N1m amplitudes elicited by stimuli located at the audiometric edge and at the (surrogate) tinnitus frequency were smaller than N1m amplitudes elicited by sub-edge tones both in patients and controls. The relationship of the N1m response in the composite condition to the N1m response in the single condition indicated that reciprocal inhibition among component N1m responses was reduced in patients compared against controls. Conclusions: In the present study, no evidence was found for an N1-amplitude enhancement effect in tinnitus. Compared to controls, reciprocal inhibition is reduced in tinnitus patients. Thus, as there is no effect on N1m that could potentially be attributed to attention, it seems unlikely that the enhancement effect of tinnitus on ASSR amplitude could be accounted for in terms of attention induced by tinnitus.

Neural plasticity expressed in central auditory structures with and without tinnitus

Sensory training therapies for tinnitus are based on the assumption that, notwithstanding neural changes related to tinnitus, auditory training can alter the response properties of neurons in auditory pathways. To assess this assumption, we investigated whether brain changes induced by sensory training in tinnitus sufferers and measured by electroencephalography (EEG) are similar to those induced in age and hearing loss matched individuals without tinnitus trained on the same auditory task. Auditory training was given using a 5 kHz 40-Hz amplitude-modulated (AM) sound that was in the tinnitus frequency region of the tinnitus subjects and enabled extraction of the 40-Hz auditory steady-state response (ASSR) and P2 transient response known to localize to primary and non-primary auditory cortex, respectively. P2 amplitude increased over training sessions equally in participants with tinnitus and in control subjects, suggesting normal remodeling of non-primary auditory regions in tinnitus. However, training-induced changes in the ASSR differed between the tinnitus and control groups. In controls the phase delay between the 40-Hz response and stimulus waveforms reduced by about 10° over training, in agreement with previous results obtained in young normal hearing individuals. However, ASSR phase did not change significantly with training in the tinnitus group, although some participants showed phase shifts resembling controls. On the other hand, ASSR amplitude increased with training in the tinnitus group, whereas in controls this response (which is difficult to remodel in young normal hearing subjects) did not change with training. These results suggest that neural changes related to tinnitus altered how neural plasticity was expressed in the region of primary but not non-primary auditory cortex. Auditory training did not reduce tinnitus loudness although a small effect on the tinnitus spectrum was detected.

The effect of varying stimulus phase between frequency and amplitude modulation on auditory steady-state responses in neonates

Objective: To compare auditory steady-state responses (ASSRs) to air-conducted amplitude, frequency, and mixed modulated stimuli (AM, FM, and MM, respectively) in neonates. Design: Multiple ASSRs to AM, FM, and MM to 0.5, 1, 2, and 4 kHz tones modulated between 0.078 and 0.092 kHz were recorded and compared. MM phase settings across the cycle at 45° intervals were used and optimum phase settings were predicted using a sinusoidal model. Study sample: Twenty neonates with click ABR thresholds of ≤ 40 dB nHL. Results: ASSR amplitudes were significantly larger to AM than FM stimuli. MM phase setting had a significant effect on amplitude at 1, 2, and 4 kHz but not 0.5 kHz. MM phase settings (± 95% confidence intervals) of 276° (± 9.5°) and 270° (± 19.1°) were predicted for 1 and 2 kHz, respectively. The 0.5 and 4 kHz data were not sufficient to model any effect of phase. MM and AM response latencies increased with decreasing carrier frequency. Some MM response latencies were significantly different from AM response latencies, however no consistent trend was apparent. Test times were significantly affected by phase setting. Conclusions: MM phase settings have a significant effect on ASSR response amplitude and latencies in neonates.

Maturation of the Occlusion Effect: A Bone Conduction Auditory Steady State Response Study in Infants and Adults With Normal Hearing

The aim of this study was to investigate the maturational time course of the occlusion effect in infants with normal hearing. The objectives were (i) to investigate the occlusion effect in a larger group of young infants, (ii) to determine whether the occlusion effect is seen in bone conduction auditory steady state responses (ASSRs) for older infants, and (iii) to investigate the mechanisms that underlie bone conduction hearing in unoccluded and occluded ears in infants by measuring sound pressure in the ear canal. Experiments 1A and 1B: The SPL in the ear canal to 500, 1000, and 2000 Hz bone-conducted pure tones were compared in 22 young infants (0-7 mo), 10 older infants (10-22 mo), and 34 adults, all with normal hearing, for unoccluded and occluded ears. Experiment 2: Bone conduction behavioral thresholds in 17 adults were compared for unoccluded and occluded ears at 500, 1000, 2000, and 4000 Hz. Experiment 3: Bone conduction ASSR thresholds and amplitudes were compared in 22 young infants, 10 older infants, and 20 adults for an unoccluded and occluded test ear. Stimuli were bone-conducted amplitude/frequency-modulated tones presented simultaneously at 500, 1000, 2000, and 4000 Hz. There were significant increases in sound pressure in the ear canal for stimuli presented at 40 dB HL when ears were occluded at 500 and 1000 Hz for all age groups. Infants showed the largest increases in SPL at 500 and 1000 Hz (5-8 dB > adults). Young infants showed no significant decreases in ASSR thresholds (2-6 dB) and amplitudes (0-10 nV) across frequency with occlusion; however, a significant number of infants had an occlusion effect at 500 Hz. Older infants showed a nonsignificant decrease in ASSR thresholds with occlusion (8 dB), a significant increase in ASSR amplitudes at 1000 Hz (6-21 nV), and a significant number of infants with an occlusion effect at 1000 Hz. Adult behavioral thresholds decreased significantly when ears were occluded at 500 and 1000 Hz; for ASSRs, thresholds also decreased (6-7 dB) and amplitudes increased (3-11 nV) at both 500 and 1000 Hz, but the mean trends and statistical findings were not in agreement in all cases. A significant number of adult subjects had an occlusion effect at 500 and 1000 Hz for both behavioral and ASSR thresholds. Our findings suggest that the occlusion effect for ASSR thresholds in young infants is small but emerging at 500 Hz but negligible at 1000 Hz and that the occlusion effect in older infants is emerging at both 500 and 1000 Hz. The clinical implications of these findings are that it is appropriate to conduct bone conduction testing on young infants without compensating for an occlusion effect; however, for older infants, it is prudent to remove insert earphones during bone conduction testing. For both young and older infants, occluding the ear canal increases the sound pressure near the tympanic membrane; however, this pathway appears to contribute less to bone conduction hearing when ears are occluded compared with adults as measured by ASSRs.