Amplitude-modulated Tones Research Articles

Loudness and auditory steady-state responses in normal-hearing subjects

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the growth of loudness in listeners with normal hearing. Individual intensity functions were obtained from measures of loudness growth using the contour test and from the electrophysiological amplitude measures of multiple amplitude-modulated (77–105Hz) tones (500, 1000, 2000, and 4000Hz) simultaneously presented to both ears and recorded over the scalp. Slope analyses for the behavioural and electrophysiological intensity functions were separately performed. Response amplitudes of the ASSRs and loudness sensation judgements increase as the stimulus intensity increases for the four frequencies studied. A significant relationship was obtained between loudness and the ASSRs. The results of this study suggest that the amplitude of the ASSRs may be used to estimate loudness growth at least for individuals with normal hearing.

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This study describes the history, stimulus tones, detection techniques, source generators and clinical applications of auditory steady-state responses (ASSR). The most important benefit of ASSR in objective audiometry is that it can provide an accurate assessment of hearing at different audiometric frequencies in a frequency specific manner, if sinusoidally amplitude-modulated tones are used as tonal stimuli. Power spectrum analysis and phase coherence using fast Fourier transform are useful for the automatic detection of threshold because of the sinusoidal response waveform configuration. Because the detectability of ASSR changes under different arousal states, 40-Hz ASSR is suitable for waking adults and 80-Hz ASSR for sleeping children in the assessment of hearing. Regarding the sources of those responses, 40-Hz ASSR is considered the steady-state version of the middle latency response and 80-Hz ASSR is considered the steady-state version of auditory brainstem response (ABR). Bone conduction stimuli are also useful in the assessment of conductive hearing loss, though the responses are not reliable at the intensity level of 60dB or higher. The difficulty in predicting a hearing level of 500Hz or less using the 80-Hz ASSR threshold can be explained by auditory filter. Its advantage is that the thresholds at 4 different frequencies in both ears can be predicted more rapidly than ABR using the multiple simultaneous stimulation technique. In addition, the ASSR to clicks may provide a rapid screening technique. Finally, it may be helpful in assessing suprathreshold hearing to use ASSRs to sounds that sweep their intensity (sweep technique) or actual speech sounds. In this respect, ASSRs may contribute to the objective fitting of hearing aids in young children.

Listeners’ sensitivity to “onset/offset” and “ongoing” interaural delays in high-frequency, sinusoidally amplitude-modulated tones

The relative potency of onset/offset and envelope-based ongoing interaural time delays (ITDs) was assessed using high-frequency stimuli. A two-cue, two-alternative, forced-choice adaptive task was employed to measure threshold ITDs with 100% sinusoidally amplitude-modulated tones centered at 4 kHz. Modulation rates of 125, 250, and 350 Hz were tested with durations of 32, 90, or 240 ms. In the first experiment, ITDs to be detected were imposed only at the onset/offset, only within the ongoing portion, or within both the onset/offset and ongoing portions of the stimuli. Results indicated that ongoing ITDs dominated onset/offset ITDs. The relative potency of ongoing ITDs was directly proportional to duration and inversely proportional to modulation rate. Quantitative analysis suggested that listeners effectively combine onset/offset and ongoing ITDs. Furthermore, the data could be largely accounted for by assuming that listeners attend to the interaural decorrelation of the stimulus resulting from onset/offset and/or ongoing ITDs. A second experiment showed that, (1) overall, an ongoing ITD of one-half period of the envelope had little impact on listeners' sensitivity to delays of the onset/offset and (2) sensitivity to delays within the onset/offset portion of the waveform was reduced by roving the delay within the ongoing portion of the waveform.

The auditory continuity illusion: A parametric investigation and filter model

A sound that is briefly interrupted by a silent gap is perceived as discontinuous. However, when the gap is filled with noise, the sound may be perceived as continuing through the noise. It has been shown that this continuity illusion depends on the masking of the omitted target sound, but the underlying mechanisms have yet to be quantified thoroughly. In this article, we systematically quantify the relation between perceived continuity and the duration, relative power, or notch width of the interrupting broadband noise for interrupted and noninterrupted amplitude-modulated tones at different frequencies. We fitted the psychometric results in order to estimate the range of the noise parameters that induced auditory grouping. To explain our results within a common theoretical framework, we applied a power spectrum model to thedifferent masking resultsand estimated the critical bandwidth of the auditory filter that may be responsible for the continuity illusion. Our results set constraints on the spectral resolution of the mechanisms underlying the continuity illusion and provide a stimulus set that can be readily applied for neurophysiological studies of its neural correlates.

Neural substrate of concurrent sound perception: direct electrophysiological recordings from human auditory cortex

In everyday life, consciously or not, we are constantly disentangling the multiple auditory sources contributing to our acoustical environment. To better understand the neural mechanisms involved in concurrent sound processing, we manipulated sound onset asynchrony to induce the segregation or grouping of two concurrent sounds. Each sound consisted of amplitude-modulated tones at different carrier and modulation frequencies, allowing a cortical tagging of each sound. Electrophysiological recordings were carried out in epileptic patients with pharmacologically resistant partial epilepsy, implanted with depth electrodes in the temporal cortex. Patients were presented with the stimuli while they performed an auditory distracting task. We found that transient and steady-state evoked responses, and induced gamma oscillatory activities were enhanced in the case of onset synchrony. These effects were mainly located in the Heschl's gyrus for steady-state responses whereas they were found in the lateral superior temporal gyrus for evoked transient responses and induced gamma oscillations. They can be related to distinct neural mechanisms such as frequency selectivity and habituation. These results in the auditory cortex provide an anatomically refined description of the neurophysiological components which might be involved in the perception of concurrent sounds.

Analytical model for neuronal responses to amplitude-modulated tones

Event Abstract Back to Event Analytical model for neuronal responses to amplitude-modulated tones Moritz Bürck1*, Laurel H. Carne2 and Leo Van J. Hemmen1 1 Technische Universität München, Germany 2 University of Rochester , United States Many animals, including men, use periodicity information, e.g., amplitude modulations of acoustic stimuli, as a vital cue to auditory object formation. The underlying neuronal mechanisms, however, still remain a matter of debate. Here we present the results of a mathematical analysis based on a model that relies on the interplay of excitation and delayed inhibition. Our analysis shows explicitly how the maximal response of such a system varies systematically with the time constants of excitation and inhibition. The model reliably identifies signal periodicity in the biologically relevant range from 10 to 140 Hz. More importantly, we rely on biologically plausible parameters only and do not need unrealistically long delays to identify low frequencies. The model works best for excitatory and inhibitory neuronal couplings of equal strength, the so-called 'balanced inhibition'. We show why balanced inhibition allows identifying low-frequency signal periodicity, and tentatively explain why it may well play an important role in the formation of and oscillations in biological systems. Conference: Bernstein Symposium 2008, Munich, Germany, 8 Oct - 10 Oct, 2008. Presentation Type: Oral Presentation Topic: All Abstracts Citation: Bürck M, Carne LH and Hemmen LJ (2008). Analytical model for neuronal responses to amplitude-modulated tones. Front. Comput. Neurosci. Conference Abstract: Bernstein Symposium 2008. doi: 10.3389/conf.neuro.10.2008.01.034 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 13 Nov 2008; Published Online: 13 Nov 2008. * Correspondence: Moritz Bürck, Technische Universität München, Munich, Germany, mbuerck@ph.tum.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Moritz Bürck Laurel H Carne Leo Van J Hemmen Google Moritz Bürck Laurel H Carne Leo Van J Hemmen Google Scholar Moritz Bürck Laurel H Carne Leo Van J Hemmen PubMed Moritz Bürck Laurel H Carne Leo Van J Hemmen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Effects of level and background noise on interaural time difference discrimination for transposed stimuli

Just-noticeable interaural time differences were measured for low-frequency pure tones, high-frequency sinusoidally amplitude-modulated (SAM) tones, and high-frequency transposed stimuli, at multiple levels with or without a spectrally notched diotic noise to prevent spread of excitation. Performance with transposed stimuli and pure tones was similar in quiet; however, in noise, performance was poorer for transposed stimuli than for pure tones. Performance with SAM tones was always poorest. In all conditions, performance improved slightly with increasing level. The results suggest that the equivalence postulated between transposed stimuli and pure tones is not valid in the presence of a spectrally notched background noise.

Aging in Binaural Hearing Begins in Mid-Life: Evidence from Cortical Auditory-Evoked Responses to Changes in Interaural Phase

Older adults often have difficulty understanding speech in a noisy environment or with multiple speakers. In such situations, binaural hearing improves the signal-to-noise ratio. How does this binaural advantage change with increasing age? Using magnetoencephalography, we recorded cortical activity evoked by changes in interaural phase differences of amplitude-modulated tones. These responses occurred for frequencies up to 1225 Hz in young subjects but only up to 940 Hz in middle-aged and 760 Hz in older adults. Behavioral thresholds also decreased with increasing age but were more variable, likely because some older adults make effective use of compensatory mechanisms. The reduced frequency range for binaural hearing became significant in middle age, before decline in hearing sensation and the morphology of cortical responses, which became apparent only in the older subjects. This study provides evidence from human physiological data for the early onset of biological aging in binaural hearing.

Reliability and frequency specificity of auditory steady-state response detected by phase spectral analysis

Automatic threshold detection techniques are described for auditory steady-state response (ASSR) elicited with a sinusoidally amplitude-modulated tone. The reliability and frequency specificity of ASSR are discussed. When applied to awake adults and detected by phase spectral analysis, 40-Hz ASSR threshold patterns closely resemble their corresponding audiograms. However, 40-Hz ASSR is insufficiently reliable for determining hearing thresholds in young children during sleep. On the other hand, 80-Hz ASSR is detected clearly in sleeping children. Moreover, 80-Hz ASSR threshold patterns also closely resemble the corresponding audiograms. Therefore, 80-Hz ASSR appears to be useful for objective audiometry in children.

Detection and differentiation of sensorineural hearing loss in mice using auditory steady-state responses and transient auditory brainstem responses

Sensorineural hearing loss (SNHL) comprises hearing disorders with diverse pathologies of the inner ear and the auditory nerve. To date, an unambiguous phenotypical characterization of the specific pathologies in an affected individual remains impossible. Here, we evaluated the use of scalp-recorded auditory steady-state responses (ASSR) and transient auditory brainstem responses (ABR) for differentiating the disease mechanisms underlying sensorineural hearing loss in well-characterized mouse models. We first characterized the ASSR evoked by sinusoidally amplitude-modulated tones in wild-type mice. ASSR were robustly elicited within three ranges of modulation frequencies below 200 Hz, from 200 to 600 Hz and beyond 600 Hz in most recordings. Using phase information we estimated the apparent ASSR latency to be about 3 ms, suggesting generation in the auditory brainstem. Auditory thresholds obtained by automated and visual analysis of ASSR recordings were comparable to those found with tone-burst evoked ABR in the same mice. We then recorded ASSR and ABR from mouse mutants bearing defects of either outer hair cell amplification (KCNQ4-knockout) or inner hair cell synaptic transmission (Bassoon-mutant). Both mutants showed an increase of ASSR and ABR thresholds of approximately 40 dB versus wild-type when investigated at 8 weeks of age. Mice with defective amplification displayed a steep rise of ASSR and ABR amplitudes with increasing sound intensity, presumably reflecting a strong recruitment of synchronously activated neural elements beyond threshold. In contrast, the amplitudes of ASSR and ABR responses of mice with impaired synaptic transmission grew very little with sound intensity. In summary, ASSR allow for a rapid, objective and frequency-specific hearing assessment and together with ABR and otoacoustic emissions can contribute to the differential diagnosis of SNHL.

Human Auditory Steady-State Responses During Sweeps of Intensity

To record steady-state responses to amplitude-modulated tones that change their intensity over time and to see how well behavioral thresholds can be estimated from such responses. The intensity of the stimuli used in this experiment increased from 25 to 75 dB SPL for 8 sec and then decreased back to 25 dB HL during the subsequent 8 sec. Responses to this intensity sweep were averaged and then analyzed using a short-time Fast-Fourier Transform to measure how the amplitude and phase of the responses changed with intensity. One experimental condition presented single 2-kHz tones to the left ear; a second condition examined the use of simultaneously presented multiple tones (0.5, 1, 2, and 4 kHz) to the left ear; a third condition used multiple tones presented dichotically; and a fourth condition presented the multiple dichotic tones in masking noise to simulate either low-frequency (less than 1400 Hz) or high-frequency (greater than 1400 Hz) hearing loss. Physiological thresholds were determined using six different algorithms and the relations between physiological and behavioral thresholds were evaluated to see how well behavioral thresholds could be estimated. The amplitude-intensity functions for the 1 and 2 kHz responses both demonstrated a plateau at higher intensities in the multiple-stimulus conditions but not in the single-stimulus condition. The slope of the amplitude-intensity functions varied significantly with the carrier frequency of the stimulus: 1.30 at 500 Hz, 0.87 at 1000 Hz, 0.75 at 2000 Hz, and 1.40 at 4000 Hz. The slope of the phase-intensity function averaged 1.16 degrees per dB and did not vary with carrier frequency. Estimates of latency, however, indicated that latency increased with decreasing carrier frequency and with decreasing intensity. The performance of the threshold estimating algorithms differed between normal hearing and simulated hearing loss, since the amplitude- and phase-intensity functions in the latter condition were not linear. Physiological-behavioral threshold differences were generally greater for normal hearing than for simulated hearing loss. Linear regression provided the least physiological-behavioral difference but was quite variable during simulated hearing loss. Simply defining threshold as the lowest intensity above which all responses were significantly different from residual EEG noise was the most accurate method in terms of yielding the least standard deviation of the physiological-behavioral difference with an average standard deviation of 10 dB, provided EEG noise levels were low enough in the normal hearing condition. Thresholds can be estimated using intensity sweeps with about the same accuracy as recording separate responses to discrete intensities. Sweep recordings provide additional information about the responses at suprathreshold intensities by clearly determining amplitude- and phase- intensity functions at these intensities.

Response Properties and Location of Neurons Selective for Sinusoidal Frequency Modulations in the Inferior Colliculus of the Big Brown Bat

Most animal vocalizations, including echolocation signals used by bats, contain frequency-modulated (FM) components. Previous studies have described a class of neurons in the inferior colliculus (IC) of the big brown bat that respond exclusively to sinusoidally frequency modulated (SFM) signals and fail to respond to pure tones, noise, amplitude-modulated tones, or single FM sweeps. The aims of this study were to further characterize these neurons' response properties and to determine whether they are localized within a specific area of the IC. We recorded extracellularly from 214 neurons throughout the IC. Of these, 47 (22%) responded exclusively to SFM. SFM-selective cells were tuned to relatively low carrier frequencies (9-50 kHz), low modulation rates (20-210 Hz), and shallow modulation depths (3-10 kHz). Most had extremely low thresholds, with an average of 16.5 +/- 7.6 dB SPL, and 89% had upper thresholds and closed response areas. For SFM-selective cells with spontaneous activity, the spontaneous activity was eliminated when sound amplitude exceeded their upper threshold and resumed after the stimulus was over. These findings suggest that SFM-selective cells receive low-threshold excitatory inputs and high-threshold inhibitory inputs. SFM-selective cells were clustered in the rostrodorsal part of the IC. Within this area, best modulation rate appeared to be correlated with best carrier frequency and depth within the IC.

Chopper unit responses to amplitude-modulated tones: does stochastic mode-locking theory allow a more accurate characterisation of observed temporal structure?

Responses to amplitude-modulated pure tones have been used extensively to assess temporal properties of neurons across the auditory system. The synchronisation to the modulation frequency has mostly been measured by an index called vector strength. This index is based on the distribution of spikes along the period of modulation. One obtains a low value when the spikes occur evenly across the period and a high value when they are sharply distributed around a single time. In the ventral cochlear nucleus, chopper units have been found to show band-pass temporal responses at high sound pressure level and low-pass temporal responses at low level [1]. However, the fine structure of the responses remains uncharacterised. Here, we show that data obtained from chopper neurons in response to amplitude-modulated tone exhibit more complex synchronised discharge patterns, reminiscent of mode-locked states. These responses can be organised around an Arnol'd tongue structure of a periodically forced model accounting for the sub-threshold properties of the T-multipolar cells. Numerical simulations of a stochastic version of this integrate-and-fire model give response patterns similar to the one observed experimentally. Thus, the results tend to show that care should be taken when considering the temporal properties of a neuron only on the basis of its vector strength.

Simultaneously measured behavioral and electrophysiological hearing thresholds in a bottlenose dolphin (Tursiops truncatus)

Dolphin auditory thresholds obtained via evoked potential audiometry may deviate from behavioral estimates by 20 dB or more. Differences in the sound source, stimulus presentation method, wave form, and duration may partially explain these discrepancies. To determine the agreement between behavioral and auditory evoked potential (AEP) threshold estimates when these parameters are held constant, behavioral and AEP hearing tests were simultaneously conducted in a bottlenose dolphin. Measurements were made in-air, using sinusoidal amplitude-modulated tones continuously projected via a transducer coupled to the pan region of the dolphin's lower jaw. Tone trials were presented using the method of constant stimuli. Behavioral thresholds were estimated using a 50% correct detection. AEP thresholds were based on the envelope following response and 50% correct detection. Differences between AEP and behavioral thresholds were within +/-5 dB, except at 10 kHz (12 dB), 20 kHz (8 dB), 30 kHz (7 dB), and 150 kHz (24 dB). In general, behavioral thresholds were slightly lower, though this trend was not significant. The results demonstrate that when the test environment, sound source, stimulus wave form, duration, presentation method, and analysis are consistent, the magnitude of the differences between AEP and behavioral thresholds is substantially reduced.

Phase dynamics in the 40-Hz auditory steady state response

The phase of the 40-Hz auditory steady-state response (SSR) evoked by amplitude-modulated tones and recorded by MEG decreases monotonically with increasing carrier frequency, shifting about 6.25 ms or 90° over the range 250–4000 Hz. This phase shift has been attributed to propagation delays along the basilar membrane. Here we report that the slope of the SSR phase/carrier frequency characteristic is not invariant but depends on contextual factors relating to stimulus presentation. When steady-state stimuli were presented continuously with a random change to one of four carriers occurring every second, the phase difference between 500 and 1000 Hz was amplified by a factor of 2.5 compared to when the same frequencies were presented without shifting (baseline). Changing carrier frequencies in a predictable fixed order once per second resulted in a phase separation 1.6 times larger than baseline. We also tested whether a carrier frequency change was required to generate this wider phase separation. We presented continuous 15 min blocks at constant carrier frequencies of 500 or 1000 Hz, interrupted regularly once per second by increasing the amplitude of one modulation cycle. Once about every three seconds the enhancement occurred in only one ear, yielding a distinctive percept (target). The phase difference between 500 and 1000 Hz increased by 1.54 compared to baseline, regardless of whether the subject was attending to the target. Taken together, these results suggest that any disturbance in an on-going steady stimulus is sufficient to change the phase of the SSR for a period of time lasting on the order of several seconds or longer. The mechanisms responsible are unknown although a contribution of cortical dynamics to SSR phase is implied.

Comparison of Responses of Neurons in the Mouse Inferior Colliculus to Current Injections, Tones of Different Durations, and Sinusoidal Amplitude-Modulated Tones

We made in vivo whole cell patch-clamp recordings from the inferior colliculus of young-adult, anesthetized C57/Bl6 mice to compare the responses to constant-current injections with the responses to tones of different duration or to sinusoidal amplitude-modulated (SAM) tones. We observed that voltage-dependent ion channels contributed in several ways to the response to tones. A sustained response to long tones was observed only in cells showing little accommodation during current injection. Cells showing burst-onset firing during current injection showed a small response to SAM tones, whereas burst-sustained cells showed a good response to SAM tones. The hyperpolarization-activated nonselective cation channel I(h) had a special role in shaping the responses: I(h) was associated with an increased excitability, with chopper and pauser responses, and with an afterhyperpolarization following tones. Synaptic properties were more important in determining the responses to tones of different durations. A short-latency inhibitory response appeared to contribute to the long-pass response in some cells and short-pass and band-pass neurons were characterized by their slow recovery from synaptic adaptation. Cells that recovered slowly from synaptic adaptation showed a relatively small response to SAM tones. Our results show an important role for both intrinsic membrane properties -- most notably the presence of I(h) and the extent of accommodation -- and synaptic adaptation in shaping the response to tones in the inferior colliculus.

Within- and across-listener performance variability in discrimination of interaural time and level differences

The ability to discriminate changes in interaural time differences (ITDs) and interaural level differences (ILDs) varies markedly both within and across individuals. Such variability has been frequently noted, but rarely systematically investigated. Here, both forms of variability, expressed in terms of coefficients of variation (mean divided by standard deviation), were examined across a variety of ITD and ILD conditions in a total of 84 naive listeners. Three consistent patterns emerged. First, across-listener variability was relatively constant across ITD and ILD discrimination, suggesting that this variability might be determined by factors common to the processing of both cues. Second, compared to across-listener variability, within-listener variability was larger for the ITD conditions but smaller for the ILD conditions, suggesting that, within individuals, ITD processing may have a higher level of internal noise than ILD processing. Third, stimulus frequency (ranging from 0.3 to 6 kHz) and stimulus type (pure tones and sinusoidally amplitude modulated tones) had little influence on either type of variability, suggesting that variability was determined by cue-dependent, rather than stimulus-dependent mechanisms. The systematic nature of these variability patterns illustrates that examination of variability can provide important insights into the processing of interaural cues. [Work supported by NIH.]

Similar patterns of learning and performance variability for human discrimination of interaural time differences at high and low frequencies

Sound source localization on the horizontal plane is primarily determined by interaural time differences (ITDs) for low-frequency stimuli and by interaural level differences (ILDs) for high-frequency stimuli, but ITDs in high-frequency complex stimuli can also be used for localization. Of interest here is the relationship between the processing of high-frequency ITDs and that of low-frequency ITDs and high-frequency ILDs. A few similarities in human performance with high- and low-frequency ITDs have been taken as evidence for similar ITD processing across frequency regions. However, such similarities, unless accompanied by differences between ITD and ILD performance on the same measure, could potentially reflect processing attributes common to both ITDs and ILDs rather than to ITDs only. In the present experiment, both learning and variability patterns in human discrimination of ITDs in high-frequency amplitude-modulated tones were examined and compared to previously obtained data with low-frequency ITDs and high-frequency ILDs. Both patterns for high-frequency ITDs were more similar to those for low-frequency ITDs than for high-frequency ILDs. These results thus add to the evidence supporting similar ITD processing across frequency regions, and further suggest that both high- and low-frequency ITD processing is less modifiable and more noisy than ILD processing.

Perception of roughness by listeners with sensorineural hearing loss

The perception of auditory roughness presumably results from imperfect spectral or temporal resolution. Sensorineural hearing loss, by affecting spectral resolution, may therefore alter roughness perception. In this study, normal-hearing and hearing-impaired listeners estimated the roughness of amplitude-modulated tones varying in carrier frequency, modulation rate, and modulation depth. Their judgments were expected to reflect effects of impaired spectral resolution. Instead, their judgments were similar, in most respects, to those of normally-hearing listeners, except at very slow modulation rates. Results suggest that mild-to-moderate sensorineural hearing loss increases the roughness of slowly fluctuating signals.

Physiological detection of interaural phase differences

Auditory evoked cortical responses to changes in the interaural phase difference (IPD) were recorded using magnetoencephalography (MEG). Twelve normal-hearing young adults were tested with amplitude-modulated tones with carrier frequencies of 500, 1000, 1250, and 1500 Hz. The onset of the stimuli evoked P1m-N1m-P2m cortical responses, as did the changes in the interaural phase. Significant responses to IPD changes were identified at 500 and 1000 Hz in all subjects and at 1250 Hz in nine subjects, whereas responses were absent in all subjects at 1500 Hz, indicating a group mean threshold for detecting IPDs of 1250 Hz. Behavioral thresholds were found at 1200 Hz using an adaptive two alternative forced choice procedure. Because the physiological responses require phase information, through synchronous bilateral inputs at the level of the auditory brainstem, physiological "change" detection thresholds likely reflect the upper limit of phase synchronous activity in the brainstem. The procedure has potential applications in investigating impaired binaural processing because phase statistic applied to single epoch MEG data allowed individual thresholds to be obtained.