Behavioral Sensitivity to Broadband Binaural Localization Cues in the Ferret

Although the ferret has become an important model species for studying both fundamental and clinical aspects of spatial hearing, previous behavioral work has focused on studies of sound localization and spatial release from masking in the free field. This makes it difficult to tease apart the role played by different spatial cues. In humans and other species, interaural time differences (ITDs) and interaural level differences (ILDs) play a critical role in sound localization in the azimuthal plane and also facilitate sound source separation in noisy environments. In this study, we used a range of broadband noise stimuli presented via customized earphones to measure ITD and ILD sensitivity in the ferret. Our behavioral data show that ferrets are extremely sensitive to changes in either binaural cue, with levels of performance approximating that found in humans. The measured thresholds were relatively stable despite extensive and prolonged (>16 weeks) testing on ITD and ILD tasks with broadband stimuli. For both cues, sensitivity was reduced at shorter durations. In addition, subtle effects of changing the stimulus envelope were observed on ITD, but not ILD, thresholds. Sensitivity to these cues also differed in other ways. Whereas ILD sensitivity was unaffected by changes in average binaural level or interaural correlation, the same manipulations produced much larger effects on ITD sensitivity, with thresholds declining when either of these parameters was reduced. The binaural sensitivity measured in this study can largely account for the ability of ferrets to localize broadband stimuli in the azimuthal plane. Our results are also broadly consistent with data from humans and confirm the ferret as an excellent experimental model for studying spatial hearing.     

Acoustic Cues for Sound Source Distance and Azimuth in Rabbits, a Racquetball and a Rigid Spherical Model

There are numerous studies measuring the transfer functions representing signal transformation between a source and each ear canal, i.e., the head-related transfer functions (HRTFs), for various species. However, only a handful of these address the effects of sound source distance on HRTFs. This is the first study of HRTFs in the rabbit where the emphasis is on the effects of sound source distance and azimuth on HRTFs. With the rabbit placed in an anechoic chamber, we made acoustic measurements with miniature microphones placed deep in each ear canal to a sound source at different positions (10–160 cm distance, ±150° azimuth). The sound was a logarithmically swept broadband chirp. For comparisons, we also obtained the HRTFs from a racquetball and a computational model for a rigid sphere. We found that (1) the spectral shape of the HRTF in each ear changed with sound source location; (2) interaural level difference (ILD) increased with decreasing distance and with increasing frequency. Furthermore, ILDs can be substantial even at low frequencies when distance is close; and (3) interaural time difference (ITD) decreased with decreasing distance and generally increased with decreasing frequency. The observations in the rabbit were reproduced, in general, by those in the racquetball, albeit greater in magnitude in the rabbit. In the sphere model, the results were partly similar and partly different than those in the racquetball and the rabbit. These findings refute the common notions that ILD is negligible at low frequencies and that ITD is constant across frequency. These misconceptions became evident when distance-dependent changes were examined.     

Hemispheric Asymmetry of Auditory Steady-State Responses to Monaural and Diotic Stimulation

Amplitude modulations in the speech envelope are crucial elements for speech perception. These modulations comprise the processing rate at which syllabic (∼3–7 Hz), and phonemic transitions occur in speech. Theories about speech perception hypothesize that each hemisphere in the auditory cortex is specialized in analyzing modulations at different timescales, and that phonemic-rate modulations of the speech envelope lateralize to the left hemisphere, whereas right lateralization occurs for slow, syllabic-rate modulations. In the present study, neural processing of phonemic- and syllabic-rate modulations was investigated with auditory steady-state responses (ASSRs). ASSRs to speech-weighted noise stimuli, amplitude modulated at 4, 20, and 80 Hz, were recorded in 30 normal-hearing adults. The 80 Hz ASSR is primarily generated by the brainstem, whereas 20 and 4 Hz ASSRs are mainly cortically evoked and relate to speech perception. Stimuli were presented diotically (same signal to both ears) and monaurally (one signal to the left or right ear). For 80 Hz, diotic ASSRs were larger than monaural responses. This binaural advantage decreased with decreasing modulation frequency. For 20 Hz, diotic ASSRs were equal to monaural responses, while for 4 Hz, diotic responses were smaller than monaural responses. Comparison of left and right ear stimulation demonstrated that, with decreasing modulation rate, a gradual change from ipsilateral to right lateralization occurred. Together, these results (1) suggest that ASSR enhancement to binaural stimulation decreases in the ascending auditory system and (2) indicate that right lateralization is more prominent for low-frequency ASSRs. These findings may have important consequences for electrode placement in clinical settings, as well as for the understanding of low-frequency ASSR generation.     

Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

The discrimination of a change in a stimulus is determined both by the magnitude of that change and by the variability in the neural response to the stimulus. When the stimulus is itself noisy, then the relative contributions of the neural (intrinsic) and stimulus induced variability becomes a critical question. We measured the contribution of intrinsic neural noise and interstimulus variability to the discrimination of interaural time differences (ITDs) and interaural correlation (IAC). We measured discharge rate versus characteristic frequency (CF) tone ITD functions, and CF-centered narrowband noise ITD and IAC functions in interleaved blocks in the same units in the inferior colliculus of urethane-anesthetized guinea pigs. Ten “frozen” tokens of noise were synthesized and the responses to each token were separately analyzed to allow the relative contributions of intrinsic and stimulus variability to be assessed. ITD and IAC discrimination thresholds were determined for a simulated two-interval forced-choice experiment, based on the firing rate distributions, using receiver operating characteristic analysis. On average, between stimulus variability contributed 19% (range, 1.5–30%) of the variance in noise ITD discrimination and 27% (range, 3–50%) in IAC discrimination. Noise ITD thresholds were slightly higher than tone ITD thresholds. Taking the mean of the thresholds for individual noise tokens gave a similar result to pooling across all noise tokens. This implies that although the stimulus induced variability is measurable, it is insignificant in relation to the intrinsic noise in ITD and IAC discrimination.     

Neural Coding of Sound Intensity and Loudness in the Human Auditory System

Inter-individual differences in loudness sensation of 45 young normal-hearing participants were employed to investigate how and at what stage of the auditory pathway perceived loudness, the perceptual correlate of sound intensity, is transformed into neural activation. Loudness sensation was assessed by categorical loudness scaling, a psychoacoustical scaling procedure, whereas neural activation in the auditory cortex, inferior colliculi, and medial geniculate bodies was investigated with functional magnetic resonance imaging (fMRI). We observed an almost linear increase of perceived loudness and percent signal change from baseline (PSC) in all examined stages of the upper auditory pathway. Across individuals, the slope of the underlying growth function for perceived loudness was significantly correlated with the slope of the growth function for the PSC in the auditory cortex, but not in subcortical structures. In conclusion, the fMRI correlate of neural activity in the auditory cortex as measured by the blood oxygen level-dependent effect appears to be more a linear reflection of subjective loudness sensation rather than a display of physical sound pressure level, as measured using a sound-level meter.     

Neural Correlates of an Auditory Afterimage in Primary Auditory Cortex

The Zwicker tone (ZT) is defined as an auditory negative afterimage, perceived after the presentation of an appropriate inducer. Typically, a notched noise (NN) with a notch width of 1/2 octave induces a ZT with a pitch falling in the frequency range of the notch. The aim of the present study was to find potential neural correlates of the ZT in the primary auditory cortex of ketamine-anesthetized cats. Responses of multiunits were recorded simultaneously with two 8-electrode arrays during 1 s and over 2 s after the presentation of a white noise (WN) and three NNs differing by the width of the notch, namely, 1/3 octave (NN1), 1/2 octave (NN2), and 2/3 octave (NN3). Both firing rate (FR) and peak cross-correlation coefficient () were evaluated for time windows of 500 ms. The cortical units were grouped according to whether their characteristic frequency (CF) was inside (In neurons) or outside (Out neurons) a 1-octave-wide frequency band centered on the notch center frequency. The ratios between the FRs and the s for each NN and the WN condition and for each group of neurons were then statistically evaluated. The ratios of FRs were significantly increased during and after the presentation of the NN for the In neurons. In contrast, the changes for the Out neurons were small and most often insignificant. The ratios of the values differed significantly from 1 in the In–In and In–Out groups during stimulation as well as after it. We also found that the s of Out neurons were dependent on the type of NN. Potentially, a combination of increased and increased FR might be a neurophysiological correlate of the ZT.     

Interaural Interaction in the Human Auditory Cortex

We studied the effect of binaural, contralateral and ipsilateral stimulation on middle- and long-latency auditory-evoked magnetic fields using trains of 40-Hz clicks. The stimuli evoked both a transient response (N100m) and a 40-Hz response, which presumably reflects coalescence of middle-latency responses. Binaural stimuli elicited significantly larger 40-Hz responses and sustained fields than contralateral stimuli. N100m amplitudes did not differ between binaural and contralateral stimulation; the dipole moments were even smaller to binaural than contralateral stimuli. Responses to the ipsilateral stimuli were always the smallest.     

Desynchronisation of auditory steady-state responses related to changes in interaural phase differences: an objective measure of binaural hearing

Objective: Binaural processing can be measured objectively as a desynchronisation of phase-locked neural activity to changes in interaural phase differences (IPDs). This was reported in a magnetoencephalography study for 40?Hz amplitude modulated tones. The goal of this study was to measure this desynchronisation using electroencephalography and explore the outcomes for different modulation frequencies. Design: Auditory steady-state responses (ASSRs) were recorded to pure tones, amplitude modulated at 20, 40 or 80?Hz. IPDs switched between 0 and 180° at fixed time intervals. Study sample: Sixteen young listeners with bilateral normal hearing thresholds (≤25?dB HL at 125–8000?Hz) participated in this study. Results: Significant ASSR phase desynchronisations to IPD changes were detected in 14 out of 16 participants for 40?Hz and in 8, respectively 9, out of 13 participants for 20 and 80?Hz modulators. Desynchronisation and restoration of ASSR phase took place significantly faster for 80?Hz than for 40 and 20?Hz. Conclusions: ASSR desynchronisation to IPD changes was successfully recorded using electroencephalography. It was feasible for 20, 40 and 80?Hz modulators and could be an objective tool to assess processing of changes in binaural information.     

Auditory Cortex Electrical Stimulation Suppresses Tinnitus in Rats

Recent clinical studies have demonstrated that auditory cortex electrical stimulation (ACES) has yielded promising results in the suppression of patients’ tinnitus. However, the large variability in the efficacy of ACES-induced suppression across individuals has hindered its development into a reliable therapy. Due to ethical reasons, many issues cannot be comprehensively addressed in patients. In order to search for effective stimulation targets and identify optimal stimulation strategies, we have developed the first rat model to test for the suppression of behavioral evidence of tone-induced tinnitus through ACES. Our behavioral results demonstrated that electrical stimulation of all channels (frequency bands) in the auditory cortex significantly suppressed behavioral evidence of tinnitus and enhanced hearing detection at the central level. Such suppression of tinnitus and enhancement of hearing detection were respectively demonstrated by a reversal of tone exposure compromised gap detection at 10–12, 14–16, and 26–28 kHz and compromised prepulse inhibition at 10–12 and 26–28 kHz. On the contrary, ACES did not induce behavioral changes in animals that did not manifest any behavioral evidence of tinnitus and compromised hearing detection following the same tone exposure. The results point out that tinnitus may be more related to compromised central auditory processing than hearing loss at the peripheral level. The ACES-induced suppression of behavioral evidence of tinnitus may involve restoration of abnormal central auditory processing.     

Cells in Auditory Cortex that Project to the Cochlear Nucleus in Guinea Pigs

Fluorescent retrograde tracers were used to identify the cells in auditory cortex that project directly to the cochlear nucleus (CN). Following injection of a tracer into the CN, cells were labeled bilaterally in primary auditory cortex and the dorsocaudal auditory field as well as several surrounding fields. On both sides, the cells were limited to layer V. The size of labeled cell bodies varied considerably, suggesting that different cell types may project to the CN. Cells ranging from small to medium in size were present bilaterally, whereas the largest cells were labeled only ipsilaterally. In optimal cases, the extent of dendritic labeling was sufficient to identify the morphologic class. Many cells had an apical dendrite that could be traced to a terminal tuft in layer I. Such “tufted” pyramidal cells were identified both ipsilateral and contralateral to the injected CN. The results suggest that the direct pathway from auditory cortex to the cochlear nucleus is substantial and is likely to play a role in modulating the way the cochlear nucleus processes acoustic stimuli.     

Auditory network connectivity in tinnitus patients: A resting-state fMRI study

Abstract

Objective: Resting-state functional magnetic resonance imaging (fMRI) uncovers correlated activity between spatially distinct functionally related brain regions and offers clues about the integrity of functional brain circuits in people with chronic subjective tinnitus. We chose to investigate auditory network connectivity, adopting and extending previously used analyses methods to provide an independent evaluation of replicability. Design: Independent components analysis (ICA) was used to identify coherent patterns arising from spontaneous brain signals within the resting-state data. The auditory network component was extracted and evaluated. Bivariate and partial correlation analyses were performed on pre-defined regions of bilateral auditory cortex to assess functional connectivity. Study sample: Our design carefully matched participant groups for possible confounds, such as hearing status. Twelve patients (seven male, five female; mean age 66 years) all with chronic constant tinnitus and eleven controls (eight male, three female; mean age 68 years) took part. Results: No significant differences were found in auditory network connectivity between groups after correcting for multiple statistical comparisons in the analysis. This contradicts previous findings reporting reduced auditory network connectivity; albeit at a less stringent statistical threshold. Conclusions: Auditory network connectivity does not appear to be reliably altered by the experience of chronic subjective tinnitus.     

Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains

Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners.     

Forward Masking Estimated by Signal Detection Theory Analysis of Neuronal Responses in Primary Auditory Cortex

Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.     

Higher Sensitivity of Human Auditory Nerve Fibers to Positive Electrical Currents

Most contemporary cochlear implants (CIs) stimulate the auditory nerve with trains of amplitude-modulated, symmetric biphasic pulses. Although both polarities of a pulse can depolarize the nerve fibers and generate action potentials, it remains unknown which of the two (positive or negative) phases has the stronger effect. Understanding the effects of pulse polarity will help to optimize the stimulation protocols and to deliver the most relevant information to the implant listeners. Animal experiments have shown that cathodic (negative) current flows are more effective than anodic (positive) ones in eliciting neural responses, and this finding has motivated the development of novel speech-processing algorithms. In this study, we show electrophysiologically and psychophysically that the human auditory system exhibits the opposite pattern, being more sensitive to anodic stimulation. We measured electrically evoked compound action potentials in CI listeners for phase-separated pulses, allowing us to tease out the responses to each of the two opposite-polarity phases. At an equal stimulus level, the anodic phase yielded the larger response. Furthermore, a measure of psychophysical masking patterns revealed that this polarity difference was still present at higher levels of the auditory system and was therefore not solely due to antidromic propagation of the neural response. This finding may relate to a particular orientation of the nerve fibers relative to the electrode or to a substantial degeneration and demyelination of the peripheral processes. Potential applications to improve CI speech-processing strategies are discussed.     

Electrophysiological Validation of a Human Prototype Auditory Midbrain Implant in a Guinea Pig Model

The auditory midbrain implant (AMI) is a new treatment for hearing restoration in patients with neural deafness or surgically inaccessible cochleae who cannot benefit from cochlear implants (CI). This includes neurofibromatosis type II (NF2) patients who, due to development and/or removal of vestibular schwannomas, usually experience complete damage of their auditory nerves. Although the auditory brainstem implant (ABI) provides sound awareness and aids lip-reading capabilities for these NF2 patients, it generally only achieves hearing performance levels comparable with a single-channel CI. In collaboration with Cochlear Ltd. (Lane Cove, Australia), we developed a human prototype AMI, which is designed for electrical stimulation along the well-defined tonotopic gradient of the inferior colliculus central nucleus (ICC). Considering that better speech perception and hearing performance has been correlated with a greater number of discriminable frequency channels of information available, the ability of the AMI to effectively activate discrete frequency regions within the ICC may enable better hearing performance than achieved by the ABI. Therefore, the goal of this study was to investigate if our AMI array could achieve low-threshold, frequency-specific activation within the ICC, and whether the levels for ICC activation via AMI stimulation were within safe limits for human application. We electrically stimulated different frequency regions within the ICC via the AMI array and recorded the corresponding neural activity in the primary auditory cortex (A1) using a multisite silicon probe in ketamine-anesthetized guinea pigs. Based on our results, AMI stimulation achieves lower thresholds and more localized, frequency-specific activation than CI stimulation. Furthermore, AMI stimulation achieves cortical activation with current levels that are within safe limits for central nervous system stimulation. This study confirms that our AMI design is sufficient for ensuring safe and effective activation of the ICC, and warrants further studies to translate the AMI into clinical application.Minoo Lenarz and Hubert H. Lim contributed equally as first authors.     

The choice of stimulation strategy affects the ability to detect pure tone inter-aural time differences in children with early bilateral cochlear implantation

Objectives: To investigate if the interaural time difference (ITD) ability is dependent of stimulation strategy. To examine the correlation between ITD, interaural level differences (ILD) and the ability to localize different sounds.

Methods: Thirty subjects aged 8–13 who were implanted bilaterally before 3 years of age were tested. Twenty of the subjects used processors programmed with fine structure (FS) strategy on both sides. ITD and ILD just noticeable difference (JND) of a 250?Hz pure tone was measured using their clinical processors. Furthermore, their ability to localize sound in the horizontal plane was measured using eye tracking.

Results: Ten of the 20 subjects with FS obtained an ITD threshold compared to none in the group without FS (0/10). ILD JND was correlated to localization ability of the broadband (BB) sound. Mean absolute error of the localization of a low-frequency (LF) sound was larger than that of a BB sound.

Conclusions: The ability to detect ITD was present only when the cochlear implant stimulation had FS. The LF sound was more difficult to localize than the BB sound and ITD ability of FS strategies did not affect the localization ability of either sound. A low ILD seems necessary to improve the localization ability.     

Concurrent Development of the Head and Pinnae and the Acoustical Cues to Sound Location in a Precocious Species,the Chinchilla (<Emphasis Type="Italic">Chinchilla lanigera)</Emphasis>

Sounds are filtered in a spatial- and frequency-dependent manner by the head and pinna giving rise to the acoustical cues to sound source location. These spectral and temporal transformations are dependent on the physical dimensions of the head and pinna. Therefore, the magnitudes of binaural sound location cues—the interaural time (ITD) and level (ILD) differences—are hypothesized to systematically increase while the lower frequency limit of substantial ILD production is expected to decrease due to the increase in head and pinna size during development. The frequency ranges of the monaural spectral notch cues to source elevation are also expected to decrease. This hypothesis was tested here by measuring directional transfer functions (DTFs), the directional components of head-related transfer functions, and the linear dimensions of the head and pinnae for chinchillas from birth through adulthood. Dimensions of the head and pinna increased by factors of 1.8 and 2.42, respectively, reaching adult values by ~6 weeks. From the DTFs, the ITDs, ILDs, and spectral shape cues were computed. Maximum ITDs increased by a factor of 1.75, from ~160 μs at birth (P0-1, first postnatal day) to 280 μs in adults. ILDs depended on source location and frequency exhibiting a shift in the frequency range of substantial ILD (>10 dB) from higher to lower frequencies with increasing head and pinnae size. Similar trends were observed for the spectral notch frequencies which ranged from 14.7–33.4 kHz at P0-1 to 5.3–19.1 kHz in adults. The development of the spectral notch cues, the spatial- and frequency-dependent distributions of DTF amplitude gain, acoustic directionality, maximum gain, and the acoustic axis were systematically related to the dimensions of the head and pinnae. The dimension of the head and pinnae in the chinchilla as well as the acoustical properties associated with them are mature by ~6 weeks.     

Magnetic Fields from the Auditory Cortex of a Deaf Human Individual Occurring Spontaneously or Evoked by Stimulation through a Cochlear Prosthesis

In a postlingually deaf individual, the magnetic field evoked by stimulation through a cochlear prosthesis (extracochlear electrodes) as well as of the spontaneous magnetoencephalogram was measured over the hemisphere contralateral to the prosthesis (CP), and the results were compared with those obtained from normal-hearing subjects. The latency of the 2 best developed waves M₁₀₀ and M₂₀₀ turned out to be prolonged in the CP patient by approximately 40 ms. The amplitude of wave M₁₀₀ was significantly diminished, while wave M₂₀₀ was only poorly developed. Location and direction of the equivalent current dipole (ECD) calculated for wave M₁₀₀ was in good agreement with normal data, whereas the dipole moment was only about one third of the average dipole moment found in normals. Furthermore, evidence was obtained for another magnetic field wave, preceding the delayed auditory wave M₁₀₀, which exhibits the same latency, ECD location and direction as reported in the literature for the somatosensory evoked magnetic field. This wave probably results from stimulation, through the intratympanic electrodes, of somatosensory nerves innervating the tympanic cavity. A potential clinical application of neuromagnetic measurements is discussed: The calculation of the ECD moment from the auditory cortical magnetic field evoked by electrical stimulation at the promontory would allow to estimate, prior to CP implantation, the number of persisting, excitable nerve fibres.     

Auditory brainstem,middle and late latency responses to short gaps in noise at different presentation rates

Objective: The effects of rate on auditory-evoked potentials (AEP) to short noise gaps (12?ms) recorded at high sampling rates using wide-band filters were investigated. Design: Auditory brainstem (ABR), middle latency (MLR), late latency (LLR) and steady-state (ASSR) responses were simultaneously recorded in adult subjects at four gap rates (0.5, 1, 5 and 40?Hz). Major components (V, Na, Pa, Nb, Pb, N1 and P2) were identified at each rate and analysed for latency/amplitude characteristics. Gap responses at 40?Hz were recovered from Quasi-ASSRs (QASSR) using the CLAD deconvolution method. Study Sample: Fourteen right ears of young normal hearing subjects were tested. Results: All major components were present in all subjects at 1?Hz. P1 (P50) appeared as a low-pass filtered component of Pa and Pb waves. At higher rates, N1 and P2 disappeared completely while major ABR-MLR components were identified. Peak latencies were mostly determined by noise onsets slightly delayed by offset responses. Conclusions: Major AEP components can be recorded to short gaps at 1?Hz using high sampling rates and wide-band filters. At higher rates, only ABR and MLRs can be recorded. Such simultaneous recordings may provide a complete assessment of temporal resolution and processing at different levels of auditory pathways.