The auditory evoked "off" response: sources and comparison with the "on" and the "sustained" responses

OBJECTIVE

It is well known that tone bursts elicit a prominent N1/P2 complex in the auditory evoked potential (the on-response), but less is known about a morphologically similar complex (the off-response) that can be recorded under suitable stimulus conditions. The interaction between the two responses indicated that the responses were not physiologically independent. The present experiment employed neuromagnetic methods to determine the cortical sources of N1 and P2 on- and off-responses and their relation to other events observed in the auditory evoked field.

DESIGN

Five female and five male subjects with no history of otologic or neurological disorders and with normal audiological status participated in this study. Tone bursts of 2 sec duration (10 msec rise and decay time, cosine function), carrier frequency of 1 kHz, and intensity of 60 dB nHL (normative hearing level) were presented 512 times to the subject's right ear (contralateral to the investigated hemisphere) with an interstimulus interval randomized between 5 and 7 sec.

RESULTS

The present study is unique in that several components of the complex auditory evoked response (P1, N1on, P2on, sustained-field, N1off, P2off) were recorded and localized in the same subjects and in the same experiment. The source coordinates obtained for N1 and P2 on- and off-responses indicated that the two responses are generated by overlapping cortical regions. Sources for the P2 components were situated anterior and medial to sources for the N1 components and were indistinguishable from sources for the auditory sustained-field. An early P1on event preceded the N1on (but not the N1off) response and was spatially indistinguishable from the N1on. The equivalent source strength was greater for N1on and P2on sources compared with N1off and P2off sources.

CONCLUSIONS

The recoding process signaled by on-and off-responses may be a dynamic form of plasticity in the auditory cortex with a time constant on the order of hundreds of milliseconds, corresponding to the duration of sustained-responses released by acoustic changes and to the duration of the acoustic foreperiod that is necessary before on-and off-responses to acoustic changes can be observed.

High-frequency cortical responses reflect lexical processing: an MEG study

Meaningful words and matched pseudowords, such as moon vs. noom, are of equal perceptual complexity, but invoke different cognitive processes. To investigate high-frequency cortical responses to these stimuli, biomagnetic signals were recorded simultaneously over both hemispheres of right-handed individuals listening to words and pseudowords. Consistent with earlier EEG studies, evoked spectral responses recorded from the left hemisphere revealed depression of spectral power in the low gamma band (around 30 Hz) after pseudowords but not after words. Similar differences between stimulus categories were present in the beta range. These results indicate that distinct patterns of high-frequency cortical responses correspond to the different cognitive processes invoked by words and pseudowords. It is hypothesized that differential high-frequency cortical responses signal the activation or activation failure of distributed Hebbian cell assemblies representing words and other elements of cognitive processing.

Cortical reorganization in human amputees and mislocalization of painful stimuli to the phantom limb

In human arm amputees, a significant relationship was found between the amount of reorganization in the primary somato-sensory cortex, and the amount of body surface from which painful stimuli evoked sensations that were perceived to be emanating from the now missing extremity, i.e. the phantom limb. This mislocalization could be evoked almost equally from stimulation of either side of the body. Based on these findings obtained by magnetic source imaging and psychophysical testing in eight amputees, it is concluded that the extent of the generally known cortical reorganization contralateral to the amputation is an indicator of more widespread plastic changes in the brain involving bilateral pathways.

Increased cortical representation of the fingers of the left hand in string players

Magnetic source imaging revealed that the cortical representation of the digits of the left hand of string players was larger than that in controls. The effect was smallest for the left thumb, and no such differences were observed for the representations of the right hand digits. The amount of cortical reorganization in the representation of the fingering digits was correlated with the age at which the person had begun to play. These results suggest that the representation of different parts of the body in the primary somatosensory cortex of humans depends on use and changes to conform to the current needs and experiences of the individual.

Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation

Although phantom-limb pain is a frequent consequence of the amputation of an extremity, little is known about its origin. On the basis of the demonstration of substantial plasticity of the somatosensory cortex after amputation or somatosensory deafferentation in adult monkeys, it has been suggested that cortical reorganization could account for some non-painful phantom-limb phenomena in amputees and that cortical reorganization has an adaptive (that is, pain-preventing) function. Theoretical and empirical work on chronic back pain has revealed a positive relationship between the amount of cortical alteration and the magnitude of pain, so we predicted that cortical reorganization and phantom-limb pain should be positively related. Using non-invasive neuromagnetic imaging techniques to determine cortical reorganization in humans, we report a very strong direct relationship (r = 0.93) between the amount of cortical reorganization and the magnitude of phantom limb pain (but not non-painful phantom phenomena) experienced after arm amputation. These data indicate that phantom-limb pain is related to, and may be a consequence of, plastic changes in primary somatosensory cortex.

Evoked and induced gamma-band activity of the human cortex

The evoked gamma-band activity is an event related rhythmic response which persists within the first 100 ms after the stimulus onset. It shows spectral peaks between 30 and 40 Hz in the auditory, between 45 and 55 Hz in the somatosensory and between 100 and 110 Hz in the visual system. After separation of the wide-band activity in slow and gamma-band activity, a moving single equivalent current dipole model accounts for each activity almost completely. The induced gamma-band activity is not phase-locked to the stimulus or it is strongly gittering and thus it cannot be extracted in time domain. In this case we are using signal analysis methods in frequency domain. The evaluation of the induced brain gamma-band activity around 30 Hz shows differences to word and nonword stimuli. It was supposed that the induced gamma-band activity represents the synchronized activity of Hebbian cell assemblies correlated to words.

Magnetic and electric brain activity evoked by the processing of tone and vowel stimuli

Sustained magnetic and electric brain waves may reflect linguistic processing when elicited by auditory speech stimuli. In the present study, only in the latency interval subsequent to the N1m/N1 has a sensitivity of brain responses to features of speech been demonstrated. We conclude this from studying the auditory-evoked magnetic field (AEF) and the corresponding evoked potential (AEP) in response to vowels and a tone. Brain activity was recorded from the left and the right hemisphere of 11 subjects. Three aspects of transient activity were examined: (1) the amplitudes and source characteristics of the N1m component of the AEF; (2) the amplitudes and source characteristics of the sustained field (SF), and (3) the corresponding amplitude characteristics of the AEP. Sustained potential amplitudes and SF root mean square amplitudes, as well as the dipole strength of the SF source, were found to be larger for vowel-evoked signals than for signals elicited by the tone stimulus. The amplitude and dipole strength effects had an interaction with hemisphere, with larger interhemispheric differences for the vowel condition, as well as larger tone-vowel differences of these parameters in the speech-dominant left hemisphere. No statistically significant hemisphere-by-stimulus-type interactions were found in N1/N1m amplitudes and N1m source parameters.

Specific tonotopic organizations of different areas of the human auditory cortex revealed by simultaneous magnetic and electric recordings

This paper presents data concerning auditory evoked responses in the middle latency range (wave Pam/Pa) and slow latency range (wave N1m/N1) recorded from 12 subjects. It is the first group study to report multi-channel data of both MEG and EEG recordings from the human auditory cortex. The experimental procedure involved potential and current density topographical brain mapping as well as magnetic and electric source analysis. Responses were compared for the following 3 stimulus frequencies: 500, 1000 and 4000 Hz. It was found that two areas of the auditory cortex showed mirrored tonotopic organization; one area, the source of N1m/N1 wave, exhibited higher frequencies at progressively deeper locations, while the second area, the source of the Pam/Pa wave, exhibited higher frequencies at progressively more superficial locations. The Pa tonotopic map was located in the primary auditory cortex anterior to the N1m/N1 mirror map. It is likely that N1m/N1 results from activation of secondary auditory areas. The location of the Pa map in A1, and its N1 mirror image in secondary auditory areas is in agreement with observations from animal studies.

Comparison of magnetic and metabolic brain activity during a verb generation task

The magnetic and metabolic activational patterns of the brain during the perception, generation and silent articulation of words overlap to some extent, yet also measure concrete activational patterns. In the present study, auditory evoked magnetic fields (MEG) and changes in regional cerebral blood flow (PET) were examined in healthy subjects during a verb generation task. The aim of the study was to determine whether the advantages of both recording techniques can be combined so as to identify distributed sources of brain activity during particular tasks such as language processing. Given the currently observed disparity of the results from the two types of brain imaging we conclude that PET data will most likely not provide physiologically meaningful constraints for the distributed source analysis of MEG data, and may not necessarily validate results of distributed source analyses.

The magnetic counterpart of the contingent negative variation

The magnetic counterpart of the CNV, the contingent magnetic variation (CMV), was investigated in an Go/No Go design: subjects moved their index finger to the offset of a 4 sec tone of a certain frequency in the Go condition and were asked not to move during presentation of a 4 sec tone of different frequency in the No Go condition. During the preparatory interval, both the CMV and the electrical wave form followed a similar time course and both produced an equally pronounced statistical difference between conditions (Go and No Go). Compared to the variability in the auditory evoked fields, the CMV showed considerably more variance in the field distribution across subjects. The polarity reversal across the temporal surface of the head and the pronounced amplitudes over inferior temporal areas led us to conclude that a significant temporal activity contributes to both the late and the early CMV. However, neither for the early nor for the late CMV component did a single equivalent dipole prove to be a satisfying model. The data are consistent with the suggestion that the earlier as well as the later aspects of the CMV are fed through distributed sources in motoric, sensory and association areas, a distribution with considerable intersubject variability.

Intrasubject reliability and validity of somatosensory source localization using a large array biomagnetometer

Neuromagnetic fields were evoked by tactile stimuli and detected with a multi-channel biomagnetometer through 72 independent repetitive measurements on a single subject. Each measurement consisted of a somatosensory evoked response (N = 256 stimuli) using a single probe placement. These fields were then analyzed for source localization using an equivalent current dipole model and demonstrated highly reliable localizations. The 3 major neuromagnetic somatosensory response components peaking at 35, 65 and 110 msec all localized to the same area of cortex. The relative contributions of intrinsic brain activity, habituation, probe placement, and choice of fiduciary points for headframe determination were quantified. Intrinsic factors were found to constitute the major source of inter-measurement error. Sources localized by magnetic source imaging (MSI) appeared valid relative to neuroanatomical estimation of the central fissure on MRI. Non-invasive presurgical biomagnetic localization of somatosensory cortex produces reliable and valid functional localizations which can be of potential value in risk assessment and may provide a useful guide for invasive functional mapping.

The auditory evoked sustained field: origin and frequency dependence

A sound lasting for several seconds is known to elicit a baseline shift in electrical and magnetic records. We have studied the dependence of the magnetic field distribution of this "per-stimulatory" sustained field (SF) on tone frequency. Tone bursts of 2 sec duration and 60 dB nHL intensity were presented to 11 subjects at varying interstimulus intervals between 5 and 7 sec. The carrier frequencies of 250, 1000 and 4000 Hz varied randomly from trial to trial. The field distributions obtained are consistent with the view that the auditory evoked sustained field activity originates in the supratemporal cortex. Differences in the locations of equivalent current dipoles of the SF from those of the M100 wave of the slow auditory evoked field are consistent across subjects. The SF source locations corresponding to stimulus frequencies over an extended frequency range are arranged in a tonotopic manner and support the idea that the sources of the M100 and the SF are current dipole sheets located on the superior surface of the primary auditory cortex.

Modulation of early sensory processing in human auditory cortex during auditory selective attention

Neuromagnetic fields were recorded from human subjects as they listened selectively to sequences of rapidly presented tones in one ear while ignoring tones of a different pitch in the opposite ear. Tones in the attended ear evoked larger magnetic brain responses than did unattended tones in the latency ranges 20-50 msec and 80-130 msec poststimulus. Source localization techniques in conjunction with magnetic resonance imaging placed the neural generators of these early attention-sensitive brain responses in auditory cortex on the supratemporal plane. These data demonstrate that focused auditory attention in humans can selectively modulate sensory processing in auditory cortex beginning as early as 20 msec poststimulus, thereby providing strong evidence for an "early selection" mechanism of auditory attention that can regulate auditory input at or before the initial stages of cortical analysis.

Relationship of transient and steady-state auditory evoked fields

Transient and steady-state auditory evoked fields (AEFs) to brief tone pips were recorded over the left hemisphere at 7 different stimulus rates (0.125-39 Hz) using a 37-channel biomagnetometer. Previous observations of transient auditory gamma band response (GBR) activity were replicated. Similar rate characteristics and equivalent dipole locations supported the suggestion that the steady-state response (SSR) at about 40 Hz represents the summation of successive overlapping (10 Hz) middle latency responses (MLRs). On the other hand, differences in equivalent dipole locations and habituation effects suggest that the magnetically recorded GBR is a separate phenomenon which occurs primarily at low stimulus rates and is unrelated to either the magnetically recorded MRL or SSR.

Magnetoencephalography using high temperature rf SQUIDs

We have developed high-critical-temperature radio-frequency Super conducting QUantum Interference Devices (SQUIDs) with step-edge grain-boundary Josephson junctions and large flux focusers. These planar devices were fabricated from epitaxial YBa2Cu3O7 films and operated in the magnetometer and first-order gradiometer configurations while immersed in liquid nitrogen. At the temperature of 77K, we have attained a magnetic field resolution for the magnetometer better than 200 fT/Hz1/2 down to less than 1 Hz, i.e., over the low signal frequency range important for medical diagnostics. The results to date show a high promise for biomagnetic diagnostics. For the first time, we recorded the evoked responses from human brains using a high-temperature magnetometer and a first-order electronic gradiometer channel simultaneously. These results were obtained in a magnetically shielded room. An improvement in the magnetic field resolution by another order of magnitude is possible and probable.

Evoked magnetic responses of the human auditory cortex to minor pitch changes: localization of the mismatch field

The neuromagnetic source localizations of the auditory M100 and the mismatch field (MMF) were studied using a large-array biomagnetometer. Standard tones of 1000 Hz and deviant tones of 1050 Hz were delivered with 90% and 10% probability, respectively. Wave forms of the derived MMF were computed by examining difference wave forms between the responses to the deviants and the responses to the standards preceding (D-P) and following (D-F) the deviants as well as to all remaining standards (D-A). The subset of standards preceding the deviants was used for a more realistic comparison with the set of deviants (having the same number of epochs and a similar signal-to-noise ratio), while the subset of standards following the deviants served to answer the question whether those standards also elicit an MMF. The MMF deflections were compared with each other, with the "native" MMF occurring in response to the deviants, and with wave M100. (The MMF as it appears in the unprocessed response to the deviants was termed "native" for an easy distinction from the "derived" MMF.) Our results demonstrate a distinct MMF deflection, corresponding in latency to the simultaneously recorded fronto-central electrical MMN. Source analysis, using a single moving dipole model, showed the same spatial localization for the native MMF and for the different derived MMFs. The MMF source location turned out to be significantly anterior, medial and inferior relative to the sources of the M100. The present data also demonstrate that a minor frequency deviation may not activate measurably different M100 generators, yet be sufficient to trigger the nearby but spatially distinct mismatch generator.

Human auditory evoked gamma-band magnetic fields

We have discovered a ca. 40-Hz transient magnetic oscillatory response, evoked in the human brain by the onset of auditory stimuli, consisting of four or more cycles locked in phase to stimulus onset in approximately the 20- to 130-ms poststimulus interval. The response originates in the supratemporal auditory cortex, some millimeters deeper and anterior to the source of the larger-amplitude slow-wave M100 component of the evoked magnetic field and moves in a posterior arcing trajectory 1 cm or more in length. The oscillatory cortical activation elicited by auditory stimuli may be similar to the gamma-band cortical oscillations elicited by olfactory and visual stimuli and may represent an essential component of auditory perceptual processing.

A timesaving BERA technique for frequency-specific assessment of the auditory threshold through tone-pulse series stimulation (TOPSTIM) with simultaneous gliding high-pass noise masking (GHINOMA)

A new stimulation paradigm is described for eliciting frequency-specific auditory brainstem responses (ABR) by stimulation with a series of seven Gaussian-shaped tone pulses with carrier frequencies descending, in half-octave steps, from 4,000 to 500 Hz, and an interstimulus interval between consecutive pulses of 18 ms. The pause between two consecutive series is 54 ms so that the interval between two tone pulses of the same frequency is 162 ms (stimulus repetition rate approximately 6/s). Simultaneously a high-pass noise masker is presented whose lower cut-off frequency is continuously diminished in such a way that, when a tone pulse is presented, the cut-off frequency of the masker is exactly one octave above the carrier frequency of the pulse. Forward masking effects of preceding tone pulses as well as forward and simultaneous masking effects of the high-pass noise suppress activity originating from those regions of the cochlea which are located basalwards to the region to be stimulated by the respective pulse, thus enhancing the frequency specificity, especially for low-frequency stimuli of higher intensity. The new stimulation paradigm was tested in 12 normal hearing subjects and turned out to be suitable to elicit frequency-specific ABR with frequencies as low as 500 Hz and intensities as low as 10 dB nHL. The main advantage of the described technique is that the time required for a complete assessment of the auditory threshold at seven test frequencies (covering the relevant speech frequency range) is substantially shorter as compared to conventional techniques so that it can routinely be employed in pedaudiology, where infants usually have to be investigated in sedation.

Auditory cortical basis of tinnitus

The waveforms of the auditory evoked magnetic field (AEF) in normal-hearing individuals and patients suffering from tinnitus are distinctly different. In tinnitus patients, the magnetic wave M200 (corresponding to the electric wave P200, or P2) is delayed and only poorly developed or even completely missing, while the amplitude of the magnetic wave M100 (corresponding to the electric wave N100, or N1) is significantly augmented. A very characteristic feature turned out to be the amplitude ratio of the two waves M200 and M100. Below the age of 50, the amplitude ratio M200/M100 represents a clear-cut criterion to distinguish between tinnitus patients and individuals without tinnitus. In tinnitus patients, the ratio is less than 0.5 independent of age, whereas, in young and middle-aged normal-hearing individuals, it is greater than 0.5. Since in normal-hearing individuals the average amplitude ratio decreases linearly with age, the clusters of amplitude ratios of the two groups begin to overlap beyond the age of 50. The hypothesis is put forward that the decrease of the average amplitude ratio in normal-hearing individuals reflects a degenerative process probably initiated by multiple exogenous and endogenous factors, which leads to both an increased excitability of the generators of a particular component of wave M100 and a sustained neural activity in the generators of one particular component of wave M200 and eventually gives rise to the sensation of tinnitus. The absence or poor development of wave M200 is a concomitant phenomenon, resulting from the involved generators being less responsive to external stimuli. Our hypothesis has been supported by one exemplary case in which we were able to trace the process of tinnitus remission during a period of 256 days after acute onset of tinnitus (due to an acute noise trauma), showing a recovery of the amplitude ratio from an initial value of 0 to a normal value of approximately 1.

Neuromagnetic evidence of functional organization of the auditory cortex in humans

The influence of two physical stimulus parameters (frequency and intensity) and of one sensation parameter (pitch) on the auditory evoked magnetic field (AEF) was quantified by approximating the measured magnetic field distribution by that of an equivalent current dipole (ECD) embedded in a homogeneous semi-infinite volume conductor. The main results are as follows: The depth of the ECD increases with increasing frequency, but decreases with increasing intensity. In the case of a complex tone with missing fundamental it is the virtual pitch that determines the ECD location and not the spectral contents of the stimulus.

On the biomagnetic inverse problem in the case of multiple dipoles

Series of Monte Carlo simulations have been carried out which were based on the assumption that two dipoles with a distance of 0.5-2 cm are located in a homogeneous semi-infinite volume conductor (depth 3 cm), and that the magnetic field component perpendicular to the surface of the volume conductor is recorded by means of a magnetometer with infinitesimal coil diameter. Moving-dipole models (all parameters time-dependent), rotating-dipole models (dipole locations fixed, dipole orientation and amplitudes time-dependent) as well as fixed-dipole models (dipole locations and orientations fixed, amplitudes time-dependent) were considered. The algorithm used to retrieve the model parameters from the simulated field distributions (biomagnetic inverse procedure) was based on a transformation of the standard least-squares fit procedure into a minimization procedure with respect to the nonlinear parameters (dipole locations and orientations), which was solved iteratively by means of the Fletcher-Powell algorithm. It was found that the resolving power of the biomagnetic inverse procedure is highly dependent on the relative orientation of the two dipoles, the temporal overlap of the dipole moments, and the correlation of successive samples of the superimposed noise. The results obtained in this study suggest that the resolving power of the biomagnetic inverse procedure for conditions typically found in the case of auditory evoked magnetic fields is not better than 2 cm for the moving-dipole approach, and not better than 1 cm for the fixed-dipole approach, provided that no additional a priori information is available. In practice, the situation is probably even worse since the depth of the generators is usually larger than assumed in this study.

Identification of sources of brain neuronal activity with high spatiotemporal resolution through combination of neuromagnetic source localization (NMSL) and magnetic resonance imaging (MRI)

The locations of the origin of wave M100 of the auditory evoked magnetic field in response to tone bursts of different carrier frequencies, obtained through dipole localization methods (DLM), were related to cerebral structures, displayed by coronal MRI (magnetic resonance imaging) tomograms of the respective subjects. This was done by displaying the landmarks which served as reference for the neuromagnetic measurements in MRI tomogram (reference plane). All calculated source locations project exactly onto the transverse temporal gyri (Heschl) in which the primary auditory cortex, the supposed origin of wave M100, is located. The results highlight the exceptional capabilities of a combination of these 2 non-invasive, high-resolution techniques for functional diagnosis.