Sequential source of the M100 exhibits inter-hemispheric asymmetry

Overlapping auditory M100 and M200 abnormalities in schizophrenia and bipolar disorder: a MEG study

OBJECTIVES

Schizophrenia and bipolar disorder share common etiological factors and pathophysiological pathways and have overlapping clinical features. Only few studies have directly compared early auditory information processing in the two disorders. The objective of this study was to investigate the M100 and M200 auditory responses in patients with schizophrenia and bipolar disorder and compare them with healthy controls using magnetoencephalography (MEG).

METHODS

Whole-head MEG data were acquired during an auditory oddball paradigm in 24 schizophrenia patients, 26 bipolar I disorder patients, and 31 healthy controls. The strengths and latencies of M100 and M200 in both hemispheres and the dipole source localizations were investigated from the standard stimuli.

RESULTS

The M100 and M200 dipolar sources were localized to the left and right posterior portion of the superior temporal gyrus (STG) in all the subjects. An asymmetric pattern of M100 and M200 auditory response with more anterior sources in the right STG was observed in the healthy controls. However, both the schizophrenia and bipolar disorder patients showed a symmetric M100 and M200 source pattern. When compared with the healthy control group, both patient groups showed significantly reduced M100 and M200 source strength in both hemispheres.

CONCLUSIONS

Our study suggests that early auditory information processing deficits may be similar in schizophrenia and bipolar disorder and may be related to abnormalities of the STG.

Auditory M50 and M100 sensory gating deficits in bipolar disorder: a MEG study

OBJECTIVES

Auditory sensory gating deficits have been reported in subjects with bipolar disorder, but the hemispheric and neuronal origins of this deficit are not well understood. Moreover, gating of the auditory evoked components reflecting early attentive stage of information processing has not been investigated in bipolar disorder. The objectives of this study were to investigate the right and left hemispheric auditory sensory gating of the M50 (preattentive processing) and M100 (early attentive processing) in patients diagnosed with bipolar I disorder by utilizing magnetoencephalography (MEG).

METHODS

Whole-head MEG data were acquired during the standard paired-click paradigm in 20 bipolar I disorder patients and 20 healthy controls. The M50 and the M100 responses were investigated, and dipole source localizations were also investigated. Sensory gating were determined by measuring the strength of the M50 and the M100 response to the second click divided by that of the first click (S2/S1).

RESULTS

In every subject, M50 and M100 dipolar sources localized to the left and right posterior portion of superior temporal gyrus (STG). Bipolar I disorder patients showed bilateral gating deficits in M50 and M100. The bilateral M50 S2 source strengths were significantly higher in the bipolar I disorder group compared to the control group.

LIMITATIONS

The sample size was relatively small. More studies with larger sample sizes are warranted. Bipolar subjects were taking a wide range of medications that could not be readily controlled for.

CONCLUSIONS

These findings suggest that bipolar I disorder patients have auditory gating deficits at both pre-attentive and early attentive levels, which might be related to STG structural abnormality.

Absence of auditory M100 source asymmetry in schizophrenia and bipolar disorder: a MEG study

Background

Whether schizophrenia and bipolar disorder are the clinical outcomes of discrete or shared causative processes is much debated in psychiatry. Several studies have demonstrated anomalous structural and functional superior temporal gyrus (STG) symmetries in schizophrenia. We examined bipolar patients to determine if they also have altered STG asymmetry.

Methods

Whole-head magnetoencephalography (MEG) recordings of auditory evoked fields were obtained for 20 subjects with schizophrenia, 20 with bipolar disorder, and 20 control subjects. Neural generators of the M100 auditory response were modeled using a single equivalent current dipole for each hemisphere. The source location of the M100 response was used as a measure of functional STG asymmetry.

Results

Control subjects showed the typical M100 asymmetrical pattern with more anterior sources in the right STG. In contrast, both schizophrenia and bipolar disorder patients displayed a symmetrical M100 source pattern. There was no significant difference in the M100 latency and strength in bilateral hemispheres within three groups.

Conclusions

Our results indicate that disturbed asymmetry of temporal lobe function may reflect a common deviance present in schizophrenia and bipolar disorder, suggesting the two disorders might share etiological and pathophysiological factors.

Absence of M100 source asymmetry in autism associated with language functioning

Various clinical populations display atypical volume asymmetry of language structures and also the auditory M100 source. Although such atypical volume asymmetries have also been observed in autism, M100 source asymmetries have not yet been investigated. We examined M100 asymmetry in autism and its relationship with language functioning. Evoked neural activity to a 1 kHz tone was recorded using whole-cortex 151-channel magnetoencephalography in three groups of individuals. A single-dipole model identified the M100 generator in auditory cortex in each hemisphere. Healthy adults and control children displayed the expected right-sided M100 anteriority, whereas children with autism showed no such asymmetry. An association was found between language functioning and the degree of asymmetry across the two groups of children, suggesting a possible relationship between functional-structural asymmetry and language ability.

Hemispheric asymmetry in mid and long latency neuromagnetic responses to single clicks

We examine lateralization in the evoked magnetic field response to a click stimulus, observing that lateralization effects previously demonstrated for tones, noise, frequency modulated sweeps and certain syllables are also observed for (acoustically simpler) clicks. These effects include a difference in the peak latency of the M100 component of the evoked field waveform such that the peak consistently appears earlier in the right hemisphere, as well as rightward lateralization of field amplitude during the rise of the M100 component. Our review of previous findings on M100 lateralization, taken together with our data on the click-evoked response, leads to the hypothesis that these lateralization effects are elicited by stimuli containing a sharp sound energy onset or acoustic transition rather than specific types of stimuli. We argue that both the latency and the amplitude lateralization effects have a common origin, namely, hemispheric asymmetry in the amplitude of the magnetic field generated by one or more sources active during the M100 rise. While anatomical asymmetry cannot be excluded as the cause of the amplitude difference, we propose that the difference reflects a rightward asymmetry in the processing of sound energy onsets that potentially underlies the lateralization of several functions.

MEG auditory evoked fields suggest altered structural/functional asymmetry in primary but not secondary auditory cortex in bipolar disorder

OBJECTIVES

Objective physiological indices independently characterizing affective and schizophreniform psychoses would contribute to our understanding of the nature of their relationships. Magnetoencephalography (MEG)-based metrics of altered structural/functional asymmetry in the superior temporal gyrus have previously been found to characterize schizophrenia at the level of both the primary auditory (AI) and the secondary auditory (AII) cortex. This study examines these markers in patients with bipolar disorder, with the goal of improved understanding of the patterns of brain asymmetry that may independently characterize affective and schizophreniform psychosis.

METHODS

We studied 17 euthymic bipolar subjects and 17 matched controls. Auditory evoked fields were generated by both 40 Hz auditory stimuli eliciting steady state gamma band (SSR), activating the AI cortex, and discrete 1 kHz tone pips, activating the AII cortex. MEG was recorded from the hemisphere contralateral to the ear stimulated using a 37-channel MEG system. Source location estimates were calculated in both left and right hemispheres. Neuroanatomical location estimates for medial Heschl's gyri were determined from magnetic resonance images for correlation with MEG source locations.

RESULTS

Bipolar subjects failed to demonstrate normal laterality of SSR AI responses, indicating altered patterns of asymmetry at the level of AI cortex, but demonstrated normal asymmetry of AII responses (right anterior to left). Medial Heschl's gyri centroids were similarly lateralized in both groups, however (right anterior to left), dissociating function from structure in the AI cortex in the bipolar group.

CONCLUSIONS

The findings are compatible with altered functional/structural relationships, including diminished left-right hemisphere asymmetry of the AI, but not the AII cortex in bipolar disorder. In schizophrenia, both the AI and AII cortices exhibit such derangements; thus, the findings support both shared and nonshared features of auditory cortical disruption between the two disorders. This functional disorganization may help explain previously reported decreases in amplitude and phase synchrony of SSR gamma band responses in bipolar subjects, suggesting impaired neocortical synchrony in AI, possibly at a cortico-thalamic level, but perhaps not extending to heteromodal association cortex, and may relate to the cognitive impairments found in bipolar disorder.

Neuromagnetic evidence of broader auditory cortical tuning in schizophrenia

Deficits in basic auditory perception have been described in schizophrenia. Previous electrophysiological imaging research has documented a structure-function disassociation in the auditory system and altered tonotopic mapping in schizophrenia. The present study examined auditory cortical tuning in patients with schizophrenia. Eighteen patients with schizophrenia and 15 comparison subjects were recorded in a magnetoencephalographic (MEG) experiment of auditory tuning. Auditory cortical tuning at 1 kHz was examined by delivering 1 kHz pure tones in conjunction with pure tones at 5 frequencies surrounding and including 1 kHz. Source reconstruction data were examined for evidence of frequency specificity for the M100 component. There was a significant broadening of tuning in the schizophrenia group evident for the source amplitude of the M100. The frequently reported reduction in anterior-posterior source asymmetry for individuals with schizophrenia was replicated in this experiment. No relationships between symptom severity ratings and MEG measures were observed. This finding suggests that the frequency specificity of the M100 auditory evoked field is disturbed in schizophrenia, and may help explain the relatively poor behavioral performance of schizophrenia patients on simple frequency discrimination tasks.

Reduced hemispheric asymmetry of the auditory N260m in dyslexia

Dyslexia seems to be related to a lack of planum temporale (PT) asymmetry that is accompanied by functional differences to control subjects in both left and right hemispheric temporal regions during language tasks. PT asymmetry has been found to correlate with phonological and verbal skills. In accordance, reduced asymmetry of the auditory N100m sources in dyslexic adults and P100m sources in dyslexic children has been reported. These results might also be related to an atypical PT symmetry or the recruitment of other structures than the PT for speech processing in dyslexia. In the present study we tried to replicate and extend previous findings by examining a sample of 64 dyslexic and 22 control children in the MEG. We measured cortical activity during a passive auditory oddball-paradigm and localised ERF sources evoked by the standard stimulus /ba/. Reduced hemispheric asymmetry in the localisation of the auditory N260m was revealed. While control children displayed a typical asymmetrical pattern with more anterior sources in the right hemisphere, this asymmetry was not present for the dyslexic children. Further, a correlation between N260m asymmetry and spelling test performance was found. Our results suggest that localisation of ERF components is indeed an applicative tool for investigating cortical deviances in dyslexia. A lack of source localisation asymmetry in dyslexia appears to be a robust finding across different samples of dyslexic children and adults. It appears that cortical auditory (language) processing is organised differently in dyslexic subjects than in controls. This might be the consequence of a more symmetrical PT organisation, which in turn might be the result of maturational delay.

Distinct M50 and M100 auditory gating deficits in schizophrenia

The time course of the schizophrenia auditory gating deficit may provide clues to mechanisms of impaired cognition. Magnetoencephalography was recorded during a standard paired-click paradigm. Using source strength of the M50 and M100 components for each click, calculated from dipole locations identified as underlying each component for the first click, a ratio of the second divided by the first was used to measure gating. Patients showed a left-hemisphere gating deficit in M50 and a bilateral gating deficit in M100. Hypothesizing that an early deficit may affect later processing, hierarchical regression was used to examine variance shared between the components. A left-hemisphere M100 gating deficit was coupled with the left M50 gating deficit. In contrast, a right-hemisphere M100 gating deficit was unrelated to M50 gating in either hemisphere. Investigations of interhemisphere gating relations may clarify group differences in regional connectivity and their role in gating.

Reduced auditory M100 asymmetry in schizophrenia and dyslexia: applying a developmental instability approach to assess atypical brain asymmetry

Although atypical structural and functional superior temporal gyrus (STG) asymmetries are frequently observed in patients with schizophrenia and individuals with dyslexia, their significance is unclear. One possibility is that atypical asymmetries reflect a general risk factor that can be seen across multiple neurodevelopmental conditions--a risk factor whose origins are best understood in the context of Developmental Instability (DI) theory. DI measures (minor physical anomalies (MPAs) and fluctuating asymmetries (FAs)) reflect perturbation of the genetic plan. The present study sought to assess whether the presence of peripheral indices of DI predicts anomalous functional auditory cortex asymmetry in schizophrenia patients and dyslexia subjects. The location of the auditory M100 response was used as a measure of functional STG asymmetry, as it has been reported that in controls (but not in subjects with schizophrenia or dyslexia) the M100 source location in the right hemisphere is shifted anterior to that seen for the left hemisphere. Whole-brain auditory evoked magnetic field data were successfully recorded from 14 male schizophrenia patients, 21 male subjects with dyslexia, and 16 normal male control subjects. MPA and FA measures were also obtained. Replicating previous studies, both schizophrenia and dyslexia groups showed less M100 asymmetry than did controls. Schizophrenia and dyslexia subjects also had higher MPA scores than normal controls. Although neither total MPA nor FA measures predicted M100 asymmetry, analyses on individual MPA items revealed a relationship between high palate and M100 asymmetry. Findings suggest that M100 positional asymmetry is not a diagnostically specific feature in several neurodevelopmental conditions. Continued research examining DI and brain asymmetry relationships is warranted.

Distinguishing Between Moving and Stationary Sources Using EEG/MEG Measurements With an Application to Epilepsy

Performances of electroencephalography (EEG) and magnetoencephalography (MEG) source estimation methods depend on the validity of the assumed model. In many cases, the model structure is related to physical information. We discuss a number of statistical selection methods to distinguish between two possible models using least-squares estimation and assuming a spherical head model. The first model has a single moving source whereas the second has two stationary sources; these may result in similar EEG/MEG measurements. The need to decide between such models occurs for example in Jacksonian seizures (e.g., epilepsy) or in intralobular activities, where a model with either two stationary dipole sources or a single moving dipole source may be possible. We also show that all of the selection methods discussed choose the correct model with probability one when the number of trials goes to infinity. Finally we present numerical examples and compare the performances of the methods by varying parameters such as the signal-to-noise ratio, source depth, and separation of sources, and also apply the methods to real MEG data for epilepsy.

Attentional influences on functional mapping of speech sounds in human auditory cortex

Background

The speech signal contains both information about phonological features such as place of articulation and non-phonological features such as speaker identity. These are different aspects of the 'what'-processing stream (speaker vs. speech content), and here we show that they can be further segregated as they may occur in parallel but within different neural substrates. Subjects listened to two different vowels, each spoken by two different speakers. During one block, they were asked to identify a given vowel irrespectively of the speaker (phonological categorization), while during the other block the speaker had to be identified irrespectively of the vowel (speaker categorization). Auditory evoked fields were recorded using 148-channel magnetoencephalography (MEG), and magnetic source imaging was obtained for 17 subjects.

Results

During phonological categorization, a vowel-dependent difference of N100m source location perpendicular to the main tonotopic gradient replicated previous findings. In speaker categorization, the relative mapping of vowels remained unchanged but sources were shifted towards more posterior and more superior locations.

Conclusions

These results imply that the N100m reflects the extraction of abstract invariants from the speech signal. This part of the processing is accomplished in auditory areas anterior to AI, which are part of the auditory 'what' system. This network seems to include spatially separable modules for identifying the phonological information and for associating it with a particular speaker that are activated in synchrony but within different regions, suggesting that the 'what' processing can be more adequately modeled by a stream of parallel stages. The relative activation of the parallel processing stages can be modulated by attentional or task demands.

The dipole location shift within the auditory evoked neuromagnetic field components N100m and mismatch negativity (MMNm)

OBJECTIVE

To investigate whether within the latency range of the neuromagnetic mismatch negativity (MMNm) a similar dipole shift can be observed as found in previous studies for the auditory evoked field component N100m.

METHODS

For this purpose selected neuromagnetic data of 29 subjects were analysed in a time window of 15 ms before and 15 ms beyond the peak maximum of both components. In our oddball paradigm, we applied frequency, duration and intensity deviants within one block. The time course of dipole location was analysed by means of a repeated measure analysis of variance (ANOVA).

RESULTS

As expected, N100m dipoles shifted significantly from superior to inferior and from posterior to anterior within their latency range. In analogy, the MMNm dipoles of duration and intensity deviants also exhibited a significant shift from posterior to anterior within their latency range. However, the MMNm dipoles of all 3 deviants did not shift significantly from superior to inferior. Concerning this direction, the ANOVA revealed a significant TIME*COMPONENT interaction between the N100m and the MMNm of duration and intensity deviants.

CONCLUSIONS

The finding of a different time course of N100m and MMNm dipoles does not support the assumption that the MMN represents an amplitude- and latency-modulated auditory N100 response.

Differences between auditory evoked responses recorded during spatial and nonspatial working memory tasks

Results from several recent studies suggest that neuronal processing of sound content and its spatial location may be dissociated. The use of modern neuroimaging techniques has allowed for the determination that different brain structures may be specifically activated during working memory processing of pitch and location of sound. The time course of these task-related differences, however, remains uncertain. In the present study, we performed simultaneous whole-head electroencephalogram and magnetoencephalogram recordings, using a new behavioral paradigm, to investigate the dynamics of differences between "what" and "where" evoked responses in the auditory system as a function of memory load. In the location task the latency of the N1m was shorter and its generator was situated more inferiorly than in the pitch task. Working memory processing of the tonal frequency enhanced the amplitude of the N2 component, as well as the negative-going deflection at a latency around 400 ms. A memory-load-dependent task-related difference was found in the positive slow wave which was higher during the location than pitch task at the low load. Late slow waves were affected by memory load but not type of task. These results suggest that separate neuronal networks are involved in the attribute-specific analysis of auditory stimuli and their encoding into working memory, whereas the maintenance of auditory information is accomplished by a common, nonspecific neuronal network.

Reduced laterality of the source locations for generators of the auditory steady-state field in schizophrenia

BACKGROUND

Reduced hemispheric asymmetry of evoked field generators in schizophrenia in secondary auditory cortex has been replicated with mixed success. This disparity is possibly due to the complex nature of the 100-msec generators typically investigated. In this study, we evaluated a source located in primary auditory cortex, responsible for the steady-state response, to determine whether similar asymmetries were present.

METHODS

Contralateral evoked fields were recorded with a 37-channel neuromagnetometer in response to 40-Hz pulse trains. Dipole models were used to localize the sources in both hemispheres of schizophrenic subjects (n = 17) and normal control subjects (n = 18).

RESULTS

Control subjects were asymmetrically lateralized, with sources in the right hemisphere approximately 6 mm further anterior than those on the left. In contrast, schizophrenic subjects showed no significant difference between hemispheres.

CONCLUSIONS

The findings support theories linking schizophrenia to disturbed asymmetry of temporal lobe function. The steady-state response, having a more discrete Heschl's gyrus generator location, may be better suited for evaluating auditory asymmetry.

The auditory evoked magnetic fields to very high frequency tones

We studied the auditory evoked magnetic fields (AEFs) in response to pure tones especially at very high frequencies (from 4000 Hz to 40,000 Hz). This is the first systematic study of AEFs using tones above 5000 Hz, the upper audible range of humans, and ultrasound. We performed two experiments. In the first, AEFs were recorded in 12 subjects from both hemispheres under binaural listening conditions. Six types of auditory stimulus (pure tones of five different frequencies: 4000 Hz, 8000 Hz, 10,000 Hz, 12,000 Hz, 14,000 Hz, and a click sound as the target stimulus) were used. In the second experiment, we used 1000 Hz, 15,000 Hz, and two ultrasounds with frequencies of 20,000 Hz and 40,000 Hz. The subjects could detect all stimuli in the first experiment but not the ultrasounds in the second experiment. We analyzed N1m, the main response with approximately 100 ms in peak latency, and made the following findings. (1) N1m responses to the tones up to 12,000 Hz were clearly recorded from at least one hemisphere in all 12 subjects. N1m for 14,000 Hz was identified in at least one hemisphere in 10 subjects, and in both hemispheres in six subjects. No significant response could be identified to ultrasounds over 20,000 Hz. (2) The amplitude of the N1m to the tones above 8000 Hz was significantly smaller than that to 4000 Hz in both hemispheres. There was a tendency for the peak latency of the N1m to be longer for the tones with higher frequencies, but no significant change was found. (3) The equivalent current dipole (ECD) of the N1m was located in the auditory cortex. There was a tendency for the ECD for the tones with higher frequencies to lie in more medial and posterior areas, but no significant change was found. (4) As for the interhemispheric difference, the N1m amplitude for all frequency tones was significantly larger and the ECDs were estimated to be located more anterior and medial in the right hemisphere than the left. The priority of the right hemisphere, that is the larger amplitude, for very high frequency tones was confirmed. (5) The orientation of the ECD in the left hemisphere became significantly more vertical the higher the tones. This result was consistent with previous studies which revealed the sensitivity of the frequency difference in the left hemisphere. From these findings we suggest that tonotopy in the auditory cortex exists up to the upper limit of audible range within the small area, where the directly air-conducted ultrasounds are not reflected.

Evidence for dissociation of spatial and nonspatial auditory information processing

Several lines of evidence suggest that visual information processing is segregated into the ventral "what" and dorsal "where" pathways. But the question whether information processing in the auditory system is also parceled to spatial and nonspatial domains remains open. In the present study, we performed simultaneous EEG and MEG recordings during auditory location and pitch delayed matching-to-sample tasks to find out whether working memory processing of the auditory stimulus attribute affects the transient components of the evoked potentials. In both tasks, identical blocks of tone stimuli of one of two frequencies were presented in one of two locations; the only difference between the tasks was the instruction to attend either to the frequency or to the location. In the match condition, the N1 latency was shorter and the N1m amplitude larger in the location task compared to the pitch task. Furthermore, the right-hemisphere generator of N1m elicited in the match condition of the location task was situated significantly medially to the N1m generator in the match condition of the pitch task. Latency and amplitude task-related differences in the N1/N1m components as well as the source location differences indicate at least partial segregation of neuronal mechanisms involved in working memory processing of spatial and nonspatial auditory information.

Fine structure of the auditory M100 in schizophrenia and schizoaffective disorder

BACKGROUND

Several studies have demonstrated anomalous asymmetry of the 100-msec latency auditory-evoked field (M100) in schizophrenia. Recent evidence suggests this may be a compound component, however. Our study examines the localization of two M100 subcomponents in patients with schizophrenia and schizoaffective disorder.

METHODS

Magnetoencephalographic recordings of auditory-evoked fields were obtained for 14 subjects with schizophrenia, 12 with schizoaffective disorder, and 23 control subjects. Two M100 subcomponents were identified and localized in each hemisphere.

RESULTS

Both patient groups exhibited different lateralization compared with control subjects, with the second subcomponent tending to be less lateralized.

CONCLUSIONS

The second subcomponent may be the major contributor to previously reported laterality differences. Future studies might benefit by separating M100 subcomponents so that specific functions could be addressed.

Tonotopy of the Auditory-Evoked Field Component N100m in Patients with Schizophrenia

Abstract A number of clinical studies on the auditory neuromagnetic evoked field (AEF) component N100m have reported an altered lateralization in schizophrenic patients. This study addresses the problem of a possible functional reorganization of the temporal cortex in schizophrenia by examining the tonotopic organization of the N100m. Thirty-two patients with schizophrenia and 33 healthy controls of both sexes took part. Two tone frequencies (1000 and 5000 Hz tone) were applied for auditory stimuli, and AEF were recorded over both hemispheres successively using a 31-channel biomagnetometer. The comparison of N100m dipole location and orientation between hemispheres revealed no alterations in male or female patients. Between tone frequencies highly significant differences were found for N100m peak latency, mean global field power, dipole orientation, and dipole location in the anterior-posterior direction. Although the main effects of frequency were found to be the same in patients and controls, the balance between hemispheres was altered in patients with schizophrenia, with respect to the dependence between frequency and dipole location in the anterior-posterior direction as well as between frequency and latency. In patients, the influence of frequency on these variables was more pronounced in the right hemisphere and less pronounced in the left, compared to controls.

Latency of the auditory evoked neuromagnetic field components: stimulus dependence and insights toward perception

This review will focus on investigations of the auditory evoked neuromagnetic field component, the M100, detectable in the magnetoencephalogram recorded during presentation of auditory stimuli, approximately 100 milliseconds after stimulus onset. In particular, the dependence of M100 latency on attributes of the stimulus, such as intensity, pitch and timbre will be discussed, along with evidence relating M100 latency observations to perceptual features of the stimuli. Comparison with investigation of the analogous electrical potential component, the N1, will be made. Parametric development of stimuli from pure tones through complex tones to speech elements will be made, allowing the influence of spectral pitch, virtual pitch and perceptual categorization to be delineated and suggesting implications for the role of such latency observations in the study of speech processing. The final section will deal with potential clinical applications offered by M100 latency measurements, as objective indices of normal and abnormal cortical processing.

Magnetoencephalography: applications in psychiatry

Magnetoencephalography (MEG) measures the extracranial magnetic fields produced by intraneuronal ionic current flow within appropriately oriented cortical pyramidal cells. Based upon superconducting quantum interference device technology operating at liquid helium temperatures (4 K), MEG offers excellent temporal and spatial resolution for selected sources, and complements information obtained from electroencephalograms and other functional imaging strategies. Current instrumentation permits recording up to several hundred channels simultaneously with head-shaped dewars, although the cost of such systems is high. The fact that magnetic fields fall off with the square of the distance from the source is both a benefit (when separating activity in the two hemispheres) and a limitation (when attempting to record deep sources). The lack of skin contact facilitates using MEG to record direct current and very high frequency (> 600 Hz) brain activity. The clinical utility of MEG includes presurgical mapping of sensory cortical areas and localization of epileptiform abnormalities, and localization of areas of brain hypoperfusion in stroke patients. MEG studies in psychiatric disorders have contributed materially to improved understanding of anomalous brain lateralization in the psychoses, have suggested that P50 abnormalities may reflect altered gamma band activity, and have provided evidence of hemisphere-specific abnormalities of short-term auditory memory function.