Interpreting abnormality: an EEG and MEG study of P50 and the auditory paired-stimulus paradigm

Functional and structural maturation of auditory cortex from 2 months to 2 years old

In school-age children, the myelination of the auditory radiation thalamocortical pathway is associated with the latency of auditory evoked responses, with the myelination of thalamocortical axons facilitating the rapid propagation of acoustic information. Little is known regarding this auditory system function-structure association in infants and toddlers. The present study tested the hypothesis that maturation of auditory radiation white-matter microstructure (e.g., fractional anisotropy (FA); measured using diffusion-weighted MRI) is associated with the latency of the infant auditory response (P2m measured using magnetoencephalography, MEG) in a cross-sectional (2 to 24 months) as well as longitudinal cohort (2 to 29 months) of typically developing infants and toddlers. In the cross-sectional sample, non-linear maturation of P2m latency and auditory radiation diffusion measures were observed. After removing the variance associated with age in both P2m latency and auditory radiation diffusion measures, auditory radiation still accounted for significant variance in P2m latency. In the longitudinal sample, latency and FA associations could be observed at the level of a single child. Findings provide strong support for a contribution of auditory radiation white matter to rapid cortical auditory encoding processes in infants.

Differential Maturation of Auditory Cortex Activity in Young Children with Autism and Typical Development

Maturation of auditory cortex neural encoding processes was assessed in children with typical development (TD) and autism. Children 6-9 years old were enrolled at Time 1 (T1), with follow-up data obtained ~ 18 months later at Time 2 (T2), and ~ 36 months later at Time 3 (T3). Findings suggested an initial period of rapid auditory cortex maturation in autism, earlier than TD (prior to and surrounding the T1 exam), followed by a period of faster maturation in TD than autism (T1-T3). As a result of group maturation differences, post-stimulus group differences were observed at T1 but not T3. In contrast, stronger pre-stimulus activity in autism than TD was found at all time points, indicating this brain measure is stable across time.

Maturation of auditory cortex neural responses during infancy and toddlerhood

The infant auditory system rapidly matures across the first years of life, with a primary goal of obtaining ever-more-accurate real-time representations of the external world. Our understanding of how left and right auditory cortex neural processes develop during infancy, however, is meager, with few studies having the statistical power to detect potential hemisphere and sex differences in primary/secondary auditory cortex maturation. Using infant magnetoencephalography (MEG) and a cross-sectional study design, left and right auditory cortex P2m responses to pure tones were examined in 114 typically developing infants and toddlers (66 males, 2 to 24 months). Non-linear maturation of P2m latency was observed, with P2m latencies decreasing rapidly as a function of age during the first year of life, followed by slower changes between 12 and 24 months. Whereas in younger infants auditory tones were encoded more slowly in the left than right hemisphere, similar left and right P2m latencies were observed by ∼21 months of age due to faster maturation rate in the left than right hemisphere. No sex differences in the maturation of the P2m responses were observed. Finally, an earlier left than right hemisphere P2m latency predicted better language performance in older infants (12 to 24 months). Findings indicate the need to consider hemisphere when examining the maturation of auditory cortex neural activity in infants and toddlers and show that the pattern of left-right hemisphere P2m maturation is associated with language performance.

P50 sensory gating, cognitive deficits and depressive symptoms in first-episode antipsychotics-naïve schizophrenia

OBJECTIVE

Sensory gating P50 (SG-P50) may be involved in the pathophysiological mechanisms of impaired cognition in schizophrenia (SCZ). Comorbid depressive symptoms are common in SCZ patients and are also found to be associated with their cognitive impairment. However, it is unclear whether SG-P50 is abnormal in first episode antipsychotics naïve (FEAN) SCZ patients with depressive symptoms. Our aimed to investigate the relationships between SG-P50, depressive symptoms and neurocognition in FEAN-SCZ patients.

METHODS

We recruited 103 FEAN-SCZ patients (depression: n = 63; non-depression: n = 40) and 55 healthy controls. SG-P50 was measured using the standard auditory dual-click (S1&S2) paradigm. Clinical symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS) and the Hamilton Depression Rating Scale-17 (HDRS-17). Cognitive performance was evaluated using the MATRICS Consensus Cognitive Battery (MCCB).

RESULTS

Compared with non-depressive patients, depressive patients had a significantly larger S2 amplitude (p = 0.005) and a higher S2/S1 ratio at trend level (p = 0.075) after corrected. There were significant differences in the scores of CPT-IP and Mazes (NAB) between depressive and non-depressive FEAN-SCZ patients (both p values < 0.05). For all patients, the SG-P50 S2/S1 ratio was significantly correlated with HDRS-17 score (r = 0.23, p = 0.020) and MCCB-Symbol coding (r = -0.16, p = 0.043). For depressive FEAN-SCZ patients, S2 amplitude was an independent predictor of the MCCB-Mazes (NAB) (β = -0.31, t = -2.52, p = 0.015).

CONCLUSIONS

SG-P50 deficit may be an informational biomarker for depressive symptoms and neurocognitive impairments in FEAN-SCZ patients.

Modulation in alpha band activity reflects syntax composition: an MEG study of minimal syntactic binding

Successful sentence comprehension requires the binding, or composition, of multiple words into larger structures to establish meaning. Using magnetoencephalography, we investigated the neural mechanisms involved in binding at the syntax level, in a task where contributions from semantics were minimized. Participants were auditorily presented with minimal sentences that required binding (pronoun and pseudo-verb with the corresponding morphological inflection; "she grushes") and pseudo-verb wordlists that did not require binding ("cugged grushes"). Relative to no binding, we found that syntactic binding was associated with a modulation in alpha band (8-12 Hz) activity in left-lateralized language regions. First, we observed a significantly smaller increase in alpha power around the presentation of the target word ("grushes") that required binding (-0.05 to 0.1 s), which we suggest reflects an expectation of binding to occur. Second, during binding of the target word (0.15-0.25 s), we observed significantly decreased alpha phase-locking between the left inferior frontal gyrus and the left middle/inferior temporal cortex, which we suggest reflects alpha-driven cortical disinhibition serving to strengthen communication within the syntax composition neural network. Altogether, our findings highlight the critical role of rapid spatial-temporal alpha band activity in controlling the allocation, transfer, and coordination of the brain's resources during syntax composition.

From Sound Perception to Automatic Detection of Schizophrenia: An EEG-Based Deep Learning Approach

Deep learning techniques have been applied to electroencephalogram (EEG) signals, with promising applications in the field of psychiatry. Schizophrenia is one of the most disabling neuropsychiatric disorders, often characterized by the presence of auditory hallucinations. Auditory processing impairments have been studied using EEG-derived event-related potentials and have been associated with clinical symptoms and cognitive dysfunction in schizophrenia. Due to consistent changes in the amplitude of ERP components, such as the auditory N100, some have been proposed as biomarkers of schizophrenia. In this paper, we examine altered patterns in electrical brain activity during auditory processing and their potential to discriminate schizophrenia and healthy subjects. Using deep convolutional neural networks, we propose an architecture to perform the classification based on multi-channels auditory-related EEG single-trials, recorded during a passive listening task. We analyzed the effect of the number of electrodes used, as well as the laterality and distribution of the electrical activity over the scalp. Results show that the proposed model is able to classify schizophrenia and healthy subjects with an average accuracy of 78% using only 5 midline channels (Fz, FCz, Cz, CPz, and Pz). The present study shows the potential of deep learning methods in the study of impaired auditory processing in schizophrenia with implications for diagnosis. The proposed design can provide a base model for future developments in schizophrenia research.

Revisiting the relationship between neural correlates of sensory gating and self-reported sensory gating inventory: An MEG investigation

BACKGROUND

Accumulated evidence has revealed that bilateral superior temporal gyrus (STG), inferior frontal gyrus (IFG), and inferior parietal lobule (IPL) are involved in the processes of sensory gating (SG). However, it remains unknown which neural correlate(s) of SG specifically reflect individuals' perceptual experiences, as measured by the Sensory Gating Inventory (SGI). Thus, this study aims to investigate the relationship of SGI with cortical SG-related regions. Furthermore, we examine whether SG hemispheric asymmetry exists, which is still an inconclusive issue.

METHODS

Twenty-two healthy young adults performed the auditory paired-stimulus paradigm during magnetoencephalographic recordings. SG of M50 and M100 was measured as ratios (S2/S1) and differences (S1-S2). They were also evaluated with SGI, which factored into three categories of Perceptual Modulation, Distractibility, and Over-Inclusion. SG in the STG, IFG, and IPL were compared between left and right hemispheres, and were used to determine the relationship with SGI.

RESULTS

Only M100 SG differences (S1-S2) of the right IFG were significantly correlated with scores of Perceptual Modulation (partial r = -0.392, p = 0.040) and total SGI scores (partial r = -0.387, p = 0.041). However, we did not find significant lateralization of M50 SG and M100 SG in any studying region.

CONCLUSIONS

The individual's perceptual experience is specifically related to electrophysiological SG function of the right IFG.

Neuromagnetic evidence of abnormal automatic inhibitory function in subjective memory complaint

Subjective memory complaint (SMC), a self-perceived worsening in memory capacity concurrent with normal performance on standardized cognitive assessments, is considered a risk factor for the development of Alzheimer's disease (AD). Deficient sensory gating (SG), referring to the lack of automatic inhibition of neural responses to the second identical stimulus, has been documented in prodromal and incident AD patients. However, it remains unknown whether the cognitively normal elderly with SMC demonstrate alterations of SG function compared with those without SMC. A total of 19 healthy controls (HC) and 16 SMC subjects were included in the present study. Neural responses to the auditory paired-stimulus paradigm were recorded by the magnetoencephalography and analyzed by the distributed source imaging method of minimum norm estimate. The SG of M50 and M100 components were measured using the amplitude ratio of the second response over the first response at the cortical level. Compared to HC, subjects with SMC showed significantly increased M50 SG ratios in the inferior parietal lobule (IPL). Furthermore, M50 SG ratios in the right IPL yielded an acceptable discriminative ability to distinguish SMC from HC. However, we did not find a significant association between SG ratios and cognitive function requiring inhibitory control either in the HC or SMC group. In conclusion, although SMC subjects have intact cognitive functioning revealed by objective neuropsychological tests, their deficits in automatic inhibitory function could be detected through neurophysiological recordings. Our results suggest that altered brain function occurs in SMC prior to the obvious decline of cognitive performance.

Oscillatory connectivity as a mechanism of auditory sensory gating and its disruption in schizophrenia

Although innumerable studies using an auditory sensory gating paradigm have confirmed that individuals with schizophrenia (SZ) show less reduction in brain response to the second in a pair of clicks, this large literature has not yielded consensus on the circuit(s) responsible for gating nor for the gating difference in SZ. Clinically stable adult inpatients (N = 157) and matched community participants (N = 90) participated in a standard auditory sensory gating protocol. Responses to paired clicks were quantified as peak-to-peak amplitude from a response at approximately 50 ms to a response at approximately 100 ms in MEG-derived source waveforms. For bilateral sources in each of four regions near Heschl's gyrus, the gating ratio was computed as the response to the second stimulus divided by the response to the first stimulus. Spectrally resolved Granger causality quantified effective connectivity among regions manifested in alpha-band oscillatory coupling before and during stimulation. Poorer sensory gating localized to A1 in SZ than in controls confirmed previous results, here found in adjacent brain regions as well. Spontaneous, stimulus-independent effective connectivity within the hemisphere from angular gyrus to portions of the superior temporal gyrus was lower in SZ and correlated with gating ratio. Significant involvement of frontal and subcortical brain regions previously proposed as contributing to the auditory gating abnormality was not found. Findings point to endogenous connectivity evident in a sequence of activity from angular gyrus to portions of superior temporal gyrus as a mechanism contributing to normal and abnormal gating in SZ and potentially to sensory and cognitive symptoms.

The Influence of Background Auditory Noise on P50 and N100 Suppression Elicited by the Paired-Click Paradigm

Abstract. Auditory sensory gating is commonly assessed using the Paired-Click Paradigm (PCP), an electroencephalography (EEG) task in which two identical sounds are presented sequentially and the brain’s inhibitory response to the second sound is measured. Many clinical populations demonstrate reduced P50 and/or N100 suppression. Testing sensory gating in children may help to identify individuals at risk for neurodevelopmental disorders earlier, including autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD), which could lead to more optimal outcomes. Minimal research has been done with children because of the difficulty of performing lengthy EEG experiments with young children, requiring them to sit still for long periods of time. We designed a modified, potentially child-friendly version of the PCP and evaluated it in typically developing adults. The PCP was administered twice, once in a traditional silent room (silent movie condition) and once with an audible movie playing (audible movie condition) to minimize boredom and enhance behavioral compliance. We tested whether P50 and N100 suppression were influenced by the presence of the auditory background noise from the movie. N100 suppression was observed in both hemispheres in the silent movie condition and in the left hemisphere only during the audible movie condition, though suppression was attenuated in the audible movie condition. P50 suppression was not observed in either condition. N100 sensory gating was successfully elicited with an audible movie playing during the PCP, supporting the use of the modified task for future research in both children and adults.

Magnetoencephalography for Schizophrenia

Schizophrenia (Sz) is a chronic mental disorder characterized by disturbances in thought (such as delusions and confused thinking), perception (hearing voices), and behavior (lack of motivation). The lifetime prevalence of Sz is between 0.3% and 0.7%, with late adolescence and early adulthood, the peak period for the onset of psychotic symptoms. Causal factors in Sz include environmental and genetic factors and especially their interaction. About 50% of individuals with a diagnosis of Sz have lifelong impairment.

Dysfunction of Inferior Parietal Lobule During Sensory Gating in Patients With Amnestic Mild Cognitive Impairment

Patients with amnestic mild cognitive impairment (aMCI) demonstrate significant cognitive deficits, especially in the memory aspect. The memory deficiency might be attributed to the difficulties in the inhibitory function to suppress redundant stimuli. Sensory gating (SG) refers to the attenuation of neural responses to the second identical stimulus in a paired-click paradigm, in which auditory stimuli are delivered in pairs with inter-stimulus intervals (ISI) of 500 ms and inter-pair intervals of 6-8 s. It is considered as an electrophysiological signal to reflect the brain's automatic response to gate out repetitive sensory inputs. However, there has been no study systematically investigating SG function in aMCI patients. Thus, the present study used magnetoencephalography (MEG) to record neuromagnetic responses to a paired-click paradigm in 23 healthy controls (HC) and 26 aMCI patients. The Stimulus 2/Stimulus 1 (S2/S1) amplitude ratio was used to represent the SG function. Compared to HC, aMCI patients showed M50 SG deficits in the left inferior frontal gyrus (IFG) and right inferior parietal lobule (IPL). M100 SG defects were also observed in the right IPL. Based on the ROIs showing significant between-group SG differences, we found that a more deficient M50 SG function in the right IPL was associated with poorer performance in the immediate recall of Logic Memory (LM), Chinese Version Verbal Learning Test (CVVLT) and Digit Span Backward (DSB) Test. Furthermore, the M50 SG ratios of the right IPL together with the neuropsychological performance of LM and CVVLT demonstrated very good accuracy in the discrimination of aMCI from HC. In conclusion, compared to HC, aMCI patients showed a significant SG deficit in the right IPL, which was correlated with the auditory short-term memory function. We suggest the combination of SG in the right IPL, LM and CVVLT to be sensitive indicators to differentiate aMCI patients from HC.

Maturation of Auditory Cortex Neural Activity in Children and Implications for Auditory Clinical Markers in Diagnosis

Functional brain markers that can inform research on brain abnormalities, and especially those ready to facilitate clinical work on such abnormalities, will need to show not only considerable sensitivity and specificity but enough consistency with respect to developmental course that their validity in individual cases can be trusted. A challenge to establishing such markers may be individual differences in developmental course. The present study examined auditory cortex activity in children at an age when developmental changes to the auditory cortex 50 ms (M50) and 100 ms (M100) components are prominent to better understand the use of auditory markers in pediatric clinical research. MEG auditory encoding measures (auditory evoked fields in response to pure tone stimuli) were obtained from 15 typically developing children 6-8 years old, with measures repeated 18 and 36 months after the initial exam. MEG analyses were conducted in source space (i.e., brain location), with M50 and M100 sources identified in left and right primary/secondary auditory cortex (Heschl's gyrus). A left and right M50 response was observed at all times (Time 1, Time 2, Time 3), with M50 latency (collapsing across hemisphere) at Time 3 (77 ms) 10 ms earlier than Time 1 (87 ms; p < 0.001) and with M50 responses on average (collapsing across time) 5 ms earlier in the right (80 ms) than left hemisphere (85 ms; p < 0.05). In the majority of children, however, M50 latency changes were not constant across the three-year period; for example, whereas in some children a ~10 ms latency reduction was observed from Time 1 to Time 2, in other children a ~10 ms latency reduction was observed from Time 2 to Time 3. M100 responses were defined by a significant "peak" of detected power with magnetic field topography opposite M50 and occurring 50-100 ms later than the M50. Although M100s were observed in a few children at Time 1 and Time 2 (and more often in the right than left hemisphere), M100s were not observed in the majority of children except in the right hemisphere at Time 3. In sum, longitudinal findings showed large between- and within-subject variability in rate of change as well as time to reach neural developmental milestones (e.g., presence of a detectable M100 response). Findings also demonstrated the need to examine whole-brain activity, given hemisphere differences in the rate of auditory cortex maturation. Pediatric research will need to take such normal variability into account when seeking clinical auditory markers.

Magnetic Source Imaging and Infant MEG: Current Trends and Technical Advances

Magnetoencephalography (MEG) is known for its temporal precision and good spatial resolution in cognitive brain research. Nonetheless, it is still rarely used in developmental research, and its role in developmental cognitive neuroscience is not adequately addressed. The current review focuses on the source analysis of MEG measurement and its potential to answer critical questions on neural activation origins and patterns underlying infants’ early cognitive experience. The advantages of MEG source localization are discussed in comparison with functional magnetic resonance imaging (fMRI) and functional near-infrared spectroscopy (fNIRS), two leading imaging tools for studying cognition across age. Challenges of the current MEG experimental protocols are highlighted, including measurement and data processing, which could potentially be resolved by developing and improving both software and hardware. A selection of infant MEG research in auditory, speech, vision, motor, sleep, cross-modality, and clinical application is then summarized and discussed with a focus on the source localization analyses. Based on the literature review and the advancements of the infant MEG systems and source analysis software, typical practices of infant MEG data collection and analysis are summarized as the basis for future developmental cognitive research.

Attention modulates topology and dynamics of auditory sensory gating

This work challenges the widely accepted model of sensory gating as a preattention inhibitory process by investigating whether attention directed at the second tone (S2) within a paired-click paradigm could affect gating at the cortical level. We utilized magnetoencephalography, magnetic resonance imaging and spatio-temporal source localization to compare the cortical dynamics underlying gating responses across two conditions (passive and attention) in 19 healthy subjects. Source localization results reaffirmed the existence of a fast processing pathway between the prefrontal cortex (PFC) and bilateral superior temporal gyri (STG) that underlies the auditory gating process. STG source dynamics comprised two gating sub-components, Mb1 and Mb2, both of which showed significant gating suppression (>51%). The attention directed to the S2 tone changed the gating network topology by switching the prefrontal generator from a dorsolateral location, which was active in the passive condition (18/19), to a medial location, active in the attention condition (19/19). Enhanced responses to the attended stimulus caused a significant reduction in gating suppression in both STG gating components (>50%). Our results demonstrate that attention not only modulates sensory gating dynamics, but also exerts topological rerouting of information processing within the PFC. The present data, suggesting that the cortical levels of early sensory processing are subject to top-down influences, change the current view of gating as a purely automatic bottom-up process.

Abnormal frontal generator during auditory sensory gating in panic disorder: An MEG study

Patients with panic disorder (PD) exhibit abnormalities in early-stage information processing, even for the nonthreatening stimuli. A previous event-related potential study reported that PD patients show a deficit in sensory gating (SG), a protective mechanism of the brain to filter out irrelevant sensory inputs. However, there is no clear understanding about the neural correlates of SG deficits in PD. Moreover, whether SG deficits, if any, are associated with clinical manifestations remain unknown. In this study, 18 patients with PD and 20 age- and gender-matched healthy controls were recruited to perform auditory paired-stimulus paradigm using magnetoencephalographic (MEG) recordings. Results showed that PD patients demonstrated significantly higher M50 SG ratios in the right inferior frontal gyrus (RIFG) and higher M100 SG ratios in both RIFG and right superior temporal gyrus (RSTG) than those of the control group. It was important to note that in the RIFG, the M50 SG ratios correlated significantly with the scores of Body Sensation Questionnaire (BSQ) and Distractibility scale of Sensory Gating Inventory among patients with PD. In conclusion, this study suggests that PD patients exhibited a deficient ability to filter out irrelevant information, and such a defect might lead to cognitive misinterpretation of somatic sensations and distractibility.

Magnetoencephalography and the infant brain

Magnetoencephalography (MEG) is a non-invasive neuroimaging technique that provides whole-head measures of neural activity with millisecond temporal resolution. Over the last three decades, MEG has been used for assessing brain activity, most commonly in adults. MEG has been used less often to examine neural function during early development, in large part due to the fact that infant whole-head MEG systems have only recently been developed. In this review, an overview of infant MEG studies is provided, focusing on the period from birth to three years. The advantages of MEG for measuring neural activity in infants are highlighted (See Box 1), including the ability to assess activity in brain (source) space rather than sensor space, thus allowing direct assessment of neural generator activity. Recent advances in MEG hardware and source analysis are also discussed. As the review indicates, efforts in this area demonstrate that MEG is a promising technology for studying the infant brain. As a noninvasive technology, with emerging hardware providing the necessary sensitivity, an expected deliverable is the capability for longitudinal infant MEG studies evaluating the developmental trajectory (maturation) of neural activity. It is expected that departures from neuro-typical trajectories will offer early detection and prognosis insights in infants and toddlers at-risk for neurodevelopmental disorders, thus paving the way for early targeted interventions.

By our bootstraps: Comparing methods for measuring auditory 40 Hz steady-state neural activity

Although the 40 Hz auditory steady-state response (ASSR) is of clinical interest, the construct validity of EEG and MEG measures of 40 Hz ASSR cortical microcircuits is unclear. This study evaluated several MEG and EEG metrics by leveraging findings of (a) an association between the 40 Hz ASSR and age in the left but not right hemisphere, and (b) right- > left-hemisphere differences in the strength of the 40 Hz ASSR. The contention is that, if an analysis method does not demonstrate a left 40 Hz ASSR and age relationship or hemisphere differences, then the obtained measures likely have low validity. Fifty-three adults were presented 500 Hz stimuli modulated at 40 Hz while MEG and EEG were collected. ASSR activity was examined as a function of phase similarity (intertrial coherence) and percent change from baseline (total power). A variety of head models (spherical and realistic) and a variety of dipole source modeling strategies (dipole source localization and dipoles fixed to Heschl's gyri) were compared. Several sensor analysis strategies were also tested. EEG sensor measures failed to detect left 40 Hz ASSR and age associations or hemisphere differences. A comparison of MEG and EEG head-source models showed similarity in the 40 Hz ASSR measures and in estimating age and left 40 Hz ASSR associations, indicating good construct validity across models. Given a goal of measuring the 40 Hz ASSR cortical microcircuits, a source-modeling approach was shown to be superior in measuring this construct versus methods that rely on EEG sensor measures.

The maturation of auditory responses in infants and young children: a cross-sectional study from 6 to 59 months

Background: An understanding of the maturation of auditory cortex responses in typically developing infants and toddlers is needed to later identify auditory processing abnormalities in infants at risk for neurodevelopmental disorders. The availability of infant and young child magnetoencephalography (MEG) systems may now provide near optimal assessment of left and right hemisphere auditory neuromagnetic responses in young populations. To assess the performance of a novel whole-head infant MEG system, a cross-sectional study examined the maturation of left and right auditory cortex responses in children 6- to 59-months of age.

Methods: Blocks of 1000 Hz (1st and 3rd blocks) and 500 Hz tones (2nd block) were presented while MEG data were recorded using an infant/young child biomagnetometer (Artemis 123). Data were obtained from 29 children (11 males; 6- to 59-months). Latency measures were obtained for the first positive-to-negative evoked response waveform complex in each hemisphere. Latency and age associations as well as frequency and hemisphere latency differences were examined. For the 1000 Hz tone, measures of reliability were computed.

Results: For the first response—a response with a “P2m” topography—latencies decreased as a function of age. For the second response—a response with a “N2m” topography—no N2m latency and age relationships were observed. A main effect of tone frequency showed earlier P2m responses for 1st 1000 Hz (150 ms) and 2nd 1000 Hz (148 ms) vs. 500 Hz tones (162 ms). A significant main effect of hemisphere showed earlier N2m responses for 2nd 1000 Hz (226 ms) vs. 1st 1000 Hz (241 ms) vs. 500 Hz tones (265 ms). P2m and N2m interclass correlation coefficient latency findings were as follows: left P2m (0.72, p < 0.001), right P2m (0.84, p < 0.001), left N2m (0.77, p < 0.001), and right N2m (0.77,p < 0.01).

Conclusions: Findings of strong age and latency associations, sensitivity to tone frequency, and good test-retest reliability support the viability of longitudinal infant MEG studies that include younger as well as older participants as well as studies examining auditory processing abnormalities in infants at risk for neurodevelopmental disorders.

Habituation of Auditory Steady State Responses Evoked by Amplitude-Modulated Acoustic Signals in Rats

Generation of the auditory steady state responses (ASSR) is commonly explained by the linear combination of random background noise activity and the stationary response. Based on this model, the decrease of amplitude that occurs over the sequential averaging of epochs of the raw data has been exclusively linked to the cancelation of noise. Nevertheless, this behavior might also reflect the non-stationary response of the ASSR generators. We tested this hypothesis by characterizing the ASSR time course in rats with different auditory maturational stages. ASSR were evoked by 8-kHz tones of different supra-threshold intensities, modulated in amplitude at 115 Hz. Results show that the ASSR amplitude habituated to the sustained stimulation and that dishabituation occurred when deviant stimuli were presented. ASSR habituation increased as animals became adults, suggesting that the ability to filter acoustic stimuli with no-relevant temporal information increased with age. Results are discussed in terms of the current model of the ASSR generation and analysis procedures. They might have implications for audiometric tests designed to assess hearing in subjects who cannot provide reliable results in the psychophysical trials.

1H-MRS glutamate level predicts auditory sensory gating in alcohol dependence: Preliminary results

Background:

Impairment in auditory sensory gating (ASG) has been documented in alcohol dependence [1]. Likewise, it has been shown that ASG becomes abnormal during alcohol administration in otherwise healthy individuals [2]. Patterns of gating abnormality associated with alcohol use are likely associated with an alcohol responsive neurochemical like glutamate (Glu), particularly since it is well-established that alcohol affects NMDA receptors and that glutamatergic functioning is abnormal in both acute alcohol use and in alcohol dependence [3]. Hence, a link between Glu metabolite levels and ASG was hypothesized. It was first hypothesized that Glu and ASG abnormality would be found in groups with alcohol dependence. A second hypothesis was that across groups, greater Glu would predict reduced ASG.

Methods:

Groups were comprised of healthy, non-drinking controls (Controls, N = 4), individuals with current alcohol dependence (AUD-current, N = 6), and with alcohol dependence in remission for at least 1 year (AUD-remission, N = 6). Participants underwent a diagnostic assessment for alcohol consumption, MRI, 1H-MRS for in vivo assessment of Glu and other metabolites, and MEG scanning during a paired click protocol. ASG was computed as the ratio of the source strength of the 50 ms component in the event related field (ERF) to the second click in the pair divided by the source strength of the 50 ms component to the first click in the pair.

Results:

Univariate MANOVAs controlling for age and gender revealed a significant effect for group on Glu and ASG, such that ASG ratios were significantly elevated, implying weakened gating. Glu concentration was reduced in AUD-current relative to the other two groups. Further analysis revealed that when additionally controlling for the group effect, reduced Glu predicted increasing impairment in ASG.

Conclusions:

The overall results were consistent with the hypothesis that differences in Glu metabolite levels associated with alcohol dependence result in impaired ASG.

Missing and delayed auditory responses in young and older children with autism spectrum disorders

Background: The development of left and right superior temporal gyrus (STG) 50 ms (M50) and 100 ms (M100) auditory responses in typically developing (TD) children and in children with autism spectrum disorder (ASD) was examined. Reflecting differential development of primary/secondary auditory areas and supporting previous studies, it was hypothesized that whereas left and right M50 STG responses would be observed equally often in younger and older children, left and right M100 STG responses would more often be absent in younger than older children. In ASD, delayed neurodevelopment would be indicated via the observation of a greater proportion of ASD than TD subjects showing missing M100 but not M50 responses in both age groups. Missing M100 responses would be observed primarily in children with ASD with language impairment (ASD + LI) (and perhaps concomitantly lower general cognitive abilities).

Methods: Thirty-five TD controls, 63 ASD without language impairment (ASD − LI), and 38 ASD + LI were recruited. Binaural tones were presented. The presence or absence of a STG M50 and M100 was scored. Subjects were grouped into younger (6–10 years old) and older groups (11–15 years old).

Results: Although M50 responses were observed equally often in older and younger subjects and equally often in TD and ASD, left and right M50 responses were delayed in ASD − LI and ASD + LI. Group comparisons showed that in younger subjects M100 responses were observed more often in TD than ASD + LI (90 versus 66%, p = 0.04), with no differences between TD and ASD − LI (90 versus 76%, p = 0.14) or between ASD − LI and ASD + LI (76 versus 66%, p = 0.53). In older subjects, whereas no differences were observed between TD and ASD + LI, responses were observed more often in ASD − LI than ASD + LI. Findings were similar when splitting the ASD group into lower- and higher-cognitive functioning groups.

Conclusion: Although present in all groups, M50 responses were delayed in ASD. Examining the TD data, findings indicated that by 11 years, a right M100 should be observed in 100% of subjects and a left M100 in 80% of subjects. Thus, by 11 years, lack of a left and especially right M100 offers neurobiological insight into sensory processing that may underlie language or cognitive impairment.

Modulatory role of the prefrontal generator within the auditory M50 network

The amplitude variability of the M50 component of neuromagnetic responses is commonly used to explore the brain's ability to modulate its response to incoming repetitive or novel auditory stimuli, a process conceptualized as a gating mechanism. The goal of this study was to identify the spatial and temporal characteristics of the cortical sources underlying the M50 network evoked by tones in a passive oddball paradigm. Twenty elderly subjects [10 patients diagnosed with mild cognitive impairment (MCI) or probable Alzheimer disease (AD) and 10 age-matched controls] were examined using magnetoencephalographic (MEG) recordings and the multi-dipole Calibrated Start Spatio-Temporal (CSST) source localization method. We identified three cortical regions underlying the M50 network: prefrontal cortex (PF) in addition to bilateral activation of the superior temporal gyrus (STG). The cortical dynamics of the PF source within the 30-100 ms post-stimulus interval was characterized and was found to be comprised of two subcomponents, Mb1c and Mb2c. The PF source was localized for 10/10 healthy subjects, whereas 9/10 MCI/AD patients were lacking the PF source for both tone conditions. The selective activation of the PF source in healthy controls along with the inactivation of the PF region for MCI/AD patients, enabled us to examine the dynamics of this network of activity when it was functional and dysfunctional, respectively. We found significantly enhanced activity of the STG sources in response to both tone conditions for all subjects who lacked a PF source. The reported results provide novel insights into the topology and neurodynamics of the M50 auditory network, which suggest an inhibitory role of the PF source that normally suppresses activity of the STG sources.

Artemis 123: development of a whole-head infant and young child MEG system

BACKGROUND

A major motivation in designing the new infant and child magnetoencephalography (MEG) system described in this manuscript is the premise that electrophysiological signatures (resting activity and evoked responses) may serve as biomarkers of neurodevelopmental disorders, with neuronal abnormalities in conditions such as autism spectrum disorder (ASD) potentially detectable early in development. Whole-head MEG systems are generally optimized/sized for adults. Since magnetic field produced by neuronal currents decreases as a function of distance(2) and infants and young children have smaller head sizes (and thus increased brain-to-sensor distance), whole-head adult MEG systems do not provide optimal signal-to-noise in younger individuals. This spurred development of a whole-head infant and young child MEG system - Artemis 123.

METHODS

In addition to describing the design of the Artemis 123, the focus of this manuscript is the use of Artemis 123 to obtain auditory evoked neuromagnetic recordings and resting-state data in young children. Data were collected from a 14-month-old female, an 18-month-old female, and a 48-month-old male. Phantom data are also provided to show localization accuracy.

RESULTS

Examination of Artemis 123 auditory data showed generalizability and reproducibility, with auditory responses observed in all participants. The auditory MEG measures were also found to be manipulable, exhibiting sensitivity to tone frequency. Furthermore, there appeared to be a predictable sensitivity of evoked components to development, with latencies decreasing with age. Examination of resting-state data showed characteristic oscillatory activity. Finally, phantom data showed that dipole sources could be localized with an error less than 0.5 cm.

CONCLUSIONS

Artemis 123 allows efficient recording of high-quality whole-head MEG in infants four years and younger. Future work will involve examining the feasibility of obtaining somatosensory and visual recordings in similar-age children as well as obtaining recordings from younger infants. Thus, the Artemis 123 offers the promise of detecting earlier diagnostic signatures in such neurodevelopmental disorders.

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.

Prepulse inhibition of change-related P50m no correlation with P50m gating

Both prepulse inhibition (PPI) of the startle response and P50 sensory gating are important tools to investigate the inhibitory mechanisms of sensory processing. However, previous studies found no or a weak association between these two measures, which may have been due to the different indexes used. We examined the relationship between P50 sensory gating and P50 PPI. P50m sensory gating and PPI of Change-related P50m were assessed in 14 subjects using magnetoencephalography. Concerning P50m sensory gating, the amplitudes of the response to the second click relative to that to the first one were reduced by 43 and 47% for the left and right hemisphere, respectively. Change-related P50m was evoked by an abrupt sound pressure increase by 10 dB in a continuous click train of 70 dB. When this test stimulus was preceded by a click (prepulse) with a weaker sound pressure increase (5 dB) at a prepulse-test interval of 30, 60, or 90 ms, Change-P50m was suppressed by 33 ~ 65% while the prepulse itself elicited no or very weak P50m responses. Although the amplitude of the P50m response to the first click and the amplitude of the Change-P50m test alone response were positively correlated (r = 0.6), the degree of the inhibition of the two measures was not (r = -0.06 ~ 0.14). The neural origin was estimated to be located in the supratemporal plane around the superior temporal gyrus or Heschl’s gyrus and did not differ between P50m and Change-P50m. The present results suggest that P50m and Change-P50m are generated by a similar group of neurons in the auditory cortex, while the mechanisms of P50m sensory gating and Change-P50m PPI are different.

Atypical brain lateralisation in the auditory cortex and language performance in 3- to 7-year-old children with high-functioning autism spectrum disorder: a child-customised magnetoencephalography (MEG) study

BACKGROUND

Magnetoencephalography (MEG) is used to measure the auditory evoked magnetic field (AEF), which reflects language-related performance. In young children, however, the simultaneous quantification of the bilateral auditory-evoked response during binaural hearing is difficult using conventional adult-sized MEG systems. Recently, a child-customised MEG device has facilitated the acquisition of bi-hemispheric recordings, even in young children. Using the child-customised MEG device, we previously reported that language-related performance was reflected in the strength of the early component (P50m) of the auditory evoked magnetic field (AEF) in typically developing (TD) young children (2 to 5 years old) [Eur J Neurosci 2012, 35:644-650]. The aim of this study was to investigate how this neurophysiological index in each hemisphere is correlated with language performance in autism spectrum disorder (ASD) and TD children.

METHODS

We used magnetoencephalography (MEG) to measure the auditory evoked magnetic field (AEF), which reflects language-related performance. We investigated the P50m that is evoked by voice stimuli (/ne/) bilaterally in 33 young children (3 to 7 years old) with ASD and in 30 young children who were typically developing (TD). The children were matched according to their age (in months) and gender. Most of the children with ASD were high-functioning subjects.

RESULTS

The results showed that the children with ASD exhibited significantly less leftward lateralisation in their P50m intensity compared with the TD children. Furthermore, the results of a multiple regression analysis indicated that a shorter P50m latency in both hemispheres was specifically correlated with higher language-related performance in the TD children, whereas this latency was not correlated with non-verbal cognitive performance or chronological age. The children with ASD did not show any correlation between P50m latency and language-related performance; instead, increasing chronological age was a significant predictor of shorter P50m latency in the right hemisphere.

CONCLUSIONS

Using a child-customised MEG device, we studied the P50m component that was evoked through binaural human voice stimuli in young ASD and TD children to examine differences in auditory cortex function that are associated with language development. Our results suggest that there is atypical brain function in the auditory cortex in young children with ASD, regardless of language development.

Abnormal pre-attentive arousal in young children with autism spectrum disorder contributes to their atypical auditory behavior: an ERP study

Auditory sensory modulation difficulties and problems with automatic re-orienting to sound are well documented in autism spectrum disorders (ASD). Abnormal preattentive arousal processes may contribute to these deficits. In this study, we investigated components of the cortical auditory evoked potential (CAEP) reflecting preattentive arousal in children with ASD and typically developing (TD) children aged 3-8 years. Pairs of clicks (‘S1’ and ‘S2’) separated by a 1 sec S1-S2 interstimulus interval (ISI) and much longer (8-10 sec) S1-S1 ISIs were presented monaurally to either the left or right ear. In TD children, the P50, P100 and N1c CAEP components were strongly influenced by temporal novelty of clicks and were much greater in response to the S1 than the S2 click. Irrespective of the stimulation side, the ‘tangential’ P100 component was rightward lateralized in TD children, whereas the ‘radial’ N1c component had higher amplitude contralaterally to the stimulated ear. Compared to the TD children, children with ASD demonstrated 1) reduced amplitude of the P100 component under the condition of temporal novelty (S1) and 2) an attenuated P100 repetition suppression effect. The abnormalities were lateralized and depended on the presentation side. They were evident in the case of the left but not the right ear stimulation. The P100 abnormalities in ASD correlated with the degree of developmental delay and with the severity of auditory sensory modulation difficulties observed in early life. The results suggest that some rightward-lateralized brain networks that are crucially important for arousal and attention re-orienting are compromised in children with ASD and that this deficit contributes to sensory modulation difficulties and possibly even other behavioral deficits in ASD.

Frontal and superior temporal auditory processing abnormalities in schizophrenia

BACKGROUND

Although magnetoencephalography (MEG) studies show superior temporal gyrus (STG) auditory processing abnormalities in schizophrenia at 50 and 100 ms, EEG and corticography studies suggest involvement of additional brain areas (e.g., frontal areas) during this interval. Study goals were to identify 30 to 130 ms auditory encoding processes in schizophrenia (SZ) and healthy controls (HC) and group differences throughout the cortex.

METHODS

The standard paired-click task was administered to 19 SZ and 21 HC subjects during MEG recording. Vector-based Spatial-temporal Analysis using L1-minimum-norm (VESTAL) provided 4D maps of activity from 30 to 130 ms. Within-group t-tests compared post-stimulus 50 ms and 100 ms activity to baseline. Between-group t-tests examined 50 and 100 ms group differences.

RESULTS

Bilateral 50 and 100 ms STG activity was observed in both groups. HC had stronger bilateral 50 and 100 ms STG activity than SZ. In addition to the STG group difference, non-STG activity was also observed in both groups. For example, whereas HC had stronger left and right inferior frontal gyrus activity than SZ, SZ had stronger right superior frontal gyrus and left supramarginal gyrus activity than HC.

CONCLUSIONS

Less STG activity was observed in SZ than HC, indicating encoding problems in SZ. Yet auditory encoding abnormalities are not specific to STG, as group differences were observed in frontal and SMG areas. Thus, present findings indicate that individuals with SZ show abnormalities in multiple nodes of a concurrently activated auditory network.

Language performance and auditory evoked fields in 2- to 5-year-old children

Language development progresses at a dramatic rate in preschool children. As rapid temporal processing of speech signals is important in daily colloquial environments, we performed magnetoencephalography (MEG) to investigate the linkage between speech-evoked responses during rapid-rate stimulus presentation (interstimulus interval < 1 s) and language performance in 2- to 5-year-old children (n = 59). Our results indicated that syllables with this short stimulus interval evoked detectable P50m, but not N100m, in most participants, indicating a marked influence of longer neuronal refractory period for stimulation. The results of equivalent dipole estimation showed that the intensity of the P50m component in the left hemisphere was positively correlated with language performance (conceptual inference ability). The observed positive correlations were suggested to reflect the maturation of synaptic organisation or axonal maturation and myelination underlying the acquisition of linguistic abilities. The present study is among the first to use MEG to study brain maturation pertaining to language abilities in preschool children.

Auditory magnetic mismatch field latency: a biomarker for language impairment in autism

BACKGROUND

Auditory processing abnormalities are frequently observed in autism spectrum disorders (ASD), and these abnormalities may have sequelae in terms of clinical language impairment (LI). The present study assessed associations between language impairment and the amplitude and latency of the superior temporal gyrus magnetic mismatch field (MMF) in response to changes in an auditory stream of tones or vowels.

METHODS

Fifty-one children with ASD, and 27 neurotypical control subjects, all aged 6 to 15 years, underwent neuropsychological evaluation, including tests of language function, as well as magnetoencephalographic recording during presentation of tones and vowels. The MMF was identified in the difference waveform obtained from subtraction of responses to standard from deviant stimuli.

RESULTS

Magnetic mismatch field latency was significantly prolonged (p < .001) in children with ASD, compared with neurotypical control subjects. Furthermore, this delay was most pronounced (∼50 msec) in children with concomitant LI, with significant differences in latency between children with ASD with LI and those without (p < .01). Receiver operator characteristic analysis indicated a sensitivity of 82.4% and specificity of 71.2% for diagnosing LI based on MMF latency.

CONCLUSIONS

Neural correlates of auditory change detection (the MMF) are significantly delayed in children with ASD, and especially those with concomitant LI, suggesting a neurobiological basis as well as a clinical biomarker for LI in ASD.

Specific cognitive training normalizes auditory sensory gating in schizophrenia: a randomized trial

BACKGROUND

The ratio of scalp-recorded brain responses occurring 50 msec after paired clicks (S2-evoked P50/S1-evoked P50) serves as a measure of sensory gating. An abnormally large ratio is commonly found in schizophrenia and is considered as a sign of reduced sensory gating or otherwise dysfunctional organization of the auditory/verbal system as a factor contributing to psychopathology and cognitive dysfunction in schizophrenia. This initial randomized clinical trial compared the efficacy of two 4-week, computer-based cognitive training methods that emphasize either auditory discrimination and verbal memory or a broader range of cognitive functions in schizophrenia.

METHODS

Thirty-nine schizophrenia patients (ICD-F20.0 diagnosis) were assigned to Cognitive Exercises (CE) or Cognitive Package (Cogpack). The M50, the magnetoencephalographic analogue of electroencephalographic P50, and performance on verbal learning and memory tests were used to evaluate training effects.

RESULTS

As expected, patients exhibited higher pretreatment gating ratios than 28 age-matched healthy comparison participants. Gating ratios decreased after CE but not after Cogpack. Cognitive test performance improved more after CE than after Cogpack.

CONCLUSIONS

Appropriately specific psychological training changes the neural performance in schizophrenia, normalizing sensory and cognitive function.

Gamma synchrony: towards a translational biomarker for the treatment-resistant symptoms of schizophrenia

The lack of efficacy for antipsychotics with respect to negative symptoms and cognitive deficits is a significant obstacle for the treatment of schizophrenia. Developing new drugs to target these symptoms requires appropriate neural biomarkers that can be investigated in model organisms, be used to track treatment response, and provide insight into pathophysiological disease mechanisms. A growing body of evidence indicates that neural oscillations in the gamma frequency range (30-80 Hz) are disturbed in schizophrenia. Gamma synchrony has been shown to mediate a host of sensory and cognitive functions, including perceptual encoding, selective attention, salience, and working memory - neurocognitive processes that are dysfunctional in schizophrenia and largely refractory to treatment. This review summarizes the current state of clinical literature with respect to gamma-band responses (GBRs) in schizophrenia, focusing on resting and auditory paradigms. Next, preclinical studies of schizophrenia that have investigated gamma-band activity are reviewed to gain insight into neural mechanisms associated with these deficits. We conclude that abnormalities in gamma synchrony are ubiquitous in schizophrenia and likely reflect an elevation in baseline cortical gamma synchrony ('noise') coupled with reduced stimulus-evoked GBRs ('signal'). Such a model likely reflects hippocampal and cortical dysfunction, as well as reduced glutamatergic signaling with downstream GABAergic deficits, but is probably less influenced by dopaminergic abnormalities implicated in schizophrenia. Finally, we propose that analogous signal-to-noise deficits in the flow of cortical information in preclinical models are useful targets for the development of new drugs that target the treatment-resistant symptoms of schizophrenia.

Spatiotemporal signatures of an abnormal auditory system in stuttering

People who stutter (PWS) can reduce their stuttering rates under masking noise and altered auditory feedback; such a response can be attributed to altered auditory input, which suggests that abnormal speech processing in PWS results from abnormal processing of auditory input. However, the details of this abnormal processing of basic auditory information remain unclear. In order to characterize such abnormalities, we examined the functional and structural changes in the auditory cortices of PWS by using a 306-channel magnetoencephalography system to assess auditory sensory gating (P50m suppression) and tonotopic organization. Additionally, we employed voxel-based morphometry to compare cortical gray matter (GM) volumes on structural MR images. PWS exhibited impaired left auditory sensory gating. The tonotopic organization in the right hemisphere of PWS is expanded compared with that of the controls. Furthermore, PWS showed a significant increase in the GM volume of the right superior temporal gyrus, consistent with the right tonotopic expansion. Accordingly, we suggest that PWS have impaired left auditory sensory gating during basic auditory input processing and that some error signals in the auditory cortex could result in abnormal speech processing. Functional and structural reorganization of the right auditory cortex appears to be a compensatory mechanism for impaired left auditory cortex function in PWS.

Cognitive abilities and 50- and 100-msec paired-click processes in schizophrenia

OBJECTIVE

Abnormal 50- and 100-msec event-related brain activity derived from paired-click procedures are well established in schizophrenia. There is little agreement on whether group differences in the ratio score, i.e., the ratio of EEG amplitude after the second stimulus (S2) to the amplitude after the first stimulus (S1), reflect an encoding or gating abnormality. In addition, the functional implications remain unclear. In the present study, EEG and magnetoencephalography (MEG) were used to examine paired-click measures and cognitive correlates of paired-click activity.

METHOD

EEG and whole-cortex MEG data were acquired during the standard paired-click paradigm in 73 comparison subjects and 79 schizophrenia patients. Paired-click ratio scores were obtained at 50 msec (P50 evoked potential at Cz, M50 at left and right superior temporal gyrus [STG]) and 100 msec (N100 at Cz, M100 at left and right STG). A cognitive battery assessing attention, working memory, and long-delay memory was administered. IQ was also estimated.

RESULTS

Groups differed on ratio score and amplitude of S1 response. Ratio scores at 50 msec and 100 msec and S1 amplitude predicted variance in attention (primarily S1 amplitude), working memory, and long-delay memory. The attention findings remained after removal of variance associated with IQ.

CONCLUSIONS

Associations between paired-click measures and cognitive performance in patients support 50-msec and 100-msec ratio and amplitude scores as clinically significant biomarkers of schizophrenia. In general, cognitive performance was better predicted by the ability to encode auditory information than the ability to filter redundant information.

MEG does not reveal impaired sensory gating in first-episode schizophrenia

OBJECTIVE

The inability to adequately suppress the second of two identical stimuli is called sensory gating deficit and can be studied by recording evoked potentials to auditory stimuli, e.g. the P50 and the N100. It has been considered the physiological correlate of schizophrenia patients' perception of being flooded by sensory impressions. According to the notion that the gating deficit constitutes a genetic trait, we expected to demonstrate the phenomenon in first-episode schizophrenia patients by using Magnetencephalography (MEG).

METHODS

Eighteen inpatients in remission of their first psychotic episode and 24 healthy, age- and sex-matched control subjects participated in the study. Diagnoses, psychopathology, and handedness were assessed with established instruments. Stimulation was performed with the double click paradigm (ISI 500 ms, ITI 9-10 s). MEG recordings of 15 patients and 18 controls entered further analyses with the software BESA for spatio-temporal source analyses and statistical analyses with MATLAB.

RESULTS

Neither P50 nor N100 responses differed statistically between the groups, which means that gating was not impaired in this sample of first-episode schizophrenia patients.

CONCLUSIONS

These results are not in line with the majority of studies on sensory gating in schizophrenia, however, studies on first-episode patients are scarce. The most likely reasons for not observing a gating deficit in our study are patients' first-episode status and atypical antipsychotic medication.

MEG detection of delayed auditory evoked responses in autism spectrum disorders: towards an imaging biomarker for autism

Motivated by auditory and speech deficits in autism spectrum disorders (ASD), the frequency dependence of superior temporal gyrus (STG) 50 msec (M50) and 100 msec (M100) neuromagnetic auditory evoked field responses in children with ASD and typically developing controls were evaluated. Whole-cortex magnetoencephalography (MEG) was obtained from 17 typically developing children and 25 children with ASD. Subjects were presented tones with frequencies of 200, 300, 500, and 1,000 Hz, and left and right STG M50 and M100 STG activity was examined. No M50 latency or amplitude Group differences were observed. In the right hemisphere, a Group x Frequency ANOVA on M100 latency produced a main effect for Group (P=0.01), with an average M100 latency delay of 11 msec in children with ASD. In addition, only in the control group was the expected association of earlier M100 latencies in older than younger children observed. Group latency differences remained significant when hierarchical regression analyses partialed out M100 variance associated with age, IQ, and language ability (all P-values <0.05). Examining the right-hemisphere 500 Hz condition (where the largest latency differences were observed), a sensitivity of 75%, a specificity of 81%, and a positive predictive value (PPV) of 86% was obtained at a threshold of 116 msec. The M100 latency delay indicates disruption of encoding simple sensory information. Given similar findings in language impaired and non-language impaired ASD subjects, a right-hemisphere M100 latency delay appears to be an electrophysiological endophenotype for autism.

MEG in the macaque monkey and human: distinguishing cortical fields in space and time

Magnetoencephalography (MEG) is an increasingly popular non-invasive tool used to record, on a millisecond timescale, the magnetic field changes generated by cortical neural activity. MEG has the advantage, over fMRI for example, that it is a direct measure of neural activity. In the current investigation we used MEG to measure cortical responses to tactile and auditory stimuli in the macaque monkey. We had two aims. First, we sought to determine whether MEG, a technique that may have low spatial accuracy, could be used to distinguish the location and organization of sensory cortical fields in macaque monkeys, a species with a relatively small brain compared to that of the human. Second, we wanted to examine the temporal dynamics of cortical responses in the macaque monkey relative to the human. We recorded MEG data from anesthetized monkeys and, for comparison, from awake humans that were presented with simple tactile and auditory stimuli. Neural source reconstruction of MEG data showed that primary somatosensory and auditory cortex could be differentiated and, further, that separate representations of the digit and lip within somatosensory cortex could be identified in macaque monkeys as well as humans. We compared the latencies of activity from monkey and human data for the three stimulation types and proposed a correspondence between the neural responses of the two species. We thus demonstrate the feasibility of using MEG in the macaque monkey and provide a non-human primate model for examining the relationship between external evoked magnetic fields and their underlying neural sources.

Auditory gating deficit to human voices in schizophrenia: a MEG study

BACKGROUND

Patients with schizophrenia have auditory gating deficits; however, little is known about P50 auditory gating to human voices and its association with clinical symptoms. We examined the functioning of auditory gating and its relationship with the clinical symptoms in schizophrenia.

METHODS

Auditory evoked magnetoencephalography responses to the first and the second voices stimuli were recorded in 22 schizophrenia patients and 28 normal control subjects. The auditory gating ratios of P50m and N100m were investigated and P50m-symptom correlations were also investigated.

RESULTS

Patients showed significantly higher P50m gating ratios to human voices specifically in the left hemisphere. Moreover, patients with higher left P50m gating ratios showed more severe auditory hallucinations, while patients with higher right P50m gating ratios showed more severe negative symptoms.

CONCLUSIONS

The present study suggests that schizophrenia patients have auditory gating deficits to human voices, specifically in the left hemisphere and auditory hallucinations of schizophrenia may be associated with sensory overload to human voices in the auditory cortex.

Intracranial recording and source localization of auditory brain responses elicited at the 50 ms latency in three children aged from 3 to 16 years

Maturational studies of the auditory-evoked brain response at the 50 ms latency provide an insight into why this response is aberrant in a number of psychiatric disorders that have developmental origin. Here, using intracranial recordings we found that neuronal activity of the primary contributors to this response can be localised at the lateral part of Heschl's gyrus already at the age of 3.5 years. This study provides results to support the notion that deviations in cognitive function(s) attributed to the auditory P50 in adults might involve abnormalities in neuronal activity of the frontal lobe or in the interaction between the frontal and temporal lobes. Validation and localisation of progenitors of the adults' P50 in young children is a much-needed step in the understanding of the biological significance of different subcomponents that comprise the auditory P50 in the adult brain. In combination with other approaches investigating neuronal mechanisms of auditory P50, the present results contribute to the greater understanding of what and why neuronal activity underlying this response is aberrant in a number of brain dysfunctions. Moreover, the present source localisation results of auditory response at the 50 ms latency might be useful in paediatric neurosurgery practice.

Schizophrenia diagnosis and anterior hippocampal volume make separate contributions to sensory gating

Impaired P50 gating is thought to reflect a core deficit in schizophrenia, but the relevant neural network is not well understood. The present study used EEG and MEG to assess sensory gating and volumetric MRI to measure hippocampal volume to investigate relationships between them in 22 normal controls and 22 patients with schizophrenia. In the schizophrenia group, anterior but not posterior hippocampal volume was smaller, and both the P50 and M50 gating ratios were larger (worse) than in controls. Independent of group, left-hemisphere M50 gating ratio correlated negatively with left anterior hippocampal volume, and right-hemisphere M50 gating ratio correlated negatively with right anterior hippocampal volume. Schizophrenia diagnosis predicted M50 gating independent of hippocampal volume. These results are consistent with the finding that hippocampus is a critical part of a fronto-temporal circuit involved in auditory gating.

Auditory sensory gating to the human voice: a preliminary MEG study

The ability of the brain to suppress incoming irrelevant sensory input is termed 'sensory gating,' and auditory sensory gating is often indexed by the auditory evoked response. We recorded the auditory evoked magnetic fields to the human voice, using the conditioning-testing paradigm, to investigate whether or not healthy subjects show less activation to the second voice stimulus. Seventeen healthy adults (mean age 27.9+/-4.8 years, 9 males and 8 females) participated in the experiment. The auditory stimuli were presented monaurally as a series of 120 paired voices, with 500-ms interstimulus intervals and 6-s interpaired stimulus intervals. The P50m and the N100m responses were investigated, and dipole source localization was performed. Root mean squares of both P50m and N100m were significantly suppressed to the second stimulus bilaterally, and the suppression was more significant in N100m. The N100m was located significantly more laterally than the P50m for both hemispheres. These results therefore demonstrate the presence of sensory gating for auditory inputs of the human voice in the primary auditory cortex and the auditory association area.

The neural networks underlying auditory sensory gating

One of the most consistent electrophysiological deficits reported in the schizophrenia literature is the failure to inhibit, or properly gate, the neuronal response to the second stimulus of an identical pair (i.e., sensory gating). Although animal and invasive human studies have consistently implicated the auditory cortex, prefrontal cortex and hippocampus in mediating the sensory gating response, localized activation in these structures has not always been reported during non-invasive imaging modalities. In the current experiment, event-related FMRI and a variant of the traditional gating paradigm were utilized to examine how the gating network differentially responded to the processing of pairs of identical and non-identical tones. Two single-tone conditions were also presented so that they could be used to estimate the HRF for paired stimuli, reconstructed based on actual hemodynamic responses, to serve as a control non-gating condition. Results supported an emerging theory that the gating response for both paired-tone conditions was primarily mediated by auditory and prefrontal cortex, with potential contributions from the thalamus. Results also indicated that the left auditory cortex may play a preferential role in determining the stimuli that should be inhibited (gated) or receive further processing due to novelty of information. In contrast, there was no evidence of hippocampal involvement, suggesting that future work is needed to determine what role it may play in the gating response.