Neuroimaging investigations of dorsal stream processing and effects of stimulus synchrony in schizophrenia.

Impairments in auditory and visual processing are common in schizophrenia (SP). In the unisensory realm visual deficits are primarily noted for the dorsal visual stream. In addition, insensitivity to timing offsets between stimuli are widely reported for SP. The aim of the present study was to test at the physiological level differences in dorsal/ventral stream visual processing and timing sensitivity between SP and healthy controls (HC) using MEG and a simple auditory/visual task utilizing a variety of multisensory conditions. The paradigm included all combinations of synchronous/asynchronous and central/peripheral stimuli, yielding 4 task conditions. Both HC and SP groups showed activation in parietal areas (dorsal visual stream) during all multisensory conditions, with parietal areas showing decreased activation for SP relative to HC, and a significantly delayed peak of activation for SP in intraparietal sulcus (IPS). We also observed a differential effect of stimulus synchrony on HC and SP parietal response. Furthermore, a (negative) correlation was found between SP positive symptoms and activity in IPS. Taken together, our results provide evidence of impairment of the dorsal visual stream in SP during a multisensory task, along with an altered response to timing offsets between presented multisensory stimuli.

MEG biomarker of Alzheimer's disease: Absence of a prefrontal generator during auditory sensory gating.

Magnetoencephalography (MEG), a direct measure of neuronal activity, is an underexplored tool in the search for biomarkers of Alzheimer's disease (AD). In this study, we used MEG source estimates of auditory gating generators, nonlinear correlations with neuropsychological results, and multivariate analyses to examine the sensitivity and specificity of gating topology modulation to detect AD. Our results demonstrated the use of MEG localization of a medial prefrontal (mPFC) gating generator as a discrete (binary) detector of AD at the individual level and resulted in recategorizing the participant categories in: (1) controls with mPFC generator localized in response to both the standard and deviant tones; (2) a possible preclinical stage of AD participants (a lower functioning group of controls) in which mPFC activation was localized to the deviant tone only; and (3) symptomatic AD in which mPFC activation was not localized to either the deviant or standard tones. This approach showed a large effect size (0.9) and high accuracy, sensitivity, and specificity (100%) in identifying symptomatic AD patients within a limited research sample. The present results demonstrate high potential of mPFC activation as a noninvasive biomarker of AD pathology during putative preclinical and clinical stages. Hum Brain Mapp 38:5180-5194, 2017. © 2017 Wiley Periodicals, Inc.

Magnetoencephalographic and functional MRI connectomics in schizophrenia via intra- and inter-network connectivity.

Examination of intrinsic functional connectivity using functional MRI (fMRI) has provided important findings regarding dysconnectivity in schizophrenia. Extending these results using a complementary neuroimaging modality, magnetoencephalography (MEG), we present the first direct comparison of functional connectivity between schizophrenia patients and controls, using these two modalities combined. We developed a novel MEG approach for estimation of networks using MEG that incorporates spatial independent component analysis (ICA) and pairwise correlations between independent component timecourses, to estimate intra- and intern-network connectivity. This analysis enables group-level inference and testing of between-group differences. Resting state MEG and fMRI data were acquired from a large sample of healthy controls (n=45) and schizophrenia patients (n=46). Group spatial ICA was performed on fMRI and MEG data to extract intrinsic fMRI and MEG networks and to compensate for signal leakage in MEG. Similar, but not identical spatial independent components were detected for MEG and fMRI. Analysis of functional network connectivity (FNC; i.e., pairwise correlations in network (ICA component) timecourses) revealed a differential between-modalities pattern, with greater connectivity among occipital networks in fMRI and among frontal networks in MEG. Most importantly, significant differences between controls and patients were observed in both modalities. MEG FNC results in particular indicated dysfunctional hyperconnectivity within frontal and temporal networks in patients, while in fMRI FNC was always greater for controls than for patients. This is the first study to apply group spatial ICA as an approach to leakage correction, and as such our results may be biased by spatial leakage effects. Results suggest that combining these two neuroimaging modalities reveals additional disease-relevant patterns of connectivity that were not detectable with fMRI or MEG alone.

Multimodal Classification of Schizophrenia Patients with MEG and fMRI Data Using Static and Dynamic Connectivity Measures.

Mental disorders like schizophrenia are currently diagnosed by physicians/psychiatrists through clinical assessment and their evaluation of patient's self-reported experiences as the illness emerges. There is great interest in identifying biological markers of prognosis at the onset of illness, rather than relying on the evolution of symptoms across time. Functional network connectivity, which indicates a subject's overall level of "synchronicity" of activity between brain regions, demonstrates promise in providing individual subject predictive power. Many previous studies reported functional connectivity changes during resting-state using only functional magnetic resonance imaging (fMRI). Nevertheless, exclusive reliance on fMRI to generate such networks may limit the inference of the underlying dysfunctional connectivity, which is hypothesized to be a factor in patient symptoms, as fMRI measures connectivity via hemodynamics. Therefore, combination of connectivity assessments using fMRI and magnetoencephalography (MEG), which more directly measures neuronal activity, may provide improved classification of schizophrenia than either modality alone. Moreover, recent evidence indicates that metrics of dynamic connectivity may also be critical for understanding pathology in schizophrenia. In this work, we propose a new framework for extraction of important disease related features and classification of patients with schizophrenia based on using both fMRI and MEG to investigate functional network components in the resting state. Results of this study show that the integration of fMRI and MEG provides important information that captures fundamental characteristics of functional network connectivity in schizophrenia and is helpful for prediction of schizophrenia patient group membership. Combined fMRI/MEG methods, using static functional network connectivity analyses, improved classification accuracy relative to use of fMRI or MEG methods alone (by 15 and 12.45%, respectively), while combined fMRI/MEG methods using dynamic functional network connectivity analyses improved classification up to 5.12% relative to use of fMRI alone and up to 17.21% relative to use of MEG alone.

Multisensory stimuli elicit altered oscillatory brain responses at gamma frequencies in patients with schizophrenia.

Deficits in auditory and visual unisensory responses are well documented in patients with schizophrenia; however, potential abnormalities elicited from multisensory audio-visual stimuli are less understood. Further, schizophrenia patients have shown abnormal patterns in task-related and task-independent oscillatory brain activity, particularly in the gamma frequency band. We examined oscillatory responses to basic unisensory and multisensory stimuli in schizophrenia patients (N = 46) and healthy controls (N = 57) using magnetoencephalography (MEG). Time-frequency decomposition was performed to determine regions of significant changes in gamma band power by group in response to unisensory and multisensory stimuli relative to baseline levels. Results showed significant behavioral differences between groups in response to unisensory and multisensory stimuli. In addition, time-frequency analysis revealed significant decreases and increases in gamma-band power in schizophrenia patients relative to healthy controls, which emerged both early and late over both sensory and frontal regions in response to unisensory and multisensory stimuli. Unisensory gamma-band power predicted multisensory gamma-band power differently by group. Furthermore, gamma-band power in these regions predicted performance in select measures of the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) test battery differently by group. These results reveal a unique pattern of task-related gamma-band power in schizophrenia patients relative to controls that may indicate reduced inhibition in combination with impaired oscillatory mechanisms in patients with schizophrenia.

Same task, different strategies: how brain networks can be influenced by memory strategy.

Previous functional neuroimaging studies demonstrated that different neural networks underlie different types of cognitive processing by engaging participants in particular tasks, such as verbal or spatial working memory (WM) tasks. However, we report here that even when a WM task is defined as verbal or spatial, different types of memory strategies may be used to complete it, with concomitant variations in brain activity. We developed a questionnaire to characterize the type of strategy used by individual members in a group of 28 young healthy participants (18-25 years) during a spatial WM task. A cluster analysis was performed to differentiate groups. We acquired functional magnetoencephalography and structural diffusion tensor imaging measures to characterize the brain networks associated with the use of different strategies. We found two types of strategies were used during the spatial WM task, a visuospatial and a verbal strategy, and brain regions and time courses of activation differed between participants who used each. Task performance also varied by type of strategy used with verbal strategies showing an advantage. In addition, performance on neuropsychological tests (indices from Wechsler Adult Intelligence Scale-IV, Rey Complex Figure Test) correlated significantly with fractional anisotropy measures for the visuospatial strategy group in white matter tracts implicated in other WM and attention studies. We conclude that differences in memory strategy can have a pronounced effect on the locations and timing of brain activation and that these differences need further investigation as a possible confounding factor for studies using group averaging as a means for summarizing results.

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.

Unisensory processing and multisensory integration in schizophrenia: a high-density electrical mapping study.

In real-world settings, information from multiple sensory modalities is combined to form a complete, behaviorally salient percept - a process known as multisensory integration. While deficits in auditory and visual processing are often observed in schizophrenia, little is known about how multisensory integration is affected by the disorder. The present study examined auditory, visual, and combined audio-visual processing in schizophrenia patients using high-density electrical mapping. An ecologically relevant task was used to compare unisensory and multisensory evoked potentials from schizophrenia patients to potentials from healthy normal volunteers. Analysis of unisensory responses revealed a large decrease in the N100 component of the auditory-evoked potential, as well as early differences in the visual-evoked components in the schizophrenia group. Differences in early evoked responses to multisensory stimuli were also detected. Multisensory facilitation was assessed by comparing the sum of auditory and visual evoked responses to the audio-visual evoked response. Schizophrenia patients showed a significantly greater absolute magnitude response to audio-visual stimuli than to summed unisensory stimuli when compared to healthy volunteers, indicating significantly greater multisensory facilitation in the patient group. Behavioral responses also indicated increased facilitation from multisensory stimuli. The results represent the first report of increased multisensory facilitation in schizophrenia and suggest that, although unisensory deficits are present, compensatory mechanisms may exist under certain conditions that permit improved multisensory integration in individuals afflicted with the disorder.

Aging-related changes in auditory and visual integration measured with MEG.

As noted in the aging literature, processing delays often occur in the central nervous system with increasing age, which is often attributable in part to demyelination. In addition, differential slowing between sensory systems has been shown to be most discrepant between visual (up to 20ms) and auditory systems (<5ms). Therefore, we used MEG to measure the multisensory integration response in auditory association cortex in young and elderly participants to better understand the effects of aging on multisensory integration abilities. Results show a main effect for reaction times (RTs); the mean RTs of the elderly were significantly slower than the young. In addition, in the young we found significant facilitation of RTs to the multisensory stimuli relative to both unisensory stimuli, when comparing the cumulative distribution functions, which was not evident for the elderly. We also identified a significant interaction between age and condition in the superior temporal gyrus. In particular, the elderly had larger amplitude responses (∼100ms) to auditory stimuli relative to the young when auditory stimuli alone were presented, whereas the amplitude of responses to the multisensory stimuli was reduced in the elderly, relative to the young. This suppressed cortical multisensory integration response in the elderly, which corresponded with slower RTs and reduced RT facilitation effects, has not been reported previously and may be related to poor cortical integration based on timing changes in unisensory processing in the elderly.

Somatosensory responses in normal aging, mild cognitive impairment, and Alzheimer's disease.

As a part of a larger study of normal aging and Alzheimer's disease (AD), which included patients with mild cognitive impairment (MCI), we investigated the response to median nerve stimulation in primary and secondary somatosensory areas. We hypothesized that the somatosensory response would be relatively spared given the reported late involvement of sensory areas in the progression of AD. We applied brief pulses of electric current to left and right median nerves to test the somatosensory response in normal elderly (NE), MCI, and AD. MEG responses were measured and were analyzed with a semi-automated source localization algorithm to characterize source locations and timecourses. We found an overall difference in the amplitude of the response of the primary somatosensory source (SI) based on diagnosis. Across the first three peaks of the SI response, the MCI patients exhibited a larger amplitude response than the NE and AD groups (P < 0.03). Additional relationships between neuropsychological measures and SI amplitude were also determined. There was no significant difference in amplitude for the contralateral secondary somatosensory source across diagnostic category. These results suggest that somatosensory cortex is affected early in the progression of AD and may have some consequence on behavioral and functional measures.

Frequency-following and connectivity of different visual areas in response to contrast-reversal stimulation.

The sensitivity of visual areas to different temporal frequencies, as well as the functional connections between these areas, was examined using magnetoencephalography (MEG). Alternating circular sinusoids (0, 3.1, 8.7 and 14 Hz) were presented to foveal and peripheral locations in the visual field to target ventral and dorsal stream structures, respectively. It was hypothesized that higher temporal frequencies would preferentially activate dorsal stream structures. To determine the effect of frequency on the cortical response we analyzed the late time interval (220-770 ms) using a multi-dipole spatio-temporal analysis approach to provide source locations and timecourses for each condition. As an exploratory aspect, we performed cross-correlation analysis on the source timecourses to determine which sources responded similarly within conditions. Contrary to predictions, dorsal stream areas were not activated more frequently during high temporal frequency stimulation. However, across cortical sources the frequency-following response showed a difference, with significantly higher power at the second harmonic for the 3.1 and 8.7 Hz stimulation and at the first and second harmonics for the 14 Hz stimulation with this pattern seen robustly in area V1. Cross-correlations of the source timecourses showed that both low- and high-order visual areas, including dorsal and ventral stream areas, were significantly correlated in the late time interval. The results imply that frequency information is transferred to higher-order visual areas without translation. Despite the less complex waveforms seen in the late interval of time, the cross-correlation results show that visual, temporal and parietal cortical areas are intricately involved in late-interval visual processing.

Aging: compensation or maturation?

Neuroimaging studies of healthy aging often reveal differences in neural activation patterns between young and elderly groups for episodic memory tasks, even though there are no differences in behavioral performance. One explanation typically offered is that the elderly compensate for their memory deficiencies through the recruitment of additional prefrontal regions. The present study of healthy aging compared magnetoencephalographic (MEG) time-courses localized to specific cortical regions in two groups of subjects (20-29 years and >or=65 years) during a visual delayed-match-to-sample (DMS) task. MR morphometrics and neuropsychological test results were also examined with the hope of providing insight into the nature of the age-related differences. The behavioral results indicated no differences in performance between young and elderly groups. Although there was a main effect of age on the latency of the initial peak in primary/secondary visual cortex, these longer latencies were not correlated with the performance of elderly on the DMS task. The lateral occipital gyrus (LOG) revealed qualitatively different patterns of activity for the two age groups corroborated by neuropsychological test results. Morphometric results for the young versus elderly groups revealed less white (WM) and gray matter (GM) volumes in the frontal lobes of the elderly. When a group of middle-aged subjects (33-43 years) was included in the morphometric analyses, the middle-aged subjects revealed statistically greater WM volumes in frontal and parietal cortex suggesting immature WM tracts in the young. Perhaps our elderly utilized a different strategy compared to the young due to the different brain maturation levels of these groups.

Aging changes and gender differences in response to median nerve stimulation measured with MEG.

OBJECTIVE: The current study uses magnetoencephalography (MEG) to characterize age-related changes and gender differences in the amplitudes and timing of cortical sources evoked by median nerve stimulation. METHODS: Thirty-four healthy subjects from two age groups: 20-29 and >64 years of age were examined. After measuring the MEG responses, we modeled the data using a spatio-temporal multi-dipole modeling approach to determine the source locations and their associated timecourses. RESULTS: We found early, large amplitude responses in the elderly in primary somatosensory (approximately 20 ms) and pre-central sulcus timecourses (approximately 22 ms) and lower amplitude responses in the elderly later in primary somatosensory (approximately 32 ms) and contralateral secondary somatosensory timecourses (approximately 90 ms). In addition, females had larger peak amplitude responses than males in the contralateral secondary somatosensory timecourse (approximately 28 and 51 ms). CONCLUSIONS: These results show that the median nerve stimulation paradigm provides considerable sensitivity to age- and gender-related differences. The results are consistent with the theory that increased amplitudes identified in the elderly may be associated with decreased inhibition. SIGNIFICANCE: The results emphasize that an examination of two discrete age groups, collapsed across gender, cannot provide a complete understanding of the fundamental changes that occur in the brain across the lifetime.

Temporal dynamics of age-related differences in auditory incidental verbal learning.

Auditory response profiles for a group of ten healthy young and ten healthy elderly subjects, evoked by implicit memory and delayed verbal recognition tasks, were evaluated to determine if effects of stimulus repetition could be identified in the superior temporal gyrus (STG) and prefrontal cortical regions. We hypothesized that effects of stimulus repetition should occur both early in time and at early levels of the nervous system (STG) followed by later effects in prefrontal regions. Magnetoencephalographic (MEG) responses were recorded using a whole-head MEG system and automated, multi-start analysis methods were applied to the data in order to characterize the temporal response profiles from distributed but focal, cortical regions engaged in memory-related tasks. The findings revealed a main effect of age for early activity ( approximately 50 ms) in STG which appeared to be nonspecific for Old/New words and an Age x Task interaction for late activity ( approximately 100-800 ms) in STG which was specific to Old/New words. Although the behavioral performance measures did not reveal traditional effects of response priming, the MEG measures did reveal a reduction in amplitude with stimulus repetition in young subjects. The elderly did not reveal a reduction in amplitude concomitant with stimulus repetition for either the global attributes of words or for specific Old/New words. Long duration effects of stimulus repetition noted in the present study raise the possibility that results from sensory gating, mismatch negativity and P300 paradigms may represent a continuum of stimulus repetition effects. Two of these paradigms evoke greater enhancement to novel or infrequent stimuli, or rather, greater reduction of amplitude with repetition.

Age-related effects on superior temporal gyrus activity during an auditory oddball task.

We used magnetoencephalography in combination with magnetic resonance imaging to investigate the effects of aging on the temporal dynamics of activity localized to several brain regions during an auditory oddball task. The most interesting effects were noted in the superior temporal gyrus as follows: (1) responses were generally stronger to rare than to frequent tones throughout the entire 600-ms time interval, and (2) increases in the amplitude of the 40-ms peak and the latency of the maximum late response were evident in the elderly. Although superior temporal gyrus activity has traditionally been associated with early sensory processing, these results suggest that superior temporal gyrus activity is also important for later decision-related processing.

Differentiability of simulated MEG hippocampal, medial temporal and neocortical temporal epileptic spike activity.

Previous studies have shown that magnetoencephalography (MEG) can measure hippocampal activity, despite the cylindrical shape and deep location in the brain. The current study extended this work by examining the ability to differentiate the hippocampal subfields, parahippocampal cortex, and neocortical temporal sources using simulated interictal epileptic activity. A model of the hippocampus was generated on the MRIs of five subjects. CA1, CA3, and dentate gyrus of the hippocampus were activated as well as entorhinal cortex, presubiculum, and neocortical temporal cortex. In addition, pairs of sources were activated sequentially to emulate various hypotheses of mesial temporal lobe seizure generation. The simulated MEG activity was added to real background brain activity from the five subjects and modeled using a multidipole spatiotemporal modeling technique. The waveforms and source locations/orientations for hippocampal and parahippocampal sources were differentiable from neocortical temporal sources. In addition, hippocampal and parahippocampal sources were differentiated to varying degrees depending on source. The sequential activation of hippocampal and parahippocampal sources was adequately modeled by a single source; however, these sources were not resolvable when they overlapped in time. These results suggest that MEG has the sensitivity to distinguish parahippocampal and hippocampal spike generators in mesial temporal lobe epilepsy.

Investigation of the normal proximal somatomotor system using magnetoencephalography.

OBJECTIVE: The role of the ipsilateral cortex in proximal muscle control in normal human subjects is still under debate. One clinical finding, rapid recovery of proximal muscle relative to distal muscle use following stroke, has led to the suggestion that the ipsilateral as well as the contralateral motor cortex may be involved in normal proximal muscle control. The primary goal of this project was to identify contralateral and ipsilateral motor cortex activation associated with proximal muscle movement in normal subjects using magnetoencephalography (MEG). METHODS: We developed protocols for a self-paced bicep motor task and a deltoid, electrical-stimulation somatosensory task. The MEG data were analyzed using automated multi-dipole spatiotemporal modeling techniques to localize the sources and characterize the associated timing of these sources. RESULTS: Reliable contralateral primary motor and somatosensory sources localized to areas consistent with the homunculus. Ipsilateral M1 activation was only found in 2/12 hemispheres. CONCLUSIONS: Robust contralateral motor cortex activation and sparse ipsilateral motor cortex activation suggest that the ipsilateral motor cortex is not involved in normal proximal muscle control. SIGNIFICANCE: The results suggest that proximal and distal muscle control is similar in normal subjects in the sense that proximal muscle control is primarily governed by the contralateral motor cortex.

Central versus peripheral visual field stimulation results in timing differences in dorsal stream sources as measured with MEG.

Small, achromatic circular sinusoids were presented in the central and peripheral visual fields to investigate dorsal visual stream activation. It was hypothesized that peripheral stimulation would lead to faster onset latencies, as well as preferentially activate dorsal stream visual areas relative to central field stimulation. Although both central and peripheral stimulation activated similar areas, the onset latencies of neuromagnetic sources in two dorsal stream areas were found to be significantly shorter for peripheral versus central field stimulation. The results suggest that information from central versus peripheral fields arrives in the higher-order visual areas via different routes.