Ryouhei Ishii

Medial prefrontal cortex generates frontal midline theta rhythm

Frontal midline theta rhythm (Fm theta) is a distinct theta activity of EEG in the frontal midline area that appears during concentrated performance of mental tasks in normal subjects and reflects focused attentional processing. To tomographically visualize the source current density distributions of Fm theta, we recorded Fm theta by using a 64-channel whole-head MEG system from four healthy subjects, and applied a new analysis method, synthetic aperture magnetometry (SAM), an adaptive beam forming method. Fm theta was observed in the MEG signals over the bilateral frontal regions. SAM analysis showed bilateral medial prefrontal cortices, including anterior cingulate cortex, as the source of Fm theta. This result suggests that focused attention is mainly related to medial prefrontal cortex.

Effect of bilingualism on cognitive control in the Simon task: evidence from MEG

The present study used magneto-encephalography (MEG) to determine the neural correlates of the bilingual advantage previously reported for behavioral measures in conflict tasks. Bilingual Cantonese-English, bilingual French-English, and monolingual English speakers, performed the Simon task in the MEG. Reaction times were faster for congruent than for incongruent trials, and the Cantonese group was faster than the other two groups, which did not differ from each other. Analyses of the MEG data using synthetic aperture magnetometry (SAM) and partial last squares (PLS) showed that the same pattern of activity, involving signal changes in left and medial prefrontal areas, characterized all three groups. Correlations between activated regions and reaction times, however, showed that the two bilingual groups demonstrated faster reaction times with greater activity in superior and middle temporal, cingulate, and superior and inferior frontal regions, largely in the left hemisphere. The monolinguals demonstrated faster reaction times with activation in middle frontal regions. The interpretation is that the management of two language systems led to systematic changes in frontal executive functions.

Frontal midline theta rhythm and gamma power changes during focused attention on mental calculation: an MEG beamformer analysis

Frontal midline theta rhythm (Fmθ) appears widely distributed over medial prefrontal areas in EEG recordings, indicating focused attention. Although mental calculation is often used as an attention-demanding task, little has been reported on calculation-related activation in Fmθ experiments. In this study we used spatially filtered MEG and permutation analysis to precisely localize cortical generators of the magnetic counterpart of Fmθ, as well as other sources of oscillatory activity associated with mental calculation processing (i.e., arithmetic subtraction). Our results confirmed and extended earlier EEG/MEG studies indicating that Fmθ during mental calculation is generated in the dorsal anterior cingulate and adjacent medial prefrontal cortex. Mental subtraction was also associated with gamma event-related synchronization, as an index of activation, in right parietal regions subserving basic numerical processing and number-based spatial attention. Gamma event-related desynchronization appeared in the right lateral prefrontal cortex, likely representing a mechanism to interrupt neural activity that can interfere with the ongoing cognitive task.

Neuroimaging evidence for cortical involvement in the preparation and in the act of swallowing

This study employed whole head magnetoencephalography and synthetic aperture magnetometry to investigate the cortical topography of the preparation and the execution of volitional and reflexive water swallowing and of a simple tongue movement. Concerning movement execution, activation of the mid-lateral primary sensorimotor cortex was strongly lateralized to the left during volitional water swallowing, less strongly lateralized to the left during reflexive water swallowing, and not lateralized at all during tongue movement. In contrast, the preparation for both volitional water swallowing and tongue movement showed a bilateral activation of the primary sensorimotor cortex. No activation was seen prior to reflexive water swallowing. Activation of the left insula and frontal operculum was observed only during both the preparation and the execution of volitional water swallowing. These new findings suggest a left hemispheric dominance for the cortical control of swallowing in humans.

Determination of activation areas in the human auditory cortex by means of synthetic aperture magnetometry

In this study we applied synthetic aperture magnetometry (SAM) to investigate active cortical areas associated with magnetically recorded transient and steady-state auditory evoked responses. For transient evoked responses, SAM images reveal an activated volume of cortical tissue within the lateral aspect of the superior temporal plane. The volume of cortical activation for steady-state responses was located more medially than that for transient evoked responses. Additionally, SAM also reveals a small overlap of activated areas between transient and steady-state evoked responses, which has not be demonstrated when using equivalent current dipole (ECD) source modeling. Source waveforms from SAM and ECD analyses show comparable temporal information. Results from this study suggest that SAM is a useful technique for imaging cortical structures involved in processing perceptual information.

Resting-State EEG Source Localization and Functional Connectivity in Schizophrenia-Like Psychosis of Epilepsy

Background It is unclear whether, like in schizophrenia, psychosis-related disruption in connectivity between certain regions, as an index of intrinsic functional disintegration, occurs in schizophrenia-like psychosis of epilepsy (SLPE). In this study, we sought to determine abnormal patterns of resting-state EEG oscillations and functional connectivity in patients with SLPE, compared with nonpsychotic epilepsy patients, and to assess correlations with psychopathological deficits. Methodology/Principal Findings Resting EEG was recorded in 21 patients with focal epilepsy and SLPE and in 21 clinically-matched non-psychotic epilepsy controls. Source current density and functional connectivity were determined using eLORETA software. For connectivity analysis, a novel nonlinear connectivity measure called “lagged phase synchronization” was used. We found increased theta oscillations in regions involved in the default mode network (DMN), namely the medial and lateral parietal cortex bilaterally in the psychotic patients relative to their nonpsychotic counterparts. In addition, patients with psychosis had increased beta temporo-prefrontal connectivity in the hemisphere with predominant seizure focus. This functional connectivity in temporo-prefrontal circuits correlated with positive symptoms. Additionally, there was increased interhemispheric phase synchronization between the auditory cortex of the affected temporal lobe and the Brocas area correlating with auditory hallucination scores. Conclusions/Significance In addition to dysfunction of parietal regions that are part of the DMN, resting-state disrupted connectivity of the medial temporal cortex with prefrontal areas that are either involved in the DMN or implicated in psychopathological dysfunction may be critical to schizophrenia-like psychosis, especially in individuals with temporal lobe epilepsy. This suggests that DMN deficits might be a core neurobiological feature of the disorder, and that abnormalities in theta oscillations and beta phase synchronization represent the underlying neural activity.

Frequency-dependent spatial distribution of human somatosensory evoked neuromagnetic fields

Using synthetic aperture magnetometry (SAM), we examined the spatial distribution of frequency changes in magnetoencephalography signal rhythms on individual magnetic resonance images following somatosensory stimulation. SAM is a novel statistical spatial filtering method that uses an adaptive beamformer. Electrical stimulation of the right median nerve demonstrated high-frequency event-related synchronization (ERS) in the 50-200-Hz range, consistently localized in the contralateral primary sensorimotor area in all subjects (n=7). Event-related desynchronization (ERD) was demonstrated in the 8-13, 13-25 and 25-50-Hz ranges bilaterally in the area surrounding the central sulcus. The differences in the spatial distribution as well as the frequency bands between ERS and ERD suggest that ERS and ERD reflect the responses of different cell assemblies rather than a frequency shift of the same cell assembly.

Theta rhythm increases in left superior temporal cortex during auditory hallucinations in schizophrenia : a case report

Auditory hallucinations (AH), the perception of sounds and voices in the absence of external stimuli, remain a serious problem for a large subgroup of patients with schizophrenia. Functional imaging of brain activity associated with AH is difficult, since the target event is involuntary and its timing cannot be predicted. Prior efforts to image the patterns of cortical activity during AH have yielded conflicting results. In this study, MEG was used to directly image the brain electrophysiological events associated with AH in schizophrenia. We observed an increase in theta rhythm, as sporadic bursts, in the left superior temporal area during the AH states, whereas there was steady theta band activity in the resting state. The present finding suggests strong association of the left superior temporal cortex with the experience of AH in this patient. This is consistent with the hypothesis that AH arises from areas of auditory cortex subserving receptive language processing.

An integrative MEG-fMRI study of the primary somatosensory cortex using cross-modal correspondence analysis.

We develop a novel approach of cross-modal correspondence analysis (CMCA) to address whether brain activities observed in magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) represent a common neuronal subpopulation, and if so, which frequency band obtained by MEG best fits the common brain areas. Fourteen adults were investigated by whole-head MEG using a single equivalent current dipole (ECD) and synthetic aperture magnetometry (SAM) approaches and by fMRI at 1.5 T using linear time-invariant modeling to generate statistical maps. The same somatosensory stimulus sequences consisting of tactile impulses to the right sided: digit 1, digit 4 and lower lip were used in both neuroimaging modalities. To evaluate the reproducibility of MEG and fMRI results, one subject was measured repeatedly. Despite different MEG dipole locations and locations of maximum activation in SAM and fMRI, CMCA revealed a common subpopulation of the primary somatosensory cortex, which displays a clear homuncular organization. MEG activity in the frequency range between 30 and 60 Hz, followed by the ranges of 20-30 and 60-100 Hz, explained best the defined subrepresentation given by both MEG and fMRI. These findings have important implications for improving and understanding of the biophysics underlying both neuroimaging techniques, and for determining the best strategy to combine MEG and fMRI data to study the spatiotemporal nature of brain activity.

Resting-State Network Disruption and APOE Genotype in Alzheimer's Disease: A lagged Functional Connectivity Study

Background The apolipoprotein E epsilon 4 (APOE-4) is associated with a genetic vulnerability to Alzheimers disease (AD) and with AD-related abnormalities in cortical rhythms. However, it is unclear whether APOE-4 is linked to a specific pattern of intrinsic functional disintegration of the brain after the development of the disease or during its different stages. This study aimed at identifying spatial patterns and effects of APOE genotype on resting-state oscillations and functional connectivity in patients with AD, using a physiological connectivity index called “lagged phase synchronization”. Methodology/Principal Findings Resting EEG was recorded during awake, eyes-closed state in 125 patients with AD and 60 elderly controls. Source current density and functional connectivity were determined using eLORETA. Patients with AD exhibited reduced parieto-occipital alpha oscillations compared with controls, and those carrying the APOE-4 allele had reduced alpha activity in the left inferior parietal and temporo-occipital cortex relative to noncarriers. There was a decreased alpha2 connectivity pattern in AD, involving the left temporal and bilateral parietal cortex. Several brain regions exhibited increased lagged phase synchronization in low frequencies, specifically in the theta band, across and within hemispheres, where temporal lobe connections were particularly compromised. Areas with abnormal theta connectivity correlated with cognitive scores. In patients with early AD, we found an APOE-4-related decrease in interhemispheric alpha connectivity in frontal and parieto-temporal regions. Conclusions/Significance In addition to regional cortical dysfunction, as indicated by abnormal alpha oscillations, there are patterns of functional network disruption affecting theta and alpha bands in AD that associate with the level of cognitive disturbance or with the APOE genotype. These functional patterns of nonlinear connectivity may potentially represent neurophysiological or phenotypic markers of AD, and aid in early detection of the disorder.