Satoru Ohtomo

Hemispheric asymmetry of the auditory evoked N100m response in relation to the crossing point between the central sulcus and Sylvian fissure

The positions of the bilateral N100m sources of the auditory evoked magnetic fields (AEFs) were measured in relation to the central sulcus (CS) using an MRI-linked whole head magnetoencephalography system in 20 right-handed normal male subjects. The location of the N20m source of the median nerve-stimulated somatosensory evoked magnetic fields (SEFs), in the left hemisphere was 3.9+/-5.4 mm (mean+/-SD) posterior to that in the right hemisphere (P < 0.005). The crossing point (CP) between the CS and Sylvian fissure in the left hemisphere was 4.3+/-4.8 mm posterior to that in the right hemisphere (P < 0.001). The N100m sources were posterior to the CP in both hemispheres. The left hemispheric N100m source was 9.4+/-6.4 mm posterior to that on the right (P < 0.0001) in absolute position. The relative distance between CP and the N100m source was 22.7+/-8.5 mm in the left hemisphere and 17.7+/-5.3 mm in the right hemisphere (P < 0.01). Comparison of positions of the AEF sources and the CS as defined by the SEF demonstrated functional asymmetry of the human temporal lobe and possible source extension of the AEF-N100m beyond the Heschl gyrus over the planum temporale.

Neuromagnetic measurement of unilateral temporo-parietal theta rhythm in patients with internal carotid artery occlusive disease

Rhythmic theta activity detected by electroencephalography (EEG) may be correlated with cerebrovascular brain diseases. Magnetoencephalography (MEG) has higher sensitivity and spatial resolution than conventional scalp EEG, so may be a better method to detect theta rhythm in patients with internal carotid artery (ICA) occlusive disease. Simultaneous EEG and MEG were performed in the awake state in 48 patients with unilateral (n = 42) or bilateral (n = 6) stenotic lesions (more than 60% occlusion) of the ICA (n = 47) or middle cerebral artery (n = 7), and in 27 age-matched healthy normal subjects. No subject had severe neurological deficits. MEG detected the theta rhythm (6-8 Hz) in 14 of 48 patients: ipsilateral to the stenotic or occluded side in 13 hemispheres and bilaterally in one patient with unilateral lesion. The source of the MEG theta rhythm was estimated in the dorsolateral temporo-parietal area, regardless of the location of infarct foci or the stenotic portion of the ICA system. The temporo-parietal theta rhythm was separated from the occipital alpha rhythm by frequency and distribution in MEG. The theta rhythm was found in only two patients by EEG, as well as by MEG. MEG provided better separation of this theta rhythm from the occipital alpha rhythm. Neither MEG nor EEG detected this theta rhythm in the normal subjects. Unilateral temporo-parietal theta rhythm is correlated with the hemisphere with ICA occlusive disease. This rhythm may indicate mild or subclinical abnormalities in the ICA system. MEG is superior to EEG for the detection and localization of theta rhythm.

Temporo-parietal theta activity correlates with misery perfusion in arterial occlusive disease

OBJECTIVE Temporo-parietal theta activity (TPTA), often detected in hemispheres with internal carotid (ICA) or middle cerebral artery (MCA) occlusive lesions, is more clearly separated from occipital alpha activity by magnetoencephalography (MEG) than electroencephalography. The present study investigated whether TPTA is correlated with misery perfusion, a surgically correctable type of hemodynamic impairment. METHODS Awake MEG was measured in 56 patients with ICA or MCA occlusive lesions. Regional cerebral blood flow (rCBF) and regional cerebrovascular reactivity (rCVR) to acetazolamide were measured in the MCA territory by xenon-133 single-photon emission computed tomography. MEG was repeated in 10 patients after vascular reconstruction surgery. RESULTS Fourteen patients showed TPTA in the lesion hemisphere (n=13) or bilaterally (n=1). The presence of TPTA was significantly correlated with both reduced rCBF and reduced rCVR (P=0.0009). After surgery, TPTA disappeared in 7 of the 10 studied patients. CONCLUSIONS The presence of TPTA suggests misery perfusion, which is characterized by reduced rCBF and reduced rCVR. SIGNIFICANCE MEG can detect the presence of reversible and surgically remediable cerebral ischemia before the development of complete infarction.

Satellite Sign: A Poor Outcome Predictor in Intracerebral Hemorrhage

Background: The presence of high-density starry dots around the intracerebral hemorrhage (ICH), which we termed as a satellite sign, is occasionally observed in CT. The relationship between ICH with a satellite sign and its functional outcome has not been identified. This study aimed to determine whether the presence of a satellite sign could be an independent prognostic factor for patients with ICH. Methods: Patients with acute spontaneous ICH were retrospectively identified and their initial CT scans were reviewed. A satellite sign was defined as scattered high-density lesions completely separate from the main hemorrhage in at least the single axial slice. Functional outcome was evaluated using the modified Rankin Scale (mRS) at discharge. Poor functional outcome was defined as mRS scores of 3-6. Univariate and multivariate logistic regression analyses were applied to assess the presence of a satellite sign and its association with poor functional outcome. Results: A total of 241 patients with ICH were enrolled in the study. Of these, 98 (40.7%) had a satellite sign. Patients with a satellite sign had a significantly higher rate of poor functional outcome (95.9%) than those without a satellite sign (55.9%, p < 0.0001). Multivariate logistic regression analysis revealed that higher age (OR 1.06; 95% CI 1.03-1.10; p = 0.00016), large hemorrhage size (OR 1.06; 95% CI 1.03-1.11; p = 0.00015), and ICH with a satellite sign (OR 13.5; 95% CI 4.42-53.4; p < 0.0001) were significantly related to poor outcome. A satellite sign was significantly related with higher systolic blood pressure (p = 0.0014), higher diastolic blood pressure (p = 0.0117), shorter activated partial thromboplastin time (p = 0.0427), higher rate of intraventricular bleeding (p < 0.0001), and larger main hemorrhage (p < 0.0001). Conclusions: The presence of a satellite sign in the initial CT scan is associated with a significantly worse functional outcome in ICH patients.

Neuromagnetic Identification of the Somatosensory Cortex in Cases with Arteriovenous Malformation Adjacent to the Central Sulcus

It is critical to preserve eloquent cortices during surgery of cerebral arteriovenous malformations (AVMs). Although MRI scans may identify “anatomical” central suclus, it is controversial whether the “functional” central sulcus can be shifted due to AVMs. Cortical recording of somatosensory evoked potentials (SEPs) can be used to recognize the central sulcus during open surgery. However, the cortical SEPs are not available during surgery for large AVMs, intravascular surgery, or stereotaxic radiosurgery. In the present study, somatosensory evoked fields (SEFs) were measured to localize the “functional” central sulcus non-invasively in cases with AVM adjacent to the central sulcus.

Effect of Color on Visual Evoked Magnetic Fields with Pattern-Reversal Stimulation

Visual image is supposed to be processed by parallel pathways that analyze different properties of vision such as form, depth, motion, and color. Positron emission tomography (PET) studies of functional specializations of the human visual system have demonstrated that the fusiform gyrus is playing a role in color processing[1],[2]. Poor time resolution of PET, however, gave no information about the time course of the activity in the brain. Although visual evoked potentials (VEPs) studies with dipole source analysis [3],[4] supplemented PET results, no direct comparison with the anatomy of the visual cortex of subjects was made. This is because spatial resolution of source localization in VEPs is low[4]. We have been investigating visual evoked magnetic fields (VEFs), which in principle have much better spatial resolution than VEPs, and found that the most prominent response (P100) evoked with pattern-reversal stimulation was located in the primary visual cortex[5].

Visual Evoked Fields for Pattern Reversal Stimuli in Patients with Occipital Lobe Lesions

The “cruciform model” of the visual cortex suggests that the P100 generators of the visual evoked responses to pattern reversal (PR) stimuli are located throughout the entire striate cortex, including the interhemisphere surface and calcarine fissures. Our recent studies of visual evoked fields (VEFs) to PR stimuli [1–3] have localized the P100m sources near the lateral end of the calcarine fissures. We suggested that a smaller part of the striate cortex contributes to the P100m response than in the cruciform model. In the present study, we measured PR-VEFs in patients with homonymous hemianopsia due to unilateral occipital lobe lesions, to demonstrate the correlation between PI00m patterns and occipital lesions.

Visual Evoked Magnetic Fields: Bilateral Dipole Pattern for Full-Field Stimuli

Pattern reversal stimulus is most frequently used for clinical application of visual evoked potentials (VEPs). Half-field stimulus has been employed to separate the left and right hemispheric responses. In the partial-field stimulus, however, subjects have to fix their eyes on a given point during the entire procedure; if the visual fixation is not strict, bilateral occipital responses may interfere with each other. In a clinical applications, therefore, the partial-field stimulus may not be suitable for inexperienced patients.

Origin of Slow Wave Observed in Cerebrovascular Disease

Due to excellent spatio temporal resolution and noninvasive nature of the measurements, magnetoencephalography (MEG) becomes a powerful tool for clinical application. This is especially the case in evoked MEG applied for presurgical diagnosis of the patients who suffered brain diseases. However, in spontaneous activity such as epileptic discharges, applicability is not so simple. This is because, for spontaneous waves, several brain activities or noises were superimposed on the detected signal, and separation for target activity is generally difficult. Small array of sensing coils also restricted the spatial resolution and sometimes brought erroneous results simply because they could not cover the entire brain at once. The appearance of whole-head MEG directly linked to magnetic resonance imaging (MRI) reduces data acquisition time, patient fatigue and localization error. It also provides objective information about patients which is essential for correct diagnosis. Spontaneous MEG studies used to be concentrated to epileptic spike activity. However, slow wave activity, typically observed by electroencephalography (EEG), is quite common in stroke, tumor, epilepsy, cerebrovascular disease, or head injury. Earlier investigations of slow wave activity with MEG [1–3] were restricted to small (1 to 37) channel system. Here we present the results of whole head MEG and source analysis of a slow wave activity.

Normalized N100m Latency of the Auditory Evoked Fields After Surgical Removal of Temporal Lobe Gliomas

Auditory evoked potentials have been measured in patients with several temporal lobe diseases. The N100 responses disappear in patients with lesions on bilateral superior temporal gyrus[1]. However, separation of a unilateral abnormality in evoked potentials, is difficult due to the spearing effect by tissue layers with inhomogeneous electric conductivities. Activity on the normal hemisphere may interfere with abnormal activity on the diseased hemisphere. Magnetoencephalography (MEG) is known to be less influenced by the inhomogeneous head conductivity. We have found that the whole head MEG is especially suitable to identify differences in bilateral cerebral function [2–4].