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Biomedical subjects

N Nakasato

Publications and source records attributed to N Nakasato.

At least 55 records · Page 3Linked to original sources
PubMed1996

Neuromagnetic evidence of pre- and post-central cortical sources of somatosensory evoked responses.

Somatosensory evoked magnetic fields (SEFs), corresponding with electrical median nerve stimulation, were measured using an MRI-linked, whole-head MEG system. In total 184 hemispheres, SEFs were measured, involving 22 normal volunteers and 70 patients with intracranial lesions. The first SEF peak appeared 20.2 +/- 1.5 msec (mean +/- S.D.) after stimulation. The first peak was followed by a second and third peak, 7.7 +/- 3.2 msec and 14.6 +/- 5.4 msec respectively after the first. Corresponding isofield maps suggested a single current dipole oriented in the anterior direction for the first peak, posterior for the second and either anterior or posterior for the third. Using a spherical model and a single current dipole source, the localization of each peak was estimated and superimposed on individually determined MRI images. On a grand average basis, the first-peak dipole was localized on the posterior surface of the central sulcus, "area 3b." The second-peak dipole was localized 3.7 mm medial (P < 0.001) and 2.0 mm superior (P < 0.001) to the first-peak dipole, on the anterior wall of the central sulcus, "area 4." This study with a large number of subjects was able to statistically distinguish two adjacent cortical sources, located on opposite walls of the central sulcus, and within a few millimeters of each other.

Adult↗
PubMed1996

[Right hemispheric dominancy in the auditory evoked magnetic fields for pure-tone stimuli].

It is well known that the auditory evoked N100m response is dominant in the contralateral hemisphere to the stimulated ear: N100m latency is shorter and N100m amplitude is larger in the contralateral hemisphere than the ipsilateral hemisphere. Interhemispheric asymmetry of the N100m responses were reported in terms of the equivalent dipole localization: the right hemispheric N100m dipole is more anterior to the left hemispheric dipole. In the present study, we investigated interhemispheric asymmetry of the auditory evoked P50m and N100m response in terms of signal amplitude. A helmet-shaped whole head magnetoencephalography system was employed to see small but significant differences between two hemispheres. Thirty-seven right-handed healthy subjects, 31 males and 6 females, participated in the study. The helmet-shaped whole head MEG system (CTF Systems-Osaka Gas) consists of an array of 66 MEG sensor sites uniformly distributed over the entire head. The monaural auditory stimuli, 2000 Hz tone burst of 50 msec duration were delivered to the left and then the right ear sequentially, with an intensity of 80 dB SPL at the ear. White noise of 50 dB SPL was delivered to the opposite ear. The inter-stimulus intervals were randomized in the range from 2.5 to 4.7 sec. In a total of 37 subjects, N100m dipole pattern appeared on both hemispheres either left or right ear stimuli. When P50m peak amplitude was compared between contralateral responses, the right-hemispheric response (141.5 +/- 16.3 fT, mean +/- s.e.m.) was slightly larger than the left-hemispheric response (134.3 +/- 10.2 fT). Differences, however, were not significant. When P50m peak amplitude was compared between ipsilateral responses, the right-hemispheric response (135.1 +/- 11.0 fT) was slightly larger than the left-hemispheric response (115.3 +/- 10.9 fT). Differences were not significant, either. When N100m peak amplitude was compared between contralateral responses, the right-hemispheric response (484.4 +/- 28.4 fT) was statistically (p < 0.01) larger than the left-hemispheric response (375.6 +/- 28.0 fT). When N100m peak amplitude was compared between ipsilateral responses, the right-hemispheric response (405.8 +/- 27.9 fT) was statistically (p < 0.02) larger than the left-hemispheric response (315.1 +/- 24.8 fT). Previously, we reported that the right hemispheric latency is shorter than the left hemispheric latency in the auditory evoked field N100m responses. Present study indicated another evidence of the right hemispheric dominancy in the auditory evoked responses, namely the lager amplitude in the right hemispheric response at least for pure-tone stimuli. The whole-head MEG system is suitable to analyze differences of auditory function between two hemispheres.

Acoustic Stimulation↗
PubMed1996

Neuromagnetic evidence that the P100 component of the pattern reversal visual evoked response originates in the bottom of the calcarine fissure.

Visual evoked magnetic fields due to pattern reversal stimuli were measured in 5 normal subjects using a helmet-shaped 66 channel magnetoencephalography system linked to MRI. The magnetic topography of the prominent 100 ms response (P100m) evoked by fullfield visual showed a double-dipole pattern in the occipital areas of all subjects. Right or left half-field stimuli and upper or lower quadrant-field stimuli evoked a single-dipole pattern in the contralateral occipital area. The P100m sources were then localized using a current dipole model and superimposed on MRI images of each subject. The visual cortex was morphologically variable among the subjects, but the P100m dipoles were all localized at the lateral bottom of the calcarine fissure. Moreover, these P100m dipoles had similar orientations for both half-or quadrant-field stimuli. These results suggest that the P100m is located in a smaller part of the striate cortex than previously reported.

Adult↗
PubMed1996

[Auditory evoked magnetic fields in cases with temporal lobe glioma].

Auditory evoked magnetic field (AEF) is known to be suitable to separate left and right hemispheric activities while auditory evoked potential is not. To evaluate cortical auditory function in ten patients with temporal lobe gliomas, we measured AEF for monaural tone stimuli using a helmet-shaped 66-channel MEG system. Latency of the N 100 m, the most prominent peak with a latency around 90 ms, was measured in the hemisphere contralateral to the stimulus onset. In five patients, the N 100 m latency was within our normal range (mean +/- 2 s.d.). In these five cases, tumor was located in the anterior or the inferior part of the temporal lobe. We observed significant delay of the N 100 m latency in four patients and disappearance of the N 100 m in another patient. In the later five patients, tumor extended to the superior and posterior part of the temporal lobe. AEF can be used to evaluate cortical auditory function noninvasively in cases with temporal lobe gliomas.

Adult↗
PubMed1996

Clinical application of visual evoked fields using an MRI-linked whole head MEG system.

We have demonstrated anatomo-functional correlation of the brain visual function using an MRI-linked whole-head magnetoencephalography system. Visual evoked magnetic fields (VEFs) due to pattern reversal stimuli were measured in seven healthy subjects and 13 patients with intracranial structural lesions. Full-field stimuli evoked the most prominent peak around 100 ms latency (P100m) as a two-dipole pattern over the occipital area in all of the normal subjects and four patients with mild and partial hemianopsia. In five patients with homonymous hemianopsia due to unilateral occipital lesion, a single-dipole pattern of the P100m appeared over the normal occipital area only. In four patients with bitemporal hemianopsia due to pituitary tumors, a single-dipole pattern of the P100m appeared only over the ipsilateral occipital area to the stimulated eye. Using current dipole models, P100m sources were localized at the lateral bottom of the calcarine fissures. Although human occipital lobes are known to be morphologically variable, our results indicate excellent correlation of the cortical anatomy and human visual function. Full-field stimuli, requiring no strict visual fixation, are especially useful for clinical application. A clear two-dipole pattern in the normal VEFs enables us to compare two hemispheric activities.

Adult↗
PubMed1995

MEG and ECoG localization accuracy test.

We tested the localization accuracy of magnetoencephalography (MEG) and electrocorticography (ECoG) for a current dipole in a saline filled sphere at depths ranging from 1 to 6 cm at 1 cm intervals. We used standard neuromagnetometer placements and subdural electrode grids, previously employed for patient studies, with precise measurements of sensor and electrode locations with a 3-dimensional spatial digitizer. MEG and ECoG had comparable accuracy with mean errors of 1.5 and 1.8 mm, respectively. It appears that use of the spatial digitizer increases accuracy for both MEG and EGoG localizations. The larger errors in the ECoG with increasing depths could be attributed to under-sampling of the spatial pattern of the field which spreads out with deeper sources. It should be noted that in clinical applications a grid of the dimensions used here would most typically be used for superficial sources on the cortex with depth recordings being preferred for investigations of deep epileptogenic activity. Results are encouraging for continued development of non-invasive MEG methods for further definition of epileptogenic zones in the brain.

Cerebral Cortex↗
PubMed1995

Functional localization of bilateral auditory cortices using an MRI-linked whole head magnetoencephalography (MEG) system.

In 20 healthy subjects, auditory evoked magnetic fields were measured over the entire head, using a helmet-shaped 66-channel MEG system linked to MRI. When the left or right ear was stimulated by 60 msec 2 kHz tones, the prominent 100 msec response (N100m) appeared significantly earlier in the contralateral hemisphere than in the ipsilateral one. In 16 cases, the N100m dipolar field patterns were clear in both hemispheres, overlapping each other across the midline. The N100m sources were estimated using a 2-dipole model in a spherical conducting medium with the size and location of the sphere determined individually according to the MRI images. No differences were found between the contralateral and ipsilateral N100m dipole positions in one hemisphere. When superimposed on MRI, the N100m dipoles were located precisely on the upper surface of bilateral temporal lobes with a standard deviation of 2.2 mm in the superior-inferior direction. In 16 right handed males, the right hemispheric N100m dipoles were 6 mm anterior to the left hemispheric dipoles. The whole head MEG is suitable to see small but significant differences of bilateral cerebral function, with exceptionally high spatial resolution, confirmed by the MRI-linked system.

Adult↗
PubMed1995

[Interhemispheric differences of the middle latency auditory evoked magnetic fields revealed by a whole head magnetoencephalography system].

It is known that the auditory evoked N100m response appears faster in the contralateral hemisphere than in the ipsilateral hemisphere to the stimulated ear. For the middle latency response named P50m, however, there have been only few studies with controversial findings. In the present study, auditory evoked responses to the unilateral tone stimuli were measured in 36 right-handed normal subjects, using a helmet-shaped whole head magnetoencephalography system. In a total of 72 hemispheres of 36 subjects, P50m dipolar patterns were observed statistically (p > 0.02) frequently in the contralateral hemispheres (63.9%) compared to the ipsilateral hemispheres (43.0%). N100m dipolar patterns were observed over both left and right hemispheres either by left or right ear stimuli. Peak latency of the P50m was statistically (p < 0.02) shorter in the contralateral hemisphere (45.0 +/- 8.5ms, mean +/- s.d.) compared to the ipsilateral hemisphere (54.4 +/- 11.5ms). Peak latency of the N100m was also statistically (p < 0.001) shorter in the contralateral hemisphere. Moreover, when compared between contralateral responses, the left-ear stimulated right-hemispheric response (84.6 +/- 8.4) was statistically (p < 0.05) faster than the right-ear stimulated left-hemispheric response (89.3 +/- 10.2ms). These results indicate that contralateral auditory cortex dominates the ipsilateral cortex at the middle as well as the long latency components. Moreover, at least for pure-tone stimuli, right hemispheric dominance was indicated for the N100m responses. The whole-head MEG system is suitable to analyze small but significant interhemispheric difference of auditory function.

Adult↗
PubMed1994

Stereotactic endoscopic resection of angiographically occult vascular malformations.

Total resection of angiographically occult vascular malformations was performed in four patients using stereotactic open-system endoscopy. The lesions were relatively small but localized deep in the brain or involving eloquent cortex. Fiber-guided YAG laser coagulation was efficient for haemostasis and no significant complication was seen in any case. Endoscopic stereotactic laser surgery by open-system endoscopy is considered a safe and promising treatment for small AOVM's located in deep or eloquent neural structures.

Adult↗
PubMed1994

Comparisons of MEG, EEG, and ECoG source localization in neocortical partial epilepsy in humans.

In order to delineate the characteristics of epileptic spikes, 1946 different spikes were studied in 6 patients with complex partial epilepsy. Non-invasive MEG and EEG source analysis of interictal spikes were contrasted to ECoG localization, surgical outcome and presence of lesions on MRI. Results indicated that: (1) using the most frequent occurring spike topography patterns from a large sample of spikes improved goodness-of-fit values for both MEG and EEG localization, (2) when spike patterns could be appropriately matched on several successive MEG measurements to provide an adequate matrix (3 of 6 subjects), there was excellent agreement between MEG dipole sources and ECoG sources as well as surgical outcome and presence of MRI lesions, (3) EEG source analyses also gave good results but not as consistently as MEG.

Adult↗
PubMed1994

[Whole head measurement of auditory evoked magnetic field using a MR-linked helmet shaped MEG system].

Authors compared MR anatomy and magneto-encephalographic (MEG) functional methods in locating auditory cortex in 20 normal volunteers and 56 patients with intracranial structural lesions. Auditory evoked response was measured over the entire head, using a helmet shaped 64 channel MEG system. In each subject, three dimensional MR image was obtained with identical coordinate system of MEG. Best fit sphere to each subject's head was calculated from MRI data. Signal source of auditory evoked MEG was estimated using a two dipole model in the best fit sphere. Position and orientation of dipoles were superimposed on each subject's MRI. Both left and right dipolar patterns appeared about 100 ms after unilateral auditory stimuli (N100m) in 19 normal subjects and 47 patients. These dipolar patterns overlap each other across the midline. A two dipole model successfully localized bilateral N100m dipoles in the upper surface of superior temporal planes. N100m response was missing unilaterally in 1 normal subjects and 9 patients inspite of normal audiometry findings, suggesting sub-clinical auditory dysfunction. The MR-linked whole head MEG is a non-invasive tool to localize source of auditory evoked N100m response even in the case with intracranial structural lesions.

Acoustic Stimulation↗