Akitake Kanno

Neuromagnetic localization of N15, the initial cortical response to lip stimulus.

The initial cortical response of the trigeminal somatosensory evoked magnetic fields was measured for electrical stimulation of the lower lip in eleven subjects. The stimulus frequency was 0.7 Hz and stimulus intensity was nine times sensory threshold. The initial contralateral response was detected in 20 hemispheres at a latency of 14.6 ± 1.3 ms and was named N15m. The equivalent current dipole of N15m was localized at the posterior bank of the central sulcus with anterior–superior orientation, and inferior to the dipole of N20m for median nerve stimulation.

Surgical Implications of Neuromagnetic Spike Localization in Temporal Lobe Epilepsy

Summary:  Purpose: To investigate the clinical usefulness of magnetoencephalography (MEG) as a guide to the surgical treatment of temporal lobe epilepsy (TLE).

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.

Ipsilateral Area 3b Responses to Median Nerve Somatosensory Stimulation

Magnetoencephalography investigation of the somatosensory evoked fields for median nerve stimulation detected ipsilateral area 3b responses in 18 hemispheres of 14 (1 normal subject and 13 patients with brain diseases) among 482 consecutive subjects. The major three peaks in the ipsilateral response were named iP50m, iN75m, and iP100m, based on the current orientation in the posterior, anterior, and posterior directions and the latency of 52.7 +/- 6.2, 74.1 +/- 9.4, and 100.2 +/- 15.8 ms (mean +/- standard deviation), respectively. The moment of the iP50m dipole (9.4 +/- 5.7 nAm) was significantly smaller than that of the N20m dipole of the contralateral response (cN20m, 27.5 +/- 10.5 nAm, P < 0.0001). Dipoles of iP50m and cN20m were similarly localized on the posterior bank of the central sulcus. iP50m in the present study had the same current orientation as and peak latency similar to that of the first ipsilateral primary somatosensory response to lip stimulation in our previous report. Therefore, the somatosensory afferent pathway from the hand may reach directly to the ipsilateral area 3b at least in part of the human population.

Middle and long latency peak sources in auditory evoked magnetic fields for tone bursts in humans

The relative position of the P50m and the N100m sources of the auditory evoked magnetic field remains unclear. Magnetoencephalography was performed in 24 normal subjects. Contralateral P50m to left and right ear stimulus was observed in 21 and 19hemispheres, respectively. Ipsilateral P50m to left and right ear stimulus was observed in 17 and 16hemispheres, respectively. N100m was observed in all subjects for all stimuli. Relative position of the equivalent current dipole of the P50m was 1.0+/-7.6 (mean+/-SD) mm posterior, 2.0+/-5.8mm inferior and 1.8+/-8.0mm medial to the N100m dipole position considering all observations. We suggest that the P50m and N100m sources are colocated in an extended area of the cortex.

Somatosensory-evoked Fields for Gingiva, Lip, and Tongue

To localize the oral primary somatosensory cortex, we measured somatosensory-evoked fields for the lip, gingiva, and tongue in six healthy subjects. The latency of the first peak of the posterior-oriented current in the contralateral hemisphere was 50.9 ± 8.3 ms for the gingiva, significantly shorter than those for the lip and tongue peaks. The equivalent current dipole was localized on the central sulcus. The gingival dipole was localized significantly inferior to the lip dipole but not different from the tongue dipole. The moment of the gingival dipole was significantly smaller than that of the lip dipole but not different from that of the tongue dipole. Differences in the above parameters were negligible between the left and right, anterior and posterior, and upper and lower locations within the same organ, except that the dipole location for the anterior upper tongue was significantly inferior to that for the lower tongue.

Gustatory evoked magnetic fields in humans

Magnetic fields evoked by taste stimuli of the human tongue were measured over the whole head using a helmet-shaped 64 channel magnetoencephalography system in five normal subjects. The stimuli were 10% glucose and 0.3 M NaCl solutions and distilled water. The most prominent peak (N175m) appearing over the bilateral hemispheres had a latency of 150-210 ms. The N175m sources were located using a two-dipole model in a spherical conducting medium based on the individual head dimensions and superimposed on magnetic resonance images. The N175m dipoles due to 10% glucose and 0.3 M NaCl stimuli were located at the operculum and circum-insular areas in both hemispheres, but those due to distilled water could not be located accurately.

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 localization of spike sources in perilesional, contralateral mirror, and ipsilateral remote areas in patients with cavernoma.

Purpose: To assess neuromagnetic spike localization as an indication for extended lesionectomy of cavernoma.

Electro- and magneto-encephalographic spike source localization of small focal cortical dysplasia in the dorsal peri-rolandic region

OBJECTIVE Small focal cortical dysplasia (FCD) may be ambiguous or overlooked on magnetic resonance (MR) imaging. Source localization of EEG and magnetoencephalography (MEG) spikes was evaluated to confirm the diagnosis of small FCD. METHODS This study included 6 epilepsy patients with a single small lesion on MR imaging suggesting FCD within a single gyrus among 181 consecutive epilepsy patients admitted to our epilepsy monitoring unit over 27 months. Stereotypical interictal spikes were detected on simultaneous EEG and MEG recordings and the onset-related source of averaged spikes was estimated. RESULTS All 6 patients had unique clinical characteristics as follows: leg sensori-motor seizures in 5 patients and eye version in 1 patient; a small MR imaging lesion suggesting FCD in the dorsal peri-rolandic region, which had been overlooked until our evaluation; and both EEG and MEG dipoles were estimated adjacent to the MR imaging lesion. CONCLUSIONS Source localization of EEG and MEG spikes can confirm the diagnosis of FCD based on a single small MR imaging lesion, which was overlooked by previous examination of MR images. SIGNIFICANCE Examination of MR images should be based on spike source localization as well as seizure semiology to identify subtle MR imaging abnormalities.