Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section最新文献

Event-related potentials (ERPs) were recorded during a continuous recognition memory paradigm in patients with left-sided (LTLE; n=8) or right-sided temporal lobe epilepsy (RTLE; n=6), and in healthy control subjects (n=24). Control subjects and both patient groups exhibited consistent OLD/NEW ERP-differences from 200–600 ms after stimulus onset. ERPs did not differ significantly between LTLE and RTLE patients, with respect to OLD/NEW distinction or the type of presented material (verbal vs. non-verbal). However, ERP topography showed significant differences between LTLE and RTLE patients: in lateral fronto-temporal recordings, patients showed larger negativities contralateral to the seizure focus, whereas we found larger negativities ipsilateral to the seizure focus in parietal recordings. Differences between the groups were significant from 300 to 600 ms post-stimulus. As a consequence, the amplitude gradient from fronto-temporal to parietal recordings was higher on the right side in LTLE patients and on the left side in RTLE patients. Again, differences between LTLE and RTLE patients were highly significant. We assume that ERPs reflect disturbances of a cortico-cortical network dependent on the side of the seizure focus in temporal lobe epilepsy. Furthermore, scalp-recorded ERPs might be a useful tool in the prediction of the side of the seizure focus in patients with temporal lobe epilepsy.

Tibial nerve somatosensory evoked potentials (SEPs) show higher amplitudes ipsilateral to the side of stimulation, whereas subdural recordings revealed a source in the foot area of the contralateral hemisphere. We now investigated this paradoxical lateralization by performing a brain electrical source analysis in the P40 time window (34–46 ms). The tibial nerve was stimulated behind the ankle (8 subjects). On each side, 2048 stimuli were applied twice. SEPs were recorded using 32 magnetic resonance imaging (MRI)-verified electrode positions (bandpass 0.5–500 Hz). In each case, the P40 amplitude was higher ipsilaterally (0.45±0.14 μV) than contralaterally (−0.49±0.16 μV). The best fitting regional source, however, was always located in the contralateral hemisphere with a mean distance of 8.2±4.3 mm from the midline. The positivity pointed ipsilaterally shifting from a frontal orientation (P37) to a parietal direction (P40). The P40 dipole moment was 2.5 times stronger than the dipole moment of P37, which makes P40 most prominent in EEG recordings. However, with its oblique dipole orientation compared to the tangential P37 dipole, it is systematically underestimated in MEG. Dipole orientations explained interindividual variability of scalp potential distribution. SEP amplitudes were smaller when generated in the dominant (left) hemisphere. This is explained by deeper located sources (5.4±1.6 mm) with a more tangential orientation (Δϑ=17.5±2.3°) in the left hemisphere.

Results: In a magnetoencephalographic investigation of the auditory evoked field (AEF) in 17 schizophrenics and 17 controls, 37% of the schizophrenics and 12% of the controls showed eye artifacts in every second trial or even more frequently. In the uncorrected average fields, the ratio between the power of artifacts and the power of the magnetoencephalogram (MEG) exceeded the value of 0.1 for 48% of the schizophrenics and for 29% of the controls. Ocular artifacts biased the locations of equivalent current dipoles of the M100 component towards deeper positions. A regression algorithm for the correction of ocular artifacts in raw data and an identification technique of ocular artifacts based on the topography of transmission coefficients is described.

Conclusions: A linear dependence of ocular artifacts in AEF on the electrooculogram (EOG) was confirmed. Possible errors introduced by the correction are discussed. Transmission coefficients should be calculated for several individual trials with the same type of artifact. Errors due to evoked potentials in the EOG were found to be comparable in amplitude to noise in the AEF. Examples of transmission coefficients from the EOG to the MEG are given.

Objective: To evaluate the characteristics of high frequency (HF) components of the early cortical somatosensory evoked potentials (SEPs).

Methods: We recorded 8-channel SEPs from the frontal and left centro-parietal scalp after right median nerve stimulation with a wide band-pass (0.5–2000 Hz) and digitized at 40 kHz sampling rate in 12 healthy subjects. HF components were analyzed after digital band-pass filtering (300–1000 Hz). The power spectrum was obtained by a maximum entropy method.

Results: HF oscillations (maximum power at 600–800 Hz) consisting of 5 to 8 peaks were discriminated from the preceding P14 far-field in all cases and their phases were reversed between the frontal and contralateral parietal regions. In addition, in subjects with a high amplitude central P22 potential in original wide-band recordings, a single HF oscillation with a maximum at the central region was present. Furthermore, this component showed no phase reversal over the centro-parietal area.

Conclusion: We therefore conclude that HF oscillations are superimposed not only on the tangential N20-P20 but on the radial P22 potential, and are generated from both tangential (area 3b) and radial (area 1) current sources.