Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section最新文献

Joint ECG and EEG measurements were performed in 22 healthy subjects under standardized laboratory conditions. Averaged EEG potentials were computed using the R-peaks in the ECG as reference events. Spatio-temporal potential patterns of heart action-related EEG activity were obtained from 26 scalp channels. A heart action-related positive potential was found, peaking over the parietal scalp regions. Its independence from the cardiac electrical field, the source of an EEG artifact that may be confounded with heart action-related brain potentials, is demonstrated. The potential reaches its maximum amplitude of about 0.5 μV at a latency of about 500 ms after the R-peak. Its topography, with peak amplitudes at the parietal electrode locations, is different from the topography of potentials observed in the few comparable experimental studies published so far. This suggests the presence of somatosensory-evoked components in heart action-related potentials and indicates that a renewed discussion of the underlying neuronal processes is necessary.

Flash visual evoked potentials (VEP) were simultaneously recorded from the primary visual cortex and the dorsal lateral geniculate nucleus in freely-moving WAG/Rij rats, to investigate whether the thalamic VEP shows the same state-dependent alterations as the cortical VEP. VEPs obtained during active and passive wakefulness (AW and PW), slow-wave sleep (SWS), REM sleep and during the occurrence of spike-wave discharges (SWD), a specific trait of the genetically epileptic WAG/Rij rat, were compared. The general architecture of the thalamic VEP resembles the cortical VEP, although its polarity is reversed. This facilitated the interpretation of components in terms of underlying neuronal events. The primary excitation peak is differently modulated in cortex and thalamus. Whereas the thalamic component (P30) is not affected by brain-state, the cortical component (N1) shows a strong increase in latency during SWS and SWD. In contrast, the modulation of later components is highly similar for cortex and thalamus. VEPs obtained during AW and REM resemble each other. During SWS and SWD there is a considerable, and during PW a moderate, enlargement of primarily inhibitory components. After-discharges are enhanced during SWS, SWD and REM. No evidence is found for a major impairment of sensory transmission during SWD.

Magnetoencephalographic source localisation techniques were used to measure oesophageal evoked magnetic fields from the cerebral cortex in 3 subjects. By using rapid balloon distension as a stimulus, a comparison of proximal and distal oesophageal cortical representation was made. The distal oesophagus was represented bilaterally in the insular cortex and SII as well as the inferior aspect of SI. The proximal oesophagus was represented unilaterally in superior and inferior SI, insular cortex and SII. Significantly, the superior portion of SI was consistently activated in subjects following stimulation of the proximal oesophagus, but similar activation was not found in response to distal stimulation. This may reflect the contribution from somatic afferent fibres in the striate muscle of the proximal segment. In conclusion, vagal afferents appear to contribute more to cortical activation following stimulation of the distal rather than the proximal oesophagus, while spinal afferents appear to be activated by both proximal and distal oesophageal stimulation.

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.

Cortical evoked responses to median nerve stimulation were recorded from 21 subjects during sevoflurane anaesthesia at the level of burst suppression in EEG. The N20/P22 wave had the typical form of a negative wave postcentrally, and positive precentrally. The amplitude exceeded 4 μV in all patients, making it easily visible without averaging on the low-amplitude suppression. These results show that two kinds of somatosensory evoked potential can be studied without averaging during EEG suppression in deep anaesthesia. One is the localised N20/P22 wave, which is seen regularly during suppression after stimuli with intervals exceeding 1 s. The other is the burst, involving the whole cortex, which is not evoked by every stimulus. We suggest that somatosensory evoked potentials can be monitored during sevoflurane-induced EEG suppression, and often can be evaluated reliably from a couple of single sweeps with stimulation interval exceeding 1 s. The enhancement of early cortical components of SEP, their adaptation to repeated stimuli, and the disappearance of later polysynaptic components during EEG suppression, give new possibilities to study the generators of SEP and the different effects of anaesthetics.

The aim of the present study was to explore the utility of dipole tracing (DT) of a scalp-skull-brain (SSB) head model in preoperative functional localization of the human brain. Nine patients who underwent surgery of mass lesions around the central sulcus (CS) were employed. By using SSB/DT, dipole source location of early cortical components of the somatosensory evoked potential (SEP) was estimated before surgery. Motor cortex, CS and primary somatosensory cortex were determined by cortical SEP during surgery. After surgery precise functional mapping was reproduced in MRI, and the accuracy of DT was evaluated by measuring the distance between estimated dipole source and the posterior bank of the CS. We defined this distance as localization error of DT. In 4 cases without structural change around the sensorimotor cortex, localization error ranged from 1 to 4 mm with an average of 2 mm. In 5 cases with structural alteration of sensorimotor cortex, localization error ranged from 6 to 10 mm with an average of 8 mm. The difference in localization error between the two groups was statistically significant, and may have been caused by changes of conductance near sensorimotor cortex in the latter group. Functional localization by DT was accurate and useful. But localization error could not be ignored in cases with structural alteration in the sensorimotor cortex.

The P3 event-related brain potential (ERP) is a positive-going voltage change of scalp-recorded electroencephalographic activity that occurs between 300–500 ms after stimulus onset. It is elicited when a stimulus is perceived, memory operations are engaged, and attentional resources are allocated toward its processing. Because this ERP component reflects fundamental cognitive processing, it has found wide utility as an assessment of human mental function in basic and clinical studies. In particular, P3 attributes are heritable and have demonstrated considerable promise as a means to identify individuals at genetic risk for alcoholism. We have conducted a quantitative linkage analysis on a large sample from families with a high density of affected individuals. The analyses suggest that several regions of the human genome contain genetic loci related to the generation of the P3 component of the ERP, which are possible candidate loci underlying the functional organization of human neuroelectric activity.

We investigated the replicability of the source location, amplitude and latency measures of the auditory evoked N1 (EEG) and N1m (MEG) responses. Each of the 5 subjects was measured 6 times in two recording sessions. Responses to monaural stimuli were recorded from 122 MEG and 64 EEG channels simultaneously. The EEG data were modeled with a symmetrically-located dipole pair. For the MEG data, one dipole in each hemisphere was located independently using a subset of channels. Standard deviation (SD) was used as a measure for replicability. The average SD of the x, y and z coordinates of the contralateral N1m dipole was about 2 mm, whereas the corresponding figures for the ipsilateral N1m and the contra- and ipsilateral N1 were about twice as large. The SDs of the dipole amplitudes and latencies were almost equal with MEG and EEG. The amplitude and latency measures of the MEG field gradient waveforms were almost as replicable as those of the dipole models. The results suggest that both MEG and EEG can be used for investigating the simultaneous activity of the left and right auditory cortices independently, MEG being superior in certain experimental setups.

Parietal positivities of the `slow wave' type are known to emerge after the P300 whenever target detection leads to a complex subsidiary task. Although the functional correlates of these `positive slow waves' (PSW) are not known, it has been suggested that they may index (a) the selection or decision processes, (b) the preparation of the response or (c) the evaluation of its correctness. We investigated whether PSW could be dissociated from each of these putative steps of information processing by means of a paradigm devoid of motor components and needing very long reaction times. In our protocol, target stimuli acted as the triggering signal to perform silently one of 4 different tasks, namely (a) simple updating of a target count; (b) counting backward in threes; (c) simultaneous updating of two items (day of the week and ordinal of the month) and (d) updating of 3 items (the two above plus the month of the year). Reaction times to the same stimuli were obtained in 5 subjects during separate sessions. The different tasks did not modify the latencies of N2 or P3b components, but attenuated the amplitude of P3 as a mirror image of the subjective difficulty scores. A conspicuous parietal PSW appeared in conditions where two or 3 items had to be updated. This PSW developed 1–2 s earlier than the reaction times to the same experiments and could be therefore dissociated from the selection and decision processes. PSW latency was correlated with the number of items to be updated, but not with subjective difficulty. In the present paradigm PSW appeared to index the retrieval of information from working memory; however, in more general terms our results suggest that PSW is a non-specific activity that signals the completion of any synchronized operation immediately following target detection. Our data suggest a functional link between P3 and PSW, also supported by the similarity of their respective scalp topographies. The present paradigm proved to be easy to implement and suitable to study the `executive' functions governing attentional and working-memory control during the performance of multiple tasks.

The present study uses the N400 component of event-related potentials (ERPs) as a processing marker of single spoken words presented during sleep. Thirteen healthy volunteers participated in the study. The auditory ERPs were registered in response to a semantic priming paradigm made up of pairs of words (50% related, 50% unrelated) presented in the waking state and during sleep stages II, III–IV and REM. The amplitude, latency and scalp distribution parameters of the negativity observed during stage II and the REM stage were contrasted with the results obtained in the waking state. The `N400-like' effect elicited in these stages of sleep showed a mean amplitude for pairs of unrelated words significantly greater than for related pairs and an increment of latency. These results suggest that during these sleep stages a semantic priming effect is maintained actively although the lexical processing time increases.