Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section最新文献

Frontal visual evoked potentials (VEPs) were studied in order to extend the application of a recently introduced algorithm for selective averaging of evoked potentials. This algorithm is based on the inverse relationship between amplitudes of alpha or theta components of the spontaneous EEG activity and evoked potential (EP) amplitudes. Stimuli were only applied if the root mean square (RMS) value of the ongoing EEG at the lead F₄ was below an individual threshold level (`selective stimulation'). For this comparison, the EEG was filtered in one of the frequency ranges `alpha', `theta' and `alpha and theta', respectively. `Alpha' and `alpha-and-theta-dependent' selective stimulation conditions resulted in significant amplitude increases (P<0.05) at the input reference channel F₄ and partly at ipsihemispherical temporal and parietal leads and at Cz. The largest increase of 35% at F₄ (P<0.01) was obtained with visual stimulation during low prestimulus theta activity. We conclude that spontaneous theta activity of the frontal cortex may be a factor influencing the amplitudes of frontal VEPs. A sophisticated analysis of frontal EPs, mainly in the framework of cognitive studies, should consider the theta activity prior to stimulation.

A sudden visual onset is thought to `attract attention to its location' within less than 100 ms. We attempted to measure the effect of this attentional process on the event-related potential (ERP) to a probe presented about 140 ms after the onset, and to delineate the spatio-temporal characteristics of such an effect, if any. ERPs were recorded from 30 channels from 6 subjects while they performed a target detection task. Both targets and probes could be located in each of the 4 quadrants (eccentricities 6.1° and 7°, respectively). For a given single target, the subsequent probe was either presented near the location of the target (`valid target') or at the diagonal opposite (`invalid target'). Appropriate `neutral' conditions (probes preceded by no target, or by simultaneous targets in all quadrants) were applied, and ERPs to probes were corrected for the contribution of the ERPs to targets. The earliest effect of (in)validity was found at about 120 ms after probe onset for lower field probes. This effect consisted of enhanced posterior positivity for valid relative to neutral relative to invalid conditions. This positivity was superposed on a contralateral, extrastriate negative ongoing wave peaking at about 150 ms (`N150'). Source localization suggested that the (in)validity effects originate from deep medial parietal areas. The source corresponding to the N150 activity was not influenced by (in)validity. An earlier deflection to the probe at 80 ms (`NP80') depended on location, but not on (in)validity, and seemed to be of striate origin. Results are discussed in terms of a model postulating an attention-independent `input module' from which activation is fed to a `location module' embodying the actual attention mechanism.

The study examined age-related changes in the perception of olfactory and trigeminal chemical stimuli using chemosensory event-related potentials (CSERP). Three groups of healthy volunteers, each comprised of 8 men and 8 women, were tested (age ranges 15–34, 35–54, and 55–74 years). Subjects underwent extensive psychological testing focusing on impairments of memory and attention. In addition, odor identification and discrimination ability was evaluated, as well as detection threshold sensitivity for two odorants. Odor discrimination scores exhibited a significant age-related decrease. Significant age-related changes were also observed for CSERP N1P2 and P2 amplitudes, and for the N1 peak latency. The age-related decrease of CSERP amplitudes appeared to follow a different time course for responses to trigeminal and olfactory stimulants.

The goal of this study was to test the influence of two widespread techniques of general anesthesia on motor evoked potentials (MEP) in response to transcranial and direct cortical high frequency repetitive electrical stimulation. Total intravenous anesthesia (TIVA) based on propofol and alfentanil was examined in 17 patients (group A), and balanced anesthesia (BA), based on nitrous oxide, isoflurane and fentanyl, was studied in 13 patients (group B). Distinct motor responses were available in 15 of 17 patients (88%) of group A, and in one of 13 patients (8%) of group B. Amplitudes increased significantly with increasing stimulus intensity and number of pulses under conditions of TIVA. At the same time, latencies decreased significantly with increasing stimulus intensity and decreasing interstimulus interval, but not with increasing number of pulses. It is hypothesized that propofol suppresses corticospinal I-waves at the cortical level, resulting in a conduction block at the level of the α-motoneuron, and that this effect may be overcome by high frequency repetitive stimulation. In contrast, nitrous oxide and isoflurane seem to have an additional suppressive effect on corticospinal D-waves, which may be overcome by higher stimulation intensity. In conclusion, transcranial high frequency repetitive stimulation and TIVA provide a feasible setting for intraoperative MEP monitoring, while higher doses of nitrous oxide and isoflurane are not compatible with recording of muscular activity elicited by the stimulation technique as described.

We recorded upper and lower limb MEPs and SEPs in 55 patients with clinically suggestive and myelography-documented cervical cord compression due to spondylotic changes. MEPs were abnormal in biceps brachii of 21 patients (38%), in first dorsal interosseous muscle of the hand of 49 patients (89%) and in tibialis anterior of 47 patients (85%). Overall, MEP abnormalities were present in at least one muscle of 51/55 patients (93%). Median SEPs were abnormal in 20 cases (36%), ulnar SEPs in 24 (44%) and posterior tibial SEPs in 40 (73%). Overall incidence of SEP alterations was 73% (40/55) and SEPs detected clinically silent sensory dysfunction in 10 patients (18%). Among the 43 patients who underwent surgical decompression, first dorsal interosseous (FDI) MEPs and tibial SEPs remained abnormal in most cases 1 year after surgery, independently of clinical outcome. On the other hand, serial EP studies seemed useful to confirm and monitor the clinical evolution of unoperated patients.

Single-pulse transcranial magnetic stimulation (TMS) is a safe and useful tool for investigating various aspects of human neurophysiology, particularly corticospinal function, in health and disease. Repetitive TMS (rTMS), however, is a more powerful and potentially dangerous modality, capable of regionally blocking or facilitating cortical processes. Although there is evidence that rTMS is useful for treating clinical depression, and possibly other brain disorders, it had caused 7 known seizures by 1996 and could have other undesirable effects. In June 1996 a workshop was organized to review the available data on the safety of rTMS and to develop guidelines for its safe use. This article summarizes the workshop's deliberations. In addition to issues of risk and safety, it also addresses the principles and applications of rTMS, nomenclature, and potential therapeutic effects of rTMS. The guidelines for the use of rTMS, which are summarized in an appendix, cover the ethical issues, recommended limits on stimulation parameters, monitoring of subjects (both physiologically and neuropsychologically), expertise and function of the rTMS team, medical and psychosocial management of induced seizures, and contraindications to rTMS.

The aim of our work was to evaluate if an optic nerve involvement (multiple sclerosis patients previously affected by optic neuritis) may induce any change in visual evoked potential (VEP) after photostress response. VEP in basal conditions and after photostress were assessed in 10 patients with defined multiple sclerosis without a history of optic neuritis (MSWO); in 14 patients with defined multiple sclerosis previously affected by optic neuritis but with complete recovery of the visual acuity (MSON) and in 14 age-matched controls. In order to complete the investigation of the retinal function, Transient Pattern electroretinogram (PERG) and steady-state focal-ERG (counterphased gratings presented at 8 Hz in the macular region) were performed in MSON patients only. In MSWO eyes VEP parameters in basal condition and after photostress did not undergo significant changes compared to controls (ANOVA; P>0.05). In MSON eyes we observed basal VEP with delayed P100 peak latency and reduced N75-P100 amplitude when compared with the control ones (P<0.01). In MSON eyes the parameters of VEP after photostress underwent larger changes and longer recovery time (RT) than in control and MSWO eyes (P<0.01). In addition; in MSON eyes we found increased transient PERG P50 latency (P<0.01) and reduced P50-N95 amplitude (P<0.01); Focal-ERG (that displays a major component at 16 Hz; 2nd harmonic:2P) with reduced 2P amplitudes and delayed 2P phases (P<0.01). Our results indicate that patients previously affected by optic neuritis present an abnormal VEP after photostress response and this may be ascribed predominantly to an involvement of the inner retinal layers as indicated by the concomitant impairment of PERG and focal-ERG responses.

This study provides evidence that far field potentials (FFPs) are generated from the trigeminal nerve in the cat. By stimulating the main mental nerve, three components (component 1, 0.59±0.06 ms; component 2, 0.81±0.06 ms; and component 3, 0.98±0.07 ms) were identified from surface electrodes. These three components were thought to be positive and negative FFPs because each component had a stationary peak and was distributed on the head being divided into positive and negative fields. Results of a study of lesions and recording compound action potentials (CAPs) defined the neural origins of those potentials as follows: component 1, the mandibular nerve at the mandibular foramen; component 2, the mandibular nerve at the foramen ovale; and component 3, somewhere between the gasserian ganglion and the trigeminal root. The amplitude of component 2 decreased when the mandibular nerve at the foramen ovale was immersed in cerebrospinal fluid (CSF) after opening the foramen and recovered to the prior level after closing the foramen with beeswax. We concluded that this transformation resulted in the change in electrical resistance of the volume conductor around the nerve.

Based on recent research that indicated that P300 scalp topography varies as a function of task and/or information to be processed, this study examined scalp-recorded magnetic fields correlated with the P300 by means of whole-head magnetoencephalography. Subjects performed two discrimination tasks, in which targets, defined on either object or spatial characteristics of the same visual stimuli, had to be discriminated. Based on the across-subject root mean square (RMS) functions a sequence of 4 components could be identified in both tasks, N1m, P3m, and two later components, which, based on their estimated neuronal sources, were classified as representing motor processes during and following the manual responses to target stimuli. Reliable between-task differences in source localization were obtained for the P3m component, but not for the other components. Inferior-medial sources were found for the P3m evoked by both spatial and object targets, with these sources being located about 3.5 cm more anterior for object targets. These results suggest that different neuronal sources, possibly located in subcortical regions in the vicinity of the thalamus, contribute to the P3m evoked by target stimuli defined by either object or spatial stimulus characteristics.

The aim of this study is to localize the primary sensory cortex of urogenital organs in the human brain. Using a newly developed MRI-linked magnetoencephalography system, we measured somatosensory evoked magnetic fields (SEFs) for unilateral stimuli on the dorsal penile nerve (DPN), posterior tibial nerve (PTN) and median nerve (MN). In five healthy male subjects, SEFs were clearly observed. Peak latencty of the first cortical components were 63.8±9.2 ms for DPN, 39.8±3.0 ms for PTN and 20.7±0.7 ms for MN stimuli. Peak amplitude of the first cortical components were 63.1±10.8 fT for DPN, 160.2±50.1 fT for PTN and 335.2±70.3 fT for MN stimuli. Isofield map for the peak latencies indicated a single dipolar pattern for DPN as well as for PTN and MN stimuli. Using a single current dipole model, all SEF sources were localized on the contralateral central sulcus to the stimuli, indicating the primary sensory cortex. The DPN sources were localized on the interhemispheric surfaces, corresponding to previous speculations by direct cerebral stimulation. This non-invasive SEF technique promises further brain functional mapping for the urogenital organs.