Abstract: I-waves refer to high-frequency (approximately 600 Hz) repetitive discharge of corticospinal fibers produced by single-pulse stimulation of the motor cortex. First detected in animal preparations, this multiple discharge can also be recorded in humans with epidural electrodes over the spinal cord, and with recently developed noninvasive paired-pulse transcranial magnetic stimulation protocols. The exact nature of the generation of I-waves is still unclear, but there is convincing evidence that they originate in the motor cortex, mainly through activation of corticocortical projections onto corticospinal neurons. The ability to measure I-waves in human motor cortex allows one to test the integrity and excitability of the underlying corticocortical circuits in health and disease.

Abstract: SummaryClinical and neuroimaging observations of the cortical network implicated in tactile attention have identified foci in parietal somatosensory, posterior parietal, and superior frontal locations. Tasks involving intentional hand–arm movements activate similar or nearby parietal and frontal foc

Abstract: A brief taxonomy of neurologic disorders resulting in global impairments of consciousness is presented. Particular emphasis is placed on focal injuries of subcortical structures that may produce disorders that are otherwise associated to large bilateral cortical injuries. A distinction between subcortical arousal and "gating" systems is developed. Both clinical and experimental studies are reviewed in the context of these disorders and their possible underlying mechanisms.

Abstract: SummarySomatosensory evoked human EEG and magnetoencephalographic (MEG) responses comprise a brief burst of low-amplitude, high-frequency (approximately 600 Hz) spikelike wavelets (“ς-bursts”) superimposed on the primary cortical response (e.g., the N20 to electrical median nerve stimulation). The r

Abstract: SummaryThe use of functional brain imaging techniques has led to considerable advances in our understanding of brain processing of human visceral sensation. The use of complementary techniques such as functional MRI, positron emission tomography, magnetoencephalography, and EEG has led to the identi

Abstract: The introduction of whole-head magnetoencephalography (MEG) systems facilitating simultaneous recording from the entire brain surface has established MEG as a clinically feasible method for the evaluation of patients with temporal lobe epilepsy (TLE). In mesial TLE, two types of MEG spike dipoles could be identified: an anterior vertical and an anterior horizontal dipole. Dipole orientations can be used to attribute spike activity to temporal lobe subcompartments. Whereas the anterior vertical dipole is compatible with epileptic activity in the mediobasal temporal lobe, the anterior horizontal dipole can be explained by epileptic activity of the temporal tip cortex. In nonlesional TLE, medial and lateral vertical dipoles were found which could distinguish between medial and lateral temporal seizure onset zones as evidenced from invasive recordings. In lesional TLE, MEG could clarify the spatial relationship of the structural lesion to the irritative zone. Evaluation of patients with persistent seizures after epilepsy surgery may represent another clinical important application of MEG because magnetic fields are less influenced than electric fields by the prior operation. Simultaneous MEG and invasive EEG recordings indicate that epileptic activity restricted to mesial temporal structures cannot reliably be detected on MEG and that an extended cortical area of at least 6 to 8 cm2 involving also the basal temporal lobe is necessary to produce a reproducible MEG signal. In lateral neocortical TLE MEG seems to be more sensitive than scalp-EEG which further underlines the potential role of MEG for the study of nonlesional TLE. Whole-head MEG therefore can be regarded as a valuable and clinically relevant noninvasive method for the evaluation of patients with TLE.

Abstract: The authors reviewed basic and clinical reports of pain-related somatosensory evoked potentials (SSEP) after high-intensity electrical stimulation [pain SSEP(E)] and painful laser stimulation [pain SSEP(L)]. The conduction velocity of peripheral nerves for both pain SSEP(E) and pain SSEP(L) is approximately 10 to 15 m/second, in a range of Adelta fibers. The generator sources are considered to be the secondary somatosensory cortex and insula, and the limbic system, including the cingulate cortex, amygdala, or hippocampus of the bilateral hemispheres. The latencies and amplitudes are clearly affected by vigilance, attention-distraction, and various kinds of stimulation applied simultaneously with pain. Abnormalities of pain SSEP(L) reflect an impairment of pain-temperature sensation, probably relating to dysfunction of A5 fibers of the peripheral nerve and spinothalamic tract. In contrast, conventional SSEP after nonpainful electrical stimulation reflects an impairment of tactile, vibratory, and deep sensation, probably relating to dysfunction of Aalpha or Abeta fibers of the peripheral nerve and dorsal column. Therefore, combining the study of pain SSEP(L) and conventional SSEP is useful to detect physiologic abnormalities, and sometimes subclinical abnormalities, of patients with peripheral and central nervous system lesions.

Abstract: Epilepsy surgery candidates with extratemporal foci represent a particular diagnostic and therapeutic challenge, because of anatomic and functional features of the pertaining areas. In the last decade, novel developments in the field of electrophysiological techniques have offered new approaches to detailed localization of specific epileptic discharges as well as eloquent regions. Magnetoencephalography, in combination with neuroimaging data and simultaneously recorded EEG, yields promising results to clarify centers of epileptic activity and their relationship to structural abnormalites and functionally significant areas. Examples are given to illustrate the range of applications of this method as a contribution to routine presurgical evaluation.

Abstract: The ability to trigger functional magnetic resonance imaging (fMRI) acquisitions related to the occurrence of EEG-based physiologic transients has changed the field of fMRI into a more dynamically based technique. By knowing the temporal relationship between focal increases in neuronal firing rates and the provoked focal increase in blood flow, investigators are able to maximize the fMR-linked images that show where the activity originates. Our mastery of recording EEG inside the bore of a MR scanner has also allowed us to develop cognitive paradigms that record not only the fMR BOLD images, but also the evoked potentials (EPs). The EPs can subsequently be subjected to localization paradigms that can be compared to the localization seen on the BOLD images. These two techniques will most probably be complimentary. BOLD responses are dependent on a focal increase in metabolic demand while the EPs may or may not be related to energy demand increases. Additionally, recording EPs require that the source or sources of that potential come from an area that is able to generate far-field potentials. These potentials are related to the laminar organization of the neuronal population generating that potential. As best we know the BOLD response does not depend on any inherent laminar neuronal organization. Therefore, by merging these two recording methods, it is likely that we will gain a more detailed understanding of not only the areas involved in certain physiologic events, e.g. focal epilepsy or cognitive processing, but also on the sequencing of the activation of the various participating regions.

Abstract: Objectives : In order to study the interaction between left- and right-sided stimuli on the activation of cortical somatosensory areas, we recorded somatosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122 channel whole-scalp SQUID gradiometer. Methods : Right and left median nerves were stimulated either alternately within the same run, with interstimulus intervals (ISIs) of 1.5 and 3 s, or separately in different runs with a 3 s ISI. In all conditions 4 cortical source areas were activated: the contralateral primary somatosensory cortex (SI), the contra- and ipsilateral secondary somatosensory cortices (SII) and the contralateral posterior parietal cortex (PPC). Results : The earliest activity starting at 20 ms was generated solely in the SI cortex, whereas longer-latency activity was detected from all 4 source areas. The mean peak latencies for SII responses were 86–96 ms for contralateral and 94–97 ms for ipsilateral stimuli. However, the activation of right and left SII areas started at 61±3 and 62±3 ms to contralateral stimuli and at 66±2 and 63±2 ms to ipsilateral stimuli, suggesting a simultaneous commencing of activation of the SII areas. PPC sources were activated between 70 and 110 ms in different subjects. The 1.5 s ISI alternating stimuli elicited smaller SII responses than the 3 s ISI non-alternating stimuli, suggesting that a considerable part of the neural population in SII responds both to contra- and ipsilateral stimuli. The earliest SI responses did not differ between the two conditions. There were no significant differences in source locations of SII responses to ipsi- and contralateral stimuli in either hemisphere. Subaverages of the responses in sets of 30 responses revealed that amplitudes of the SII responses gradually attenuated during repetitive stimulation, whereas the amplitudes of the SI responses were not changed. Conclusions : The present results implicate that ipsi- and contralateral SII receive simultaneous input, and that a large part of SII neurons responds both to contra- and ipsilateral stimulation. The present data also highlight the different behavior of SI and SII cortices to repetitive stimuli.

Abstract: SummaryMagnetoencephalography has proven to be a powerful noninvasive tool for investigating the functional organization of the human auditory cortex and its plastic changes. The first part of this review summarizes some recent experiments on the tonotopic organization, which can be observed not onl

Abstract: SummaryThis study was conducted to investigate the reliability of magnetoencephalography in lateralizing and localizing brain activity associated with receptive language function. Sixteen right-handed adults with no history of neurologic disorder engaged in a continuous recognition memory task for v

Abstract: A short review of previous studies is presented on somatic, evoked high-frequency oscillations. Also described briefly is recent data on the detection of high-frequency oscillations to posterior tibial nerve stimulation, and also on both tangential (area 3b) and radial (area 1) dipoles to median nerve stimulation. The findings show that high-frequency oscillations are not specific to median nerve stimulation but represent ubiquitous activity in the primary somatosensory cortex. Modulation of high-frequency oscillations versus electric and magnetic N20, N20 (m), primary response by a wake-sleep cycle, by attention or interference, by aging, and in central nervous system diseases such as Parkinson's disease and myoclonus epilepsy are also presented. Finally, a gamma-aminobutyric acid inhibitory interneuron hypothesis is presented for high-frequency oscillations based primarily on the findings regarding reciprocal modulation of the high-frequency oscillations and the underlying magnetic N20 (N20m) by a wake-sleep cycle and by attention or interference.

Abstract: Most patients who are comatose a few hours after a period of global cerebral ischemia have a poor prognosis. In a series of studies selected with strict criteria for study design, the median prevalence of death or survival in a vegetative state was 78% (range, 56-90%) (Zandbergen et al., 1998). Most nonsurvivors die within the first weeks, not from brain damage, but from cardiac or pulmonary complications. Uncertainty about treatment and nontreatment decisions is therefore most critical during this period. To reduce this uncertainty among caregivers, and the related anxiety among family members, early identification of patients with such a poor prognosis is desirable.

Abstract: SummaryThe features of somatosensory (SEFs), auditory (AEFs), and visual evoked fields (VEFs) in healthy subjects and patients with brain diseases provide the basis for clinical investigations using magnetoencephalography (MEG). The SEFs provide clinically useful information to identify the central

Abstract: The interfering influences by which the different components of the early somatosensory evoked potentials are modified are reviewed from both neurophysiologic and clinical perspectives. Special consideration is given to the specific differences between sensory and motor interferences. In this context, the specific effect of the mental movement simulation task on the frontal N30 component is discussed in relation to the involvement of this evoked wave as a physiologic index of the dopaminergic motor pathways. Relevant interfering approaches, including concurrent events ranging from tactile stimulation to locomotion, are reviewed and discussed insofar as these data provide insights into the neurophysiologic processes of interaction between competing internal models controlling motor acts and sensory information.

Abstract: Positron emission tomography (PET) is a relatively noninvasive neuroimaging method by means of which a large variety of human brain functions can be assessed. Localized neurochemical abnormalities detected by PET were found in patients with partial epilepsy and suggested the use of this modality for localizing epileptogenic regions of the brain. The clinical usefulness of PET is determined by its sensitivity and specificity for identifying epileptogenic areas as defined by ictal surface and intracranial EEG recordings. The findings obtained from comparative EEG and glucose PET data are reviewed with special emphasis on patients undergoing presurgical evaluation because of medically intractable temporal and extratemporal lobe epilepsy. The utility of glucose PET studies for identifying regions of seizure onset is presented, and the limited specificity of glucose metabolic abnormalities for the detection of various EEG patterns in clinical epilepsy is discussed. The authors review the available intracranial EEG and PET comparisons using [11C]flumazenil (FMZ) PET, a tracer for the assessment of tau-amino-butyric acid/benzodiazepine receptor function. They also summarize their experience with [11C]flumazenil PET in identifying cortical regions that show various ictal and interictal cortical EEG abnormalities in patients with extratemporal seizure origin. Finally, the authors demonstrate that further development of new PET tracers, such as alpha-[11C]methyl-L-tryptophan, is feasible and clinically useful and may increase the number of patients in whom PET studies can replace invasive EEG monitoring.

Abstract: Microelectrode studies of single units in the human thalamus during stereotactic surgery offer a unique opportunity to study the organization and plasticity of the sensory thalamus. In this review the authors present results using single-unit microelectrode recording in the mapping of human sensory thalamus in a variety of patients. First they outline the overall organization of the human sensory thalamus, including both somatosensory and pain pathways. They also show that the sensory maps for receptive and projection fields can be altered during pathologic states such as amputation and spinal transection. Additionally, the sensory maps show plasticity during states with abnormal patterns of motor activity, like dystonia. Lastly, they discuss the processing of painful and emotionally laden sensory experiences through the thalamus. The physiologic results of thalamic pain processing are discussed in relation to the sensory-limbic model of pain. The studies reviewed demonstrate the spectrum of stimulus processing and plasticity of both painful and nonpainful signals by the human thalamus.

Abstract: This review outlines the rationale for the use of magnetoencephalography (MEG) or magnetic source imaging (MSI), a noninvasive functional imaging technique, and the features that any imaging method should display to make a substantial contribution to cognitive neuroscience. After a brief discussion of the basic experimental approach used in the authors' studies, the use of early sensory components of brain magnetic responses is reviewed to address issues of the functional organization of the primary sensory cortices, followed by a comment on the clinical use of these components. Second, normative studies focusing on the late components of magnetic responses for establishing the validity and reliability of MSI maps of the language-specific cortex in normal subjects are reviewed. Third, the authors' investigations of fine spatiotemporal features of brain activation maps, specific to receptive language and to reading, are reviewed. Fourth, experience with presurgical mapping of the language-specific cortex in neurosurgery candidates and in patients undergoing the "Wada" procedure is summarized followed by a comment on the perfect agreement of the MSI maps with those derived by more direct invasive brain mapping procedures. Fifth. MSI-derived evidence of often dramatic, functional reorganization of brain areas subserving both simple sensory and linguistic functions is summarized along with comments on the use of MSI as a means for investigating brain plasticity. Finally, in the sixth section of this review, the authors relate their experience with the use of MSI in deriving brain activation profiles during silent reading of real words and pseudowords that are specific to dyslexic children. The review concludes with a discussion on the further use of MSI in assessing, among other issues, the effectiveness of intervention strategies designed to improve reading fluency in dyslexic children.

Abstract: Single photon emission computed tomography (SPECT) has increasingly been used as a diagnostic procedure for localizing epileptic seizure foci and as a research tool for investigating the physiologic mechanisms underlying seizure activity. With increasing use of SPECT in localizing the seizure focus for epilepsy surgery, there arises a need to critically assess its current role in the evaluation of patients for epilepsy surgery, especially as it relates to other clinical and laboratory data used in presurgical evaluation. Ictal EEG discharge has traditionally been used as the "gold standard" against which SPECT studies are compared in assessing the latter's localizing value. However, this practice presents a major challenge because SPECT studies are often reserved for patients with nonlocalizing EEG or magnetic resonance imaging findings. Nonetheless, SPECT studies in evaluation for epilepsy surgery should always be performed with the knowledge of the patient's EEG activity preceding, during, and after the injection of the radiotracer. The advent of techniques such as subtraction SPECT with co-registration on magnetic resonance imaging (SISCOM) and computer image-guided surgery has great potential in enhancing the clinical electrophysiologic evaluation of SPECT-detected abnormalities in epilepsy. These techniques permit accurate spatial correlation between intracranial EEG activity and SPECT perfusion patterns. The techniques can also be used to evaluate the effect of the extent of EEG focus resection compared with that of SISCOM focus resection to determine which has more prognostic importance in postsurgical control of seizures. Both animal and human studies are warranted to advance our knowledge of the electrophysiology associated with the various SPECT perfusion patterns.

Abstract: After stimulation of the lower extremity nerve (tibial nerve), N21 and N23 are recorded from L4 and T12 spine respectively. The far-field potentials of P31 and N35 are registered from Fpz-C5s (fifth cervical spine) or CPi (ipsilateral with respect to the side of stimulation)-ear derivation. Additional far-field potentials of P17 and P24 may be recorded from the scalp when a noncephalic (knee) reference is used. The major positive peak, P40, is registered at the vertex and the CPi. Preceding P40, there is a small negative peak, N37, recorded at the contralateral (CPc) hemisphere. Neuroanatomic substrates of these somatosensory evoked potential (SSEP) components are less well clarified compared with those of upper extremity (median nerve) SSEPs, primarily because clinical application of lower extremity SSEPs is more difficult, and all of the aforementioned potentials but one (P40) are not obligatory components. The concept of "paradoxical lateralization" complicates the issue further. Accumulating evidence, however, suggests that the far-field potentials of P17 and P31 arise from the distal portion of the sacral plexus and brainstem respectively. These correspond to P9 and P14 of the median nerve SSEPs respectively. The spinal potential of N23 is equivalent to the N13 cervical potential of the median nerve SSEP. N35 recorded from the ipsilateral hemisphere is analogous to N18 of the median nerve. Paradoxically lateralized P40 has been thought to represent the positive end of a dipole field, reflected by the negativity at the mesial surface of the contralateral hemisphere, and has commonly been considered to be equivalent to the first cortical potentials (N20) of the median nerve SSEP. However, more recent evidence suggests that the primary positivity is at the mesial cortical surface, and it more likely corresponds to P26 of the median nerve SSEP. Thus the first cortical potential corresponding to N20 is probably a small and inconsistent N37 recorded on the contralateral hemisphere. These assumptions need to be verified further by more extensive clinical studies applied to various neurologic disorders.

Abstract: Early diagnosis of diabetic autonomic neuropathy contributes to the prevention of serious complications and improves the prognosis of patients with diabetes. Common tests of peripheral autonomic function are the quantitative sudomotor axon reflex test or the sympathetic skin response (SSR). Quantitative sudomotor axon reflex test is quantifiable but technically demanding. Sympathetic skin response cannot be quantified easily. To study whether measurement of skin vasomotion is suited to assess early sympathetic peripheral neuropathy, we monitored skin blood flow at the index finger pulp using laser Doppler flowmetry before and after electrical stimulation. We assured that the stimulus was sufficient to elicit an efferent sympathetic response by monitoring palmar SSR ipsilateral to the flow measurement. In 21 diabetic patients with at least stage one polyneuropathy and 21 age-matched controls, SSR was recorded from one palm and sole following electrical stimulation at the contralateral wrist. Sympathetic skin response was present at the palms in all patients and controls and absent at the sole of two patients only. Eight patients (38.9%) had abnormal SSR, with absent plantar responses in two patients, prolonged plantar latencies in six patients, and prolonged volar SSR latencies in two patients. Skin blood flow responses were more often abnormal (46.1%) than SSR (P < 0.05), responses were delayed in two patients and absent in another 8 patients. Skin blood flow retest reliability was high with a repeatability coefficient of 10.64% in controls and 12.34 % in patients. Skin blood flow monitoring after sympathetic stimulation provides a reproducible parameter of sympathetic vasomotor control and complements the diagnostic value of SSR testing.

Abstract: To determine current practices in the provision of video-EEG services, the authors conducted an international survey by post. The aim of the survey was to evaluate, by reference to other centres, how and why certain things are done, be assured that their own center is providing a quality service, identify weaknesses in their service, and from this, set improvement goals and objectives. A purposive sampling method was used by sending questionnaires to 78 hospitals where it was believed a long-term video-EEG monitoring service existed. Completed survey questionnaires were returned from 42 centers. Although the survey mechanism may have resulted in self-selection bias, evaluation of the responses provides information on patient management, staffing levels, equipment, and equipment management. Ultimately, these data may aid in identifying a minimum set of requirements for the provision of a video-EEG telemetry service.

Abstract: Summary:The purpose of this study was to develop a reliable method of EEG analysis during carotid endarterectomy. EEGs of 104 patients under general anesthesia were processed by three different methods: a) “on-line” visual analysis during surgery, b) “off-line” visual analysis in laboratory, and c)

Abstract: The initial cortical component of the median nerve somatosensory evoked potential (SSEP), the parietal N20, is generated in the posterior bank of the central sulcus and inverts in polarity across the sulcus. The inversion is used to identify the central sulcus. The precentral P20 is sometimes not identifiable in scalp recordings, and this has been attributed to a dipole orientation that directs the maximum positivity downward, into the brain. The authors mapped cortical SSEPs during resection of an arteriovenous malformation in the left sensorimotor area. Preoperative scalp SSEPs over the right hemisphere were normal with a frontal P20, but those over the left hemisphere had an unusual topography with a frontal N20. Intraoperative cortical surface recordings demonstrated an N20-P20 inversion in the inferior-superior rather than the usual posterior-anterior direction. This was a result of the trajectory of the central sulcus over the surface of the brain. The section containing the hand representation was coursing in an anterior-posterior direction. This anatomic variant is an additional cause of absent frontal P20 in scalp recordings. Variations in central sulcus anatomy may cause unusual SSEP topographies, but two-dimensional SSEP mapping and correlation with the sulcal anatomy can still permit localization of the central sulcus in such cases.

Abstract: A patient in whom a variety of abnormal EEG findings can be elicited by elimination of central vision and fixation demonstrates fixation-off sensitivity. The underlying mechanisms of fixation-off sensitivity and its relationship with alpha rhythm remain unclear. To obtain a better understanding of this issue, we used a whole-head magnetoencephalograph to study an epileptic child with fixation-off sensitivity resulting in a 3-Hz, large-amplitude oscillation (300 microV) over the occipital regions on the EEG. Magnetic source localization revealed alpha activity around the calcarine fissure and surrounding parieto-occipital areas. Magnetic sources of abnormalities relating to fixation-off sensitivity, however, usually were located deeper in the brain, suggesting more extensively distributed sources, with involvement of the cingulate gyrus and the basomesial occipitotemporal region. Distributions of the sources of both types of activities show independent clusters but also an appreciable domain of overlap. Our findings indicate that abnormalities related to fixation-off sensitivity can emerge in thalamocortical networks, with larger and more anterior cortical distribution than those that generate alpha rhythm. Transition in the type of oscillation appears not only to depend on a change in cellular dynamics but also to be reflected in a different spatial distribution of the underlying neuronal networks.

Abstract: Median-nerve evoked somatosensory evoked potentials (SEPs) and brainstem auditory evoked potentials (BAEPs), examined early in the course of patients suffering from cerebrovascular disease, correlate statistically significantly with outcome. Little is known about the changes of evoked potentials in the course of disease and their correlation to outcome. In a series of 215 patients (75 supratentorial infarctions, 36 infratentorial infarctions, 58 supratentorial hemorrhages, 18 infratentorial hemorrhages, and 28 aneurysmatic subarachnoid hemorrhages) requiring neurologic intensive care treatment, we prospectively examined the correlation between the findings of serial SEPs and BAEPs and outcome at 4 weeks. Evoked potentials were examined after admission, after 1 week, and after 2 weeks. The findings were classified in 4 categories (normal, unilateral or bilateral pathologic findings, unilaterally attenuated, and bilaterally attenuated). Clinical outcome was determined by classification according to the Glasgow Outcome Scale (death, persistent vegetative state, severely incapacitated, mildly incapacitated, and recovery). Statistical evaluation was performed using Fisher's exact test for all variables. In all subgroups, SEPs correlated statistically significantly with outcome at all three examinations. No correlation was found for BAEPs at first examination in infratentorial disease, nor at second examination in subarachnoid hemorrhages. In all other cases, SEPs and BAEPs were correlated statistically significantly with outcome at all three examination timepoints.