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At least 631 records · Page 35Linked to original sources
PubMed1998

Focusing and targeting of magnetic brain stimulation using multiple coils.

Neurones can be excited by an externally applied time-varying electromagnetic field. Focused magnetic brain stimulation is attained using multiple small coils instead of one large coil, the resultant induced electric field being a superposition of the fields from each coil. In multichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusing of the stimulating field, and independent coil channels allow targeting of the stimuli on a given spot without moving the coils. The problem of shaping the stimulating field in multichannel stimulation is analysed, and a method is derived that yields the driving currents required to induce a field with a user-defined shape. The formulation makes use of lead fields and minimum-norm estimation from magneto-encephalography. Using these methods, some properties of multichannel coil arrays are examined. Computer-assisted multichannel stimulation of the cortex will enable several new studies, including quick determination of the cortical regions, the stimulation of which disrupts cortical processing required by a task.

Brain↗
PubMed1989

Suprasellar germinoma--occult presentation with hypothalamic failure 18 months before diagnosis.

Hormonal evidence of widespread hypothalamic failure, including diabetes insipidus, hyperprolactinaemia, growth failure, hypothyroidism, and adrenal failure, was present in a boy of 14 years. Investigation with computerised axial tomography (CT) and metrizamide encephalography failed to identify any anatomical abnormality at the time of presentation. However, 18 months later papilloedema was noted at which time a suprasellar mass and gross dilatation of the ventricular system were readily identified using CT and a germinoma was subsequently diagnosed and treated. In the light of this experience we recommend that when "idiopathic" hypothalamic failure is diagnosed, imaging procedures, including CT or nuclear magnetic resonance examinations, should be repeated on at least one occasion, after an interval of approximately 6 months.

Adolescent↗
PubMed2000

Serial lumbar CSF presure measurements and cranial computed tomographic findings in childhood tuberculous meningitits.

Intracranial pressure (ICP) was monitored in 218 consecutive children with hydrocephalus secondary to tuberculous meningitis (TBM). All children underwent cranial computerized tomographic (CT) scanning and continuous lumbar cerebrospinal (CSF) pressure monitoring on admission. Noncommunicating hydrocephalus (37 children), as determined by air encephalography, was treated by ventriculoperitoneal (VP) shunting and communicating hydrocephalus (181 children), by means of daily acetazolamide and frusemide. Response of ICP to treatment in the group with communicating hydrocephalus was assessed by means of repeated CSF pressure monitoring and CT scanning. One hundred and eighty-five of the 218 patients survived the 1st month of treatment. The aim of this study was the retrospective determination of (1) the relationship between ICP measurements and CT findings on admission and (2) the characteristics of the ICP recording which correlated best with the CT criteria of compensated hydrocephalus after the 1st month of treatment. No relationship was found between the level of baseline CSF pressure and the degree of hydrocephalus, as demonstrated by CT scanning, on admission. Seventy-five per cent of the patients with communicating hydrocephalus that survived the 1st month of treatment complied with the CT criteria for compensated hydrocephalus. All these patients had a baseline CSF pressure below 15 mmHg and absence of high-amplitude B waves on the pressure recording done at the end of the 1st month. In this study repeated lumbar CSF pressure monitoring proved to be an effective instrument to assess the response of communicating tuberculous hydrocephalus to medical treatment and also accurately predicted the timing of compensation of the hydrocephalus.

Acetazolamide↗
PubMed2005

A model for the mechanism of generating the auditory evoked field.

The auditory evoked potentials and fields, recorded with electro- and magneto-encephalography, respectively, have the prominent peak at the latency of about 100 ms, named N100(m). The mechanism for generating the peak is not yet fully elucidated, however. Firstly, this study reviewed empirical results on the mechanism that had been previously published by the same author. The results showed that phases of the ongoing oscillations at around 6 Hz above the auditory cortex are locked with respect to onsets of the auditory stimuli during the period of N100 m, and also that powers of the oscillations are increased during the same period. Taking into account these results as well as another evidence that the peak arises from multiple sources within the auditory cortex, a model for the mechanism of generating the auditory N100 m was proposed. The model assumed several oscillators in the auditory cortex, whose phases were locked during the period of N100 m, but not locked at other periods. Computer simulation validated the model. The model could represent a starting point for further experiment and simulation.

Acoustic Stimulation↗
PubMed2004

Displacement of central sulcus in cerebral arteriovenous malformation situated in the peri-motor cortex as assessed by magnetoencephalographic study.

BACKGROUND: Intra-operative monitoring of the position of the central sulcus (CS) is indispensable to properly treat a peri-motor cortex lesion. Noninvasive preoperative studies for identification of CS are also needed for choosing the optimal operative procedure. Magneto-encephalography (MEG) has recently been introduced for non-invasive preoperative investigation and cortical functional mapping. METHODS: Stereotactic mapping of functional CS anatomy was performed preoperatively on 13 subjects using somatosensory evoked fields with MRI-linked whole head MEG system. All subjects had a left sided peri-motor cortex lesion with diagnoses including the following conditions: three arteriovenous malformations (AVM), seven gliomas, three meningiomas. FINDINGS: Functional CS in supratentorial brain tumor and parietal AVM cases corresponded with anatomical CS identified by MRI. But the AVM cases in whom the nidus was situated within the peri-motor cortex showed discrepancies between anatomical CS and functional CS. INTERPRETATION: Careful consideration of the operative procedure combined with non-invasive cortical functional mapping is needed to optimally treat AVM and congenital brain lesions situated in the anatomical peri-motor cortex.

Adolescent↗
PubMed1998

An increase in extracellular glutamate is a sensitive method of detecting ischaemic neuronal damage during cranial base and cerebrovascular surgery. An in vivo microdialysis study.

All patients undergoing neurological surgery are at risk for serious complications. Ischaemic damage presenting with hemiparesis or speech difficulties occurs in up to 6% of patients undergoing cerebral bypass procedures and other complicated neurosurgical procedures. Currently available methods for detection of such damage include the use of somatosensory evoked potentials (SSEPs) and electro-encephalography (EEG). Unfortunately, these techniques have false positives and may remain normal in the presence of severe focal neurological deficits. Early detection of potential deficits may prevent or minimize damage through a change in operative or anaesthetic strategy. With the availability of several potential neuroprotective compounds, it is also possible to treat patients at risk of developing ischaemic complications if the individuals are identified early. The excitatory neurotransmitter glutamate is not only a metabolic product, but is also thought to promote ischaemia induced cell injury if released into the extracellular space. It may be a significant parameter for ischaemic brain metabolism. In this report we describe 10 patients who underwent extracranial-intracranial (EC-IC) high flow bypass procedures with routine intra-operative monitoring (IOM) as well as intra-operative in-vivo microdialysis measurement of glutamate. Our aim was to compare intra-operative microdialytic findings and IOM findings with respect to patients' early postoperative clinical courses. Three patients had significant intra-operative glutamate increases indicating ischaemia. Two of these patients awoke with a new neurological deficit (hemiparesis). Routine IOM findings were either normal or showed only transient changes during the time the glutamate levels were high. Our study shows that an increase in extracellular glutamate, as monitored by in-vivo microdialysis, is an excellent early market of neuronal damage. While our glutamate measurements were done off-line, it may be possible to get in future continuous on-line measurements to serve as an early warning system for potential ischaemic damage.

Adult↗
PubMed2006

Inertia and memory in ambiguous visual perception.

Perceptual multistability during ambiguous visual perception is an important clue to neural dynamics. We examined perceptual switching during ambiguous depth perception using a Necker cube stimulus, and also during binocular rivalry. Analysis of perceptual switching time series using variance-sample size analysis, spectral analysis and time series shuffling shows that switching times behave as a 1/f noise and possess very long range correlations. The long memory feature contrasts sharply with the traditional satiation models of multistability, where the memory is not incorporated, as well as with recently published models of multistability and neural processing, where memory is excluded. On the other hand, the long memory feature favors the concept of "dynamic core" or coalition of neurons, where neurons form transient coalitions. Perceptual switching then corresponds to replacement of one coalition of neurons by another. The inertia and memory measures the stability of a coalition: a strong and stable coalition has to be won over by another similarly strong and stable coalition, resulting in long switching times. The complicated transient dynamics of competing coalitions of neurons may be addressable using a combination of functional imaging, measurement of frequency-tagged magnetoencephalography and frequency-tagged encephalography, simultaneous recordings of groups of neurons in many areas of the brain, and concepts from statistical mechanics and nonlinear dynamics theory.

Data Interpretation, Statistical↗
PubMed1994

Effects of sleep on the antidepressant response to sleep deprivation.

The effect of a 90-min nap period on mood was studied in 22 sleep-deprived patients with a diagnosis of major depression. All patients remained awake from 7 AM until 12 noon the following day at which time they were permitted to nap while being monitored by sleep encephalography. Fifteen subjects showed a significant response to sleep deprivation as defined by a 35% improvement on the Hamilton Rating Scale for Depression. After the nap a relapse of depressive symptoms occurred which was significantly related to the amount of non-rapid eye movement sleep time.

Adult↗
PubMed1982

Paramedian hourglass epidermoid extending above and below the tentorium.

The case of a 22-year-old woman with a large hourglass epidermoid tumor extending both below and above the tentorium is presented. The tumor was located in the suprasellar region, left middle cranial fossa, and posterior fossa on the left side. Conventional computerized tomographic (CT) scanning was inconclusive as to the actual extension of the tumor, whereas air encephalography was more helpful. The subtemporal transtentorial approach made it possible to achieve a seemingly radical excision of the tumor. Nevertheless, the CT scan one year later showed the presence of a small tumor mass. The diagnostic and surgical implications are discussed in light of few similar cases reported in the literature.

Adult↗
PubMed2006

Resting state oscillatory brain dynamics in Parkinson's disease: an MEG study.

OBJECTIVE: The pathophysiological mechanisms of cognitive dysfunction and dementia in Parkinson's disease (PD) are still poorly understood. Altered resting state oscillatory brain activity may reflect underlying neuropathological changes. The present study using magneto encephalography (MEG) was set up to study differences in the pattern of resting state oscillatory brain activity in groups of demented and non-demented PD patients and healthy, elderly controls. METHODS: The pattern of MEG background oscillatory activity was studied in 13 demented PD patients, 13 non-demented PD patients and 13 healthy controls. Whole head MEG recordings were obtained in the morning in an eyes closed and an eyes open, resting state condition. Relative spectral power was calculated using Fast Fourier Transformation in delta, theta, alpha, beta and gamma frequency bands. RESULTS: In the non-demented PD patients, relative theta power was diffusely increased and beta power concomitantly decreased relative to controls. gamma Power was decreased in central and parietal channels. In the demented PD patients, a diffuse increase in relative delta and to lesser extent theta power and a decrease in relative alpha, beta and to lesser extent gamma power were found in comparison to the non-demented PD group. In addition, reactivity to eye opening was much reduced in the demented PD group. CONCLUSIONS: Parkinson's disease is characterized by a slowing of resting state brain activity involving theta, beta and gamma frequency bands. Dementia in PD is associated with a further slowing of resting state brain activity, additionally involving delta and alpha bands, as well as a reduction in reactivity to eye-opening. SIGNIFICANCE: The differential patterns of slowing of resting state brain activity in demented and non-demented PD patients suggests that, in conjunction with a progression of the pathological changes already present in non-demented patients, additional mechanisms are involved in the development of dementia in PD.

Aged↗
PubMedDate not supplied

Intracranial EEG and human brain mapping.

This review is an attempt to highlight the value of human intracranial recordings (intracranial electro-encephalography, iEEG) for human brain mapping, based on their technical characteristics and based on the corpus of results they have already yielded. The advantages and limitations of iEEG recordings are introduced in detail, with an estimation of their spatial and temporal resolution for both monopolar and bipolar recordings. The contribution of iEEG studies to the general field of human brain mapping is discussed through a review of the effects observed in the iEEG while patients perform cognitive tasks. Those effects range from the generation of well-localized evoked potentials to the formation of large-scale interactions between distributed brain structures, via long-range synchrony in particular. A framework is introduced to organize those iEEG studies according to the level of complexity of the spatio-temporal patterns of neural activity found to correlate with cognition. This review emphasizes the value of iEEG for the study of large-scale interactions, and describes in detail the few studies that have already addressed this point.

Brain↗
PubMedDate not supplied

The parietal distance effect appears in both the congenitally blind and matched sighted controls in an acoustic number comparison task.

Visual world experience is thought to play a significant role in the development of an abstract representation of quantity in the human brain. Nevertheless, some congenitally blind individuals demonstrate excellent numerical abilities. We show that blind adults have a phenomenologically normal semantic representation of number. Electro-encephalography data demonstrate that the numerical distance effect has similar parietal correlates both in the blind and in matched sighted controls. Our interpretation is that number comparison in the blind relies on a compensation network in the initial phase of number comparison. In a second phase, an evolutionarily hardwired parietal system is exploited. The representation of number meaning has both plastic and evolutionarily hardwired components.

Acoustic Stimulation↗
PubMed2005

Effect of bilingualism on cognitive control in the Simon task: evidence from MEG.

The present study used magneto-encephalography (MEG) to determine the neural correlates of the bilingual advantage previously reported for behavioral measures in conflict tasks. Bilingual Cantonese-English, bilingual French-English, and monolingual English speakers, performed the Simon task in the MEG. Reaction times were faster for congruent than for incongruent trials, and the Cantonese group was faster than the other two groups, which did not differ from each other. Analyses of the MEG data using synthetic aperture magnetometry (SAM) and partial last squares (PLS) showed that the same pattern of activity, involving signal changes in left and medial prefrontal areas, characterized all three groups. Correlations between activated regions and reaction times, however, showed that the two bilingual groups demonstrated faster reaction times with greater activity in superior and middle temporal, cingulate, and superior and inferior frontal regions, largely in the left hemisphere. The monolinguals demonstrated faster reaction times with activation in middle frontal regions. The interpretation is that the management of two language systems led to systematic changes in frontal executive functions.

Adult↗
PubMed1980

The clinical and radiological features of the empty sella syndrome.

The importance of the empty sella syndrome has increased with recent interest in pituitary micro-adenomas, trans-sphenoidal hypophysectomy and computerised tomography. Four cases of the empty sella syndrome which illustrate the clinical and radiological features are reported, and the literature is reviewed. Headache, visual symptoms and endocrine disturbances are commonly found in association with the empty sella syndrome, and the coexistence of pituitary micro-adenomas is well recognised. Symmetrical ballooning of the sella turcica without changes in configuration is the typical finding with plain radiography, but occasionally more extensive destructive changes in the region of the pituitary fossa may occur. Provided that the correct angulation, collimation and level of cut are chosen, computerised tomography now may be used to make a definitive diagnosis, making air encephalography unnecessary.

Adenoma↗
PubMed1971

Cerebral atrophy in young cannabis smokers.

Evidence of cerebral atrophy was demonstrated by air encephalography in ten patients with histories of consistent cannabis smoking over a period of 3-11 years. The average age of the patients was 22 years; all were males. Amphetamines and lysergide (L.S.D.) had also been taken, but in much smaller amounts. Measurements of the lateral and third ventricles were significantly different from those in thirteen controls of a similar age-group.

Adolescent↗
PubMed1977

Echo-encephalographic measurements (A-scan) of the lateral ventricles in children; its reliability.

A positioning of the probe on the skull has been developed to increase the diagnostic accuracy of hydrocephalus. Echo-encephalography in the trigonum region of the ventricles is usually possible in children up to 13 years of age. In this region sources of error as the medial wall of the temporal lobe or the lateral wall of the temporal horn are eliminated. In hydrocephalic ventricles the ultrasound measurements in this region corroborate very well with the measurements found in the pneumo-encephalograms. This in contrast to the measurements found with the probe placed in the more conventional region. As the largest diameter of the skull, corresponds with the trigonum region, a very practical topographic mark for a reliable standardized method has been found. With more confidence the ventricle indices of 93 normal children, in the age of 1 to 13 years, were measured.

Adolescent↗
PubMed1992

Transcranial Doppler for detection of cerebral ischaemia during carotid endarterectomy.

We evaluated transcranial Doppler sonography (TCD) for the detection of cerebral ischaemia during carotid endarterectomy in 30 male and 14 female patients with ipsilateral focal cerebro-vascular symptoms. Surgery was performed during halothane-nitrous oxide anaesthesia with moderate hypocapnia. Eight patients had a temporary shunt owing to contralateral occlusion or a stump pressure below 40 mmHg, and/or EEG flattening. Transcranial Doppler sonography was followed intra-operatively together with electro-encephalography (EEG), internal carotid artery (ICA) pressures and cerebral blood flow (CBF). Middle cerebral artery mean flow velocity (Vmean) was 38 (22-96) cm s-1 (median and range) and decreased during cross-clamping to 28 (10-60) cm s-1 (p less than 0.0001). After removal of the clamp it increased to 42 (20-102) cm s-1 (p less than 0.0001). AVmean clamp of less than 30 cm s-1 together with a Vmean clamp: Vmean pre-clamp ratio of less than 0.6 showed an accuracy with respect to CBF below 20 ml 100 g-1 min-1 of 89%. AVmeanclamp:Vmean pre-clamp ratio below 0.4 detected all all patients with EEG flattening (n = 3) (accuracy 97%). The corresponding level of accuracy obtained with stump pressure was 80%. The results indicate that middle cerebral flow velocity enables an increase in the accuracy of detecting cerebral ischaemia during carotid endarterectomy.

Aged↗
PubMed2003

A study of nonepileptic seizures in an Indian population.

OBJECTIVE: The goal of this study was to evaluate the background and the clinical profile of nonepileptic seizures (NESs) confirmed by short-term video encephalography (ST-VEEG) recording in an Indian population. METHODS: Seventy-one patients with NESs were enrolled. A complete history was taken and the recorded event was reviewed to define the ictal events. Patients were divided into two groups, Group 1 with a paucity of movements and Group 2 with an excess of movement, and results were analyzed. RESULTS: The mean age was 22.9 (9.6); there were 15 males (21.1%) and 56 females (78.9%). Twenty-four patients (33.8%) were receiving antiepileptic drugs. The majority of the patients (42, 59.1%) were students. All patients were amnesic for the event and were unresponsive during the event. The other characteristics were similar events in 98.6%, hyperventilation in 58 (81.7%). Forty-two patients (59.2%) were in Group 1 and 29 patients (40.8%) were in Group 2. CONCLUSION: NES is a disease of the young and can affect the student or professional. A wrong diagnosis can result in inappropriate treatment. Awareness of this entity is critical to ensuring prompt diagnosis and early intervention.

Adult↗