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L Narici

Publications and source records attributed to L Narici.

36 records · Page 2Linked to original sources
PubMed1995

Discrimination and study of rhythmical brain activities in the alpha band: a neuromagnetic frequency responsiveness test.

In this paper we propose a fast procedure--stimulation paradigm, data analysis, data presentation--that permits the study of frequency selective enhancements or suppressions of the cortical responses in selected frequency windows. These responses can be used to classify the underlying activity. This novel procedure combines, in one single test, multichannel detection, finely frequency scanned stimulation, a paradigm that permits to study the effect of the stimulation on the underlying activity (during the non-stimulated periods intermingled with the burst of stimuli) and a compact presentation to appreciate the whole rate/frequency/topographical dependency of the spectral responses. In particular we applied this procedure to non-invasively study for the first time the frequency responsiveness within the extended alpha band (6 Hz to 14 Hz) under separate visual and somatosensory stimulation. The test proved robust and the responses quite stationary. We discriminated the lower and higher alpha band through their different topographical and frequency responsiveness features and suggested them to be the same sub-bands recently discriminated via cognitive experiments on the basis of their functional correlates. We measured a rate dependent alpha suppression (both in the somatosensory and visual modality) that is suggested to be linked to the decoding of the stimulus rate. A slight decrease in the frequency of the spontaneous activity following stimulations at any rate is suggested to be connected to attentional load. We stressed the importance of our simple sensorial discrimination of those activities already evidenced on the basis of their functional correlates, as well as the possible clinical uses of the test: on epilepsy, Alzheimer's disease (recently described to have effects on the responsiveness of the cortex to sensory stimuli at different frequencies) as well as other disabling pathologies.

Adult↗
PubMed1994

Short-term brain 'plasticity' in humans: transient finger representation changes in sensory cortex somatotopy following ischemic anesthesia.

Transient rearrangements of finger representation in primary somatosensory cortex induced by an anesthetic block of the sensory information from adjacent fingers have been shown invasively in animals. Such a phenomenon has been now replicated in seven healthy human volunteers. Somatosensory Evoked Fields (SEFs) have been recorded during separate electrical stimulation of the 1st, 3rd, or 5th finger. Recordings were obtained in control conditions (stage A), following complete ischemic anesthesia of the 4 non-stimulated fingers (stage B), and after regaining sensation (stage C). SEFs were recorded using a 28-channel DC-SQUID magnetometer; a single position of the sensor was enough to identify the source of N20m, P30m and following components using the Equivalent Current Dipole (ECD) model. The amount of afferent input during stages A through C was monitored with surface electrodes placed on the nerve at wrist and elbow. No variation of the nerve compound potential was observed during stages A through C. In stage A, the localizing algorithm was able to discriminate the individual finger representation in accordance with the somatotopic organisation of the sensory homunculus. It was observed that the ECDs responsible for the cortical responses from the unanesthetized finger were significantly changing following a relatively brief period of sensory deprivation from the adjacent fingers. Such changes of the ECDs with respect to the control conditions were characterized by an increase in strength and deepening for the middle finger, and by a shift on the coronal plane for the thumb and the little finger (medial for the former, lateral for the latter). Such changes became progressively evident in stage B, but were persisting in stage C.

Action Potentials↗
PubMed1994

Analysis of interhemispheric asymmetries of somatosensory evoked magnetic fields to right and left median nerve stimulation.

This paper represents the first neuromagnetic systematic investigation of the asymmetries between the sources activated in the right and left hemispheres after electric median nerve stimulation. We focused our attention on the location and strength of the equivalent sources activated in the primary somatosensory cortex contralateral to the stimulated nerve in the 50 msec post-stimulus epoch. The spatial coordinates of the equivalent sources did not differ statistically significantly in the two hemispheres. Minor individual asymmetries are shown to be related to the interhemispheric differences in the position of the central sulcus as revealed by MRI investigation. The equivalent sources were significantly stronger in the left hemisphere. When comparing the location of the generators across individuals, we show that interhemispheric differences fluctuate less than absolute values. A quantitative evaluation of these findings is also given. Based on these results, a normative data set has been established, to be used as a baseline in following up changes of interhemispheric asymmetries due to hemispheric lesions and subsequent cortical reorganization.

Adult↗
PubMed1992

The dynamic behaviour of the evoked magnetoencephalogram detected through parametric identification.

A single trial technique has been used for the analysis of neuromagnetic signals. This method, applied to evoked magnetoencephalograms, permitted us to evaluate the dynamics of brain responses under repetitive somatosensory stimulation. Two different phenomena have been investigated: synchronized spontaneous activity (SSA), elicited by bursts of stimuli at a given frequency; and somatosensory evoked fields (SEF), the transient response to a single stimulus. A transient period of adaptation of the SSA response to the particular stimulation paradigm is reported at specific stimulation frequencies; no significant decrease of the response amplitude even after 300 repetitions is observed. The amplitudes of single SEFs also appear to be constant. Possible physiological implications of the SSA response parameters studied (amplitude, frequency and decay time), and a preliminary discussion of the relationship between SEF and SSA, are presented.

Brain↗
PubMed1991

Neuromagnetic somatosensory homunculus: a non-invasive approach in humans.

The somatosensory homunculus has been identified during stimulation of median (at wrist and elbow), femoral, tibial and pudendal nerves of the left hemibody via the neuromagnetic imaging technique. The somatic representations of different body districts have been localized in the somatosensory cortex, by means of an equivalent dipole localization algorhythm. Dipole locations agree with the well-known somatotopic organization obtained with invasive techniques. The proposed method is, therefore, an important investigating tool for studies on normal and diseased subjects.

Evoked Potentials, Somatosensory↗
PubMed1991

Analysis of the neuromagnetic data on the frequency responsivity of the human brain: a progress report.

The study of the responsivity of the human brain at different rates of sensory stimulation has provided several pieces of information on brain functionality. The new analysis technique used in this study permits us to investigate the dynamics of these responses and to obtain a larger amount of statistical data to allow for fine frequency discrimination. This allows us to suggest, for example, phase locked systems as models for synchronisable spontaneous activity. Absence of subject fatigue during the repetition of the stimulation paradigm, as well as the presence of a 'learning period' characterised by transient dynamics of the responses, are also discussed.

Acoustic Stimulation↗
PubMed1990

Evoked alpha- and mu-rhythm in humans: a neuromagnetic study.

We present the results of a neuromagnetic study on the spatial structure of brain rhythms enhanced by photic and somatosensory stimulation, as measured on the occipital, rolandic and frontorolandic regions in humans. It emerges that, while it is always possible to drive the cerebral activity during sustained stimulation at any given frequency, only certain specific frequencies can produce prolonged synchronization (i.e. the oscillating activity elicited by the repetitive stimulation continues well beyond its termination). In both studied modalities we were able to localize equivalent sources for the synchronized responses; their relationship with the known evoked responses is discussed. In the visual modality the synchronization was characterized by a potentiation of the subjects' alpha-rhythm. In the somatosensory modality synchronization was reflecting two different activities: one probably related to the rolandic mu-rhythm, the second suggesting the presence of two widely separated and time correlated sources possibly driven by a unique, deep clock. Possible implications for other studies of the dominant brain rhythms, or experimental checks on specific brain models, as well as of the visual and somatosensory evoked responses are discussed.

Adult↗
PubMed1990

Neuromagnetic topography of photoconvulsive response in man.

The neuromagnetic method was applied to the study of photoconvulsive responses. The identification of specific magnetic field distributions over the scalp was achieved by; (a) a stimulation paradigm consisting of series of trains of flicker stimuli randomly presented to the epileptic patient, after eye closure, to get epileptic responses while avoiding seizures; (b) a novel procedure for data analysis, to select consistent responses. These patterns, when sufficiently stable in time and dipolar in shape, were used for source localization in the usual biomagnetic framework of the equivalent current dipole source representation. The results of this approach suggest that different specific cortical areas are repeatedly and randomly activated, involving mainly the frontal, occipital and temporal areas, often with a hemispheric prevalence.

Brain↗
PubMed1989

Neuromagnetic imaging studies discriminate proprioceptive and cutaneous cortical inputs during median nerve stimulation in man.

The localizing capabilities of the neuromagnetic imaging technique have been used to non-invasively discriminate mainly proprioceptive from cutaneous afferences to the contralateral hemisphere in human volunteers during separate median nerve stimulation at wrist and digits. The high time-resolution achieved in the localization of the equivalent sources of the early latency (15-30 ms) evoked fields permitted to follow their apparent movement as represented by an early and deep dipole (15 ms, about 5 cm deep, probably a subcortical source), a shallower one (20 ms, about 1.5 cm), and an intermediate one (23 ms, about 3 cm). These sources are supposed to be the manifestation of time-lagged parallel proprioceptive and cutaneous afferences from the thalamic ventro basal complex to different primary sensory areas in the postcentral gyrus. Both deeper and shallower dipoles could not be identified during selective stimulation of cutaneous afferents from the digital branches of the median nerve.

Cerebral Cortex↗
PubMed1989

Simultaneous motor output and sensory input: cortical interference site resolved in humans via neuromagnetic measurements.

Neuromagnetic measurements were carried out during median nerve stimulation at the wrist in complete relaxation (a) and during active contraction of the hand muscles (b). Firstly, activity of the generator source responsible for the major component of the sensory evoked magnetic fields (SEFs) was mapped and localized during condition (a) and (b). Then the subtraction maps ((a)-(b)) were obtained and the virtual 'dipole' responsible for the 'interference' (less than or equal to SEFs amplitude) between the afferent input and the motor output was tridimensionally localized in a position compatible with the knee and the convexity of the postcentral gyrus.

Cerebral Cortex↗
PubMed1989

Short latency somatosensory evoked responses to median nerve stimulation in healthy humans: electric and magnetic recordings.

Somatosensory Evoked Potentials (SEPs) and Somatosensory Evoked magnetic Fields (SEFs) to median nerve stimulation at wrist were recorded in 5 healthy subjects and the components between 15 and 30 ms after the stimulus were evaluated on the hemiscalp contralateral to the stimulated wrist. SEPs were measured by means of a 32-channel recorder and compared with SEFs obtained via multiple measurements with a 4-channel sensor. Equivalent dipole localization was carried out for the magnetic components peaking at about 15, 20 and 24 ms. The scalp distribution of SEPs, illustrated by bit mapped color images, were qualitatively explained by three separate sources. The first is described as a tangentially oriented dipole placed behind the Central Sulcus and responsible for the parietal N20-"late P25" waves and for the frontal P20-N30 ones. The second is represented by a radieal dipole placed just in front of the Central Sulcus and pointing towards the motor strip, responsible for the rolandic P22 component. The third is just behind the Central Sulcus and is radieally oriented towards the surface of the postcentral sensory area for the "early P25" parietal wave. The SEFs distributions, illustrated by color isofield contour maps, were quantitatively explained by a unique tangential dipole localized, with good resolution, well behind the Sulcus for the 15 ms waves and slightly frontal to this site for the waves peaking at around 20 and 24 ms. The equivalent dipole has been localized at a depth of about 5 cm (15 ms component), 2 cm (20 ms components) and 4 cm (24 ms component), across the studied subjects. It is stressed that the dipole responsible for the magnetic pattern is likely to be the same tangential dipole responsible for a part of the electric pattern. Due to their radieal orientation, the other two dipoles, proposed for the SEPs maps, would be mostly undetectable by a magnetic investigation.

Adult↗
PubMed1987

Novel data analysis for synchronised spontaneous neuromagnetic activity.

A novel approach to neuromagnetic data analysis is presented. This technique is aimed at studying synchronised spontaneous activity (SSA) and has been used to resolve two different signals from one single evoked response, providing evidence for two possibly distinct sources. The data presented are consistent with a model that permits the generators of spontaneous activity to be synchronised by sensory stimuli.

Brain↗
PubMed1987

Improved procedure for neuromagnetic localisation.

We describe an improved algorithm for localising equivalent sources of biomagnetic fields in the human brain. The algorithm is an improvement over the sphere model in that it considers two distinct surfaces: an ellipsoid, to model the region of the skull on which the sensors are placed, and a sphere as the medium in which the current dipole model is considered. This allows us to easily correct the formula of the magnetic field in order to take better account of the true position of the sensor with respect to the subject's head.

Algorithms↗
PubMed1987

Neuromagnetic evidence of synchronized spontaneous activity in the brain following repetitive sensory stimulation.

Neuromagnetic measurements in the visual and somatosensory modalities reveal that, following repetitive stimulation, the brain persists in emitting synchronized after-discharges in the form of oscillations with highly specific spectral composition. In the visual modality, this rhythmic activity is centered at the frequency of the resting alpha rhythm and it is most readily induced by stimulation of the same frequency. This suggests that the phenomenon is due to synchronization of the generators responsible for the natural rhythms by the steady-state stimulation and that these generators behave as resonant oscillators. The phenomenon could then be referred to as Synchronized Spontaneous Activity (SSA). The discovery of this phenomenon has important implications for modelling the dynamics of normal evoked and spontaneous cerebral activity as well as for the understanding of pathological conditions such as photically induced epilepsies.

Adult↗
PubMed1987

Neuromagnetic characterization of the cortical response to median nerve stimulation in the steady state paradigm.

Magnetic methods for localizing evoked neural activity in the human brain have been used to search for evidence of a functional organization in primary somatosensory cortex. We have found that in response to median nerve stimulation the deduced source for the evoked component with a latency of 45 ms lies at a depth in the central sulcus that depends on stimulus repetition rate, whereas the sources of the 20 ms and 24 ms components lie at a fixed depth. Moreover we have identified this 45 ms component with the dominant sinusoidal feature characteristic of steady state response.

Brain Mapping↗
PubMed1986

Principles and clinical validity of the biomagnetic method.

We present a review of the basic principles of the biomagnetic method, and of the most significant results obtained in the measurement of magnetic fields of the human heart and brain. Particular emphasis is given to the applications of the technique which are providing important progress to clinical investigation. We also point out the present technological perspectives and suggest areas where further efforts can be particularly convenient.

Brain↗
PubMedDate not supplied

Neuromagnetic investigation of synchronized spontaneous activity.

We present an overview of the investigations on Synchronized Spontaneous Activity (SSA) in the somatosensory and visual modality. The novelty of the stimulation paradigm and of the off-line data analysis, as well as the capability of the neuromagnetic technique to localize and follow in time the generators responsible for the SSA are used to characterize SSA and to study the underlying spontaneous activity. Synchronization of alpha and mu rhythms have been recognized and studied. The possibilities opened by the technique are discussed.

Brain↗