Biomedical subjects

F Babiloni

Publications and source records attributed to F Babiloni.

At least 37 records · Page 2Linked to original sources

PubMed2000

High-resolution electro-encephalogram: source estimates of Laplacian-transformed somatosensory-evoked potentials using a realistic subject head model constructed from magnetic resonance images.

A novel high-resolution electro-encephalographic (EEG) procedure is proposed, including high spatial sampling (128 channels), a realistic magnetic resonance-constructed subject head model, a multi-dipole cortical source model and regularised weighted minimum-norm linear inverse source estimation (WMN). As an innovation, EEG potentials (two healthy subjects; median-nerve, short-latency somatosensory-evoked potentials (SEPs)) are preliminarily Laplacian-transformed (LT) to remove brain electrical activity generated by subcortical sources (i.e. not represented in the source model). LT-WMN estimates are mathematically evaluated by figures of merit (WMN estimates as a reference). Results show higher dipole identifiability (0.69; 0.088), lower dipole localisation error (0.6 mm; 7.8 mm) and lower spatial dispersion (8.6 mm; 24 mm) in LT-WMN than in WMN estimates (Bonferroni corrected p < 0.001). These estimates are presented on the subject modelled cortical surface to highlight the increased spatial information content in LT-WMN compared with WMN estimates. The proposed high-resolution EEG technique is useful for the study of somatosensory functions in basic research and clinical applications.

Electroencephalography↗

PubMed2000

Linear classification of low-resolution EEG patterns produced by imagined hand movements.

Electroencephalograph (EEG)-based brain-computer interfaces (BCI's) require on-line detection of mental states from spontaneous EEG signals. In this framework, surface Laplacian (SL) transformation of EEG signals has proved to improve the recognition scores of imagined motor activity. The results we obtained in the first year of an European project named adaptive brain interfaces (ABI) suggest that: 1) the detection of mental imagined activity can be obtained by using the signal space projection (SSP) method as a classifier and 2) a particular type of electrodes can be used in such a BCI device, reconciling the benefits of SL waveforms and the need for the use of few electrodes. Recognition of mental activity was attempted on both raw and SL-transformed EEG data from five healthy people performing two mental tasks, namely imagined right and left hand movements.

Adult↗

PubMed2000

Integration of high resolution EEG and functional magnetic resonance in the study of human movement-related potentials.

Cortical sources of human movement-related potentials (i.e. unilateral finger extension) were modeled using functional magnetic resonance imaging (fMR) data as a constraint of a linear inverse source estimation from highly sampled (128 channels) EEG data. Remarkably, this estimation was performed within realistic subject's MR-constructed head models by boundary element techniques. An appropriate figure of merit served to set the optimal amount of fMR constraints. With respect to standard linear inverse source estimates, fMR-constrained ones presented increased spatial detail and provided a more reliable timing of activation in bilateral sensorimotor cortical regions of interest.

Adult↗

PubMed1999

"Gating" of human short-latency somatosensory evoked cortical responses during execution of movement. A high resolution electroencephalography study.

The present study aimed at investigating gating of median nerve somatosensory evoked cortical responses (SECRs), estimated during executed continuous complex ipsilateral and contralateral sequential finger movements. SECRs were modeled with an advanced high resolution electroencephalography technology that dramatically improved spatial details of the scalp recorded somatosensory evoked potentials. Integration with magnetic resonance brain images allowed us to localize different SECRs within cortical areas. The working hypothesis was that the gating effects were time varying and could differently influence SECRs. Maximum statistically significant (p<0. 01) time-varying gating (magnitude reduction) of the short-latency SECRs modeled in the contralateral primary motor and somatosensory and supplementary motor areas was computed during the executed ipsilateral movement. The gating effects were stronger on the modeled SECRs peaking 30-45 ms (N30-P30, N32, P45-N45) than 20-26 ms (P20-N20, P22, N26) post-stimulus. Furthermore, the modeled SECRs peaking 30 ms post-stimulus (N30-P30) were significantly increased in magnitude during the executed contralateral movement. These results may delineate a distributed cortical sensorimotor system responsible for the gating effects on SECRs. This system would be able to modulate activity of SECR generators, based on the integration of afferent somatosensory inputs from the stimulated nerve with outputs related to the movement execution.

Adult↗

PubMed1999

Bilateral neuromagnetic activation of human primary sensorimotor cortex in preparation and execution of unilateral voluntary finger movements.

Extracranial magnetoencephalographic activity was separately recorded (25 channels) from bilateral primary sensorimotor cortex (M1-S1) of normal right-handers during unilateral finger movements. Standard dipole analysis indicated only a contralateral M1-S1 source for first movement-evoked field (MEF1) peaking at about 115 ms after electromyographic onset. However, the subtraction of the magnetic field generated by this source from the recorded magnetic field disclosed a low-amplitude ipsilateral central-parietal MEF1 that was explained by an ipsilateral M1-S1 source.

Adult↗

PubMed1999

Human movement-related potentials vs desynchronization of EEG alpha rhythm: a high-resolution EEG study.

Movement-related potentials (MRPs) and event-related desynchronization (ERD) of alpha rhythm were investigated with an advanced high-resolution electroencephalographic technology (128 channels, surface Laplacian estimate, realistic head modeling). The working hypothesis was that MRPs and alpha ERD reflect different aspects of sensorimotor cortical processes. Both MRPs and alpha ERD modeled the responses of primary sensorimotor (M1-S1), supplementary motor (SMA), and posterior parietal (PP, area 5) areas during the preparation and execution of unilateral finger movements. Maximum responses were modeled in the contralateral M1-S1 during both preparation and execution of the movement. The SMA and PP responses were modeled mainly from the MRPs and alpha ERD, respectively. The modeled ipsilateral M1-S1 responses were larger and stronger in the alpha ERD than MRPs. These results may suggest that alpha ERD reflects changes in the background oscillatory activity in wide cortical sensorimotor areas, whereas MRPs represent mainly increased, task-specific responses of SMA and contralateral M1-S1.

Adult↗

PubMed1998

Improved realistic Laplacian estimate of highly-sampled EEG potentials by regularization techniques.

In this study we investigated the effects of lambda correction, generalized cross-validation (GCV), and Tikhonov regularization techniques on the realistic Laplacian (RL) estimate of highly-sampled (128 channels) simulated and actual EEG potential distributions. The simulated EEG potential distributions were mathematically generated over a 3-shell spherical head model (analytic potential distributions). Noise was added to the analytic potential distributions to mimic EEG noise. The magnitude of the noise was 20, 40 and 80% that of the analytic potential distributions. Performance of the regularization techniques was evaluated by computing the root mean square error (RMSE) between regularized RL estimates and analytic surface Laplacian solutions. The actual EEG data were human movement-related and short-latency somatosensory-evoked potentials. The RL of these potentials was estimated over a realistically-shaped, magnetic resonance-constructed model of the subject's scalp surface. The RL estimate of the simulated potential distributions was improved with all the regularization techniques. However, the lambda correction and Tikhonov regularization techniques provided more precise Laplacian solutions than the GCV computation (P < 0.05); they also improved better than the GCV computation the spatial detail of the movement-related and short-latency somatosensory-evoked potential distributions. For both simulated and actual EEG potential distributions the Tikhonov and lambda correction techniques provided nearly equal Laplacian solutions, but the former offered the advantage that no preliminary simulation was required to regularize the RL estimate of the actual EEG data.

Adult↗

PubMed1998

Dynamic functional coupling of high resolution EEG potentials related to unilateral internally triggered one-digit movements.

Between-electrode cross-covariances of delta (0-3 Hz)- and theta (4-7 Hz)-filtered high resolution EEG potentials related to preparation, initiation. and execution of human unilateral internally triggered one-digit movements were computed to investigate statistical dynamic coupling between these potentials. Significant (P < 0.05, Bonferroni-corrected) cross-covariances were calculated between electrodes of lateral and median scalp regions. For both delta- and theta-bandpassed potentials, covariance modeling indicated a shifting functional coupling between contralateral and ipsilateral frontal-central-parietal scalp regions and between these two regions and the median frontal-central scalp region from the preparation to the execution of the movement (P < 0.05). A maximum inward functional coupling of the contralateral with the ipsilateral frontal-central-parietal scalp region was modeled during the preparation and initiation of the movement, and a maximum outward functional coupling during the movement execution. Furthermore, for theta-bandpassed potentials, rapidly oscillating inward and outward relationships were modeled between the contralateral frontal-central-parietal scalp region and the median frontal-central scalp region across the preparation, initiation, and execution of the movement. We speculate that these cross-covariance relationships might reflect an oscillating dynamic functional coupling of primary sensorimotor and supplementary motor areas during the planning, starting, and performance of unilateral movement. The involvement of these cortical areas is supported by the observation that averaged spatially enhanced delta- and theta-bandpassed potentials were computed from the scalp regions where task-related electrical activation of primary sensorimotor areas and supplementary motor area was roughly represented.

Adult↗

PubMed1998

Responses of human primary sensorimotor and supplementary motor areas to internally triggered unilateral and simultaneous bilateral one-digit movements. A high-resolution EEG study.

We modelled the responses of human primary sensorimotor areas and supplementary motor area to simple, self-initiated unilateral and simultaneous bilateral middle finger movements using a novel high-resolution electroencephalography technology. The results support the view that these cortical motor areas are involved in parallel and present similar activity in the preparation, initiation, and execution of the contralateral and bilateral movements. Furthermore, the left primary sensorimotor area (dominant hemisphere) appears to be activated more than the right primary sensorimotor area during the preparation and performance of the ipsilateral movements.

Adult↗

PubMed1997

Human short latency cortical responses to somatosensory stimulation. A high resolution EEG study.

Human short-latency cortical responses to median nerve stimulation were investigated with a new high resolution electroencephalography technology that markedly enhanced spatial details of somatosensory-evoked potentials (SEPs). Maximum amplitude potentials were estimated over contralateral and/or frontal-mesial scalp regions about 20, 22, 24, 26, 30, 32 and 45 ms following the stimulation. Frontal-lateral P20-N24-N30-P45 and parietal-lateral N20-P24-P30-N45 showed dipolar patterns, whereas frontal-mesial N24-N30-P45 and central-lateral P22-N26-N32-P45 presented no clearcut dipole counterpart. Plausibly, the spatially enhanced frontal-parietal SEP components were generated (tangential dipoles) within the lateral central sulcus cortex, and anticipated the central-lateral and frontal-mesial components generated (radial dipoles) from the crown of the pre- and/or post-central gyri and the supplementary motor area, respectively.

Cerebral Cortex↗

PubMed1997

Topography of spatially enhanced human short-latency somatosensory evoked potentials.

In the present study modern high resolution electroencephalography (EEG) was used to spatially enhance human median nerve short-latency somatosensory evoked potentials (SEPs). It was shown that the spatially enhanced N30 consisted of two frontal subcomponents, one located in the frontal-lateral area of the scalp, the other located in the frontal-mesial area. Both of these subcomponents were most reduced in amplitude (or disappeared) during concomitant hand movement ipsilateral to the stimulus, but were differentially influenced by executed contralateral movement and imagined ipsilateral movement. These results support the hypothesis of an involvement of the frontal-mesial cortex (including the supplementary motor area) in the generation of the frontal N30.

Brain↗

PubMed1997

A high resolution EEG method based on the correction of the surface Laplacian estimate for the subject's variable scalp thickness.

To improve the spatial resolution of human event-related potentials, we developed a new high resolution EEG method based on the improved estimate of the realistic surface Laplacian (SL). The novelty of this method consisted in the computation of the local scalp resistance that was assumed to be inversely proportional to the local scalp thickness measured from magnetic resonance images of the subject's head. The local scalp thickness was then multiplied by the SL estimate of the potential over a realistic magnetic resonance-constructed model of the subject's scalp surface. The new method was applied on human movement-related and somatosensory-evoked potentials, the SL estimate at a constant scalp thickness being used as a reference. The locally-predicted scalp thickness was significantly (P < 0.05) higher in the temporal areas (9.5 +/- 2.6 mm) than in the parieto-occipital (6.6 +/- 1.3 mm) and frontal (4.8 +/- 1.1 mm) areas. Compared to the SL estimate at constant scalp thickness, the improved SL estimate enhanced the spatial detail of both movement-related and somatosensory-evoked potentials.

Brain Mapping↗

PubMed1997

High resolution EEG: a new model-dependent spatial deblurring method using a realistically-shaped MR-constructed subject's head model.

This paper presents a new model-dependent method for the spatial deblurring of scalp-recorded EEG potentials based on boundary-element and cortical imaging techniques. This model-dependent spatial deblurring (MDSD) method used MR images for the reconstruction of the subject's head model, and a layer of 364 radially-oriented equivalent current dipoles as a source model. The validation of the MDSD method was performed on simulated potential distributions generated from equivalent dipoles oriented radially, obliquely, and tangentially to the head surface. Furthermore, this method was used to localize neocortical sources of human movement-related and somatosensory-evoked potentials. It was shown that the new MDSD method improved markedly the spatial resolution of the simulated surface potentials and scalp-recorded event-related potentials. The spatial information content of the scalp-recorded EEG potentials increased progressively by increasing the spatial sampling from 28 to 128 channels. These results indicate that the new method could be satisfactorily used for high resolution EEG studies.

Brain↗

PubMed1996

Human cortical activity related to unilateral movements. A high resolution EEG study.

In the present study a modern high resolution electroencephalography (EEG) technique was used to investigate the dynamic functional topography of human cortical activity related to simple unilateral internally triggered finger movements. The sensorimotor area (M1-S1) contralateral to the movement as well as the supplementary motor area (SMA) and to a lesser extent the ipsilateral M1-S1 were active during the preparation and execution of these movements. These findings suggest that both hemispheres may cooperate in both planning and production of simple unilateral volitional acts.

Adult↗

PubMed1996

Spline Laplacian estimate of EEG potentials over a realistic magnetic resonance-constructed scalp surface model.

This paper presents a realistic Laplacian (RL) estimator based on a tensorial formulation of the surface Laplacian (SL) that uses the 2-D thin plate spline function to obtain a mathematical description of a realistic scalp surface. Because of this tensorial formulation, the RL does not need an orthogonal reference frame placed on the realistic scalp surface. In simulation experiments the RL was estimated with an increasing number of "electrodes" (up to 256) on a mathematical scalp model, the analytic Laplacian being used as a reference. Second and third order spherical spline Laplacian estimates were examined for comparison. Noise of increasing magnitude and spatial frequency was added to the simulated potential distributions. Movement-related potentials and somatosensory evoked potentials sampled with 128 electrodes were used to estimate the RL on a realistically shaped, MR-constructed model of the subject's scalp surface. The RL was also estimated on a mathematical spherical scalp model computed from the real scalp surface. Simulation experiments showed that the performances of the RL estimator were similar to those of the second and third order spherical spline Laplacians. Furthermore, the information content of scalp-recorded potentials was clearly better when the RL estimator computed the SL of the potential on an MR-constructed scalp surface model.

Brain Mapping↗

PubMed1995

Performances of surface Laplacian estimators: a study of simulated and real scalp potential distributions.

This paper presents a study of the performance of various local and spherical spline methods currently in use for the surface Laplacian (SL) estimate of scalp potential distributions. The SL was estimated from simulated instantaneous event-related scalp potentials generated over a three-shell spherical head model. Laplacian estimators used planar and spherical scalp models. Noise of increasing magnitude and spatial frequency was added to the potential distributions in order to simulate noise presumed to contaminate scalp-recorded event-related potentials. A comparison of noise effects on various Laplacian estimates was made for increasing number of "electrode" positions in variants of the 10-20 system. Furthermore, to evaluate the error due to the use of unrealistic scalp models, the matching between SL estimates of human scalp-recorded movement-related potentials computed on spherical and realistically-shaped MRI-constructed models of the scalp was examined. With all methods the error of the SL estimate increased proportionally with the magnitude and spatial frequency of noise. Increased number of "electrodes" up to 256 significantly reduced the error (p < 0.05). In general, the best SL estimates were computed by second and third order splines including lambda correction, the performances of the second order spline being better with more than 64 "electrodes". Compared with spline Laplacians, the best local methods provided nearly equal estimates with low spatial sampling (19 and 28 "electrodes"), as well as high spatial frequency noise. The error of the SL estimate due to unrealistic scalp model was significant, and it augmented with increased spatial sampling from 64 to 128 electrodes.

Brain↗

PubMed1995

Reduction of ocular artefacts in source current density brain mappings by ARX2 filtering.

Sweep by sweep analysis of event-related potentials (ERP) of the human scalp represents a reliable tool for both the diagnosis of neurologic diseases and the study of the central nervous system during cognitive tasks. The off-line procedure based on stochastic parametric identification and filtering herewith described, allows an accurate analysis of single-sweep ERP and a drastic reduction of ocular artefacts variously propagating through the skull. Moreover, the spatial distribution of the recorded ERP in bidimensional form was enhanced by using the Laplacian operator in order to get an estimate of the source current density (SCD) flow from the skull into the scalp. Complete single-trial signals were filtered according to an autoregressive model of signal generation with 2 exogenous inputs (ARX2). The ARX2 procedure models the recorded signal as the sum of three signals: (a) the background EEG activity, modelled as an autoregressive process driven by a white noise; (b) a filtered version of a reference signal carrying the average information contained in each sweep; (c) a signal due to the ocular artefact propagation. The evaluation of the effect of artefact suppression on those channels close to the eyes was compared with standard ordinary least squares method (OLS) based on a linear model of the influence of EOG on ERP. Finally, the better results obtainable through ARX filtering on sweep-by-sweep brain mappings are also presented.

Adult↗

PubMed1994

Statistical analysis of topographic maps of short-latency somatosensory evoked potentials in normal and parkinsonian subjects.

This work had the following objectives: i) to integrate temporal analysis (N30 peak) with power-spectrum topographic mapping of short-latency somatosensory evoked potentials (SEP's) recorded in parkinsonian and normal control subjects; and ii) to analyze with a new statistical approach the between-group topographical differences in both the time and frequency domains. The principal aim was to better determine the topography of the scalp frontal areas where the amplitude of the N30 wave was previously found to be significantly reduced in parkinsonians. The statistical procedure was based on the combined use of descriptive data analysis (DDA) and multivariate analysis. In the context of DDA, an improved version of significance probability mapping (SPM) was used by which it is possible to evaluate homo- and nonhomoscedastic data with parametric tests. The statistical evaluation of between-group differences was performed with the multivariate Hotelling's T2 test and the associated post hoc test. With this statistical procedure, it was possible to determine that the between-group statistical differences in both the temporal and power spectrum distributions were localized only in midline and contiguous contralateral frontal areas of the scalp.

Brain Mapping↗