Biomedical subjects

F Varela

Publications and source records attributed to F Varela.

15 recordsLinked to original sources

PubMed2001

Anticipation of epileptic seizures from standard EEG recordings.

BACKGROUND: New methods derived from non-linear analysis of intracranial recordings permit the anticipation of an epileptic seizure several minutes before the seizure. Nevertheless, anticipation of seizures based on standard scalp electroencephalographical (EEG) signals has not been reported yet. The accessibility to preictal changes from standard EEGs is essential for expanding the clinical applicability of these methods. METHODS: We analysed 26 scalp-EEG/video recordings, from 60 min before a seizure, in 23 patients with temporal-lobe epilepsy. For five patients, simultaneous scalp and intracranial EEG recordings were assessed. Long-term changes before seizure onset were identified by a measure of non-linear similarity, which is very robust in spite of large artifacts and runs in real-time. FINDINGS: In 25 of 26 recordings, measurement of non-linear changes in EEG signals allowed the anticipation of a seizure several minutes before it occurred (mean 7 min). These preictal changes in the scalp EEG correspond well with concurrent changes in depth recordings. INTERPRETATION: Scalp-EEG recordings retain sufficient dynamical information which can be used for the analysis of preictal changes leading to seizures. Seizure anticipation strategies in real-time can now be envisaged for diverse clinical applications, such as devices for patient warning, for efficacy of ictal-single photon emission computed tomography procedures, and eventual treatment interventions for preventing seizures.

Electroencephalography↗

PubMed2001

The brainweb: phase synchronization and large-scale integration.

The emergence of a unified cognitive moment relies on the coordination of scattered mosaics of functionally specialized brain regions. Here we review the mechanisms of large-scale integration that counterbalance the distributed anatomical and functional organization of brain activity to enable the emergence of coherent behaviour and cognition. Although the mechanisms involved in large-scale integration are still largely unknown, we argue that the most plausible candidate is the formation of dynamic links mediated by synchrony over multiple frequency bands.

Animals↗

PubMed2001

Purification and characterization of insulin-mimetic inositol phosphoglycan-like molecules from grass pea (Lathyrus sativus) seeds.

BACKGROUND: Signal transduction through the hydrolysis of glycosyl-phosphatidylinositol (GPI) leading to the release of the water-soluble inositol phosphoglycan (IPG) molecules has been demonstrated to be important for mediating some of the actions of insulin and insulin-like growth factor-I (IGF-I). MATERIALS AND METHODS: In the present study, GPI from grass pea (Lathyrus sativus) seeds has been purified and partially characterized on the basis of its chromatographic properties and its compositional analysis. RESULTS: The results indicate that it shows similarities to GPI previously isolated from other sources such as rat liver. IPG was generated from L. sativus seed GPI by hydrolysis with a GPI-specific phospholipase D (GPI-PLD). This IPG inhibited protein kinase A (PKA) in an in vitro assay, caused cell proliferation in explanted cochleovestibular ganglia (CVG), and decreased 8-Br-cAMP-induced phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression in cultured hepatoma cells. CONCLUSIONS: Our data indicate that L. sativus seed IPG possess insulin-mimetic activities. This may explain why L. sativus seeds have been used in some traditional medicines to ameliorate diabetic symptoms.

Animals↗

PubMed2000

Spatio-temporal characterizations of non-linear changes in intracranial activities prior to human temporal lobe seizures.

Recent studies have shown that non-linear analysis of intracranial activities can detect a 'pre-ictal phase' preceding the epileptic seizure. Nevertheless, the dynamical nature of the underlying neuronal process and the spatial extension of this pre-ictal phase still remain unknown. In this paper, we address these aspects using a new non-linear measure of dynamic similarity between different parts of intracranial recordings of nine patients with medial temporal lobe epilepsy recorded during transitions to seizure. Our results confirm that non-linear changes in neuronal dynamics allow, in most cases (16 out of 17), a seizure anticipation several minutes in advance. Furthermore, we show that the spatial distribution of pre-ictal changes often involves an extended network projecting beyond the limits of the epileptogenic region. Finally, the pre-ictal phase could frequently (13 out of 17) be characterized with a marked shift toward slower frequencies in upper delta or theta frequency range.

Cerebral Cortex↗

PubMed1999

Anticipating epileptic seizures in real time by a non-linear analysis of similarity between EEG recordings.

In a previous publication we showed that non-linear analysis can extract spatio-temporal changes of brain electrical activity prior to epileptic seizures. Here we describe a new method to analyze this long-term non-stationarity in the EEG by a measure of dynamical similarity between different parts of the time series. We apply this method to the study of a group of patients with temporal lobe epilepsy recorded intracranially during transitions to seizure. We show that the method, which can be implemented on a personal computer, can track in real time spatio-temporal changes in brain dynamics several minutes prior to seizure.

Algorithms↗

PubMed1999

[Interactions between the epileptic network and brain function: an approach by nonlinear analysis of intracranial EEG].

Recent advances in the non-linear dynamics analysis have made it possible to identify hidden recurrences in EEG signals that could be missed by more traditional linear techniques such as power spectrum or coherence analysis. This is particularly true for epileptic EEG recordings either in animals or in humans as epileptic phenomena are usually concomitant with the emergence a strong non-linear EEG behavior. Non-linear dynamical analysis techniques quantify the relations between EEG signals. The literature concerning the spatio-temporal characteristics of the epileptic processes during seizures and interictal periods is reviewed. Our attention has been mainly focused on the interdependences between brain structures or on the dynamical changes of one particular brain region during intracranial recordings. These data could explain in part the dysfunctioning of the cerebral cortex induced by epileptic activities and provide an insight into the spatio-temporal organization of the epileptic network. Futhermore, by tracking the time variation of non-linear indices, one can anticipate the occurrence of seizures in temporal lobe epilepsies. All this information could contribute to improve definitions of the epileptogenic zone in partial epilepsy and also open the way to preventive interventions.

Brain↗

PubMed1997

[Predictive variables of early mortality after orthotopic heart transplant in adults].

OBJECTIVE: The purpose of the study was to analyze some variables of donors, recipients and surgical procedures in order to discover factors that could predict mortality during the early stage (< 30 days) of orthotopic heart transplants. MATERIAL AND METHOD: 125 consecutive orthotopic heart transplants in adults were analyzed. The average age was 51 +/- 11 (range: 12-67), 109 (87%) were men, 16 were women (13%). Two groups were compared: 15 patients who died within 30 days after heart transplant and 110 who survived during that period. Immunosuppressive protocol: preoperative: Cyclosporin + Azathioprine. Intraoperative: Methylprednisolone Postoperative: Methylprednisolone (first 24 h), antilymphocyte monoclonal antibodies (7-10 days after heart transplant) + Cyclosporin + Azathioprine + Corticoids. The following parameters of the recipient were analyzed: sex, age, weight, size, thoracic perimeter, pretransplant cardiopathy, previous thoracic operations, functional stage or need for catecholamines during the days prior to the transplant, pulmonary artery pressure and resistance, history of systemic arterial hypertension, elevation of creatinine, blood type, urgent transplant indication, receptor/donor weight relationship. The following parameters of donors and operation were analyzed: sex, age, weight, thoracic perimeter, period in intensive care unit, dose of dopamine and dobutamine, blood type, origin of the organ, cause of death, ischaemia time, cardiopulmonary by-pass time and cardioplegia type. RESULTS: The rate of early mortality was 12%. The univariate analysis showed differences in: prior cardiovascular surgery, receptor blood type, need for urgent transplantation, pulmonary artery resistance > 2.5 Wood Units, cardiopulmonary by-pass time, weight relationship between receptor and donor. The death cause of the donor proved significant. On multivariate analysis, the following parameters independently predicted early mortality: history of operation with extracorporeal circulation, high pulmonary artery resistance, urgent transplant, receptor/donor weight relation and time of extracorporeal circulation. CONCLUSIONS: We believe that the results of our experience can help to stratify the risk in the orthotopic heart transplant recipient and even to contraindicate the procedure in some cases showing an accumulation of poor prognostic factors in borderline recipients.

Adolescent↗

PubMed1996

Depression as a dynamical disease.

Mathematical models are helpful in the understanding of diseases through the use of dynamical indicators. A previous study has shown that brain activity can be characterized by a decrease of dynamical complexity in depressive subjects. The present paper confirms and extends these conclusions through the use of recent methodological advances: first episode and recurrent patients strongly differ in their dynamical response to therapeutic interventions. These results emphasize the need for clinical follow-ups to avoid recurrence and the necessity of specific therapeutic intervention in the case of recurrent patients.

Adult↗

PubMed1994

Development of an idiotypic network in shape space.

Based upon the shape-space formalism, a model of an idiotypic network including both bound and free immunoglobulins is simulated. Our point of interest is the network development in the context of self antigens. The investigations are organized around simulations initiated by various spatial configurations of antigens; the behavior of the system with respect to antigens is analyzed in terms of morphogenetic processes occurring in the shape space. For certain values of the parameters, the network expands by traveling waves. The resulting spatial pattern is a partition of the shape space into zones where introduction of an antigen entails an infinite growth of the clones binding to it, and into zones where, on the contrary, the anti-antigen idiotypes decrease. Among the parameter combinations tested, some produce a partition that remains static whereas others produce a partition that changes in time. For other values of the parameters, the patterns generated do not partition shape space into zones; in these cases, it is observed that the system systematically explodes when an antigen is present.

Animals↗

PubMed1991

Population dynamics of natural antibodies in normal and autoimmune individuals.

We have measured the quantities of naturally occurring autoantibodies in the serum of normal, unmanipulated individuals. These changes over time following broad-band complex dynamical patterns that are similar in mouse and man. The patterns more likely reflect the network architecture of the natural antibody repertoire, regulating the activation and decay of individual clones. The temporal changes of both disease-specific and nonspecific autoantibodies are consistently modified in autoimmune individuals.

Animals↗

PubMed1991

Role of the nucleus geniculatus lateralis ventralis (GLv) in the optokinetic reflex: a lesion study in the pigeon.

Several functions have been proposed for the avian GLv (color vision, pupillary reflex, optomotor mechanisms). In the present paper we have examined the role of the GLv in optomotor responses. For this purpose, horizontal and vertical optokinetic nystagmus (OKN) were quantified in response to different stimulation velocities, before and after chemical (kainic acid) lesions. Unilateral lesion of the GLv produced a marked increase of the horizontal OKN gain when the eye contralateral to the lesion was stimulated in the temporonasal (T-N) direction and, to a lesser extent, when the ipsilateral eye was stimulated in the naso-temporal (N-T) direction. Biocular integration was reduced after the lesion, since the biocular stimulation corresponding to these two monocular stimulations (ipsiversive to the lesion) produced only a moderate gain increase. When stimulations were delivered in the opposite direction (contraversive to the lesion), the horizontal OKN gain was slightly increased for the N-T monocular stimulation of the eye contralateral to the lesion, but was unchanged for other stimulations. A bilateral lesion of the GLv provoked only a slight increase of the horizontal OKN gain. The vertical OKN was not affected by the GLv lesions. Thus, the GLv system is probably involved in the modulation of optomotor responses and could mediate visuo-optokinetic interactions, each nucleus (and its associated system) exerting an inhibitory (or disfacilitatory) effect on the horizontal OKN in one direction.

Animals↗

PubMed1991

[Method for measuring sensitivity to periodic spectral energy modulations in human].

The distribution of spectral energy of a visual stimulus can be subject to Fourier analysis. In this perspective, we have built a device which produces periodic variations in energy (square waves) over the visible spectrum (400-700 nm), and where the amplitude, phase and frequency of the stimuli can be independently controlled. From the non-modulated spectrum, supplying a white spot, for a given frequency and phase, there is a minimal amplitude modulation (contrast threshold) for which the spot becomes chromatic. As an illustrative example we present here a curve of optimal sensitivity values (inverse of contrast) as a function of frequency (from 0.5 to 3.6 cycles/300 nm) for a normal subject.

Color Perception↗

PubMed1989

Evidence for a functional idiotypic network among natural antibodies in normal mice.

We monitored in normal adult BALB/c mice the serum concentrations of four natural IgM antibodies, two of which show idiotypic complementarity in in vitro assays. In each individual, serum concentration of all four idiotypes were found to fluctuate in complex dynamical patterns with low correlation. The spectral power of some such patterns was found to be compatible with the existence of a chaotic regime. Groups of normal adult mice were injected intravenously with low (10 ng) or moderate (10 micrograms) doses of either of the two complementary idiotypes in saline. This treatment resulted in a pronounced inhibition of the fluctuation in the serum concentration of both complementary idiotypes for periods up to 3 months. Such compensations were not detected for the two unrelated natural idiotypes and were specifically induced, for they did not occur following the injection of unrelated antibodies. These results indicate the functional operation of an idiotypic network among natural antibodies.

Animals↗

PubMed1984

Why the cortical magnification factor in rhesus is isotropic.

We examine the arguments advanced recently [by Sakitt (Vision Res. 22, 417-421, 1982), and Tootell et al. (Science 218, 902-904, 1982)] leading to the conclusion that the retino-cortical magnification factor cannot be isotropic. The relevance of the issue is discussed through the use of complex function theory as the appropriate tool to express the geometry of interconnections between neural lamina. A general expression is obtained to compute the length of a curve under a conformal transformation. When this result is applied to circles in the visual field, it is shown that Sakitt's findings imply radial asymmetry of the retino-cortical map, and not local anisotropy, as claimed. In fact, by taking Schwartz's (Vision Res. 20, 645-670, 1980) proposal that the complex log map including a constant eccentricity term be used to represent the retino-cortical mapping, we show that both local isotropy and radial asymmetry are simultaneously valid. Furthermore, the values of cortical length predicted from this approach account exactly for the data shown by Sakitt. A quantitative prediction is offered also for the ratio between the magnification factor of vertical and horizontal meridians at different eccentricities, which is precisely the value reported by Tootell et al. We conclude that these results show that (1) conformal transformations are appropriate tools to describe the geometrical interconnections between neural lamina; (2) for the case of the retino-cortical mapping the appropriate conformal transform is an eccentric complex logarithm.

Animals↗

PubMedDate not supplied

Thrombocytopathia and light-chain proteinuria in a dog naturally infected with Ehrlichia canis.

A 6-year-old dog was presented for evaluation of recurrent epistaxis. Platelet counts, biochemical tests, and coagulation tests were within the normal range, but a mucosal bleeding time was prolonged; there was hyperproteinemia and a monoclonal gammopathy. Heterogeneity of light chains appeared in urine, however, thus suggesting that the gammopathy was polyclonal. Platelet aggregation tests showed decreased responsiveness to collagen. An Ehrlichia canis indirect fluorescent-antibody titer was positive (1:40). Treatment with tetracycline, melphalan, and prednisone resulted in a rapid clinical improvement that persisted for at least 3 years.

Animals↗