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Biomedical subjects

F Lopes da Silva

Publications and source records attributed to F Lopes da Silva.

9 recordsLinked to original sources
PubMed1991

Neural mechanisms underlying brain waves: from neural membranes to networks.

In this review, a number of experimental findings and theoretical concepts that have led to new insights into the mechanisms underlying brain waves are presented. At the cellular level, the new evidence that certain types of neuron have intrinsic oscillatory properties that may underlie rhythmic EEG activities is discussed. In particular, the question of whether spindle oscillations are autonomous or input-dependent is addressed. At the neural network level, the main circuits of the thalamus and cortex that are responsible for the occurrence and modulation of spindles and alpha activity are described. In addition, the properties of rhythmic activities outside the alpha band are considered, particularly in relation to the prominent beta activity of the visual cortex. At the theoretical level, the possibility that neural networks may behave as complex dynamic systems with the properties of deterministic chaos is discussed. Finally, the fact that brain rhythms may have functional implications for the working of neural networks is examined in relation to 2 cases: the possibility that oscillations may subserve a gating function, and that oscillations may play a role in the formation of assemblies of neurons that represent given stimulus patterns.

Alpha Rhythm↗
PubMed1990

The role of hippocampal commissures in the interhemispheric transfer of epileptiform afterdischarges in the rat: a study using linear and non-linear regression analysis.

The role of the forebrain commissures and the septal area in the interhemispheric transfer of hippocampal afterdischarges (ADs) was investigated in the rat under halothane anesthesia. Electrical seizures were elicited from the dorsal hippocampus before and after commissurotomy. The degree of relatedness between EEG signals recorded from homologous sites of both hippocampi was quantified using two approaches: (i) a time domain analysis considering an AD as a succession of discrete bursts; the onset times of such bursts were measured and used to estimate interhemispheric onset delays; (ii) using signal analysis the linear (r2) and non-linear (h2) regression coefficients between pairs of EEG signals were computed as a function of time shift between the two signals. In this way the values of association (linear and non-linear) and the corresponding time delays were measured. In general a tetanus applied unilaterally to the dorsal CA3 field resulted in bilaterally synchronous ADs. The estimated interhemispheric time delay was in most cases zero. This bilateral synchrony disappeared after section of a specific part of the ventral hippocampal commissure (VHC), the dorso-caudal third, but was not affected by section of other commissural fibers or by a lesion of the septal area. This study also allowed evaluation of different methods of quantification of the association between EEG signals, namely the linear (r2) and the non-linear (h2) regression coefficients. The latter was shown to be a more robust measure than the former and to yield values of association even in cases in which r2 was at noise level. The experimental findings allow the conclusion that ADs elicited from an epileptogenic focus spread to homologous sites in the contralateral hemisphere following commissural systems that may be strong enough to ensure the forming of one bilateral oscillating system.

Animals↗
PubMed1983

Spike detection based on a pattern recognition approach using a microcomputer.

A system for the detection of spikes and sharp waves (SSWs) in EEGs of epileptic patients is described. The system uses a pattern recognition technique and it is implemented in a hybrid microcomputer. It performs the on-line simultaneous analysis of 4 EEG channels and thus it is appropriate for long-term EEG recording. The performance of the system was considered satisfactory when compared with that of a panel of electroencephalographers.

Action Potentials↗
PubMed1981

Telemetric EEG and video monitoring in epilepsy.

We report a clinical evaluation of EEG and video monitoring. In 181 consecutive records, the most common clinical indications were differentiation of epileptic and nonepileptic events (99), seizure recordings for locating a possible focus (23), seizure frequency determination (32), and investigation of possible trigger factors (19). Overall, useful information was obtained in 72% and the clinical question was answered in 67% of records. Prolonged EEG and video monitoring is therefore an important diagnostic aid in patients with proven or suspected epilepsy.

Electroencephalography↗
PubMedDate not supplied

Interdependence of EEG signals: linear vs. nonlinear associations and the significance of time delays and phase shifts.

To investigate the degree of interdependence of EEG signals, we have to use signal analysis methods. Three of these are described and their performance is compared: the cross-correlation (coherence and phase), the average amount of mutual information (AAMI) or the normalized AAMI, also called transmission coefficient T, and the correlation ratio h2 that is a general measure of nonlinear fit between any two signals. The three methods were applied to simulated and real signals in order to put in evidence how nonlinear relationships may affect differently these three measures of association. The nature of the interdependence between EEG signals is not characterized only by the degree of association, but also by the corresponding phase relationship. A basic question is whether such a phase shift can be interpreted as a transmission delay. However, a fundamental problem is that a phase shift may be difficult to interpret in terms of a biophysical model. A procedure is described in order to solve this problem. This involves computing the phase spectrum between the pair of signals, estimating the gain of the corresponding linear transfer function and the associated minimum phase. By subtracting the minimum phase from the phase spectrum, a corrected phase function can be obtained. From the slope of this phase function, a transmission delay can be estimated. This procedure is illustrated by applications to simulated and real EEG signals. It is demonstrated that from phase shifts we may estimate transmission delays between at least certain classes of EEG signals. In this way we can asses, unambiguously, how the transmission of information between different brain sites develops.

Electroencephalography↗
PubMedDate not supplied

Spreading of epileptic afterdischarges between entorhinal cortex and hippocampus in acute experiments and the kindling model of epilepsy in the rat--comparing different methods of analysis.

Spreading of epileptiform activity in the central nervous system is one of the fundamental problems in epileptology. The patterns of spreading of after-discharges in the hippocampus and entorphinal cortex were studied in acute experiments and using the kindling model of epileptogenesis. Three methods were used to determine the time relations between EEG signals from different brain areas; visual inspections, average amount of mutual information (AAMI) and phase spectrum method. The analysis methods used are adequate for quantification of the degree of coupling between different EEG signals during an afterdischarge, but should be used jointly since different signal features are taken into consideration by different methods. During an afterdischarge only at the beginning the focal area is clearly leading the other brain areas; thereafter the pattern becomes more complex.

Animals↗