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G Zouridakis

Publications and source records attributed to G Zouridakis.

27 records · Page 2Linked to original sources
PubMed1997

A fuzzy clustering approach to EP estimation.

The problem of extracting a useful signal (a response) buried in relatively high amplitude noise has been investigated, under the conditions of low signal-to-noise ratio. In particular, we present a method for detecting the "true" response of the brain resulting from repeated auditory stimulation, based on selective averaging of single-trial evoked potentials. Selective averaging is accomplished in two steps. First, an unsupervised fuzzy-clustering algorithm is employed to identify groups of trials with similar characteristics, using a performance index as an optimization criterion. Then, typical responses are obtained by ensemble averaging of all trials in the same group. Similarity among the resulting estimates is quantified through a synchronization measure, which accounts for the percentage of time that the estimates are in phase. The performance of the classifier is evaluated with synthetic signals of known characteristics, and its usefulness is demonstrated with real electrophysiological data obtained from normal volunteers.

Algorithms↗
PubMed1994

Selective averaging of evoked potentials using trajectory-based clustering.

A clustering method has been developed to group evoked potentials that display similar prestimulus dynamic behavior. The procedure involves using the method of time delay embedding to construct a trajectory in state space from a time series. Certain features that characterize the geometry of the trajectory have been defined. The trajectory-based clustering algorithm has been applied to visual evoked potentials to determine relationships between prestimulus EEG and evoked potential shape.

Cluster Analysis↗
PubMed1993

A neurophysiologically-based mathematical model of flash visual evoked potentials.

Evidence is presented that a neurophysiologically-inspired mathematical model, originally developed for the generation of spontaneous EEG (electroencephalogram) activity, can produce VEP (visual evoked potential)-like waveforms when pulse-like signals serve as input. It was found that the simulated VEP activity was mainly due to intracortical excitatory connections rather than direct thalamic input. Also, the model-generated VEPs exhibited similar relationships between prestimulus EEG characteristics and subsequent VEP morphology, as seen in human data. Specifically, the large correlation between the N1 amplitude and the prestimulus alpha phase angle, and the insensitivity of P2 to the latter feature, as observed in actual VEPs to low intensity flashes, was also found in the model-generated data. These findings provide support for the hypothesis that the spontaneous EEG and the VEP are generated by some of the same neural structures and that the VEP is due to distributed activity, rather than dipolar sources.

Cerebral Cortex↗
PubMed1993

The P50 component of the auditory evoked potential and subtypes of schizophrenia.

Decreased amplitude of the P50 component of the averaged evoked potential has been reported in schizophrenic patients. In an attempt to determine the relationship of this decrease to subtype diagnosis, we compared P50 amplitudes in 24 neuroleptic-free schizophrenic patients with paranoid (n = 13) versus nonparanoid (n = 11) subtype diagnoses. Eleven normal subjects and 11 cocaine users served as control groups. The schizophrenic patients were studied again after they had been treated with neuroleptics for 2 weeks. The control groups were studied again at least 2 weeks later. At baseline, the nonparanoid patients had significantly lower P50 amplitudes than did the normal subjects. The paranoid patients did not differ from the normal control subjects. The cocaine users had significantly decreased P50 amplitudes as compared with the normal control subjects. Neuroleptic treatment had no effect on P50 amplitudes in the paranoid patients but normalized amplitudes in the nonparanoid patients. The data suggest that P50 measurements may be useful in identifying subtypes of schizophrenia.

Acoustic Stimulation↗
PubMed1991

Test-retest reliability of the P50 mid-latency auditory evoked response.

Attenuation in mid-latency auditory evoked responses (MLAERs) can be used to study sensory gating. If paired-click stimuli (S1 and S2) are used, lower amplitude in response to S2 vs. S1 (attenuation) is considered evidence for intact sensory gating. However, the need for reliable measurements of MLAER amplitude and attenuation is a recognized problem. Ten normal volunteers were studied six times each. An S1 amplitude test-retest reliability coefficient (r) of 0.585 was obtained when means of two recordings were used vs. reliability coefficients as high as 0.809 for means of six recordings. Averaging a higher number of runs (120 vs. 60) resulted in a reliability coefficient of 0.677/recording. Similar values were obtained for S1 and S2 latencies. Reliability coefficients for S2 attenuation (S2/S1) were not nearly as high (a value of 0.138 when means of all six recordings were used). The S1 amplitude as measured in this study (with 120 averages) appears to be a reliable psychophysiologic measurement, but the S2/S1 attenuation measure is more variable, perhaps reflecting a greater sensitivity of the S2/S1 to uncontrolled variables in this study. Further research to identify such variables is necessary.

Adult↗
PubMed1991

Replication and extension of P50 findings in schizophrenia.

In a paired click "conditioning (S1), Testing (S2)" paradigm the amplitudes of responses to (S1) as well as the degree of attenuation of S2 as compared to S1 (S2/S1) were studied in two schizophrenic groups. Thirteen undifferentiated/disorganized (US) and thirteen paranoid (PS) patients were compared to thirteen age and sex matched normal volunteers. The US patients had significantly lower (S1) response amplitudes (P less than 0.001), as well as degree of attenuation of the response to (S2) (P less than 0.001) than the other two groups. No significant differences were found between the PS and N groups. Our data replicates the prior finding of decreased attenuation of the amplitude of the P50 wave in a paired click paradigm in schizophrenia. In addition, we showed that this phenomenon is significant mainly in the disorganized/undifferentiated patients as compared to the paranoid schizophrenics.

Evoked Potentials, Auditory↗
PubMedDate not supplied

Intraoperative neurophysiological monitoring. Part 1: General overview.

Intraoperative electrophysiological recordings are gradually becoming part of standard medical practice, mainly because they offer an objective and effective way to assess the functional integrity of the nervous system of a patient during the course of an orthopedic, neurological, or vascular surgery. Continuous monitoring of spontaneous and triggered bioelectrical activity not only can avert damage of neurological structures that are at risk during certain surgical maneuvers, but also allows identification of specific neuronal structures and landmarks that cannot be easily recognized on anatomical grounds only. This series on neurophysiological monitoring will introduce various techniques of monitoring available today, the rationale for their intraoperative use, and the main principles on which they are based. Recommendations for proper implementation and troubleshooting will also be given. The present article gives a general overview of the procedures.

Cost-Benefit Analysis↗
PubMedDate not supplied

Intraoperative neurophysiological monitoring. Part 2: Neurophysiological background.

Continuous intraoperative monitoring of neurophysiological signals provides a reliable way to determine the functional integrity of the nervous system during the course of neurological, orthopedic, or vascular surgery. To understand the use of electrophysiological recording in the operating room, the basic mechanisms that give rise to the recorded signals are briefly reviewed. Additionally, the equipment necessary for proper signal recording and presentation of the results is described. Finally, a short introduction to anesthesia management and some commonly induced neuroprotective conditions, such as hypothermia and hypotension, are provided, since these are the main factors affecting the signals, thus contributing to the difficulty of correct interpretation of the recordings during the course of an operation.

Anesthesia↗