PubMed Health⌕ Search

Biomedical subjects

D Popivanov

Publications and source records attributed to D Popivanov.

At least 19 recordsLinked to original sources
PubMed1996

Method for single-trial readiness potential identification, based on singular spectrum analysis.

This paper presents a brief description and the advantages of the singular spectrum analysis (SSA). SSA has recently been recommended for analysis of short, noisy time series. The method was applied to single trials of EEG activity at Cz, Fz, C4, Pz and C3, recorded 3.6 s prior to and 1.2 s after the onset of a voluntary motor act. This specific activity is known in its averaged form as readiness potential (RP) and is considered to reflect the preparation of the voluntary movement. However, some physiological investigations require identification of the parameters of single-trial RPs-their onset and successive phases. SSA is based on analysis of the principal components in vector space of delay coordinates of time series. As a result, SSA algorithm decomposes real data records to components reflecting the trend, alpha and beta frequency bands, respectively, whose oscillations appear or disappear in different time instants. The latter were interpreted to distinguish different dynamical stages of the movement preparatory process.

Adult↗
PubMed1996

Nonlinear prediction as a tool for tracking the dynamics of single-trial readiness potentials.

The performance of a voluntary act is preceded by an intrinsic process of intention and preparation accompanied by specific patterns of scalp-recorded EEG. The baseline shift to negativity and the decrease of alpha and beta oscillations prior to movement performance are considered to reflect the motor preparation and are observable even in single-trial EEG records during repetitive voluntary movements. The single-trial features of these patterns are of importance since they would reflect the dynamics of EEG activity during movement performance throughout the experimental session. Since this process is very complex and its dynamics is unclear, we applied a more general approach, i.e. the method of nonlinear prediction (NP) recommended for detection of chaos in the behavior of nonlinear dynamical systems. The NP is based on a library of past patterns of the time series used to compute the prediction of future pattern. If the time series is chaotic, the correlation coefficient between predicted and actual values would decrease as a function of the prediction step. The NP was applied to EEG records containing readiness potentials (RPs) at Fz, Cz, Pz, C3 and C4 during repetitive voluntary finger flexion. Because of nonstationarity, the prediction and the library spanned the same time period. As a result (1) chaotic segments in EEG records were detected; and (2) the sharp jumps of the absolute error between predicted and actual values indicated instances of poor prediction, which were interpreted as indicating new stages of EEG activity related to the preparatory process of motor activity.

Adult↗
PubMed1995

Single-trial readiness potentials and fatigue.

The authors propose that the cognitive processes related to internal motivation and volition (e.g., intention and preparation of a voluntary action), influenced by central fatigue, could be identified and characterized by cerebral readiness potentials (RP) using methods of chaotic dynamics. The boundaries of single-trial RP and its successive phases can be detected by tracking the data dynamics, and are represented by chaotically behaved short EEG transitions.

Animals↗
PubMed1992

Time series analysis of brain potentials preceding voluntary movements.

Brain potentials preceding voluntary movements, obtained after averaging single-trial EEG records synchronised with the start of movement, consist of slow potentials shifts SPS in the negative direction mixed with faster components like ongoing EEG activity. Two hypotheses were tested: SPS could be presented by a sum of smooth function (trends) and weakly stationary processes (residuals); the residuals and the ongoing EEG activity preceding SPS could be described by the same class of autoregressive (AR) or autoregressive moving average (ARMA) models. The trend was estimated by comparing several approximating functions in the sense of least mean square error. The SPS residuals, after subtracting the trend and background EEG activity, were estimated using AR and ARMA models of different orders. These procedures were performed on brain potentials recorded from vertex to linked ear lobes of five subjects instructed to voluntarily press a button. As a result, the hypotheses were not rejected. The trend was best approximated by a hyperbolic function or Chebyshev's second order polynomial. AR models fitted both the SPS residuals and ongoing EEG activity well enough. In conclusion, SPS were characterised by three parameters of the smooth function plus a time parameter defined by the potential boundaries.

Action Potentials↗
PubMed1991

Somatosensory evoked potentials during standing posture on different support surface.

Somatosensory evoked potentials in response to stimulation of the posterior tibial nerve at the ankle were recorded during standing on stable ground or on unstable support surface (seesaw) or on support surface short in relation to foot length. During standing on the seesaw and on the short support surface a decrease in the amplitude of the early component (N32-P39) was observed. The amplitude of N49-P58 decreased during standing on the short support surface. The amplitude of the later components (N49-P58; P58-N76; N76-P117) decreased during standing on the seesaw in comparison to that during standing on the stable ground and on the short support surface. Thus, the attenuation of the cerebral potential during standing depend on the conditions for maintenance of posture.

Adult↗
PubMed1991

Spatio-temporal characteristics of SEP to tibial nerve stimulation.

Source derivation techniques have recently been applied to analyse the spatial distribution of brain potentials. Somatosensory evoked potentials (SEPs) to tibial nerve stimulation recorded at 8 sites were analysed by using 2 source derivation techniques: Laplacian and criterion, presented by Kossev et al. (1988). First SEP recorded as a function of time were averaged and thus their basic components P39, N49, P58 were enhanced. In most of the subjects these components had maximal amplitudes at Ez rather than Cz. These sites were taken as nodes and the source derivation techniques were applied for all SEP components. Spatial derivatives (potential gradients) between adjacent leads were also computed. The current density at the nodes for different time points of SEP was obtained using Laplacian technique. For the chosen electrodes places and subjects it was found that the maximal source density did not correspond always to the SEP peak amplitudes. According to the criterion used it might be supposed that the generators of P39, N49 and P58 were located close to the scalp surface. Because of the significant individual variety of the results obtained more research is needed for choosing the appropriate scalp sites and for the contribution of the base line when measuring the SEP amplitudes.

Adult↗
PubMed1990

Effect of body and foot positions on the somatosensory evoked potentials.

Somatosensory evoked potentials in response to stimulation of the posterior tibial nerve at the ankle were recorded during sitting and standing with variable foot positions. During standing a decrease in the amplitude of the early positive component was observed. The deviation of the foot from a horizontal position was associated with an increase in the amplitude of the early negative component. The combined influence of body and foot positions showed a decrease in the amplitude of both early and late components. The standing position induced changes in more components than the varied foot positions. This suggests that maintenance of the standing posture is a more complex task than the maintenance of the foot position itself.

Adult↗
PubMed1989

Allylglycine and brain excitability. Electrophysiological investigation in cats.

The effect of DL-allylglycine (DL-AG) in two doses (40 and 60 mg/kg i.v.) was studied on cats under acute conditions. DL-AG provoked the appearance of epileptiform EEG patterns either spontaneous or in response to intermittent light stimulation with a frequency of 5-25 Hz. The effect was dose-dependent. The paroxysmal EEG was restricted to or began in the cortex and later it appeared in the thalamus and mesencephalic reticular formation. This finding together with the observation that after DL-AG the visual evoked potentials increased in the primary visual cortex and decreased in the centre median nucleus and in the reticular formation support the view about the role of the cortex in the increased brain excitability and in the increased photosensitivity after allylglycine. The photosensitive of spontaneous epileptiform EEG activity in cats pretreated with DL-allylglycine could be an useful experimental model of epilepsy for assessing the efficacy of anticonvulsant drugs.

Allylglycine↗
PubMed1986

Computer detection of EMG edges for synchronization of movement-related brain potentials.

A software-oriented technique is proposed for detection of both the start and the end of interference EMG for synchronization of single-trial movement-related brain potentials before their averaging. An iterative procedure is used on the basis of comparison of the cumulative sums of the rectified EMG segments preceding and following the time point detected by an appropriately chosen threshold. The choice of the parameters for the adjustment of the technique and the final decision for/against acceptance of the detected points are done by the experimenter in dialogue regime. In practice, after the first several trials, the procedure can be automated. In this case the number of erroneous detections, rejected by the experimenter, is small. The technique, realized on MINC 23, has been in use for a long time. Its accuracy and possibility for automation make it reliable and quick enough for various tasks related to voluntary movements and the brain potentials accompanying them.

Brain↗
PubMed1986

Somatosensory evoked potentials upon electrical stimulation triggered by voluntary movement.

The autotriggered signal was presented with a varying delay. SEP obtained without movement of the thumb (control series), SEP of autotriggered stimuli and movement-related brain potentials (MRBP) without stimuli were registered. The EEG was recorded from C3 + 2 and C3 = 2. The following results were obtained: the amplitude of all components diminished when the stimulation was autotriggered compared to the control series; the suppression was most pronounced with a stimulus delay about 150-200 ms; the effect of the suppression did not disappear up to 500 ms after the beginning of the movement.

Brain↗
PubMed1984

Changes in the Bereitschaftspotential due to subsequent voluntary autotriggered stimulus.

The Bereitschaftspotential (BP) has been studied in two different tasks: simple voluntary isometric flexion/opposition of the thumb and same movement, followed by a mild electrical shock over the median nerve, autotriggered by the movement itself. When the two tasks were organized in separate series, the subject being told about the outcome of the movement beforehand, it was found that: The BP started later; Its amplitude was smaller; The maximum negatively (N2) was smaller, when expecting electrical shock compared to simple voluntary contraction. The decrease was more pronounced over the postcentral areas. When the sequence of the autotriggered electrical shocks was equiprobably intermixed with the simple voluntary presses in single series it was found that virtually no difference existed between different potential measures. This differential influence of equal physical requirements with different subjective sequels upon the potentials related to voluntary movements implied the preferential role of the subject's attitude and state of preparedness upon the brain potentials. Both the elevated stress level and the selective suppression in specific cortical areas might take place in tasks with predictable unpleasant electrical shock.

Action Potentials↗
PubMed1983

Evoked potentials and rhythmic afterpotentials in relation to pre-stimulus EEG activity during quiet waking state.

The relationship between the frequency-characterized EEG activity preceding the stimulus presentation and the amplitude-characterized evoked potentials (EPs) and rhythmic afterreactions (RARs) was investigated. The experiments were performed on the somatosensory cortex of cats during quiet waking state. It was found that upon one and the same parameters of stimulation the amplitude of the EPs and the RARs depended significantly on the frequency of the spontaneous EEG activity recorded one second before the stimulus. A decrease of the frequency of the on-going EEG favoured the appearance of the high-amplitude early and late components of the evoked potentials and of the rhythmic afterpotentials. Conversely, the probability of appearance of low-amplitude EPs increased and that of RARs decreased when the frequency of the on-going EEG increased.

Animals↗
PubMed1983

Synchronization of movement-related cerebral potentials for averaging.

A software-oriented technique is proposed for synchronization of the individual movement-related cerebral potentials on the basis of the first derivative of the mechanogram. All procedures are computer performed using neither trigger device nor time-reverse averaging later on. The main advantages of the technique over the trigger approach are demonstrated when determining onset of both commencement and termination of the sustained contraction. Besides the objectivity and high accuracy, the proposed technique gives an additional possibility to determine movement parameters together with cerebral potential characteristics and to sort out the movement-related EEG records in greater details.

Brain↗
PubMed1983

Electrophysiological effects of propranolol and carbachol locally injected into nucleus raphe dorsalis in cats.

The effect of propranolol and carbachol injected directly into nucleus raphe dorsalis (RD) on the evoked potentials, their recovery cycles and on the spontaneous EEG activity recorded in the RD and the visual and somatosensory cortical regions was investigated. It was found that the changes in these electrophysiological parameters induced by propranolol and carbachol were similar and showed an increase in the excitability level in both the RD and the neocortex, mainly in the somatosensory cortex. The activating effect of carbachol was much more pronounced as compared with that of propranolol. The results are discussed in relation to the possible interactions between various neurotransmitter systems within the RD as well as to the data about the presumed role of this nucleus in some behavioral and/or electrophysiological manifestations. The findings suggest that raphe dorsalis participates in the regulation of the cortical excitability and apparently represents a system which modulates the transMission and integration of sensory information.

Animals↗
PubMed1982

Brain potentials related to voluntary sustained effort.

The task of the present study was the investigation of the brain potentials while performing a complex motor task, including voluntary sustained contraction (thumb opposition to the index finger). The fast press of a transducer was sustained for periods of 1.5, 5 or 10 seconds, followed by fast release. Potentials related to the start of the contraction were registered (Bereitschaftspotential, N2, etc.) with considerable interindividual variability. The end of the contraction was preceded, too, by a slow potential, similar to Bereitschaftspotential, with prevailing positivity over frontal areas and negativity over postcentral areas; N2 at the release was well expressed, regardless of the polarity of the preceding slow potential. During the sustained contraction a steady positivity was registered. The presence of potentials related to the start as well as to the end of the voluntary effort suggests the existence of two separate commands. The positivity sustained throughout the contraction, strongly expressed frontally and decreasing postcentrally, implies a decisive role for central processes including the organization and preprogramming of the motor task, but not direct reflection of feedback kinaesthetic afferentation.

Action Potentials↗
PubMed1978

Interaction of visual evoked potentials in groups of stimuli with different interstimulus intervals in cats. I. Experimental design. Dispersion analysis.

Quantitative estimation of excitability is performed at present in a pair of stimuli by studying the so-called recovery cycles. Random mixing of the interstimulus intervals and their appropriate grouping (grouped pairs) can provide new information about the course of excitability in time. An experimental setup is described in which by planning of the experiment (BIB-design) the interstimulus intervals are randomized and all components of the evoked potentials (EP) are placed under equal conditions with respect to the influence of the preceding intervals. The effects of the interstimulus intervals simultaneously on different components of visual evoked potentials in cats are estimated by means of dispersion analysis. The results show the existence of a significant effect only on definite amplitudes of the EP (predominantly with great latencies). The interactions of the effects of definite interstimulus intervals are determined, which permits the assumption of an interaction among different components of EP.

Animals↗