Biomedical subjects

V Kolev

Publications and source records attributed to V Kolev.

At least 37 records · Page 2Linked to original sources

PubMed1998

Single-sweep analysis of the theta frequency band during an auditory oddball task.

The P300 component and the oscillatory 4-7 Hz electroencephalographic activity of auditory event-related brain potentials (ERPs) were assessed to study differences between passive and oddball task conditions. Theta responses from 15 adults were analyzed for single-sweep amplitude, phase locking, and enhancement against prestimulus activity. ERPs were characterized by enhanced and strongly phase-locked theta oscillations in the early (0-300 ms) poststimulus epoch, with only the late (300-600 ms) theta responses at Fz and Pz affected by the oddball condition. P300 was strongly associated not only with the concurrent theta oscillations but also with the evoked theta activity preceding P300 (0-300 ms). It was concluded that single theta response parameters can reveal specific functional differences between passive and oddball conditions and that a strong relationship exists between the theta frequency component and the time domain P300 ERP component.

Acoustic Stimulation↗

PubMed1998

[The characteristics of the electrical activity of the human colon recorded noninvasively].

In ten healthy volunteers and ten patients with diseases involving the upper section of the gastrointestinal tract (chronic cholecystitis, chronic gastritis) the electrical activity of the colon is registered noninvasively using skin electrodes to the projection site of sigmoid and colon descendens. Recordings are taken with original gastrograph. Apart from the electrocolograms (EColG), the changes in intracolonic pressure in the group of 10 patients are also registered. Pressure recordings (mechanogram--MG) are done in physioscript "Hellige". Two types of EColG waves are identified depending on their frequency: 1) type A waves with frequency 5.86 +/- 0.59 cpm, and 2) type B waves with frequency 2.35 +/- 0.25 cpm. Type A are low-amplitude waves and correspond to the type A waves seen in the EColG of dogs (described by the authors elsewhere), characterizing the calm period of colonic motility. When A waves appear in the EColG no changes in the mechanograms are registered. Type B are high-amplitude waves, corresponding to the B waves in dogs (described elsewhere); as shown by the mechanograms, the appearance of B waves in the EColG is followed by the appearance of high-amplitude waves--the active period of colonic motility. There is a significant difference between the frequencies of either type of EColG waves and the frequency of waves in the electrogastrograms (EGG) registered synchronously with EColG.

Biomechanical Phenomena↗

PubMed1997

The phase-locking of auditory gamma band responses in humans is sensitive to task processing.

The present study assessed the effects of stimulus task-relevance and certainty on early and late 40 Hz (gamma) band responses (GBRs) in humans. Auditory GBRs of nine young adults were recorded in passive listening, simple reaction task, and choice-reaction task (target probability = 0.5) conditions and evaluated in three consecutive post-stimulus periods (0-120, 120-250, 250-400 ms) corresponding to the serial occurrence of gamma oscillation bursts. Amplitude and phase-locking of GBRs within these bursts were analysed separately at the level of single sweeps by applying a method that allows the independent quantification of between-sweep synchronization. Major results showed that the effects of stimulus certainty and task relevance on single response amplitude were specific and different from the effects on the phase-locking. The functional involvement of the early and late auditory gamma responses was distinct: early auditory gamma band responses appear primarily associated with focused attention, while the late gamma responses vary with motor-task relevance. It is concluded that along with power measures, the stability of phase-locking of gamma band responses should be regarded as a functionally meaningful parameter that varies with processing demands and recording site.

Acoustic Stimulation↗

PubMed1997

Time-frequency analysis reveals multiple functional components during oddball P300.

A time-frequency decomposition was applied to rare target and frequent non-target event-related potentials (ERPs) elicited in an oddball condition to assess whether multiple functional components occur in the P300 latency range. The wavelet transform (WT) was used because it allows capture of simultaneous or partly overlapping components in ERPs without loosing their temporal relationships. The application of a four-octave quadratic B-spline wavelet transform at the level of single-sweep data allowed us to obtain new information and revealed the presence of separate events during P300 development. Several delta, theta, and alpha frequency components in the P300 latency range differed between target and non-target processing. These findings indicate that P300 is composed of multiple functional components and that the WT method is of use for the study of P300 functional correlates more precisely.

Acoustic Stimulation↗

PubMed1997

High-pass filtering of the electrogastrogram.

The recording and processing of an electrogastrogram require adequate band-pass filtering, to suppress unwanted artefacts but preserve the original signal waveform. High-pass filtering of various types, of different time constants tau, filter order, analogue and digital implementation, have been used to obtain higher baseline stability and faster signal recovery after strong artefacts. Special attention should be given to possible signal amplitude and phase distortions due to high-pass filtering, which can strongly influence accurate amplitude measurements or studies of signal propagation from multichannel recordings. Synthesised and original signals are used to demonstrate the effect of high-pass filtering. The use of a first-order filter with tau = 5 s is recommended for EGG studies if not especially directed to investigation of bradygastria. In the opposite case, tau = 15 s should be used, and with backward filtering a full restoration of the original signal can be obtained. The same is valid for recording the electrical activity of the colon. Lower time constants (tau = 5 s or less) can be applied to acquire signals from the small intestines. A radical solution is the use of a DC amplifier with controllable subtraction of the DC component.

Electromyography↗

PubMed1997

Analysis of phase-locking is informative for studying event-related EEG activity.

A new method is presented for quantitative evaluation of single-sweep phase and amplitude electro-encephalogram (EEG) characteristics that is a more informative approach in comparison with conventional signal averaging. In the averaged potential, phase-locking and amplitude effects of the EEG response cannot be separated. To overcome this problem, single-trial EEG sweeps are decomposed into separate presentations of their phase relationships and amplitude characteristics. The stability of the phase-coupling to stimulus is then evaluated independently by analyzing the single-sweep phase presentations. The method has the following advantages: information about stability of the phase-locking can be used to assess event-related oscillatory activity; the method permits evaluation of the timing of event-related phase-locking; and a global assessment and comparison of the phase-locking of ensembles of single sweeps elicited in different processing conditions is possible. The method was employed to study auditory alpha and theta responses in young and middle-aged adults. The results showed that whereas amplitudes of frequency responses tended to decrease, the phase-locking increased significantly with age. The synchronization with stimulus (phase-locking) was the only parameter reliably to differentiate the brain responses of the two age groups, as well as to reveal specific age-related changes in frontal evoked alpha activity. Thus, the present approach can be used to evaluate dynamic brain processes more precisely.

Adult↗

PubMed1997

Is the alpha rhythm a control parameter for brain responses?

The main goal of the present study is to develop a conceptual analysis of alpha response in the brain based on single sweep evaluation. A new method was employed to estimate a set of single-sweep parameters and quantify the oscillatory behaviour of single, electroencephalograph (EEG) sweeps. It was aimed to demonstrate that brain alpha responses are governed by spontaneous alpha activity and to validate the principle of brain response excitability. Because the spontaneous alpha activity depends on both the topology of recording and the subject's age, topology and age models were used. Spontaneous and evoked alpha activity were recorded at frontal and occipital sites in three groups of subjects: 3-year-old children, young adults and middle-aged subjects. Amplitude, enhancement and phase-locking of single alpha responses to visual stimuli were analysed. Major results showed that; (1) visual alpha responses could be recorded only if the alpha rhythm was developed in the spontaneous EEG independent of electrode location; (2) middle-aged adults showed more expressed frontal spontaneous alpha activity in comparison with young adults; (3) accordingly, alpha responses with higher amplitude and stronger phase-locking were produced over the frontal brain area in middle-aged than young adults. These results validate the principle of brain response excitability and demonstrate that a shift towards frontal brain areas for both the spontaneous and evoked alpha activity occurs with increasing age in adults. The results are discussed in the context of the diffuse and distributed alpha system of the brain. Age-dependent changes in frontal alpha activity are suggested to be related to frontal brain functioning during aging.

Adult↗

PubMed1997

Alpha response system in children: changes with age.

Evoked and event-related brain potentials (Eps, ERPs) may be regarded as originating from the reorganization of the spontaneous EEG rhythms (Başar, 1980). Until now, no data is available about the development of the evoked frequency components in EPs and ERPs of children. The main objective of the present research was to study the alpha response system in 6-11-year-old children. We suggested that the ability to reorganize the alpha activity and produce repeatable alpha patterns after external stimulation might undergo developmental changes that could reflect certain changes in information processing with increasing age from childhood to adulthood. Fifty 6-11-year-old children divided into five age groups, and 10 young adults were studied in a passive and an odd-ball condition. Alpha responses in the auditory EPs and non-target ERPs at Fz, Cz and Pz were analyzed. The magnitude and phase-locking with stimulus of single alpha responses were evaluated in the first 300 ms of the post-stimulus epoch. An original method was applied to assess quantitatively the repeatability (phase-locking) of the evoked alpha oscillations. The magnitude and the phase-locking to stimulus were analyzed with respect to their dependence on the age and topography factors. Our main results show that the alpha responses in 6-11-year-old children are different from those in adults: (1) Adults had significantly lower amplitude and stronger phase-locking than children; (2) Adults had maximal alpha amplitudes and phase-locking over the vertex, whereas children displayed maximal responses over the parietal site; (3) The phase-locking of eldest (10-11-year-old) children was as strong as in adults. Whereas no difference existed between groups of children in alpha response amplitudes, a significant increase in phase-locking from 6 to 11 years was observed. Concerning the obtained results we suggest that (1) Alpha response system is functionally involved in 6-11-year-old children, though its development is not complete at the age of 11, the upper limit of our sample (2) With regard to their differential developmental time-courses, the magnitude and the phase-locking parameters might be suggested to relate to different functional aspects of the alpha response system. The applied original method makes it possible to analyze the phase-locking to stimulus (or phase-reordering) separately and independently from the amplitude (enhancement) of the frequency responses, thus providing for a deeper examination of the evoked frequency components.

Adult↗

PubMed1997

Developmental changes in the event-related EEG theta response and P300.

Event-related potentials (ERPs) from 50 children (6-11 years) and 10 adults were elicited by auditory passive, and by rare target and frequent non-target stimuli, and analyzed in the time and frequency domains. The latency of the maximal theta response (or the theta frequency component of the ERP) was evaluated with respect to age and scalp topography effects. The major findings were: (1) The latency of the maximal theta response decreased with increasing age in children, although for each stimulus type and location adults had shorter latencies than the children. (2) The developmental time course of latency reduction depended on the electrode location, with the most prominent reduction occurring at 8 years at Cz, and no differences between children groups obtained for the frontal site. (3) Maximal theta response latency was strongly associated with the latency of the late parietal P400-700 (P3b) component in children. The results suggest that the developmental latency decrease in P300 processes originate from a decrease in the preceding theta-related processes and may reflect a speeding of cognitive stimulus evaluation.

Adult↗

PubMed1997

Effects of task variables on the amplitude and phase-locking of auditory gamma band responses in human.

The present study was designed to assess the effects of stimulus certainty and motor task-relevance on auditory transient 40-Hz or gamma band responses. To study the effects of these factors in a balanced design, auditory event-related potentials (ERPs) of 9 young adults were recorded in a passive listening, simple reaction task, and choice-reaction task (target tone probability = 0.5) conditions. Amplitude and phase-locking of event-related gamma activity were analyzed separately at the level of single sweeps by applying a method that allows the independent quantification of phase synchronization between consecutive single responses. Major results demonstrated that (1) During auditory stimulus processing discernible gamma oscillation bursts were observed in three time windows of the poststimulus epoch: early (0-120) ms, middle (120-250 ms) and late (250-400 ms). (2) Early gamma response amplitudes were significantly largest for highly expected motor-task stimuli, whereas the phase-locking did not depend on either of the two variables. (3) The phase-locking of late gamma responses, however, was significantly stronger to targets than to nontargets. These results indicate that auditory gamma responses are functionally involved in the processing of task variables such as stimulus certainty and motor-task relevance. It is also demonstrated that single gamma response amplitude and phase-locking have independent functional significance as being affected in a different manner by different task conditions.

Adult↗

PubMed1996

Brain theta response predicts P300 latency in children.

The present study assessed the relationship between stimulus-induced electroencephalographic (EEG) theta activity (4-7 Hz) and the endogenous component P300 of auditory event-related potentials (ERPs) using a developmental model. ERPs were recorded from 50 children (6-11 years old) for oddball target and frequent non-target stimuli and analysed in the time and frequency domains. The latency of the maximal theta response of the digitally filtered (4-7 Hz) ERPs was evaluated with respect to age and scalp topography effects. The maximal theta response latency at Cz and Pz decreased with increasing age and entirely predicted the developmental latency reduction of the parietal P400-700 (P3b) component in children.

Acoustic Stimulation↗

PubMed1996

Extracellular action potentials of skeletal muscle fibre affected by 4-aminopyridine: a model study.

A new specially designed analytical function approximating the intracellular action potentials (ICAPs) for calculation of the extracellular potentials (ECAPs) at various radial and axial distances from the active fibre is proposed. 4-Aminopyridine (4-AP) was used to obtain ICAPs with a prolonged repolarization phase in order to investigate the influence of changes in ICAP shape on the ECAPs. From the experimentally recorded ICAPs before and after treatment of frog skeletal muscle fibres with 4-AP, approximated by the new function, the ECAPs were calculated applying the line-source model in a finite fibre. Using this function allowed calculation of the ECAPs at distances not accessible for the experimental recordings. The total ionic current (Ii) during the action potential was calculated using the cable equation. Our results showed that the ratio of the first positive to the negative phases of the ECAPs of treated fibres increased at large radial distances (3000 microns and more) and the terminal positive phase was asymmetric with an abrupt initial deflection followed by a slow inverse deflection. The calculated ECAPs at various axial distances from the fibre end (cylindrical and conical part) and at radial distances from the fibre membrane ranging from 0 to 5000 microns, corresponded in shape to the experimentally recorded potentials of untreated and 4-AP-treated muscle fibres.

4-Aminopyridine↗

PubMed1996

Evoked brain rhythms are altered markedly in middle-aged subjects: single-sweep analysis.

In this study, a new method for analysis of single frequency responses was applied to compare auditory evoked potentials between young (18-30 years old) and middle-aged (50-55 years old) adults. The method was related to the evoked brain rhythmicities concept and permitted to quantify separately the magnitude, phase-locking, and enhancement against prestimulus activity of single sweeps. Thus, a constellation of three single-sweep parameters was used. No differences could be detected between the two age groups in the conventionally averaged potentials. However, significant age-related alterations were found at the level of single sweep analysis: 1) At the three midline locations (Fz, Cz, and Pz) older adults manifested theta phase-locking and enhancement that were significantly stronger than those in young adults; 2) In contrast, the alpha responses only over the frontal brain area were stronger in phase-locking and enhancement in middle-aged than in young adults, which implies that the alpha response system may be specifically related to frontal brain functioning during aging.

Acoustic Stimulation↗

PubMed1996

Non-invasive electrocologram: non-invasive recording of the human colonic electrical activity.

The electrical activity of the descending colon of fifteen healthy volunteers was recorded non-invasively by cutaneous electrodes sticked on the abdominal wall along the descending colon projection--electrocologram (EColG). The electrical activity of the colon and stomach was recorded by an original electrogastrograph. The means, S.E.M. of the frequency of the colonic and gastric waves were calculated. Parallel studies of the EColG and the changes in the intracolonic pressure were performed on 10 patients with balloon inflated in the colon. Two kinds of EColG waves according to their frequency were identified: i) Type A waves with a frequency of 5.86 +/- 0.59 cpm and ii) Type B waves with a frequency of 2.35 +/- 0.25 cpm. There was a significant difference between the frequencies of the EColG waves and the frequency of the EGG waves -3.35 +/- 0.31 cpm. The type A waves were of low amplitude and corresponded to type A dog colonic waves i.e. they characterized the quiescent period of colonic motility. Type B waves were with a high amplitude, corresponded to type B dog colonic waves and characterised the activity periods of colonic motility. This was confirmed by the correspondence between type B waves and the changes in the intracolonic pressure. Thus the proposed non-invasive electrogastrographic method proved to be suitable for non-invasive registration of the electrical activity of the human colon.

Adult↗

PubMed1995

A new metric for analyzing single-trial event-related potentials (ERPs): application to human visual P300 delta response.

Visual P300 responses were recorded by using checkerboard-type stimuli. (1) High amplitude P300-delta responses were visible even in single trial event-related potentials (ERPs). (2) The algorithm based on the 'delta response metric' provides an efficient selection criterion for evaluation of P300 single trials in ERP ensembles. (3) The 'universal' character of the P300-delta response demonstrated in this report may open new avenues for the understanding of functional ERP components.

Adult↗

PubMed1994

EEG, auditory evoked potentials and evoked rhythmicities in three-year-old children.

According to our working hypothesis, the resonance properties of the brain systems play an important role in internal brain communications (e.g., Başar, 1992; Başar, Başar-Eroglu, Demiralp & Schürmann, 1992). It was assumed that evoked potentials (EPs) reflect brain resonance properties, showing enhancement, time and frequency-locking during the poststimulus period. All these phenomena might be referred to the spontaneous (intrinsic) EEG rhythms according to the excitability rule and the related concept of brain system response susceptibility: a brain system could react to internal or external stimuli producing those rhythms or frequency components, which have already been present in intrinsic or spontaneous activity (Başar, 1980). In order to test the hypothesis of response susceptibility, in the present paper we used an natural model--3-year-old children--to investigate how brain systems respond to external stimulation if their spontaneous rhythms are different in comparison to the spontaneous EEG rhythms in adults. For that purpose we used a combined time and frequency domain approach. The spectral characteristics of the spontaneous EEGs as well as the frequency components of auditory EPs elicited under identical auditory stimulation in 3-year-old children and adults aged 20-22 years were compared. Our observations support the hypothesis for response susceptibility; if in a given frequency channel the spontaneous brain rhythms are missing, they are also absent in the evoked and induced rhythmicities and vice versa: children at 3 years do not create alpha resonance upon sensory stimulation while they do not have developed EEG rhythms in the range of 8-15 Hz. Elicited under identical experimental conditions (auditory stimulation with fixed stimulus parameters) children and adult evoked rhythms differ. It was concluded that the AEPs recorded in 3-year-old children might be regarded mainly as a superposition of rhythmicities in delta and theta ranges. These rhythmicities are prolonged and delayed in comparison to the corresponding rhythms in adults.

Adult↗

PubMed1994

EEG rhythmicities evoked by visual stimuli in three-year-old children.

The present paper complements our investigations on children's and adult's EEGs and evoked potentials elicited by mean of auditory stimulation. We analyzed the spectral characteristics of spontaneously recorded EEGs, visual evoked potentials, and frequency responses of a group of 3-year-old children. The results were compared to the corresponding measurements in adults aged 20-22 years. Our findings show that: 1. the main frequency VEP components in 3 years old children are "delta" and "theta responses" with delayed time courses in comparison to adult's VEPs, 2. the existence of a given frequency component in the VEPs depends on the presence of the corresponding rhythm in the spontaneous EEG, and 3. the individual filtered children's VEPs show larger variability in comparison to adults, as well as different interhemispheric variability in the different frequency ranges. It was concluded that the evoked frequency components depend on the frequency content of the spontaneous EEG rhythms and their development. However, some of the children already showed alpha enhancement as well as synchronized alpha waves in the poststimulus period at the occipital and parietal leads. The possible explanation of these phenomena might be referred to early development of the posterior basic rhythms as well as to known resonance phenomena such as weak resonance and tuning.

Adult↗

PubMed1992

[An automated system for the collective protection from harmful exposure to the Veneta superhigh-frequency electromagnetic field of military units and the civilian population].

In the up-to-date exploitation of electromagnetic field generators more and more clearly stands out the problem for radiation protection of people from the biological effect of microwave radiation. In order to solve this problem an automatized system is developed, with the purpose to define according to numerical methods the areas of standardized radiation, where the stay depends on the exposure. On the basis of the developed product are statistical data of the values of the ionized radiation parameters and the algorithm for radiation and hygienic guarantee the exploitation of products emitting superhigh frequency magnetic fields, the average power and strength of the energy flow of which are determined by the impulse power of radiation and measurement in the so called "free area". The system is a first stage for group protection from superhigh frequency electromagnetic field, which afterwards to be approbated by means of apparatus dosimetric examinations.

Algorithms↗