Biomedical subjects

J Yordanova

Publications and source records attributed to J Yordanova.

At least 19 recordsLinked to original sources

PubMed2001

A transient dominance of theta ERP component characterizes passive auditory processing: evidence from a developmental study.

This study analyzed relationships among co-existent EEG frequency responses during passive auditory stimulus processing. By applying quantifiers based on wavelet entropy, it is demonstrated that a short-lasting ordering of the complex post-stimulus EEG signal occurs as a result of a transient synchronization in the theta frequency channel. Further, by using a developmental model it is shown that, independently of the frequency content of the background EEG and ERPs, a highly-ordered microstate in the ERP is always determined by theta frequency. Thus, transient dominance of synchronized theta oscillations may reflect an important functional mechanism subserving stimulus information processing.

Acoustic Stimulation↗

PubMed2001

Wavelet entropy: a new tool for analysis of short duration brain electrical signals.

Since traditional electrical brain signal analysis is mostly qualitative, the development of new quantitative methods is crucial for restricting the subjectivity in the study of brain signals. These methods are particularly fruitful when they are strongly correlated with intuitive physical concepts that allow a better understanding of brain dynamics. Here, new method based on orthogonal discrete wavelet transform (ODWT) is applied. It takes as a basic element the ODWT of the EEG signal, and defines the relative wavelet energy, the wavelet entropy (WE) and the relative wavelet entropy (RWE). The relative wavelet energy provides information about the relative energy associated with different frequency bands present in the EEG and their corresponding degree of importance. The WE carries information about the degree of order/disorder associated with a multi-frequency signal response, and the RWE measures the degree of similarity between different segments of the signal. In addition, the time evolution of the WE is calculated to give information about the dynamics in the EEG records. Within this framework, the major objective of the present work was to characterize in a quantitative way functional dynamics of order/disorder microstates in short duration EEG signals. For that aim, spontaneous EEG signals under different physiological conditions were analyzed. Further, specific quantifiers were derived to characterize how stimulus affects electrical events in terms of frequency synchronization (tuning) in the event related potentials.

Adult↗

PubMed2001

Increased frontal phase-locking of event-related alpha oscillations during task processing.

Recent findings substantiate the view that electroencephalographic (EEG) alpha rhythm (7-13 Hz) is functionally involved in cognitive stimulus processing. Our previous results have shown that enhanced alpha responses to auditory task stimuli can be well synchronized with stimulus until 800-1000 ms. The present study analyzed the effect of perceptual uncertainty and difficulty in decision making on event-related alpha oscillations in single auditory event-related brain potentials (ERPs). EEG was recorded from Fz, Cz and Pz electrodes in 10 subjects participating in two experimental sessions, in which auditory stimuli with equal physical parameters were presented under passive and task instructions. Separate measurements of single alpha response amplitude and phase-locking were performed and statistically analyzed for consecutive time windows in the post-stimulus epoch. Major results show that, during the cognitive task, the phase-locking of alpha oscillations at the frontal site is significantly increased in the time window of 500-1000 ms after stimulation. Thus, the involvement of enhanced and synchronized frontal alpha activity in higher brain processes is strongly emphasized.

Alpha Rhythm↗

PubMed2001

Abnormal early stages of task stimulus processing in children with attention-deficit hyperactivity disorder--evidence from event-related gamma oscillations.

OBJECTIVES: Attention-related differences in early stages of stimulus processing were assessed in healthy controls and children with attention-deficit hyperactivity disorder (ADHD) by analyzing phase-locked gamma band (31-63 Hz) responses to auditory stimuli in a selective-attention task. METHODS: A total of 28 children aged 9-12 years (ADHD and matched healthy controls) pressed a button in response to each target stimulus presented at the attended side (right or left). Auditory gamma band responses (GBRs) within 0-120 ms were analyzed at 8 electrodes with wavelet transform. Effects of attended channel, stimulus type, and group were evaluated for GBR power and phase-locking. RESULTS: For both groups, GBRs had a frontal-central distribution, were significantly larger and more strongly phase-locked to target than to non-target stimuli, and did not differentiate the attended from the unattended channel. ADHD children produced larger and more strongly phase-locked GBRs than controls only to right-side stimuli, irrespective of whether these were the attended or the ignored stimuli. CONCLUSIONS: The association between auditory GBR and motor task stimulus in children suggests that phase-locked gamma oscillations may reflect processes of sensory-motor integration. ADHD-related deviations of GBRs indicate that early mechanisms of auditory stimulus processing are altered in ADHD, presumably as a result of impaired motor inhibition.

Acoustic Stimulation↗

PubMed2001

Time-on-task analysis using wavelet networks in an event-related potential study on attention-deficit hyperactivity disorder.

OBJECTIVE: The aim of this event-related potential (ERP) study was to test time-on-task analysis at the level of single sweeps in a clinical trial. Since inattentiveness is one of the main symptoms of attention-deficit hyperactivity disorder (ADHD), this child psychiatric disorder was chosen as an exemplary application. METHODS: Twenty-four healthy and 24 ADHD boys, aged 9--15 years, performed an auditory selective attention task for about 5 min. ERP single trials were analyzed using wavelet networks. Time-on-task analysis was applied to omission errors, reaction time and slow ERP components (frontal negativity, parietal positivity), represented by a low-frequency wavelet component. RESULTS: Both performance and ERP measures showed distinct temporal dynamics. Time-on-task effects were not only linear, but also of higher order and started after less than 1 min. For ADHD children, earlier time-on-task effects, i.e. an earlier increase of omission errors and frontal negativity, resulted. Healthy children could allocate more attentional resources during the course of the experiment. CONCLUSION: Time-on-task analysis at the level of single trials revealed phenomena probably reflecting ADHD children's attentional deficits. Thus, a more differentiated ERP analysis may provide a better understanding of the pathophysiological background in neuropsychiatric disorders.

Acoustic Stimulation↗

PubMed2001

P300 and alpha event-related desynchronization (ERD).

In the present study we evaluated the relationships between the P300 event-related potential and event-related desynchronization (ERD) of electroencephalographic alpha activity by simultaneously analyzing P300 as well as 7-10- and 10-14-Hz alpha ERD responses from auditory passive and active oddball conditions. We compared the effects of task (target vs. nontarget) and electrode (Fz, Cz, Pz) on P300 and ERD, and correlated P300 amplitude/latency with ERD maximal amplitude/latency across individuals. The major findings were that P300 as well as slow and fast alpha ERD manifested similar task and electrode effects. P300 preceded ERD and predicted individual variance of both slow and fast alpha ERD. The relationships of P300 with alpha ERD were different for the slow and fast alpha frequencies. These findings indicate that P300 and ERD are related such that slow and fast alpha ERDs are specifically guided or modified by the internal event(s) indexed by P300.

Adolescent↗

PubMed2000

Gamma band response in children is related to task-stimulus processing.

The present study described the functional characteristics of early phase-locked electroencephalographic gamma band (30-60 Hz) responses (GBRs) in 9- to 12-year-old healthy children. GBRs were elicited in an auditory selective-attention task. Target stimuli required motor responding when presented to the right or to the left (attended side). Effects of stimulus type relevance and attended side were evaluated for GBR power and phase-locking within 0-120 ms. GBRs in children were frontally distributed, were larger and better phase-locked to targets relative to non-targets, but did not depend on the attended side. These results demonstrate that the auditory GBR is related to task-stimulus processing and imply that early target selection in children is guided by a sensory or sensorimotor preparatory set, rather than by an internal attentional focus to the side of stimulation.

Attention↗

PubMed2000

Multiple time-frequency components account for the complex functional reactivity of P300.

Consecutive and overlapping time-frequency (TF) components of auditory event-related brain potentials (ERPs) were analyzed to examine whether multiple co-existing components may account for the complex functional reactivity of P300. Auditory ERPs of 14 adult subjects were decomposed by means of the wavelet transform (WT), and TF components within P300 were tested in a systematic manner for the effects of major P300 determinants: stimulus probability, active discrimination, and mental count task. The results demonstrated that several partly or fully simultaneous delta, theta, and alpha TF components significantly depend on the factors eliciting P300, and also manifest distinct patterns of task reactivity and scalp distribution. Thus, specific functional processes that underlie the P300 ERP can be distinguished that help to account for its responsiveness to task variables.

Adolescent↗

PubMed1999

Time-frequency analysis of single-sweep event-related potentials by means of fast wavelet transform.

A time-frequency decomposition was applied to the event-related potentials (ERPs) elicited in an auditory oddball condition to assess differences in cognitive information processing. Analysis in the time domain has revealed that cognitive processes are reflected by various ERP components such as N1, P2, N2, P300, and late positive complex. However, the heterogeneous nature of these components has been strongly emphasized due to simultaneously occurring processes. The wavelet transform (WT), which decomposes the signal onto the time-frequency plane, allows the time-dependent and frequency-related information in ERPs to be captured and precisely measured. A four-octave quadratic B-spline wavelet transform was applied to single-sweep ERPs recorded in an auditory oddball paradigm. Frequency components in delta, theta, and alpha ranges reflected specific aspects of cognitive information processing. Furthermore, the temporal position of these components was related to specific cognitive processes.

Adolescent↗

PubMed1999

Event-related alpha oscillations in task processing.

OBJECTIVES: Recent findings substantiate the view that electroencephalographic (EEG) alpha rhythm (7-13 Hz) is functionally involved in information processing. However, the association of alpha rhythms with cognitive brain processes is less well understood because both augmentation and suppression of alpha oscillations have been observed to accompany task performance. The present study evaluates the effect of task processing on event-related alpha oscillations at the level of single-sweep analysis. METHODS: EEG was recorded from Fz, Cz and Pz electrodes in 10 subjects participating in two experimental sessions, in which auditory stimuli with equal physical parameters were presented under different instructions (passive and task). Separate measurements of single-sweep amplitude and phase-locking were performed and statistically analyzed for consecutive time windows in the poststimulus epoch. RESULTS: Major results show that, during the cognitive task, the phase-locking of alpha oscillations at the frontal site is significantly increased for the time window of 500-1000 ms after stimulation. CONCLUSIONS: The involvement of enhanced and synchronized frontal alpha activity in higher brain processes is strongly emphasized.

Acoustic Stimulation↗

PubMed1998

Event-related alpha oscillations are functionally associated with P300 during information processing.

Recent findings indicate that the electroencephalographic alpha (7-14 Hz) activity is functionally involved in cognitive brain functioning, but the issue of whether and how event-related alpha oscillations may relate to the processes indexed by the P300 component of the event-related brain potentials (ERPs) has not been addressed. The present study assessed the effect of auditory oddball task processing on slow (7-10 Hz) and fast (10-14 Hz) alpha activity from the P300 latency range. ERPs from mentally counted targets (20%) and uncounted non-targets (80%) were recorded at Fz, Cz, and Pz in nine subjects Single-sweep phase-locking, power of phase-locked, and power of non-phase-locked alpha responses during P300 activity were quantified. The results demonstrated that larger and more synchronized phase-locked fast alpha components at anterior (frontal-central) locations, with reduced non-phase-locked slow alpha responses at the parietal site were produced by targets relative to non-targets. Because the simultaneously recorded P300 and alpha activity manifested a similar sensitivity to the oddball task, event-related alpha appears to be functionally associated with the cognitive processing demands eliciting P300. Also, evidence is provided for the functional involvement of frontally synchronized and enhanced alpha oscillations in task processing.

Adolescent↗

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↗

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

Frontocortical activity in children with comorbidity of tic disorder and attention-deficit hyperactivity disorder.

Comorbidity of tic disorder (TD) and attention-deficit hyperactivity disorder (ADHD) in children is common but not fully understood. Thus we investigated the effect of TD and ADHD on the amplitude of the postimperative negative variation (PINV) in children with combined tic + hyperactivity symptoms (TD + HA). PINV was chosen as an indicator of frontal lobe functioning that is closely related to self-regulation of behavior. PINVs of four groups of children (healthy controls, pure TD, pure ADHD, and combined TD + HA; total number 43) were elicited in an auditory warned reaction task in three conditions (control = CC, loss-of-control = LoCC, and lack-of-control = LaCC) at midfrontal (Fz) and midcentral (Cz) leads of the scalp. Effects of TD and ADHD were revealed only in the noncontrol conditions, being independent in the LaCC, but interactive in the LoCC. Thus, the additive model of psychopathological classification concerning the comorbidity of TD and ADHD was only partially supported by the observed pattern of psychophysiological results.

Attention↗

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↗