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E Başar

Publications and source records attributed to E Başar.

At least 19 recordsLinked to original sources
PubMed2006

Increased frontal phase-locking of event-related theta oscillations in Alzheimer patients treated with cholinesterase inhibitors.

This is a pilot study describing event-related oscillations in patients with Alzheimer-type dementia (AD). Theta responses of 22 mild probable AD subjects according to NINCDS-ADRDA criteria (11 non-treated, 11 treated by cholinesterase inhibitors), and 20 healthy elderly controls were analyzed by using the conventional visual oddball paradigm. We aimed to compare theta responses of the three groups in a range between 4-7 Hz at the frontal electrodes. At F(3) location, theta responses of healthy subjects were phase locked to stimulation and theta oscillatory responses of non-treated Alzheimer patients showed weaker phase-locking, i.e. average of Z-transformed means of correlation coefficients between single trials was closer to zero. In treated AD patients, phase-locking following target stimulation was two times higher in comparison to the responses of non-treated patients. The results indicate that the phase-locking of theta oscillations at F(3) in the treated patients is as strong as the control subjects. The F(4) theta responses were not statistically significant between the groups. Our findings imply that the theta responses at F(3) location are highly unstable in comparison to F(4) in non-treated mild AD patients and cholinergic agents may modulate event-related theta oscillatory activities in the frontal regions.

Aged↗
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

Wavelet entropy in event-related potentials: a new method shows ordering of EEG oscillations.

In this work we show the application of a measure of entropy defined from the wavelet transform, namely the wavelet entropy (WS), to the study of event-related potentials (ERPs). WS was computed for ERPs recorded from nine healthy subjects with three different types of stimuli, among them target stimuli in a cognitive task. A significant decrease of entropy was correlated with the responses to target stimuli (P300), thus showing that these responses correspond to a more "ordered" state than the spontaneous EEG. Furthermore, we propose the WS as a quantitative measure for such transitions between EEG ("disordered state") and ERP ("ordered state").

Adult↗
PubMed2001

Event-related oscillations are 'real brain responses'--wavelet analysis and new strategies.

The EEG consists of the activity of an ensemble of generators producing rhythmic activity in several frequency ranges. These oscillators are active usually in a random way. However, by application of sensory stimulation these generators are coupled and act together in a coherent way. This synchronization and enhancement of EEG activity gives rise to 'evoked' or 'event-related oscillations'. The compound evoked potential manifests as superimposition of evoked rhythms in the EEG frequencies ranging from delta to gamma ('natural frequencies of the brain'). The superimposition principle is described with efficient strategies and by utilization of an efficient algorithm. The wavelet analysis confirms the results of the combined analysis procedure obtained by using the amplitude frequency characteristics (AFCs) and digital filtering. The AFC and adapted digital filtering methods are based on the first approach to analyze average evoked potentials. In contrast, the wavelet analysis is based on signal retrieval and selection among a large number of sweeps recorded in a given physiological or psychological experiment. By combining all these results and concepts, it can be stated that the wavelet analysis underlines and extends the expression that alpha-, theta-, delta-, and gamma-responses described in this report are the most important brain responses related to psychophysiological functions. The wavelet analysis confirms once more the expression 'real signals' which we attribute to EEG frequency responses of the brain. It will be demonstrated that the delta, theta, and alpha responses (i.e. the rhythms 'predicted' by digital filtering) are real brain oscillations. The frequency components of the event-related potential vary independently of each other with respect to: (a) their relation to the event; (b) their topographic distribution; and (c) with the mode of the physiological measurements.

Algorithms↗
PubMed2001

Selectively distributed gamma band system of the brain.

Experiments from the cat brain demonstrate that gamma responses can be recorded in cortex, hippocampus, cerebellum, formatio reticularis and thalamus, independent of the mode of sensory stimulation. These results support the hypothesis of a selectively distributed gamma system of the brain.

Animals↗
PubMed2001

Functional aspects of alpha oscillations in the EEG.

Experimental evidence suggests a functional significance of EEG alpha rhythms. Event-related, evoked, and induced alpha rhythms may have functional correlates in primary sensory processing and preparatory processes. These results are in accordance with the view that spontaneous and induced EEG alpha rhythms have quasi-deterministic properties.

Alpha Rhythm↗
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

The selectively distributed theta system: functions.

The present paper provides three interwoven or interrelated approaches: (1) the dependence of frontal theta response from the spontaneous activity will be pointed out. This helps in understanding that 'frontal theta' is a major oscillation of the human frontal cortex and has a response-controlling function; (2) it will be shown that complex stimulations such as bimodal stimulation enhances the theta response; and by bringing together the results outlined in a number of previous reviews the so-called 'selectively distributed theta system of the brain' is described.

Animals↗
PubMed2001

Topological distribution of oddball 'P300' responses.

This report describes the frequency response of the oddball paradigm upon auditory stimuli. Other reports related to wavelet analysis of the same ERPs (Demiralp et al., 1999) and the application of visual signals (Schürmann et al., this volume) indicate that the P300 response has a dominant delta response oscillation, independent of the modality of the stimulation. Moreover, the adaptive digital filtering and the wavelet analysis lead to very similar results, confirming that delta responses are real brain responses as already mentioned, by Başar et al. (this volume). The theta response has a second late response window in comparison to auditory evoked potentials. Moreover, the functional significance of the selectively distributed theta and delta systems of the brain will be clearly demonstrated. Signal detection, short-term memory, and decision-making processes are discussed.

Animals↗
PubMed2001

Wavelet analysis of oddball P300.

The comparative wavelet analysis presented in details by Demiralp et al. (1999), Ademoglu (1995) and by Başar et al. (2001) will be now applied to oddball P300 results (see Başar-Eroglu et al., 2001). The results obtained basically confirm those obtained by using adaptive digital filtering: The delta response dominates the P300 potential while the theta response is prolonged in a second late window.

Animals↗
PubMed2001

Delta responses and cognitive processing: single-trial evaluations of human visual P300.

Visual P300 responses were recorded by using checkerboard-type stimuli. (1) High amplitude P300-delta responses were visible even in single trial ERPs. (2) An algorithm for efficient selection of P300 single trials (based on evaluation of delta responses) is introduced. (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↗
PubMed2001

Gamma, alpha, delta, and theta oscillations govern cognitive processes.

The increased interest in gamma oscillations, now widely regarded as functionally relevant signals of the brain, underlines the importance of the concept of event-related oscillations for bridging the gap between single neurons and neural assemblies. Taking this concept further, we review experiments showing that oscillatory phenomena such as alpha, theta, and delta responses to events are, just as the gamma band, strongly interwoven with sensory and cognitive functions. This review argues that selectively distributed delta, theta, alpha and gamma oscillatory systems act as resonant communication networks through large populations of neurons. Thus, oscillatory processes might play a major role in functional communication in the brain in relation to memory and integrative functions.

Algorithms↗
PubMed2001

Bisensory stimulation increases gamma-responses over multiple cortical regions.

In the framework of the discussion about gamma (approx. 40 Hz) oscillations as information carriers in the brain, we investigated the relationship between gamma responses in the EEG and intersensory association. Auditory evoked potentials (AEPs) and visual evoked potentials (VEPs) were compared with bisensory evoked potentials (BEPs; simultaneous auditory and visual stimulation) in 15 subjects. Gamma responses in AEPs, VEPs and BEPs were assessed by means of wavelet decomposition. Overall maximum gamma-components post-stimulus were highest in BEPs (P < 0.01). Bisensory evoked gamma-responses also showed significant central, parietal and occipital amplitude-increases (P < 0.001, P < 0.01, P < 0.05, respectively; prestimulus interval as baseline). These were of greater magnitude when compared with the unisensory responses. As a correlate of the marked gamma responses to bimodal stimulation we suggest a process of 'intersensory association', i.e. one of the steps between sensory transmission and perception. Our data may be interpreted as a further example of function-related gamma responses in the EEG.

Acoustic Stimulation↗
PubMed2000

Electroencephalogram alpha (8-15 Hz) responses to visual stimuli in cat cortex, thalamus, and hippocampus: a distributed alpha network?

To investigate possible functional correlates of alpha (8-15 Hz) oscillations in the electroencephalogram (EEG) intracranial recordings in cats (from thalamus, occipital cortex, and hippocampus) were performed. In response to visual stimuli, event-related alpha oscillations were observed. Such alpha responses were found not only in a specific sensory (visual) pathway but also in the hippocampus, hinting at a possible distributed alpha response system.

Alpha Rhythm↗
PubMed2000

Odor input generates approximately 1.5 Hz and approximately 3 Hz spectral peaks in the helix pedal ganglion.

In 1999 we reported that odorants evoke in the Helix pedal ganglion (PG) activity patterns which are largely odorant-specific and related to the nature of odor and its behavioral output. Notably, some activities (for example, approximately 1.5 and approximately 3 Hz), nonspecific to odorants, were consistently evoked in PG. The present contribution goes farther in a deeper survey of the intrinsic and odorant-evoked activities of PG with special weight on the nonspecific fluctuations. We address the following questions. (i) What are the features of the activities? (ii) Are they comparable to the activities found in the motor systems of the other invertebrates? (iii) To what functions can they be related? Three main frequency components represented by power peaks at <1 Hz, 1-2 Hz and 2-8 Hz seem to feature the response activities of PG. (a) The aversive odorants induce odorant-specific patterns represented by peak power frequencies at <1 Hz. (b) The oscillation at approximately 1 Hz, which exists intrinsically in the Helix PG, can be specifically enhanced by appetitive odors. Activities induced in the procerebrum (PC), the visceral ganglion (VG) and PG by appetitive odorants, such as ethanol and apple, peak at 1.3-2 Hz, whereas those induced by aversive ones, such as formic acid and onion at <1 Hz. (c) The 2-8 Hz components always accompany the odorant-evoked activities of the PG either as the second or third strongest component, or in the form of conspicuous, long-lasting approximately 3 Hz oscillations. (d) The nonspecific odor-evoked 1-2 Hz and approximately 3 Hz activities, and the intrinsic approximately 1 Hz activity of the PG seem to be interrelated by a degree of mutual exclusion. We may therefore consider these activities as elementary, slow components that are involved in the processing of signals in this ganglion. It can be inferred from the findings in other invertebrates that the 1-3 Hz spontaneous discharge is strongly connected with motor activity that involves the feedback mechanism of the procerebro-cerebro-buccal or -procerebro-cerebro-pedal circuit. Our approach differs from most others reported so far in the following aspects: (i) use of gross steel electrodes for recording population activities; (ii) lengthy stimulation (10 min); (iii) long observation during and after stimulation; (iv) power spectral presentation of temporal evolution of activity patterns; (v) estimation of peak power frequency by Frequency-Amplitude Plot (FAP) (obtained from signals averaged in the frequency domain; a method based on systems theory).

Alcohols↗
PubMed2000

The genesis of human event-related responses explained through the theory of oscillatory neural assemblies.

The goal of the study is to investigate the contribution of delta and theta responses to N200 and P300 ERP components that are recorded from two topographical sites (Fz and Pz) under two experimental paradigms (mismatch negativity and oddball) that trigger different cognitive processes for the respective task performances. The present study was conducted on 42 normal young adults. The results showed that it is the 'interplay' between the theta and the delta oscillations that produces the morphology and the amplitude not only of the P300 but also the N200 component. The functional and physiological meaning of the delta and theta responses are discussed within the framework of the theory of oscillatory neural assemblies and the principle of superposition.

Adult↗
PubMed2000

Brain oscillations in perception and memory.

Gamma oscillations, now widely regarded as functionally relevant signals of the brain, illustrate that the concept of event-related oscillations bridges the gap between single neurons and neural assemblies. Taking this concept further, we review experiments showing that oscillatory phenomena such as alpha, theta, or delta responses to events are strongly interwoven with sensory and cognitive functions. This review argues that selectively distributed delta, theta, alpha, and gamma oscillatory systems act as resonant communication networks through large populations of neurons. Thus, oscillatory processes might play a major role in relation with memory and integrative functions. A new 'neurons-brain' doctrine is also proposed to extend the neuron doctrine of Sherrington.

Animals↗