Biomedical subjects

S Karakaş

Publications and source records attributed to S Karakaş.

10 recordsLinked to original sources

PubMed2004

Analysis of the time-varying energy of brain responses to an oddball paradigm using short-term smoothed Wigner-Ville distribution.

Cognitive brain responses to external stimuli, as measured by event related potentials (ERPs), have been analyzed from a variety of perspectives to investigate brain dynamics. Here, the brain responses of healthy subjects to auditory oddball paradigms, standard and deviant stimuli, recorded on an Fz electrode site were studied using a short-term version of the smoothed Wigner-Ville distribution (STSW) method. A smoothing kernel was designed to preserve the auto energy of the signal with maximum time and frequency resolutions. Analysis was conducted mainly on the time-frequency distributions (TFDs) of sweeps recorded during successive trials including the TFD of averaged single sweeps as the evoked time-frequency (ETF) brain response and the average of TFDs of single sweeps as the time-frequency (TF) brain response. Also the power entropy and the phase angles of the signal at frequency f and time t locked to the stimulus onset were studied across single trials as the TF power-locked and the TF phase-locked brain responses, respectively. TFDs represented in this way demonstrated the ERP spectro-temporal characteristics from multiple perspectives. The time-varying energy of the individual components manifested interesting TF structures in the form of amplitude modulated (AM) and frequency modulated (FM) energy bursts. The TF power-locked and phase-locked brain responses provoked ERP energies in a manner modulated by cognitive functions, an observation requiring further investigation. These results may lead to a better understanding of integrative brain dynamics.

Algorithms↗

PubMed2001

Gamma response of the brain: a multifunctional oscillation that represents bottom-up with top-down processing.

This paper deals with the functional correlates of the gamma response of the brain. A critical review of the literature findings reveals the existence of two types of gamma responses: an early gamma that fulfills sensory functions and a late gamma that fulfills perceptual-cognitive functions. However, even the early gamma shows individual differences. Such a finding points to the existence of top-down influences on sensory processes and to a parallel-processing model for brain function.

Brain↗

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↗

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↗

PubMed2000

A new strategy involving multiple cognitive paradigms demonstrates that ERP components are determined by the superposition of oscillatory responses.

OBJECTIVES: The goal of the present paper was to study the contribution of the delta and theta responses to two components of the event-related potential (ERP) waveform, the N200 and P300, which were recorded from 3 topographical sites of the brain. METHODS: This contribution was studied using a set of systematically varying experimental paradigms. Such a strategy enabled the demonstration of the variations in the event-related potentials and the event-related oscillations as task conditions and respective cognitive operations systematically changed. The study employed easy oddball, hard oddball, mismatch negativity and single stimulus paradigms and it was conducted on 42 healthy adults (age range 19-30 years, 26 females, 16 males) from the university student population. Data were analyzed with electrophysiological (selective averaging, amplitude frequency characteristics, digital filtering) and statistical methods (analysis of variance, multivariate step-down regression). RESULTS: The data showed that the morphology of the ERP components for different experimental paradigms represented a specific pattern of superposition of the delta and theta oscillatory responses. CONCLUSIONS: The cognitive correlates of the oscillatory responses were discussed and the results were evaluated on the basis of the superposition principle and the theory of oscillatory neural assemblies.

Acoustic Stimulation↗

PubMed1999

Are cognitive processes manifested in event-related gamma, alpha, theta and delta oscillations in the EEG?

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 concerning oscillatory responses to events (in the alpha, theta and delta ranges) as possible correlates of sensory and cognitive functions. It is argued that selectively distributed delta, theta, alpha and gamma oscillatory systems act as resonant communication networks through large populations of neurons, with functional relations to memory and integrative functions.

Alpha Rhythm↗

PubMed1998

Early gamma response is sensory in origin: a conclusion based on cross-comparison of results from multiple experimental paradigms.

The study investigates the functional correlates of the early, time-locked gamma response. The study utilized a unique experimental strategy which involved the utilization of a series of experimental paradigms to which all subjects (n = 20) were exposed to in the same recording session. These paradigms induced an increasingly complex configuration of processes for their respective task performance and also required different levels of attention allocation. In their order of administration, the paradigms were single stimulus (SS), mismatch negativity (MMN), evoked potential (EP), easy oddball (OB-EZ) and hard oddball (OB-HD). Auditory stimuli were used in the study (10 ms r/f time, 50 ms duration, 65 dB SPL) with the standards as 1000 Hz or 1900 Hz and deviants as 2000 Hz. The early gamma showed a frontocentral topography. The difference between Fz and Pz recording sites were statistically significant. A comparative analysis of the gamma responses showed that the gamma that was obtained at the early time-window of 0-150 ms as a time-locked activity occurred irrespective of experimental paradigm; the early gamma did not vary with the degree of task complexity or with attentional allocation. It was concluded from these findings that the early gamma is basically a sensory phenomenon. Various studies have previously shown that under perceptual/cognitive tasks, gamma response is obtained as a non-phase-locked activity in the late time-windows. These studies concluded that the gamma response is basically perceptual/cognitive in function. However, in these studies the early sensory gamma was also present in the data. Collectively taken, these findings may lead to the conclusion that the gamma response is a multifunctional phenomenon, with the early portion representing sensory and the late portion perceptual/cognitive processing.

Acoustic Stimulation↗

PubMed1997

Alpha oscillations in brain functioning: an integrative theory.

The old concept stating that EEG alpha (10-Hz) activity reflects passive or idling states of the brain is giving way to modern views of 10-Hz oscillations in relation to diverse brain functions comprising sensory, motor, and memory processes: (1) Spontaneous alpha activity is not pure noise as shown by methods of chaos analysis. (2) Evoked alpha oscillations patterns (precisely time-locked to a stimulus; duration approx. 200-300 ms) depend on the modality of stimulation and the recording site. (3) Induced alpha oscillations are initiated by, but not closely time-locked to a stimulus. (4) 10-Hz oscillations are recorded in nervous systems of different complexities, from the human brain to isolated ganglia of invertebrates. The neural origins of 10-Hz oscillations are demonstrated by recordings at the cellular level. (5) Rather than trying to locate a unique alpha generator, it is preferable to assume that a 'diffuse and distributed alpha system' exists. A particular support for this hypothesis is given by stimulus-dependent hippocampal alpha responses in the cat brain. (6) The major physiological meaning of 10-Hz oscillations may be comparable to the putative universal role of gamma responses in brain signaling.

Alpha Rhythm↗

PubMed1983

High-frequency components of human visual evoked potentials.

This study was aimed to further our investigations on the high frequency components of human visual evoked potentials (VEP). By using stimulations in the form of optic step functions, we measured (Cz-ear lobe) VEPs with 512 real data points in a total recording time of 20.48 msec in normal adults (N = 12). The peaks in the transient evoked potentials occurred with approximate latencies of 0.5, 1.0, 1.5, 2.5 and 5.0 msec. Transforming the individual time responses to the frequency domain by the Fourier Transform, maxima were obtained in 200, 400, 700, 1200 and 2200 Hz positions in the amplitude frequency characteristics. The findings were compared with those of a similar study on the auditory system. It is stated that the response activities of 200 Hz and 2200 Hz are unique to the visual system.

Evoked Potentials, Auditory↗