Erol Başar

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.

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.

P300-response: possible psychophysiological correlates in delta and theta frequency channels. A review

The present paper combines a review of event-related potentials (ERPs) with empirical data concerning the question: what are the differences between auditory evoked potentials (EPs) and two types of ERPs with respect to their frequency components? In this study auditory EPs were elicited by 1500 Hz tones. The first type of ERPs was responses to 3rd attended tones in an omitted stimulus paradigm where every 4th stimulus was omitted. The second type of ERPs was responses to rare 1600 Hz tones in an oddball paradigm. The amplitudes of delta and theta components of EPs and ERPs showed significant differences: in responses to 3rd attended tones there was a significant increase in the theta frequency band (frontal and parietal locations; 0-250 ms). In the delta frequency band there was no significant change. In contrast a diffuse delta increase occurred in oddball responses and an additional prolongation of theta oscillations was observed (late theta response: 250-500 ms). These results are discussed in the context of ERPs as induced rhythmicities. The intracranial sources of ERPs, their psychological correlates and the role of theta rhythms in the cortico-hippocampal interaction are reviewed. From these results and from the literature a working hypothesis is derived assuming that delta responses are mainly involved in signal matching, decision making and surprise, whereas theta responses are more related to focused attention and signal detection.

Induced rhythms in the brain

to Induced Rhythms: A Widespread, Heterogeneous Class of Oscillations.- Oscillations in the Striate Cortex.- 1 Mechanisms Underlying the Generation of Neuronal Oscillations in Cat Visual Cortex.- 2 Stimulus-Specific Synchronizations in Cat Visual Cortex: Multiple Microelectrode and Correlation Studies from Several Cortical Areas.- Cortical Rhythms, Ongoing (EEG) and Induced (ERPs).- 3 The Rhythmic Slow Activity (Theta) of the Limbic Cortex: An Oscillation in Search of a Function.- 4 Is There any Message Hidden in the Human EEG?.- 5 Event-Related Synchronization and Desynchronization of Alpha and Beta Waves in a Cognitive Task.- 6 Magnetoencephalographic Evidence for Induced Rhythms.- 7 Rostrocaudal Scan in Human Brain: A Global Characteristic of the 40-Hz Response During Sensory Input.- 8 Evoked Potentials: Ensembles of Brain Induced Rhythmicities in the Alpha, Theta and Gamma Ranges.- 9 Predictions on Neocortical Dynamics Derived from Studies in Paleocortex.- 10 A Comparison of Certain Gamma Band (40-HZ) Brain Rhythms in Cat and Man.- 11 Human Visual Evoked Potentials: Induced Rhythms or Separable Components?.- Thalamic Oscillations.- 12 Network Properties of the Thalamic Clock: Role of Oscillatory Behavior in Mood Disorders.- 13 Mesopontine Cholinergic Systems Suppress Slow Rhythms and Induce Fast Oscillations in Thalamocortical Circuits.- 14 Oscillations in CNS Neurons: A Possible Role for Cortical Interneurons in the Generation of 40-Hz Oscillations.- Cellular and Subcellular Mechanisms Based on Invertebrate and Simple Systems.- 15 Modification of Oscillator Function by Electrical Coupling to Nonoscillatory Neurons.- 16 Biological Timing: Circadian Oscillations, Cell Division, and Pulsatile Secretion.- 17 Comparison of Electrical Oscillations in Neurons with Induced or Spontaneous Cellular Rhythms due to Biochemical Regulation.- 18 Signal Functions of Brain Electrical Rhythms and their Modulation by External Electromagnetic Fields.- Theories and Models.- 19 Inhibitory Interneurons can Rapidly Phase-Lock Neural Populations.- 20 The Problem of Neural Integration: Induced Rhythms and Short-Term Correlations.- 21 Flexible Linking of Visual Features by Stimulus-Related Synchronizations of Model Neurons.- 22 Synergetics of the Brain: An Outline of Some Basic Ideas.- Epilogue.- Brain Natural Frequencies are Causal Factors for Resonances and Induced Rhythms.

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.

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.

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.

Wavelet Transform in the analysis of the frequency composition of evoked potentials

This technical paper deals with the application of the Wavelet Transform to the study of evoked potentials. In particular, Wavelet Transform gives an optimal time-dependent frequency decomposition of the evoked responses, something difficult to be achieved with previous methods such as the Fourier Transform. We describe in detail the protocol for implementing the decomposition based on the Wavelet Transform and apply it to two different types of evoked potentials. In the first case we study alpha responses in pattern visual evoked potentials and in the second case, we study gamma responses to bimodal (auditory and visual) stimulation. Although in this study we focus on methodological issues, we briefly discuss physiological implications of the present time-frequency analysis. Furthermore, we show examples of the better performance of the wavelet decomposition in comparison with Fourier-based methods.

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.

Detection of P300 Waves in Single Trials by the Wavelet Transform (WT)

The P300 response is conventionally obtained by averaging the responses to the task-relevant (target) stimuli of the oddball paradigm. However, it is well known that cognitive ERP components show a high variability due to changes of cognitive state during an experimental session. With simple tasks such changes may not be demonstrable by the conventional method of averaging the sweeps chosen according to task-relevance. Therefore, the present work employed a response-based classification procedure to choose the trials containing the P300 component from the whole set of sweeps of an auditory oddball paradigm. For this purpose, the most significant response property reflecting the P300 wave was identified by using the wavelet transform (WT). The application of a 5 octave quadratic B-spline-WT on single sweeps yielded discrete coefficients in each octave with an appropriate time resolution for each frequency range. The main feature indicating a P300 response was the positivity of the 4th delta (0.5-4 Hz) coefficient (310-430 ms) after stimulus onset. The average of selected single sweeps from the whole set of data according to this criterion yielded more enhanced P300 waves compared with the average of the target responses, and the average of the remaining sweeps showed a significantly smaller positivity in the P300 latency range compared with the average of the non-target responses. The combination of sweeps classified according to the task-based and response-based criteria differed significantly. This suggests an influence of changes in cognitive state on the presence of the P300 wave which cannot be assessed by task performance alone.