Canan Basar-Eroglu

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.

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.

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.

The associations between 40 Hz-EEG and the middle latency response of the auditory evoked potential.

The study of the 40 Hz activity of the brain which was begun by Adrian (1942) has been furthered for the past years with several new approaches: A neural model for the generation of 40 Hz activity in attention has been published by Freeman (1975) whereas new possibilities in clinical applications and exploration of cognitive processes in an extended manner was proposed by Galambos and coworkers (1981). The present study demonstrates the relation between 40 Hz spontaneous activity in human EEG-recordings and the middle latency response of the auditory evoked potentials. The applied signal analysis method allows also a single trial analysis of EEG-EP epochs which can be extended in studies on cognitive processes. A perspective concerning the middle latency response of the auditory AEP is also given.

A New Approach to Endogenous Event-Related Potentials in Man: Relation Between Eeg and P300-Wave

The family of late positive waves which occur at latencies of 250 to 500 msec (P300) has been the most widely studied endogenous event related potential of the brain. We performed experiments with human subjects by applying repetitive acoustical tone bursts; every third or fourth signal was omitted. The endogenous potential related to the omitted stimulation was first averaged. The averaged record was then transformed to frequency domain with Fourier analysis in order to obtain the frequency characteristics. We observed frequency selectivities between 1-2.5 Hz, 3-8 Hz and between 8-13 Hz. After this procedure, single epochs of prestimulus EEG and endogenous potentials were pass-band filtered in the above frequency ranges. The comparison of filtered prestimulus EEG and of single endogenous responses gave the following results: In delta and theta frequency ranges the EEG prior to omitted stimulation showed in most of the trials a phase-reordering; the P300-wave seems to be a continuation of the ordered and stationary pre-EEG. In some cases the P300-wave depicts an amplitude-enhancement against the delta and theta components of the pre-EEG. Alpha-enhancement and alpha-blocking also make important contributions to the organization of the endogenous event-related potentials.

Frontal gamma-band enhancement during multistable visual perception.

The aim of our study was to find out whether an increase in the gamma band may be related to the reversal phase during viewing of an ambiguous pattern. The present study describes the significant gamma band (30-50 Hz) activity increase in EEG during states of perceptual switching (reversal state). In our experiments the multistability was induced with an ambiguous stimulus pattern, known as stroboscopic alternative motion (SAM). The investigations carried out in 11 subjects included a measuring strategy with three different experimental conditions: (1) recording of spontaneous EEG as baseline; (2) recording of the EEG during naive observation of the ambiguous pattern; (3) recording of EEG during active observation of SAM. The results indicate that the multistable perception is one of the multifold cognitive processes giving rise to 40 Hz enhancement in the entire cortex. The most significant 40 Hz enhancements were measured in frontal areas and can reach increases of 40 to 50% in states of naive and active observations of SAM, respectively, in comparison to spontaneous EEG recordings. The results indicate that the increase of frontal gamma band is related to the destabilization of the perceptual system when viewing multistable patterns.

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.

The EEG is a Quasi-Deterministic Signal Anticipating Sensory-Cognitive Tasks

One of the main concerns of brain research is to measure the brain’s electrical activity and, in this way, to try to detect the coding of behaviorally relevant information in the central nervous system (CNS). Analyses of the EEG, of evoked potentials (EPs), and of endogenous potentials (the P300 family) are among the most fundamental research tools for understanding the sensory and cognitive information processing in the brain. Many reports in this volume show the broad extent of applications of the EEG, of sensory EPs and event-related potentials (ERPs) to the understanding of CNS information processing and behavior.

Reversal-rate dependent differences in the EEG gamma-band during multistable visual perception.

It is an often reported observation in the literature on multistable perception that the reversal rate within a given observation time is subject to a high interindividual variability. Recently, we reported frontal gamma-band enhancement during multistable visual perception. The aim of the present study was to investigate whether changes in the gamma-band correspond to the variability of the reversal rate. Therefore, a total of 25 observers were divided into two subgroups according to their reversal rate during a 400-s observation period of a reversible pattern based on apparent motion. Subjects with more than 40 reversals within the 400-s were defined as high-rate switchers (HRS). Subjects with a reversal rate below 40 switches were defined as low-rate switchers (LRS). EEG was recorded from frontal, central, parietal, and occipital locations of both hemispheres. The results showed significantly higher gamma activity for the HRS in both phase-locked and non-phase-locked oscillations. Both subgroups showed the highest gamma amplitudes at frontal locations. The results support the involvement of attentional top-down processing in figure reversal. It is concluded that the higher gamma activity for the HRS reflects states of higher arousal, alertness and/or attention according to their fast reversal rate.