Wavelet entropy analysis of event-related potentials indicates modality-independent theta dominance.

Sensory/cognitive stimulation elicits multiple electroencephalogram (EEG)-oscillations that may be partly or fully overlapping over the time axis. To evaluate co-existent multi-frequency oscillations, EEG responses to unimodal (auditory or visual) and bimodal (combined auditory and visual) stimuli were analyzed by applying a new method called wavelet entropy (WE). The method is based on the wavelet transform (WT) and quantifies entropy of short segments of the event-related brain potentials (ERPs). For each modality, a significant transient decrease of WE emerged in the post-stimulus EEG epoch indicating a highly-ordered state in the ERP. WE minimum was always determined by a prominent dominance of theta (4-8 Hz) ERP components over other frequency bands. Event-related 'transition to order' was most pronounced and stable at anterior electrodes, and after bimodal stimulation. Being consistently observed across different modalities, a transient theta-dominated state may reflect a processing stage that is obligatory for stimulus evaluation, during which interfering activations from other frequency networks are minimized.

[The neuronal background activity of the rabbit hippocampus during functional exclusion of the structures that regulate theta rhythm]. / Fonovaia aktivnost' neironov gippokampa krolika pri funktsional'nom vykliuchenii struktur, reguliruiushchikh teta-ritm.

A method of controlled successive short-term (25-30 min) stabilization or suppression of the theta rhythm by local injections of lidocaine into median raphe nucleus or medical septal area (MS-DB) was tested for further analysis of the functional significance of theta-modulation in activity of hippocampal neurons. Extracellular recording of their activity (area CAl) was performed in unanaesthetized rabbits. Injection of lidocaine into MS-DS resulted in total suppression of theta modulation in neuronal activity and EEG of the hippocampus. Mean frequency of the background activity was not changed in the majority of the units but was significantly increased in a limited group of high-frequency neurons (putative inhibitory cells). Injection into medial raphe, the source of serotonergic afferents to MS-DB and hippocampus, was followed by an increase in regularity and frequency of theta modulation, by appearance of additional group of neurons with rhythmic modulation, and by expression of continuous theta in the hippocampal EEG. Mean frequency of discharge and its regularity were significantly increased in the majority of the cells. The data confirm the presence of tonic inhibitory control of theta-generating septo-hippocampal system by the medial raphe nucleus, which can be regarded as an antagonist of the midbrain reticular formation in this respect.

[The theta rhythm of the infant EEG and the development of the mechanisms of voluntary control of attention in the 2nd half of the first year of life]. / Teta-ritm EEG mladentsev i razvitie mekhanizmov proizvol'nogo kontrolia vmimaniia na vtorom polugodii pervogo goda zhizni.

The neurophysiological basis of attention control was studied in infants at the second half-year of life, i.e. in the period when the capability for voluntary control over behavior fundamentally improves. EEG was recorded in 60 infants aed 8-11 months in three experimental conditions: 1) attention to an object in the visual field (externally controlled attention, or the baseline state), 2) anticipation of a person in the peek-a-boo game (internally controlled attention), 3) attention to the reappeared person in the peek-a-boo game (control condition). The spectral analysis of the EEG data revealed a sharp increase in the EEG theta (3.6-6.0 Hz) during internally controlled attention as compared to the baseline and control conditions. The theta1 (3.6-4.8 Hz) increase was maximal in the frontal derivations. The reactivity of the frontal theta1 during internally controlled attention discriminated infants with different abilities to maintain this type of attention. The reactivity of the theta2 (5.2-6.0 Hz) was maximal in the right temporal derivation (T6) and did not depend on stability of the anticipatory attention. The findings point to different functional significance of the theta1 and theta2 rhythms in infants. It is suggested that synchronization of the frontal theta1 rhythm in infants reflects the activity of the anterior attention system which realizes the executive attention control. The ability to maintain anticipatory attention increased with age, whereas the frontal theta1 synchronization decreased and totally disappeared at the age of 11 months. At the age of 8 months there was a positive correlation between the frontal theta1 synchronization and behavioral index of stability of the internally controlled attention. On the contrary, this correlation was negative at the age of 9 and 10 months. It is suggested that the age-dependent dynamics of the relationship between the frontal theta1 reactivity and attention reflects a leap in maturation of the anterior attention system resulting in its more economic and efficient functioning.

[The influence of hippocampal field CA1 on the theta activity during rat orienting behavior]. / Issledovanie vliianiia polia CA1 gippokampa na teta-aktivnost' pri orientirovochnom povedeniia krysy.

The influence of CA1 hippocampal field on theta-activity production depending on the efficiency of CA1 synaptic inputs was studied in freely moving rats during their orienting behaviour. The long-term potentiation of Shaffer collaterals resulted in an increase in regularity of the theta activity due to a decrease in its amplitude-frequency modulation and variability of the waveform from cycle to cycle. Additionally, an increase in amplitude and decrease in frequency were observed. The results reveal the influence of CA1 field on the work of the septal theta activity oscillator.

[Differences in the the EEG frequency characteristics during the perception of positive-emotional, negative-emotional and neutral words]. / Razlichiia chastotnykh kharakteristik EEG pri vospriiatii polozhitel'no-émotsional'nykh, otritsatel'no-émotsional'nykh i neitral'nykh slov.

Power spectral analysis of the theta- and alpha-rhythms was performed in narrow frequency bands (1 Hz) for EEGs recorded from F3, F4, C3, C4, T3, T4, P3, P4, O1, and O2 derivations in 9 healthy adults viewing emotionally positive, negative, and neutral words displayed on the screen. The relative spectral power in the 7-8 Hz band was higher during presentation of the emotionally significant words than the neutral ones. The EEG reaction to emotional words within this frequency range depended on the word valency. The relative power increase was revealed in the range of 7-7.5 Hz during presentation of emotionally positive stimuli. This reaction was most expressed in the frontal, central, and temporal cortical areas. During presentation of the negative words similar EEG reactions were observed within 7.5-8 Hz. In this case the frequency changes dominated in symmetrical frontal, right central, temporal, and parietal cortical areas.

[The regulation of the septal pacemaker of theta rhythm by the midbrain raphe nuclei]. / Reguliatsiia septal'nogo peismekera iadrami shva srednego mozga.

Neuronal activity of the medial septum (MS--DB) and hippocampal slow electroencephalogram (EEG) were recorded in unanaesthetized rabbits with stimulating electrodes implanted in the median raphe nucleus and reticular formation of midbrain and pons. In all the animals with raphe electrodes the background frequency of theta-rhythm was decreased (mean 4.7 +/- 0.15 Hz) in comparison with the intact animals and those with stimulating electrodes in the reticular formation only (5.2 +/- 0.19 Hz, p < < 0.5). Stimulation of the raphe had effects of suppression of theta-bursts, decrease in their frequency and regularity, and appearance or increase of delta-modulation in MS--DB neuronal activity. The number of spikes per burst was decreased while interburst pauses became significantly longer. At the same time, gradual decrease of theta frequency and amplitude, theta suppression, appearance of irregular delta-waves and spindles were observed in the hippocampal EEG. Stimulation of the reticular formation evoked the opposite effects in MS--DB neuronal activity, as well as in the hippocampal EEG. The data confirm the role of the midbrain raphe in constraining the reticulo-septohippocampal system of theta-rhythm generation and do not support the statements on the existence of septum-independent high-frequency serotonergic theta-rhythm. It is suggested that theta-limiting raphe influences may be important for suppression and switching of attention.

Hippocampal and cerebral cortical sequencing of cardiac activity--theta rhythm (4-10Hz).

Interaction of repetitive evoked potentials using visual and auditory stimuli has been show for the hippocampus and cerebral cortex. Selected components of the electrocardiogram (EKG) are used to stimulate certain nuclei of the amygdala. Extracellular unit potentials of the hippocampus shown repetitive activity. The micro-EEG of the cerebral cortex and dorsal hippocampus show rhythmic activity. In the hippocampus computer analysis of these analog signals show an increase of the theta rhythm related to the EKG (QRST) stimulation. Repetitive activity related to the heart rate (EKG) is evident in the cerebral cortex during stimulation.

[The theta rhythm of the rat hippocampus as a nonlinear dynamic process]. / Issledovanie teta-ritma gippokampa krys kak nelineinogo dinamicheskogo protsessa.

The theta rythm of the rat's hippocampus in paradoxical phase of sleep and in orientational behavior is investigated by estimating the attractor's dimension. It is shown that the theta rhythm of the hippocampus consists of a regular component and an irregular one of high dimension which relation is in the wide variation depending on the state of the animal. The frequency characteristics of both components are revealed as well as the mechanisms of their forming. Some hypothesis that concern their biological nature are suggested.

[The spontaneous neuronal activity of the hippocampus during the modulation of the theta rhythm by cholinergic substances]. / Spontannaia aktivnost' neironov gippokampa pri moduliatsii teta-ritma kholinergicheskimi veshchestvami.

Background activity of the hippocampal neurons, extracellularly recorded in waking chronic rabbits, was analysed in control state and after systemic injection of physostigmine and scopolamine. Similar analysis was done in the hippocampus chronically deprived of ascending brain stem afferents. Cholinergic drugs controlled the number of hippocampal neurons with theta-modulation and the degree of its stability but not the frequency. Activation of cholinergic theta-rhythm resulted also in regularization of activity with suppression of delta-modulation and complex spike discharges; its blockade was accompanied by the opposite changes. Both drugs shifted the level of background activity in the majority of neurons, but the overall mean frequency did not vary between the states. Regression analysis demonstrated significant negative correlations with dominating decrease in the level of activity in high-frequency neurons ( > 25 sp/s) and its increase in low-frequency ones ( < 25 sp/s) after injection of both drugs. Stability of the overall mean frequency and uniformity of its shifts presumably indicate that the frequency, unlike the pattern of the background activity, is not directly controlled by the cholinergic septal input.