Electroencephalographic Correlates of Brain States during Verbal Learning: II. Characteristics of EEG Spatial Synchronization

Abstract

This work continues a study of the states of the brain cortex during memorization and retrieval from memory by statistical comparison of stable mean values of EEG parameters in these states and in the resting state. Our earlier report considered the characteristics of local synchronization of the EEG (the EEG absolute power) in all EEG frequency bands [1]. This work considers the characteristics of spatial synchronization, i.e., the mean value of EEG coherence in all frequency bands. The possible participation of nonspecific modulating systems and hypothetical particular systems of memory are discussed on the basis of data on both parameters. METHODSThe method of the study was described in detail in our previous report [1]; therefore, we will only briefly outline the most important elements.The study was carried out with the participation of 57 volunteers. The subjects performed tasks based on the technique of assessing the knowledge acquisition rate (an adapted variant) [2][3][4]. The EEG was recorded in the following states: resting with open eyes, memorizing verbal bilingual semantic pairs (Latin and Russian), and retrieving (control) the learned information. A series of seven word pairs was presented for memorization; the exposure of each pair lasted 5 s. Then, after the end of the series, the control was conducted: the Latin words of the previous series were presented and the answers of the subjects were recorded.For EEG recording, 19 active electrodes were used in accordance with the complete international 10-20 system with linked earlobe reference electrodes. Quantitative characteristics of the EEG were processed after the exclusion of artifactual fragments. The averaged values of the coherence function were calculated for each subject, each particular state, and each derivation pair in the frequency bands θ (4-7 Hz), α 1 (7-10 Hz), α 2 (10 − 13 Hz), β 1 (13-18 Hz), β 2 (18-30 Hz), and γ (30 − 40 Hz). Arrays of the estimates of the EEG coherence function were normalized using the transform Y = X 2 /(1 -X 2 )) . Repeated measures analysis of variance was used for statistical analysis. The significance of differences between mean values corresponding to the examined states was assessed according to the within-subjects design. For the analysis of the EEG coherence parameters, S × Z plans were used for each frequency band separately, where S is the factor State and Z is the factor Area (factor gradations were the specified pairs of derivations). The significance of the effects of the main factors and interactions was assessed using Greenhouse-Geisser correction. Significant differences between independent pairs were revealed by means of ( log Abstract -Spatial EEG synchronization was studied using mean EEG coherences in 57 subjects in a resting state with the eyes open, during memorization of verbal bilingual semantic pairs (Latin and Russian), and during retrieval (monitoring) of the learned information. Statistical comparison of the EEG spectral power in the frequency bands θ , α 1 , α 2 , β 1 , β 2 , and γ ...