Spectral Analysis of Bioelectric Activity of the Rat Cerebral Cortex: a Functional Approach to Determining Theta-Range Boundaries

In translational studies, the amplitude–frequency parameters of the theta range of the total bioelectric brain activity in laboratory rodents are used to objectify the state of the central nervous system. In recent years, attempts have been made to standardize experimental quantitative studies of bioelectric brain activity in rats. The guidelines of the International League against Epilepsy point out that spectral analysis of rat brain bioelectric activity should not be performed using the frequency ranges recommended for analyzing human EEG. Meanwhile, no specific recommendations for the boundaries of frequency ranges have been developed. The aim of this work was to conduct a comparative spectral power analysis of bioelectric activity of the rat cerebral cortex with different boundaries of the theta and alpha ranges to substantiate the parameters of spectral analysis. The study was carried out on 42 outbred male rats, in which the electrocorticogram was recorded during exploratory activity. In 75% of cases, the peak frequency of the theta rhythm was close to 7.0 Hz. When performing spectral analysis with the right boundary of the theta range at 8.0 Hz, the power of the theta range dominated the spectral composition. In 25% of laboratory animals, a high-frequency theta rhythm (7.5–8.5 Hz) was recorded. During spectral analysis using the boundaries of the theta (4.1–8.0 Hz) and alpha (8.1–13.0 Hz) bands, commonly accepted in clinical neurophysiology, the distribution of spectral powers was inverted: the alpha range dominated the power of theta activity. Expanding the right boundary of the theta range to 9.0 Hz enabled obtaining physiologically substantiated results—the theta band dominance in the total spectral power. Thus, when performing spectral analysis of bioelectric brain activity in rats, the right boundary of the theta range should be expanded to at least 9 Hz. For a more precise determination of the theta range right boundary, the peak frequency value of the theta rhythm recorded during animal’s exploratory activity should be used as a criterion.