Age-variant and age-invariant rhythmicities in the cortical and hippocampal electrical activities of neonatal rats

The neocortical and hippocampal electrical activities were studied in 8, 15, 22, and 29-day-old rats. Segments of EEG recorded for 4 hours were analyzed by means of fast Fourier transformation. The sleep-wake activity stages were determined. The developments in the neocortical and hippocampal EEG activities were characterized by the gradual appearance of adult-like EEG waves and a shift towards higher frequencies in the theta wave range of the hippocampal electrical activity. The power value in the theta range, however, exhibited age-invariant rhythmicities with cycle lengths of about 9.5-11 and 16-22 min. The manifestation of such an age-invariant rhythm can be interpreted as an electrical manifestation of the rest-activity cycle originally described by KLEITMAN25.