Incidence and development of rhythmic episodic activity in the electroencephalogram of a large rat population under chronic conditions.

689 long-term electroencephalographic recordings from 88 rats with chronically implanted electrodes were evaluated with reference to the onset and development of rhythmic spike episodes of 8-9 Hz frequency characteristic of quiet wakefulness. Episodes occurred in the recordings of 88% of the test animals. Recordings completely devoid of episodes came from selected rats highly sensitive to an acoustic epileptogenic stimulus and from 50% of rats of a Wistar-Velaz barrier breed. These two groups differed from the other animals in respect of their greater aggressiveness and restlessness in the chronic experiment. The episodes were not evoked by the surgical procedure--implantation of a cobalt-gelatin pin in the brain--or to the actual implantation of chronic electrodes in the brain. The shortest recorded latent period for the appearance of episodes was one week after the implantation of electrodes and the longest was 6 months. The animals' mean age at the time of the onset of episodes was 3.8 months and the lowest age was 2.5 months (i.e. immediately after implantation of the electrodes). The electrodes were not implanted before the age of 2.5 months. In repeated chronic recordings during the animals' life, both the number and the duration of the episodes increased. In most of the rats, the spike frequency in the episode was 8-9 Hz and was maintained at a stable value characteristic for each rat. Episodic rhythmic activity can thus occur in a state of quiet wakefulness in the electroencephalogram of all the given groups of rats (except highly sensitive animals) from early adulthood up to the age regarded in the rat as senility. The results to date indicate that the main condition for its appearance is the attainment of an optimum state of vigilance--quiet immobile wakefulness. The mechanisms leading to this stage of vigilance display both individual and group differences and are evidently related to the ability to become habituated to the experimental environment.