Baseline transition sleep and associated sleep episodes are related to the learning ability of rats

The EEGs of 18 adult male Wistar rats were recorded during a baseline session lasting 7 h (day 1). The following day, rats were trained for a 2-way active avoidance task in an automated shuttle-box. A retention test was scheduled on the third day. On the basis of the number of avoidances scored during the training and retention sessions, rats were assigned to a fast-learning group (FL; achieving criterion during the training session), a slow-learning group (SL; achieving criterion in the retention test session), and a nonlearning group (NL; failing to achieve criterion). Vigilance states were determined by analyzing EEG data in 5-s epochs and calculating EEG power spectra of consecutive time intervals as short as 1 s. This high-resolution method led to the identification of transition sleep episodes that followed slow-wave sleep (SS) and were followed by waking (TS-->W) or by paradoxical sleep (TS-->PS). Comparison of the baseline sleep variables of the 3 behavioral groups revealed the presence of several significant differences. These observations were confirmed by the results of correlative analyses between baseline sleep variables and number of avoidances scored during the training and retention sessions. The most reliable indices of the capacity to learn the avoidance task were the amounts of SS preceding the TS-->W or the TS-->PS sequence, and the amounts of either component of the latter sequence. These variables displayed markedly higher values in FL rats. In addition, the amount of SS preceding TS-->W and the amount of TS-->(W) were significantly correlated with the number of avoidances scored during the training session. On the other hand, 1' SS-->(PS) and (SS)-->PS episodes were longer in NL rats than in SL or FL rats, respectively; and 2, the duration of SS-->(PS) episodes was inversely correlated with the number of avoidances of the first training period. The data are interpreted to suggest that TS and associated sleep episodes may predict the acquisition of the avoidance task, and the episodes of SS-->PS not associated with TS may predict the retention of innate responses, such as freezings or escapes.