Spontaneous seizure electrocortical activity in rats: relation to arousal level.

The relationship between neocortical high voltage spindle activity (HVS) and the level of arousal (state of vigilance) was investigated in imp-DAK rats. Two groups of single records (6 h and 24 h) and one group of repeated ones (6 h daily for three consecutive days) were analysed. Five levels of arousal (states) were distinguished on the basis of hippocampal and neocortical EEG: high arousal (HA), moderate arousal (MA), low arousal (LA), slow wave sleep (SWS) and paradoxical sleep (PS). The number of HVS episodes and the cumulative amount (i.e. summed duration) of each state was calculated for each successive hour of recording. It was found that the number of HVS episodes correlated positively with the amount of the MA state, but not with the amount of the remaining states. The rate of HVS occurrence during the MA state, however, was not stable. It decreased along with the decrease in amount of MA in successive recording hours of the same recording session. However, at the beginning of each successive session, the HVS episodes became more numerous, whereas the MA amount decreased. The accompanying changes in the cumulative amount of the remaining states confirm indirectly the supposition that HVS occurrence is related to transitions from one state to another rather than to a given state itself.