Relationship between photically evoked after-discharge occurrence and hippocampal EEG rhythms in restrained and unrestrained albino rats

The relationship between photically evoked after-discharge (PhAD) occurrence and dorsal hippocampal EEG patterns was examined in restrained and unrestrained albino rats during the presentation of 25 photic stimuli. Although the restrained and unrestrained conditions yielded significant differences in PhAD occurrence and type of hippocampal EEG pattern exhibited, it was determined that if the hippocampus displayed a rhythmical slow wave activity (RSA or theta) PhADs were not elicited. These hippocampal RSA periods were highly correlated with grid activity in the unrestrained condition. PhADs were elicited only during hippocampal EEG episodes that could be characterized as either large amplitude irregular slow waves or small amplitude irregular waves.