Event-related theta rhythms in cat hippocampus and prefrontal cortex during an omitted stimulus paradigm.

In a recent study on human subjects which performed a time prediction task in an omitted stimulus paradigm we found an increase in the amplitudes of evoked potentials (EPs) for the stimulus preceding the omitted stimuli, probably due to expectancy and focused attention. The amplitude increases were dominant over frontal and parietal association areas and were mainly reflected in enhancements of the theta (3-6 Hz) components. In the present study we analyse the responses of the auditory cortex (GEA, gyrus ectosylvianus anterior), different parts of hippocampus (CA1, CA3), reticular formation (RF), and motor cortex (MC) of the cat brain using a similar paradigm. Similar theta component increases of the EPs were obtained in various parts of the hippocampus, which were dominant in pyramidal cell layers (CA3, CA4), and in motor cortex. The results are interpreted as signs of a diffuse theta response system in the brain including mainly the hippocampus and frontal and parietal association areas. The diffuse theta system is probably involved in general attention and expectancy processes.