Two types of interictal transients of reversed polarity in rat hippocampus during kindling.

Two different types of spontaneous interictal transients (SITs) have been found in the course of the development of an epileptogenic focus by Schaffer collateral kindling in the CA1 region of rat hippocampus. The first type has peak amplitudes of 250 microV up to 1 mV, a duration of about 80 msec and the same polarity as field potentials recorded from the same sites evoked by Schaffer collateral stimulation. These SITs occur early during kindling, but are also observed at later stages. The second type, which is dominant in later stages of kindling, has peak amplitudes of 2-3 mV, a duration of about 20 msec and the opposite polarity; in a later stage it may also contain a clear population spike. Both types can be considered as being caused by equivalent dipoles situated across the pyramidal cell layer of CA1 but with inverted polarities.