Focal interictal epileptiform discharges (FIED) in the epicortical EEG and their relations to spinal field potentials in the rat.

To study the relations between epileptiform potentials in the surface EEG and motor phenomena, focal interictal epileptiform discharges (FIEDs) were induced by the combined application of penicillin and penicillinase to the cortical surface of the rat. Spinal field potentials (SFPs) served as an indicator of descending activity. The following results were obtained. (1) The focus induced by the technique generated epileptiform activity within a very restricted cortical region. (2) FIEDs were accompanied by synchronized SFPs in the cervical and lumbar cord if the focus was located in the corresponding cortical motor area. With FIEDs in non-motor areas SFPs failed to occur. A unilateral focus led to SFPs on both sides of the spinal cord. (3) The transverse distribution of field potentials within the spinal cord revealed that the early negative peak of the SFP reached its maximum in the dorsal horn. (4) After the application of the drugs the development of FIEDs preceded that of SFPs by ca. 5 min. When the drug effect ceased SFPs remained often fully developed up to 10 min after the FIEDs had disappeared. (5) The temporal relationship between the cortical and spinal events showed individual and interindividual variations. (6) Anodal and cathodal electrical polarization of the cortical surface by currents sufficient to alter the shape, amplitude and polarity of the FIEDs failed to change the SFPs. The present investigations demonstrate that the cortical output from a restricted focus does not correspond to a definite epicortical field potential.