We used EEG and ECoG to analyze and compare surface fields of interictal spikes from penicillin foci located on the mesial cortex of cats and from patients with midline spikes. Findings from the experimental model predicted results of the human study. When the field of a mesial cortical focus extended over the convexity, cortical scalp-recorded epileptiform discharges could be vertex negative, positive over the lateral convexity or a combination of these two in a surface dipole. Associated clinical features were in agreement with previous reports. We point out pitfalls in interpreting EEG recordings from patients with midline epileptogenic foci.