The use of 2-dimensional displays from array ECoG data is suggested as an aid for the intraoperative localization of epileptic foci. In this context, the importance of identifying the occurrence of focal epileptic activity is emphasized. A comprehensive spatiotemporal method for such identification is proposed and some preliminary offline results are shown. A transformation is proposed for the selection of data segments that are likely to show evidence of focal epileptic activity when displayed as contour level maps. These potential field maps show the characteristic 'deflection' associated with focal epileptic activity and indicate the location of the focus.