Spatio-temporal Localization And Display Of Focal Epileptic Activity From The Electrocorticogram (ECoG)
The use of 2-dimensional displays from array ECoG ahta 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 spatio-temporul method for such identijication is proposed and some preliminary off-line results are shown. The accurate localization of epileptic foci is an important stage in the surgical procedure applied to eligible patients with focal epilepsy. Intraoperative electrocorticogram (ECoG) recording allows the neurosurgeon to confirm the presence of suspected foci and tailor the resection according to their localization [3]. Our research is ultimately aimed at the development of a realtime monitoring tool that can help the neurosurgeon in locating the foci. To this effect, ECoG signals are collected from a flexible planar array of 8 by 3 electrodes placed in direct contact with the cerebral cortex [5]. Normally. 16 bipolar derivations involving a matrix of 3 by 4 electrodes are recorded and digitized at a time. The effective transversal and longitudinal distances between electrodes are 1.5 and 2.0 cm respectively. The relative proximity of the electrodes allows a reasonable reconstruction of the potential 6eld sensed by the array. The display of this field as contour level maps is an attractive complement to the “traditional” interpretation of WOG signals, based on paper-chart plots of the bipolar derivations. Furthermore, once the signals are integrated in the form of a timevarying potential field, transformations can be applied in order to emphasize the presence of focal epileptic activity. However, the transient nature of this activity makes it necessary to select only some segments of the data stream for display as raw or transformed field maps. This paper presents a transformation proposed for the selection of data segments that are likely to show evidence of focal epileptic activity when displayed as contour level maps. One the most characteristic features of epilepsy is the paroxysmal activity which produces a “spW’ in standard EEG or ECoG traces. Recordings owned from the surface and different depth levels of the cortex in animal models of focal epilepsy [4] show that (interictal) “spikes” are the manifestation of transient current sinks that appear in superficial layers of the cortex. Several methods for the detection of “spikes” from EEG recordings pursue the identification of the transient nature and specific time course of these sources, as reflected in the potential changes measured by the EEG. Particularly, several techniques ([7], [a]) attempt to identify the characteristic “temporal sharpness” of the event by measuring the second time derivative of the recorded signals. Alternatively, some methods have been proposed for the identi6cation of focal activity by measuring the “spatial sharpness” of the potential field brought about by the presence of local current sourced sinks [l]. These methods apply the Laplacian transformation to the 2-dimensional potential field Mx, y) sampled in space by an array of electrodedn either case, only one of the distinctive characteristics of focal epileptic activity, temporal sharpness or spatial sharpness is being involved in its identification.