Abstracts

Rationale: Intracranial electrographic (ECoG) monitoring of ictal and inter-ictal events has been used for the planning of surgical treatment of intractable epilepsy since the 1960s. The surgical plans have been determined by electrographic recordings of seizure onsets, and by the direction of propagation of ictal and inter-ictal activity. Methods: Here, we analyze the propagation direction of inter-ictal spikes (IISs) using a fixed-geometry micro-electrode array. In addition to a subdural ECoG grid (3-4.5 mm diameter discs, 1 cm spacing), patients undergoing ECoG monitoring for surgical planning were implanted with a micro-electrode array in neocortex. The micro-electrode array used in these studies had 100 40-µm diameter conical electrodes on 1-mm long shafts in a 10x10 configuration with 400-µm inter-electrode spacing. Data were recorded from the macro-electrode grid and the intraparenchymal micro-electrode array continuously during the patients’ clinical evaluation. Large-amplitude IISs recorded over several hours were analyzed. The local field potentials recorded by the micro-electrode array were mapped to the physical layout of the array. The slope of voltages across the micro-electrode array was computed sequentially for each voltage sample recorded (sampling rate 30kHz) during an IIS. The propagation direction of individual IISs was determined by the maximal slope of the voltages across the micro-electrode array that occurred during a given IIS. The maximal slope always occurred during the negative-going phase of the IIS. Results: Analysis of the propagation of large IISs indicated that they traveled predominately in only a few directions. The propagation of IIS recorded on the microelectrode array was correlated with the propagation of IISs recorded on the subdural macro-electrode grids. Conclusions: Directionality of IIS propagation can be detected using a microelectrode array. This approach, particularly when combined with recordings of single-neuron action potentials, should allow better analyses of the neuronal networks in human epileptic brain.