The Evaluation Period begins once the subject is implanted with the RNS® System and continues through the 4th month. Detection of epileptiform activity is enabled for all subjects within the first post-operative month. Responsive stimulation is enabled and optimized for subjects enrolled in the open label protocol or randomized to the Treatment group. Subjects randomized to the Sham group undergo simulated stimulation programming in order to maintain the treatment blind. Randomized subjects will not know whether responsive stimulation is being delivered or not.