1. A system for controlling at least one change in brain state of a subject comprising: (a) receiving unit configured to receive a signal indicative of die subject's brain activity; (b) a waveform bank containing a plurality of, or waveform components of a plurality of, stimulation waveforms; (c) output unit configured to deliver the at least one stimulation waveform to the brain of the subject; and (d) signal processing unit configured to: (1) analyze the signal indicative of the subject's brain activity to determine at least one measure of brain excitability, (2) determine when the measure of brain excitability reaches a prespecified level associated with a state change, and (3) trigger the output unit to deliver a stimulation waveform; and (e) control unit configured: (1) to select from the waveform bank, or construct from waveform components in the waveform bank, a first stimulation waveform or train of waveforms, (2) to assess therapeutic efficacy or safety of at least one stimulation with the selected waveform being delivered by the output unit to control the brain state of the subject or to control a level of brain excitability of the subject, and (3) if the therapeutic efficacy or safety is determined to be ineffective or unsafe, to select or construct a second stimulation waveform from the waveform bank for use by the output unit; and (f) wherein the output unit is configured to also determine orientation of the exogenous field in reference to the field generated by the epileptogenic zone, number and location of anodes and cathodes used by the output unit and their location in reference to the epileptogenic zone.