A method for selectively interacting with electrically excitable tissue of a patient is provided. In one configuration, an implantable pulse generator with a number of outputs and an array of electrodes with a number of electrodes being greater than the number of outputs may be implanted in a patient. An extension unit may be implanted between the implantable pulse generator and array. The extension unit acts to electrically couple the inputs of implantable pulse generator with the greater number of electrodes in the array so that the output sources are coupled to a portion of the electrodes.
