separate channels of ipg 12 connected to 16 a b and 16 b c stimulation pulses a 2 and b 2 may have different amplitudes for contacts 16 a b and 16 b c however these pulses should largely overlap in time to create an electrical field promoting the stimulation of dorsal column fibers and the inhibition of dorsal root fibers according to the concept known as u201c transverse tripolar stimulation u201d and described in u s pat no 5 501 703 and in a paper med biol eng comp 1996 34 273 279 likewise pre pulses a 1 and b 1 should largely overlap in time to promote the sensitization of dorsal column fibers and desensitization of dorsal root fibers when a third channel of ipg 12 is available this channel can be connected to a contact 16 a b c and to the metal casing of the ipg for either case for every stimulation pulse the invention anticipates the application of pre pulses it is believed to be important to have a zero net charge to and from electrodes 16 a b c for each stimulation pulse this minimizes electrode degradation and cell trauma ordinarily a zero net charge is accomplished by applying.