Matt Tarler, building on the foundation constructed by Warren Grill, performed a series of animal experiments to document selective activation of the four major fascicles in the cat sciatic nerve. He explored the tripolar configuration where the center contact was a cathode and the flanking contacts were return anodes, each carrying approximately half the magnitude of the cathodic current. In the same set of experiments he explored the monopolar configuration where the anode was at a distant location. Experimentally, the cuff was the same in the two configurations, but in the monopolar configuration the connection to the flanking electrodes was moved to a distant metal contact. Take note, the current flow still results in current exiting the axons, at now, sites called virtual anodes, that are positioned just outside the edges of the cuff. Also, as I have tried to illustrate, Rehka Chintalacharuvu’s modeling indicated that the spread of stimulating current is constrained to regions closer to the contacts for the tripolar configuration than for the monopolar configuration.
Tarler, M.D. and J.T. Mortimer, “Comparison of Joint Torque Evoked With Monopolar and Tripolar-Cuff Electrodes, IEEE Trans. on Rehabilitation Engineering, Vol 11, pp227-235. 2003.