The stability of the electrical stimulation map of the motor cortex of the anesthetized baboon

We have made repeated maps of the movements obtained by threshold electrical stimulation, with surface cathodal trains of pulses, of the motor cortex of 4 baboons, using chronically implanted arrays of electrodes. The electrode arrays were judged to remain in a constant relationship with the cortex because they did not move relative to the skull, and the arm/leg border obtained remained stable. There was evidence of a stability of response over a period of weeks. In 3 of the 4 animals, there was stability of the responding digit(s) regardless of the movement made by the digit(s), and of the digital movements performed (regardless of the digits involved). The stability of the map from minute was almost complete. Those variations that occur do so mainly by confusion within the motor homunculus, rather than by its movement as a whole across the cortex. Reasons for the imperfect long-term stability of the map are discussed in terms of variations in the physiological 'set' of the presumed polysynaptic pathways that are stimulated.