Multicontact passive semiconductor electrodes, supplied by the University of Michigan, have been successfully implanted in the arm area of the supplementary motor cortex of a primate that was trained to do a number of different wrist movement tasks. The activated iridium recording sites were discovered to have impedance characteristics that were a function of a DC bias that was applied to the electrodes. Many times, when the electrode was in the high impedance state, neuronal activity was close to the noise level of the recording system, or nonexistent. Switching the electrode to the low impedance state lower-ed the noise of the recording system and revealed neuronal activity. A single pulse of less than 0.1 micro-amperes is sufficient to shift the electrode to the low impedance state. By using this pulse and biasing technique, we have been able to extend the recording time of the semiconductor electrodes to over eight months of chronic recording. Regrowth of connective tissue at the implant site was seen to break some of the silicon ribbon cables that were an integral part of the electrode. A new electrode and connector design has been developed that should overcome the breakage problem.
