Neuronal activity evoked by chronically implanted intracortical microelectrodes

The averaged evoked compound action potentials (AECAPs) were recorded from the ipsilateral pyramidal tract of awake, unrestrained cats before, during, and after continuous electrical stimulation of the cerebral cortex via chronically implanted activated iridium or platinum-30% iridium (Pt30%Ir) microelectrodes. After stimulating 24 h at 20 pulses per second (pps), using charge-balanced, 200-μs pulse pairs of 40 to 80 μA (400 to 800 μC/cm 2, 8 to 16 nC/phase (ph), 2 to 4 A/cm 2), there was a transient elevation of the threshold of the early (direct) and of the late (transynaptic) components of the AECAP. After cessation of continuous stimulation at 80 μA, the threshold of the early component of the AECAP remained elevated for as long as 24 h and the late component as long as 4 days, indicating significant but reversible depression of the electrical excitability of cortical neurons close to the microelectrodes. In three cats stimulated 23 h/day for 1 week, the AECAP also recovered to their prestimulus threshold. In contrast, pulsing for 24 h at 320 μA (3200 μC/cm 2, 64 nC/ph, 16 A/cm 2) produced marked elevation of the threshold of the AECAPs which was not reversed by 7 to 12 days after termination of intracortical stimulation. The electrical excitability of neurons adjacent to (unpulsed) microelectrodes 2 mm from the pulsed electrode was not affected. The observations resported here, in conjunction with the histologic results reported in the companion paper, indicate that both the Pt30%Ir and the iridium microelectrodes can be operated safely at currents to at least 80 μA, charge/ph of 16 A/cm 2, and a charge density of 800 μC/cm 2 · ph. However, on the basis of the electrophysiologic criteria, both types appear to be unsafe when pulsed at 320 μA (64 nC/ph, 3200 μC/cm 2 · ph, 16 A/cm 2).