THE PULSE-CLAMP METHOD FOR ANALYZING THE ELECTROCHEMISTRY ON NEURAL STIMULATING ELECTRODES

A pulse-clamp method has been developed for measuring the electrochemical processes occurring on neural stimulating electrodes in the 1 00 mus time frame of a typical stimulus pulse. By immediately following the controlled current stimulus pulse with a controlled potential step back to the prestimulus potential, we can measure the reversal of the reversible reactions that occurred during the stimulus pulse. Using this information we can then deduce the reaction that occurred during the stimulus pulse and how much charge went into each process. This method employs an electronic instrument that can be operated in (i) regulated current mode, (ii) regulated voltage mode, and (iii) open-circuit mode with a switching time of 2 mus between modes. The frequency response of the instrument is linear up to 10(5) Hz, permitting reliable measurements in the 10 mus range. A series of experiments are described for 100 mus pulses applied to gold electrodes in 0.15 M sulfuric acid. The results of these experiments show that: (i) the kinetics of the electrochemical reactions occurring on an electrode surface cannot be neglected and (ii) limiting the electrode potential to not exceed the H-2 evolution occurring on an electrode with slow cyclic voltammetry, significantly underestimates the charge that can be injected without evolving H-2.