Effect of electrode size on brain stimulation

Behaviorally determined strength-duration curves of many brain pathways differ from those of peripheral nerves, or single fibers. The chronaxies measured using short-duration pulses are much shorter than those measured with long pulses. A suggested explanation for this was that the pathways have mixed populations of long- and short-chronaxie elements. We attempted to test this hypothesis by stimulating with electrodes of different surface area, on the assumption that they would fire different proportions of long-chronaxie elements at short pulse durations where their thresholds are high. For a given current a large electrode delivers a lower maximum current density than a small one. Electrode size was found to have no detectable effect on the shape of the strength-duration curve. An attempt to discover the characteristics needed to modulate neurons having a basically hyperbolic strength-duration curve so as to produce the empirical curve revealed that the process has a zero rheobase and is thus presumably a direct influence of current on the threshold of the stimulated axons. A probable candidate for this process is the accumulation of potassium iontophoretically driven from cells in the vicinity of the electrode.