Effective stimulation distance for current from macroelectrodes

Effective spread of stimulating current from macroelectrodes was measured using antidromic responses of axons of the pyramidal tract as an indicator of excitation. Both monopolar and concentric bipolar electrode configurations were tested with stimulating distances as large as 7 mm. The effective stimulation distance was greater from monopolar electrodes especially at greater current strengths, but differences between the two configurations were frequently small and reversals of this trend occurred. There was no statistically significant difference between the estimates of effective stimulation distance made using large and small axons. The shape of current-distance curves was approximately parabolic using both bipolar and monopolar stimulation. A current strength of 0.5 to 1.0 mA will confine effective current from a monopolar electrode to a sphere of 2-mm radius, but will not stimulate all elements within that area. Even in a brain area as homogeneous as the pyramidal tract, there is still a great deal of variability from mean values in effective stimulation distance. Presumably, the variability would be even greater in more heterogeneous regions.