Behaviorally measured refractory periods are lengthened by reducing electrode tip exposure or raising current

Small tip exposures and high currents allow the stimulation of high-threshold axons. These studies were conducted to measure the effects of current and tip surface area on stimulation parameters required to produce a constant rate of self-stimulation of lateral hypothalamic sites, or of circling when stimulating medial brain stem sites. Thresholds for circling decreased when tip surface area was decreased to 0.01 mm2. Refractory period curves for both circling and self-stimulation were shifted to slightly longer intrapair intervals when tip surface area was decreased or current was increased. This suggests that long refractory period axons make larger contributions when current density is increased. Chronaxies were not lengthened by small tip exposures but were reduced when electrodes were placed far lateral to the circling substrate.