Latency to turn ICSS off and on: On time is more sensitive than off time to frequency and current variations

A preferred duration test of ICSS yielded two measures, ON Times and OFF Times, which were compared for sensitivity to four-fold variations in pulse frequency and current. ON Times, the duration over which the stimulation train was maintained, generally decreased as pulse frequency was increased from 40 to 160 Hz. However, the inverse trend showed marked differences among the 21 sites distributed from the ventral midbrain to the rostral diencephalon. At rostral sites ON Times changed only slightly as frequency increased whereas at caudal sites they declined sharply. OFF Times, the time between trains, in contrast to ON Times generally showed less change with increased frequency and no anatomical dependence. Increased current, tested in 15 sites, generally produced shortened ON Time and the sensitivity showed a moderate anatomical dependence. OFF Times also decreased but the sensitivity to current was unrelated to the sensitivity of ON Times and showed no anatomical dependence. At the same sites and with equivalent charge, ON Times were consistently more sensitive to frequency than to current. The magnitude of decrease in OFF Times to increased frequency and current were unrelated.