Determining thresholds of aversive brain stimulation

¹ Merck Institute for Therapeutic Research, West Point, Pennsylvania

This study describes a method for determining thresholds of aversive brain stimulation. Chronically indwelling electrodes were implanted in selected brain areas of monkeys, who were then trained to press a lever in order to reduce the electric current stimulating these areas. In the absence of a response, the current rose in small steps until responding, which decreased the current in steps, was again initiated. Following the demonstration of rather precise, immediate control of lever pressing by the brain stimulation, variables affecting magnitude and variability of the threshold were explored. It was found that an increase in size of the current steps increased peak current levels and width of the oscillations. Using a variation of the technique where responding terminated brain stimulation (rather than reducing it in steps), it was shown that increasing the number of responses required for termination elevated threshold and lowered variability. An analgesic drug, anileridine, caused a threshold rise with larger doses having greater effects. In an experiment where maximum current was selectively limited, it was found that current levels the animal would tolerate continuously were levels that normally evoked responding. These data were interpreted as indicating that behavioral control by brain stimulation is complicated by discriminated avoidance behavior with lower current levels functioning as warning stimuli.