Thresholds for conditioned suppression using X-rays as the pre-aversive stimulus

Four male, 12-week-old Sprague-Dawley rats were used to determine the threshold for behavioral response to X-irradiation using the conditioned suppression technique. They were maintained at 80 per cent body weight and initially trained to stable performance on a VI 1 min schedule with 16 per cent sucrose solution as reinforcement. After a stable baseline was obtained, animals were placed in the instrumental conditioning box beneath the X-ray machine for a half-hour session each day. While subjects were actively pressing the lever for reinforcement, 15-sec X-ray exposures of 0.5 R/sec were administered, followed immediately by electric shock. After all animals had exhibited conditioned suppression, the dose-rate was decreased in subsequent sessions in an effort to establish threshold. The results indicate that X-rays at a dose-rate as low as 0.004 R/sec can be an effective pre-aversive stimulus for the rat.