p504fig13.31 Behavioral contrast can occur during reinforcement learning due to decreases in either positive or negative reinforcers.  See Figure 13.32 for illustrative operant conditioning data.<BR>||  Behavioral contrast: rebounds!  Shock level vs trials.  1. A sudden decrease in frequency or amount of food can act as a negative reinforcer: Frustration.  2. A sudden decrease in frequency or amount of shock can act as a positive reinforcer: Relief.