Blocked and overshadowed stimuli are weakened in their ability to serve asblockers and second-order reinforcers in Pavlovian fear conditioning

The ability of a blocked or overshadowed conditioned stimulus (CS) to serve as (a) a blocker or (b) a 2nd-order reinforcer in Pavlovian fear condition ing was tested in 152 albino rats. CS-evoked suppression of barpressing for food was the index of conditioned fear. Experiments 1 and 2 showed that an overshadowed CS was weakened in its ability to serve as a blocker. In Expe riment 2, a blocked CS was similarly weakened. Experiment 3 showed that an overshadowed and blocked CS was weakened in its ability to serve as a 2nd-o rder reinforcer. Experiments 4 and 5 failed to restore the blocking ability of blocked (Experiment 4) or overshadowed (Experiment 5) CSs by extinguish ing the CSs that had blocked or overshadowed them. Results favor a learning -deficit view of blocking and overshadowing.