Differential effects of reinforcement of an inhibitory feature after serial and simultaneous feature negative discrimination training.

Two experiments using a conditioned suppression procedure with rat subjects found that postconditioning reinforcement of an inhibitory feature stimulus (X) had substantially different effects depending on whether a serial (A+, X----A-) or a simultaneous (A+, AX-) feature negative discrimination procedure was used to establish the inhibition. With the simultaneous procedure, acquisition of excitation to the previously inhibitory X was retarded when X was paired with shock. Subsequent summation tests showed no evidence of inhibition to X after reinforced X presentations. However, acquisition of excitation to X was unaffected by prior serial feature negative training, and X-shock pairings had relatively little effect on X's inhibitory power in summation tests. These data suggest that the nature of inhibition established in feature negative discriminations differs substantially depending on the temporal arrangement of stimuli. One possibility is that inhibitors established using simultaneous stimulus arrangements modulate behavior by acting on a representation of the unconditioned stimulus, but inhibitors established with serial procedures act on particular conditioned stimulus-unconditioned stimulus associations.