Two ways to deepen extinction and the difference between them.

A series of experiments used rats to compare and contrast the effects of subjecting an already extinguished target conditioned stimulus (CS) to additional extinction in compound with either another extinguished or a nonextinguished CS. Exposure to either compound restored responding and their extinction deepened the loss of fear responses (freezing) to the target relative to a CS given equivalent extinction in isolation. This deepening was greater to the target extinguished in compound with the nonextinguished than with the extinguished CS. Summation tests showed that the target suppressed responding to an excitor or an excitatory context when it had been extinguished in compound with a nonextinguished but not with an extinguished CS. The results were interpreted to mean that additional extinction in compound with another extinguished CS resulted in the target acquiring the additional inhibition required to more fully oppose its original excitation. In contrast, additional extinction in compound with a nonextinguished CS resulted in the target acquiring not only the additional inhibition required to oppose its original excitation but also that of the nonextinguished CS, thereby converting the target into a net inhibitor.