Conditioning of fear and conditioning of safety in rats.

In separate groups of rats forward and backward procedures for classical defensive conditioning were superimposed on on-going bar pressing for food. The forward conditioned stimulus elicited suppression of bar presses, indicating acquisition of fear. The backward stimulus paired with identical shock elicited behaviour typical for rats in a condition of safety and caused an increase of bar press rate. Enhancement of bar presses acquired in the course of backward conditioning was stable, immune to influences from unsignalled shocks presented in the same experimental context, and resistant to extinction when all shocks were discontinued. Properties of the employed variety of the backward conditioning procedure are discussed. When a brief shock overshadowed the onset of a backward stimulus, the remaining portion of the stimulus became a signal of safety.