Bidirectional heart rate responses in rats associated with excitatory and inhibitory stimuli

Following classical conditioning to a shock-reinforced tone CS (T₁+), heart rate (HR) of three groups of rats was examined in response to a new reinforced tone (T₂+) and a nonreinforced light (L?) CS given during conditioned inhibition (T₂+ vs. T₁L?), discrimination conditioning (T₂+ vs. L?), or explicitly unpaired (T₂/L? vs. US alone) procedures. Decelerative HR reactions occurred to the reinforced T₂+ and T₂+ CSs. To the respective nonreinforced CSs, the conditioned inhibition group displayed diminished but sizable HR deceleration, the discrimination group showed near-zero responding, and the explicitly upaired group showed HR acceleration. Subsequent reversal conditioning to L was retarded in the conditioned inhibition and explicitly unpaired groups relative to the discrimination group. Group differences on combined-cue [T₂/L?) trials were not found. Both the HR responses during inhibitory training and the reversal-conditioning impairments suggest that inhibition may have been established to L? in the conditioned inhibition and explicitly unpaired groups.