Relapse of generalized bait shyness in rats after constant and graded extinction procedures

Oral ingestion of a toxic lithium chloride (LiCl) solution yields a conditioned aversion to a safe sodium chloride (NaCl) solution in rats, presumably because of the common quality of saltiness. By using this generalized bait-shyness paradigm, we examined the relapse of acquired and subsequently extinguished salty taste aversion. After conditioning by oral ingestion of a 0.15 M LiCl solution, extinction was induced by two different experimental procedures. In the constant extinction group, rats were repeatedly exposed to 0.15 M NaCl solution. For rats in the graded extinction group, the concentration of NaCl was gradually increased from a low level to 0.15 M and an additional interfering sweet taste was gradually faded out. Rats in the graded extinction group were more prone to relapse than animals that underwent the constant extinction treatment. This difference was manifested in the following three response reoccurrence phenomena: renewal upon return to the acquisition context (Experiments 1A–C), spontaneous recovery after a 23-day interval (Experiment 2), and reacquisition caused by the second LiCl intake (Experiment 3). Theoretical and clinical implications of these results are discussed.