Simultaneous and backward fear conditioning as a function of number of CS-UCS pairings

Four experiments examined the possibility that the outcome of simultaneous and backward fear conditioning procedures might depend upon the number of CS-UCS pairings. A punishment procedure will rats as subjects and shock as the UCS was used; the amount of suppression produced by response-contingent CS presentations indexed the strength of acquired fear. Experiments 1, 3, and 4 examined the suppressive tendencies of simultaneous-and backward-trained CSs after 0, 10, 20, 40, 80, and 160 pairings. The pattern of data suggested that initial pairings have the effect of increasing suppressive tendencies of the CS, while subsequent pairings decrease them. In addition, evidence of fear inhibition was found after 160 pairings in the case of the backward paradigm. Experiment 2 examined several nonassociative accounts based upon differential shock exposure. Groups given 10 pairings or 80 pairings were compared to ggroups given 10 pairings plus 70 shock-alone presentations. The results indicated that number of pairings, rather than number of UCS occurrences, is the important factor in decreasing the initial suppression. The evidence for eventual inhibition in the backward paradigm suggests that this occurs through the acquisition of inhibitory tendencies which are antagonistic to the previously conditioned excitation.