Appetitive differential conditioning of the rabbit's jaw movement response. Effects of cue similarity and US magnitude.

Two experiments examined appetitive differential conditioning of the rabbit's jaw movement response (JMR) in a two-phase procedure. The first phase entailed reinforced training with one conditioned stimulus (CS+), and the second phase involved intermixed presentations of CS+ and an unreinforced stimulus (CS-). In Experiment 1, CS+ was a 600-Hz tone, and CS- was either a 660-, 1,000-, or 2,100-Hz tone. In Experiment 2, the magnitude of the water unconditioned stimulus (US) paired with CS+ was either 1, 3, or 9 ml. The experiments revealed that 1) the level of responding to CS- rose for several days and then declined over the remainder of training; 2) the physical similarity between CS+ and CS- directly affected the level of responding to CS- but had no discernable effect on the level of responding to CS+; and 3) US magnitude positively affected the level of responding to CS+ and, to a lesser extent, CS-. The results are discussed in terms of their implications for Spence's gradient interaction theory and Pavlov's external inhibition hypothesis.