Conditioning across the duration of a backward conditioned stimulus.

Five conditioned suppression experiments examined the extent to which an appetitively motivated lever-press response can be punished by different components of a backward conditioned stimulus (CS). Using a 0-s unconditioned stimulus (US)-CS interval, Experiments 1 and 2 showed that the initial 3 s of a normally 30-s backward CS served as a more effective punisher than the CS as a whole. Experiment 3 found no such effect if the US-CS interval were 3 s rather than 0 s. Experiments 4A and 4B found that if the US-CS interval were 0 s, the initial part of the backward CS acquired excitatory properties although the CS as a whole passed a summation test for conditioned inhibition. By contrast, the 3-s US-CS interval supported inhibitory conditioning across the whole duration of the backward CS. Taken together, these findings support a modified version of Wagner's sometimes opponent process model, which suggests that different components of a backward CS become either excitatory or inhibitory depending on the components' temporal proximity to the US.