Surprise and change: variations in the strength of present and absent cues in causal learning.

It is said that "absence makes the heart grow fonder." But, when and why does an absent event become salient to the heart or to the brain? An absent event may become salient when its nonoccurrence is surprising. Van Hamme and Wasserman (1994) found that a nonpresented but expected stimulus can actually change its associative status-and in the opposite direction from a presented stimulus. Associative models like that of Rescorla and Wagner (1972) focus only on presented cues; so, they cannot explain this result. However, absent cues can be permitted to change their value by assigning different learning parameters to present and absent cues. Van Hamme and Wasserman revised the Rescorla-Wagner model so that the a parameter is positive for present cues, but negative for absent cues; now, changes in the associative strength of absent cues move in the opposite direction as presented ones. This revised Rescorla-Wagner model can thus explain such otherwise vexing empirical findings as backward blocking, recovery from overshadowing, and backward conditioned inhibition. Moreover, the revised model predicts new effects. For example, explicit information about the absence of nonpresented cues should increase their salience (that is, their negative a value should be larger), leading to stronger associative changes than when no explicit mention is made of cue absence. Support for this prediction is detailed in a new causal judgment experiment in which participants rated the effectiveness of different foods' triggering a patient's allergic reaction. Overall, these and other findings encourage us to view human causal learning from an associative perspective.