licable and careful analysis, one trial at a time, "has allowed researchers to predict and explain the occurrence of blocking, overshadowing, and unblocking" (Williams, Haddad & Strobel, 1989, p. 472). Experiments with animals have expanded on the basic blocking concept. The strength and limits of the concept have been tested, for example, by experimenting on the effects of blocking when conditioning to serial stimuli is involved. Aguado, Lopez and Lillo (1989) found that conditioning subjects to a conditioned stimulus C prior to pairing compound AB with the unconditioned stimulus resulted in the disappearance of the blocking effect.
The debate between attentional and associative causes of blocking has also been investigated. Williams et al. (1989) demonstrated the effect of blocking in the case of two-choice discrimination tasks in monkeys. But they but found no effect of unblocking when there was a decrease in the magnitude of the reward provided by the selection. Their general results were not consistent with associative learning models, howe
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