The apparent influence of informational factors on associative learning has been the focus of much attention by investigators who wished to account for this phenomenon within the classical Pavlovian conditioning paradigm. One of the most influential concepts in this area has been Kamin's (1969) notion of the blocking effect which he identified in animal experiments. Blocking describes the situation in which the prior conditioning of a subject to a conditioned stimulus (A) of a stimulus compound (AB) inhibits conditioning to the second conditioned stimulus B when the compound is paired with an unconditioned stimulus (US). The 'proof' of the blocking effect is provided by trials in which blocking does not take place when an unconditioned stimulus other than the one for which A had been previously conditioned is introduced to the compound AB (Dickinson, 1977, cited by Bonardi & Hall, 1994, p. 107). Kamin's (1969) findings were similar to the concept of overshadowing which had been described by Pavlov (1927). Overshadowing also takes place when a stimulus compound is paired with an unconditioned stimulus but without prior conditioning to either stimulus. In this case the difference arises when one stimulus is more salient than the other and the more salient stimulus may prevent conditioning to the less salient stimulus (Pavlov, 1927, cited by Hammerl, 1989, p. 74).
Basically it can be said that "the animal will not condition readily to B because it has been made redundant by A, already a perfectly good signal for reinforcement" (Bonardi & Hall, 1994, p. 107). But there is considerable debate over whether the blocking effect is related to attentional or processing factors. If learning does not occur with events that provide redundant information this provides a basis for accounts of blocking that claim that since, due to prior conditioning, A is a better predictor of the US than B the "subject rapidly learns to ignore B" (Holla...