Blocking as a function of novelty of CS and predictability of UCS

Kamin (1969) reported that after rats had been trained with one CS predicting shock, they learned virtually nothing about a second CS added to the first to form a compound--provided that the shock remained unchanged. If, however, the shock was either increased or omitted at the same time that the new CS was added, it acquired significant excitatory or inhibitory strength. Both these results were confirmed in the present experiment. In addition, however, it was found that this excitatory or inhibitory conditioning to the added element could itself be blocked if the shock remained unchanged for the first four trials after the addition of the new element. This finding implies that blocking is not due to any limitation on attentional or associative capacity, but rather occurs when subjects learn that a particular stimulus is redundant, i.e. that it predicts no change in reinforcement.