Figure 2: A classical conditioning paradigm demonstrating the insufficiency of contiguity for conditioning. In the first case (A), the CS is always paired with the US, but the US also occurs at other times during the training interval. In the second case (B), the US occurs only when the CS occurs. Although the requirement of contiguity is satisfied in both cases, conditioning is achieved only in the latter case. (Adapted from Rescorla 1988.)