Blocking

Classical conditioning is a very basic kind of learning, but experiments revealed that the mechanisms underlying it were more complex than the simple law of contiguity.  For example, one phenomenon found in classical conditioning is blocking (Kamin, 1968).  Blocking involves two conditioned stimuli, CS_A and CS_B.  Either one is capable of being conditioned to produce the CR.  However, if training begins with a phase in which only CS_A is paired with the US, and is then followed by a phase in which both CS_A and CS_B are paired with the US, then CS_B fails to produce the CR.  The prior conditioning involving CS_A blocks the conditioning of CS_B, even though in the second phase of training CS_B is contiguous with the UR.

The explanation of phenomena like blocking required a new model of associative learning.  Such a model was proposed in the early 1970s by Robert Rescorla and Allen Wagner (Rescorla & Wagner, 1972).  This mathematical model of learning has been described as being cognitive because it defines associative learning in terms of expectation.  Its basic idea is that a CS is a signal about the likelihood that a US will soon occur.  Thus the CS sets up expectations of future events.  If these expectations are met, then no learning will occur.  However, if these expectations are not met – if the agent is surprised – then associations between stimuli and responses will be modified. “Certain expectations are built up about the events following a stimulus complex; expectations initiated by that complex and its component stimuli are then only modified when consequent events disagree with the composite expectation” (Rescorla & Wagner, 1972, p. 75).

References:

1. Kamin, L. J. (1968). Attention-like processes in classical conditioning. In M. R. Jones (Ed.), Miami symposium on the prediction of behavior: Aversive stimulation (pp. 9-32). Miami: University of Miami Press.
2. Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical Conditioning II: Current Research And Theory (pp. 64-99). New York, NY: Appleton-Century-Crofts.

(Added April 2011)