Classical conditioning was first studied extensively by I. P. Pavlov, who placed it firmly in the reflex tradition codified and summarized by C. S. Sherrington (Chapter 3). These two men set the study of conditioning on a trajectory that eventually led to the split between Pavlov's classical and Skinner's operant conditioning. Chapter 19 deals with this split. In this chapter, I will describe reflex-type theoretical models for conditioning – what might be called the classical conditioning models. One of these, the Rescorla–Wagner (R–W) model, has been hugely influential. The number and complexity of proposed conditioning models has increased greatly since the original R–W chapter in 1972. I will try just to extract the essentials.

Inference and classical conditioning

But first, some comments on inference. The occurrence of a hedonic event (i.e., an unconditioned response [US]) such as food or shock poses a Bayesian problem for the animal: Which environmental feature, of those present now and in the past, is the most likely cause of the US? Is there a conditioned stimulus (CS), and if so, what is it?

The animal has two kinds of information: prior knowledge, and temporal and frequency relations between the valued event (US) and its potential cause (an environmental stimulus, the CS – or an operant response). When, and how often, did the potential CS occur in relation to the US? This chapter deals mostly with ‘how often.’

Prior knowledge is of two kinds: innate priors and previously formed representations. The animal's evolutionary history provides it with predispositions to connect certain kinds of event with certain kinds of outcome: Pain is more likely to be associated with an organism than with an inanimate stimulus – recall the hedgehog experiment. A related finding is shock-induced fighting: A pair of rats placed in a cage and briefly shocked through their feet will usually attack one another. Pain, in the presence of a probable cause, elicits the behavior that is preprogrammed for that situation: Attack the cause. Both the quality and the intensity of the US will determine the likely cause.

If the situation is familiar, or is similar (in terms of the animal's representation – see Chapter 11 for a discussion of similarity) to a familiar one, the probable cause can be inferred.