Classical conditioning, signal detection, and evolution.

Strength of classical conditioning is increased either by increasing discriminability of the conditioned stimulus (CS) from the background, or by increasing contingency between conditioned and unconditioned stimili (US). Classical conditioning can be regarded as a decision process in which the subject has to decide whether or not to respond with a conditioned response in the presence or absence of the CS. According to modern evolutionary theories, it might be assumed that this decision process maximizes the trade-off between cost and benefits. By assuming that the decision rule maximizes expected benefit, the empirical relationship between contingency and the strength of classical conditioning is theoretically derived. In addition, when the decision rule is incorporated to a signal detection paradigm, theoretical results describing the relationship between CS discriminability and CS - US contingency with the strength of classical conditioning are in agreement with experimental data.