Simple learning-operant and classical conditioning-is thought to be the key to the evolution of human intelligence, hence has been the focus of much neurobio­ logical research with vertebrate and invertebrate animals (e.g., Weinberger, McGaugh, & Lynch, 1985). Most emphasis has been placed on classical condi­ tioning, in which a neutral stimulus, the conditioned stimulus (CS), acquires the capacity to elicit a conditioned response (CR) because of a training history, in which the CS has been a reliable predictor of an unconditioned stimulus, such as food delivery, that elicits its own, unconditioned, response. Nevertheless operant conditioning-the modification of freely occurring (“emitted”) behavior by re­

warding or punishing consequences (<reinforcement)—is probably the more primi­ tive function.