Although the foregoing definitions indicate that evaluation of behavioral changes produced by response-contingent delivery or removal of such events is critical for applying the labels, the manner in which the stimuli are used in experiments may or may not involve a response contingency. For example, in studies that use instrumental learning or operant conditioning, there will be an explicit, experimenter-defined relationship between some feature of the animal’s behavior (such as whether a lever is pressed) and the delivery (or withholding) of the stimulus. In contrast, studies that use classical or Pavlovian conditioning typically do not involve a response-outcome contingency; rather, the emphasis is typically on the predictive relationship between some other stimulus (such as a light or tone) and delivery of the appetitive or aversive stimulus (Rescorla, 1988). Because brain mechanisms underlying the different types of learning may differ, the decision to present appetitive and aversive events in a response-contingent or response-noncontingent manner should be based on the scientific goals of the study.

Appetitive or aversive stimuli are typically used to motivate an animal to perform a particular behavior. However, the scientific reasons for producing that behavior can vary widely, and the overall purpose of the study will be an important consideration in the selection of the appetitive or aversive stimuli. For example, a considerable body of neuroscience research using appetitive and aversive stimuli focuses on understanding the neurobiology of basic motivational processes, such as those involved in feeding, drinking, foraging, mating, drug addiction, aggression, fear (anxiety and phobias), and the avoidance of pain or discomfort. In such cases, the selection of a particular motivational stimulus (such as salt water or a sexually receptive conspecific) is typically dictated by the specific motivational or behavioral system under study (such as sodium appetite or copulation). In other cases, however, investigators may have more leeway in the selection of motivational stimuli. For example, investigators interested in the general neural mechanisms underlying simple learning (such as classical and operant conditioning), cognition, or memory may be able to use a range of stimuli, both appetitive and aversive, to achieve their scientific aims. Similarly, investigators who simply wish to establish a reliable behavioral baseline for studying motor, sensory, attentional, or perceptual processes or for assessing the effect of various manipulations may also have some flexibility in their choice of motivational stimuli. Relevant considerations might include whether the stimulus has similar effects among species or among individuals within a species. Another consideration is the degree of variability in the efficacy of the stimulus among individuals or of repeated exposures to the stimulus in the same individual. For example, because of rapid satiation, a food rich in calories will be a poor choice as a reinforcer in a procedure that requires the animal to respond repeatedly for