and categorize these events. In general, appetitive stimuli are ones that an organism will voluntarily make contact with or approach, and aversive stimuli are ones that an organism will try to escape or avoid. Central to those definitions is the idea that the labeling of a stimulus as appetitive or aversive is based on an organism’s behavior, not on physical features of the stimuli themselves. Indeed, the same stimulus may be appetitive in some situations or to some individuals, but aversive in other situations or to other individuals. For example, in certain behavioral procedures, rats and monkeys have been shown to engage repeatedly in behaviors that produce exposure to electric shock, an event commonly assumed to be an aversive (Brown and Cunningham, 1981; Cunningham and Niehus, 1997; Cunningham et al., 1993; Kelleher and Morse, 1968). Thus, under these experimental conditions, electric shock would be labeled an appetitive stimulus, not an aversive one. Similarly counterintuitive examples can be found in the literature on behavioral effects of abused drugs. For example, the same dose of alcohol that produces a conditioned place aversion in rats will produce a conditioned place preference in mice (Cunningham et al., 1993). Moreover, a drug’s ability to produce a conditioned preference may be completely reversed (to conditioned aversion) simply by changing the temporal relationship between drug injection and the associated stimulus (e.g., Cunningham et al., 1997; Fudala and Iwamoto, 1990). It has also been shown that injection of an abused drug may concurrently induce preference for a paired spatial location, but aversion for a paired flavor solution in the same animal (e.g., Reicher and Holman, 1977). All of these examples illustrate that decisions about whether a given stimulus should be considered appetitive or aversive cannot be based solely on its physical properties, but must be informed by expert knowledge of its behavioral effects in various contexts. Importantly, those effects may vary significantly as a function of the species, genotype, sex, and past experience of each animal.

In more technical terms, the stimuli under consideration here are often referred to as either reinforcers or punishers, depending on their effects in behavioral procedures in which the response-contingent presentation or removal of a stimulus produces either increase or decrease in the rate of the target response. Stimuli that increase the rate of a contingent behavior are called reinforcers, whereas events that decrease the rate of a contingent behavior are called punishers. Both reinforcement and punishment may involve either the presentation or the removal of a stimulus (Domjan, 1998). Typically, the response-contingent presentation of an appetitive stimulus produces an increase in responding (positive reinforcement) and the response-contingent presentation of an aversive stimulus produces a decrease in responding (positive punishment). In contrast, the response-contingent removal or omission of an aversive stimulus produces an increase in responding (reinforcement based on escape or avoidance), whereas the response-contingent removal or omission of an appetitive stimulus produces a decrease in responding (punishment based on omission training).



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