a variety of behavioral baselines from which to select the one best suited for the experimental 
question. 
The basic behavioral paradigms of aversively motivated instrumental (or operant) behavior 
are escape and avoidance. An escape procedure is one in which an animal learns to make a 
particular response to terminate contact with an aversive stimulus that is already present 
(e.g., electric shock through a grid floor that can be escaped by running to another 
compartment of the apparatus or by pressing a lever that turns the shock off). An avoidance 
procedure is one in which an animal learns that making a certain response will prevent an 
encounter with an aversive stimulus. For example, in one passive avoidance procedure, a rat 
learns not to step off a platform due to experience with shock delivered through the floor 
below. In a common type of active avoidance procedure, an animal learns that steadily 
operating a lever will prevent shocks from occurring (unsignaled avoidance) or that pressing a 
lever when it hears a particular tone or sees a particular light will prevent a shock from 
occurring (signaled avoidance). 
Another behavioral paradigm is a punishment (sometimes termed conflict) procedure (Azrin 
and Holz, 1966). In this procedure, making a response occasionally produces a positive 
reinforcer (e.g., food of some sort); but some or all of the responses also produce an aversive 
stimulus, which has the effect of reducing the overall rate of responding maintained by the 
food. Different degrees of suppression can be produced by varying parameters such as inten- 
sity of the aversive stimulus, or the number of responses followed by the aversive stimulus. 
Extensive research on paradigms that use negative reinforcers revealed much about the 
behavioral processes that operate under such conditions (Azrin and Holz, 1966; Baron, 1991; 
Campbell and Church, 1969; Morse et al., 1977). Consequently, researchers who wish to 
establish reliable baselines of aversively motivated behavior to examine the effect of other 
variables (e.g., the effects of psychoactive drugs or of the modulation of particular 
neurotransmitters) can rely on that literature to determine experimental parameters that are 
most suitable. 
In studies of avoidance or punished behavior, once the animal acquires the response, it is 
common for few if any shocks to be delivered (i.e., the delivery is under the animal's control). 
The experimental focus in these studies is on the reliable performance of the response itself 
and the effects of experimental variables that will alter the probability of this response. 
The behavioral paradigms described above typically use lever operation as the response. Other 
types of behavioral research require aversive conditions but study different behaviors. An 
aversively motivated paradigm that is important in research on the neurobiology of depression 
and in research on antidepressant drugs is a forced swim test, used in rats (Lucki, 1997; 
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