Porsolt et al., 1978). Some studies use drug administration to create a noxious effect (e.g., 
nausea by lithium chloride) to study phenomena such as the development of conditioned 
aversions (e.g., avoidance of an otherwise palatable solution that had been paired with 
lithium chloride) or to study the effects of drugs on conditioned aversions. In the conditioned 
suppression paradigm, an unavoidable aversive stimulus (usually electric shock) is signaled 
by a distinctive sound or light; the animal learns to suppress ongoing behavior, typically 
responding for food, in the presence of that stimulus. 
ELECTRIC SHOCK 
Electric shock is by far the most frequently used aversive stimulus in research. Although a 
number of other aversive stimuli have been used in a variety of studies, there are 
characteristics of electric shock that have made it particularly useful as an aversive stimulus 
in a variety of laboratory research. An electric shock stimulus, whether applied through a grid 
floor or a carefully placed electrode, has several advantages from an experimental and 
humane perspective. 
In the range used for behavioral research, electric shocks do not produce tissue damage. 
Shock produces its noxious quality by directly stimulating nociceptive fibers rather than by 
producing injury. The sensation produced by electric shock does not persist beyond the period 
of stimulation, and the stimulus does not interfere with the ability to respond (e.g., under a 
punishment or conflict procedure). It is interesting to note that researchers who test the shock 
levels on themselves report that it is not clear whether shock in the intensity range typically 
used causes "pain" in the traditional sense, or if the sensation produced is more accurately 
described as a very unpleasant sensation. 
Physical aspects of the shock stimulus are specifiable and controllable by the experimenter, 
which has advantages for the subjects as well as for the experimental design. The type of 
shock, voltage, current, duration, number of shocks, and body area to which shock is applied 
all can be precisely stated and thus precisely controlled and replicated within and across 
laboratories. An extensive literature on shock parameters (Azrin and Holz, 1966) minimizes 
the amount of exploratory work needed for selecting stimulus parameters before the actual 
experiment. 
STRESS RESEARCH 
Stress research has as its purpose the production of an objectively determined stressful state 
in order to study various behavioral and physiological sequelae. For example, the research 
may investigate the behavioral and/or physiological changes involved in animal models of 
depression. Not all research that uses aversive stimuli seeks to produce stress per se, and it is 
an unresolved empirical issue whether objectively determined stressful states are necessarily 
present under all aversively motivated paradigms. An example is whether an animal that 
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