It is almost universally assumed that controllable and predictable aversive events are 
preferable to unpredictable and uncontrollable stimulation. Careful psychophysical study has 
revealed, however, that predictable shocks are perceived as more severe or intense than 
unpredictable shocks, and there are conditions in which controllable shocks are more stressful 
than uncontrollable shocks. Indeed, in many studies using shocks that are not under the 
subject's control, the shock durations are much briefer than those that are under the subject's 
control. One often uses shock durations of 0.5 to 1.0 second in classical conditioning studies. 
But, a behavioral response that requires moving from one location to another may require 
several seconds for the subject to terminate the shock. 
In certain studies, control over the stimulus entails a tradeoff for subject and investigator. If 
disturbances in catecholamine metabolism are the object of study, these disturbances come 
into play only when the aversive stimulus is of a specified intensity and uncontrollable. If 
controllable shock is used, the shock intensity required to produce measurable effects would 
be much greater than the intensity required by uncontrollable shocks. 
The effect of any given shock stimulus varies according to a wide range of variables: history of 
the subject, species used, waveform of the voltage, body region shocked, size of the electrode 
or diameter of grids, and series resistance. For example, shock stimuli that produce vigorous 
reactions in the rat are often undetected by pigeons. If electrodes are used, current density 
increases as the size of the electrodes decreases; if grids are used, current density varies as the 
animal moves across grids, with current density increasing as grid size decreases. 
Experienced investigators select shock parameters by taking account of the complexity 
inherent in these and other variables. 
CONCLUSION 
Past research on aversively motivated behavior and stress has yielded data that can inform 
researchers in designing studies that use aversive stimuli (see References). Each experimental 
procedure that uses aversive stimuli has its own set of technical methods, advantages, 
disadvantages, and cautions. In addition, methodological details of a given stressor or 
aversive stimulus differ according to the species of animal used as subjects. Investigators 
should make clear the reasons that a specific procedure is most appropriate for a given study, 
the advantages and disadvantages of the procedure, and the impact of the procedure on the 
organism under investigation. ■ 
REFERENCES 
American Association for Laboratory Animal Science (AALAS). (2000). Recognition and 
alleviation of pain and distress in laboratory animals. See AALAS web site at 
http://www.aalas.org. 
74 
