stimulus such as electric shock provides the motivating condition to learn an avoidance 
response. However, some behavioral studies are concerned with pain per se. 
Those researchers studying pain have recognized and addressed ethical issues surrounding 
this type of research. Guidelines for pain research in animals were developed early on by the 
International Association for the Study of Pain (Zimmermann, 1986) and have been updated 
by the American Association for Laboratory Animal Science (AALAS, 2000). 
Animals should be free of pain except at times when the experiment will be compromised by 
avoiding or eliminating it. Whether pain is a by-product of a research procedure or a focus of 
study, certain principles remain the same. In the latter case, the animals should be exposed 
to the minimal intensity and duration of pain necessary to carry out the experiment. A 
consensus on the application of this principle turns out to be much more difficult to achieve 
than one would think. For example, the intensity of an aversive stimulus that is suitable for 
motivating avoidance behavior may not be an intensity that is suitable for a study of stress on 
immune function or for study of analgesia. 
A committee of the International Association for the Study of Pain has defined pain in people 
as an "unpleasant sensory and emotional experience associated with actual or potential tissue 
damage, or described in terms of such damage” (Anonymous, 1979). Animals cannot give a 
verbal description of the pain, but pain can be inferred from physiological and behavioral 
changes, because animals exhibit the same motor behaviors and physiological responses as 
people in response to painful stimulation. These responses include withdrawal reflexes, 
vocalization, and learned behaviors such as pressing a bar to avoid further exposure to an 
aversive stimulus or to decrease its intensity. 
Principles developed for experimental studies of pain in humans should be applied in pain 
research on animals. Human subjects are exposed only to painful stimuli that they can 
tolerate, and they are able to remove a painful stimulus at any time (see the discussion of 
chronic pain below). Tolerance for pain needs to be clearly distinguished from the threshold 
for detecting a painful stimulus. It is when the intensity of the stimulus approaches or 
exceeds the tolerance threshold that our behavior is dominated by attempts to avoid or escape 
the stimulation. When the animal cannot control the stimulus intensity, it is critical that the 
experimenter determine the level of pain produced by stimuli. Although controllability of the 
aversive stimulus is often consistent with achieving the goals of the research in studies on 
pain, it might be inimicable to study of stress. 
PAIN ASSESSMENT METHODS 
Scales for rating clinical manifestation of animal pain have not proven to be very reliable 
(Flecknell, 1996). Thus, objective behavioral measures are employed in animal studies on 
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