serves in an avoidance procedure manifests objective indices of stress under conditions in 
which responding is so efficient as to avoid any shock deliveries. The development of reliable, 
objective indices of stress is important to stress research (i.e., those are the dependent 
variables in many studies). At the same time, information from such studies can also inform 
our understanding of the effects of other behavioral procedures that use aversive stimuli. 
Events that will serve as stressors are quite specific to species, systems, and processes, and 
thus different stressors are used for different purposes. For example, in examining the effects 
of stressors on immune function, there are several important considerations. Many of the 
dysfunctional processes that are typically associated with stress have been found to occur only 
if stress is relatively severe or prolonged. For example, depletion of norepinephrine in the 
locus coeruleus occurs only after exposure to intense stress, and increases in serum 
cholesterol are produced after exposure to repeated stressful sessions but not after a single 
session of stress. Studies of stress, then, must employ lengthier exposures to aversive stimuli 
than would occur in studies in which the primary goal is to develop behavior motivated by a 
negative reinforcer. 
In stress research, subjects often do not have control of the aversive stimulus. Many of the 
phenomena that are most relevant for human health occur only, or most readily, if the subject 
does not have control. Control is a form of coping, and the deleterious effects of exposure to 
stressors are most evident when coping is not possible. Therefore, to add the element of 
coping or control to a study on the deleterious effects of stress could be inconsistent with the 
goals of the study. 
No single physiological or behavioral measure can be taken as uniquely indicating the 
occurrence of stress response. Certain behavioral changes, if persistent, often are assumed as 
evidence of stress. These are decreases in grooming, ingestion, body weight, locomotor 
activity, exploration, aggression, or sexual behavior. Increased “freezing” is also considered 
to be indicative of stress. Although this list indicates some assessments that can be made to 
determine the existence and degree of stress, some indicators may not be useful in all 
situations. Further, these signs are not exclusive to aversive stimuli or to stressful 
environments. For instance, decreased food intake and reduced body weight are concomitants 
of illness. The relationship between aversive stimuli and the behavioral, physiological, and 
hormonal changes is a topic of ongoing research. Although corticosterone concentration in 
blood is sometimes regarded as a physiological indication of stress, no index is uniformly 
accepted as a more reliable indicator of “stress" than behavioral evidence. 
PAIN RESEARCH 
Just as many studies of aversively motivated behavior do not seek to investigate stress, many 
of those studies do not seek to investigate pain, although it is presumed that the pain of a 
70 
