pain. Latency measures often are used to assess reflex responses. For example, in the tail- 
flick reflex, a radiant heat stimulus is focused on the tail and the animal flicks its tail to 
escape the stimulus. The effectiveness of analgesic agents in this model is highly correlated 
with their effectiveness in relieving pain in humans. More recently, the tail-flick reflex has 
been used to assess pain produced by brain stimulation, stress, or the microinjection of 
opioids. Other reflex measures include the flinch-jump and the limb-withdrawal tests in 
which mechanical stimulation produces a brisk motor act. Behavioral reflexes in amphibians 
can be used to evaluate analgesics (Stevens, 1996). These simple reflex measures have 
limitations, but they all permit the animal to have control over stimulus magnitude and thus 
ensure that the animal can control the level of pain to which it is exposed. The tail-flick reflex 
has the added advantage of being functional under light anesthesia. 
More complex, organized, but unlearned behaviors are often used as measures of pain 
because they involve a purposeful act requiring supra-spinal sensory processing. A commonly 
used method is the hot-plate test in which a rat or mouse is placed on a plate preheated to 50° 
to 55°C. A paw-licking response is measured. A method has also been devised in which rats 
receive heat stimuli through a glass plate while they stand unrestrained in an experimental 
cage (Hargreaves et al., 1988). The rats withdraw their limb reflexively but also exhibit 
complex behaviors, such as paw licking and guarded behavior of the limb. A latency measure 
and the withdrawal duration (how long the limb remains off the glass plate) are used to infer 
pain. All of the above methods provide the animal with control of the intensity or duration of 
the stimulus because the motor behavior results in removal of the aversive stimulus. 
A variant of an escape procedure that has been useful in studies of analgesia is the shock 
titration procedure, in which the animal operates a lever to decrease the intensity of electric 
shock (Dykstra et al., 1993). Failing to press the lever results in increases in the intensity, 
which can then be driven down again by lever operation. In this manner, shock intensity 
thresholds can be determined. The most common and simplest escape paradigm involves the 
animal's escaping an aversive stimulus by initiating a learned behavior such as crossing a 
barrier or pressing a bar. The latency of escape is usually used as a measure of pain 
experienced. Other more complex methods include reaction time experiments in which the 
animal signals the detection of an aversive stimulus by operating a lever. 
Learned behaviors have an advantage over simpler, unlearned behaviors in that the 
magnitude of the behavioral change varies with the stimulus intensity, thus providing reliable 
evidence that a change in behavior reflects the perception of a noxious stimulus rather than 
merely a change in motor performance. Sophisticated behavioral tasks in animals also allow 
the experimenter to rule out changes in performance that are related to attentional and 
motivational variables rather than changes in pain perception (Dubner, 1994). 
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