et al., 1998; Shepherdson, 1994) and for improving the lives of animals in zoos (Markowitz, 
1982; Shepherdson, 1998). Understanding preferences, similarities, and differences among 
different species in their requirements for habitat, territory, and social interactions has greatly 
enhanced the welfare of these animals. 
REHABILITATION MEDICINE 
Nobel Laureate Charles Sherrington and his colleague (Mott and Sherrington, 1895) showed 
that sensory deafferentation — cutting the dorsal roots of the nerves supplying a forelimb — 
caused animals to stop using that limb. Later, behavioral research demonstrated that 
appropriate motivation could "rehabilitate” the deafferented forelimb to function without 
sensory feedback from the affected limb (Taub et al., 1965). 
Taub and his colleagues have since demonstrated that stroke victims can be trained to use an 
arm rendered useless by a stroke (Liepert et al., 2000; Taub et al., 1993). They accomplish 
this by restraining the normal arm and forcing the patient, through small increments of 
difficulty (a process known as shaping), to employ the affected limb for various tasks until it 
becomes useful once more, a technique learned from laborious work with deafferented 
monkeys (Taub et al., 1994). This new method is called Constraint-Induced Movement 
Therapy (Cl Therapy). “Cl Therapy changes the contingencies of reinforcement (provides 
opportunities for reinforcement of use of the more-affected arm and aversive consequences for 
its non-use by constraining the less-affected arm) so that the non-use of the more-affected arm 
learned in the acute and early sub-acute periods is counter-conditioned or lifted. Second, the 
consequent increase in more-affected arm use, involving sustained and repeated practice of 
functional arm movements, induces expansion of the contralateral cortical area controlling 
movement of the more-affected arm and recruitment of new ipsilateral areas. This use- 
dependent reorganization may serve as the neural basis for the permanent increase in use of 
the affected arm" (Taub et al., 1999, p. 241). This work has revolutionized the field of 
rehabilitation medicine. 
PAIN 
Animal research has revealed that specific pathways in the brain powerfully inhibit intense 
pain; that receptors in these same pathways bind morphine; and that the brain has its own 
opiate-like neurotransmitters, called endorphins, that function in these pathways (Basbaum 
and Fields, 1984; Mansour et al., 1995). More recently, scientists have identified molecules 
that regulate the endorphins (Mitchell et al., 2000). Targeting these molecules with selective 
antagonists may reduce the tolerance and some of the side effects typically associated with the 
use of morphine for pain control. Furthermore, research with awake, behaving animals found 
that stimulation of tiny electrodes that were implanted along pain-inhibiting pathways 
activated those pathways and effectively inhibited pain. With surgically implanted electrodes, 
some patients are able to press a button on a portable radio transmitter, activate the pain- 
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