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<p>An inescapable component to survival in a dynamic environment is detecting and
reacting to signals of danger. One of the most elegant processes animals possess to
handle this complex task is classical conditioning, wherein stimuli associated with
an aversive event acquire the capacity to elicit defensive behaviors. This process
helps ensure quick reactions prior to the occurrence of an imminent threat. A problem
of living in a dynamic environment, however, is that reliable signals of danger are
rarely re-encountered in the exact same form from one situation to the next. Thus,
to be truly adaptive it is imperative for defensive responses to extend beyond a specific
instance towards other exemplars that might portend the same negative outcome. While
the phenomenon of stimulus generalization was recognized in the earliest studies of
conditioning from Pavlov's laboratory, a century of conditioning research has not
resolved how humans and other animals actually meet this challenge. The research presented
herein employs a combination of psychophysiological and functional imaging methods
to examine how humans recruit neurocognitive systems to determine what stimuli do
(and do not) pose a threat. Results show that human fear generalization is a complex
phenomenon affected by the perceptual and conceptual nature of the stimulus. Brain
regions and functional networks involved in fear generalization comprise cortical
areas involved in coding the representation of conditioned stimuli and subcortical
regions involved conditioned learning and the production of behavioral responses,
most notably the amygdala. These results reveal the importance of stimulus-specific
factors in fear learning and generalization, provide support for anatomically constrained
models of fear generalization, and contribute to the development of model systems
of fear generalization processes in human anxiety disorders.</p>
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