Aversive conditioning is an ideal model for studying cholinergic effects on the processes of learning and memory for several reasons. First, deficits produced by selective lesions of the anatomical structures shown to be critical for Pavlovian fear conditioning and inhibitory avoidance (such as the amygdala and hippocampus) resemble those deficits seen in human pathological conditions resulting in damage to these same structures. This supports the suggestion that experimental findings on learning and memory in animal models are informative about similar processes in humans. Second, because aversive conditioning is learned rapidly, even with a single conditioning trial, the temporal dynamics of the underlying processes can be examined with a very fine degree of resolution. Third, the fear memories generated by aversive conditioning procedures tend to be very stable over long time periods. For example, we have observed robust freezing to both tone and context up to 16 mo following fear conditioning (Gale et al. 2003). Finally, fear is a motivational system that has evolved to protect an animal from danger. Fear responses observed under laboratory conditions are similar to those observed in the animal's natural habitat, meaning that the results of aversive conditioning procedures typically have external validity.