What happens in our brains when we learn? How do evolutionary changes in mechanisms underlying one form of learning impact upon the evolution of other forms of learning? In previous research this investigator used neurophysiological and behavioral studies of several present-day species (related to the popular models species, Aplysia californica) with known relationships to each other to establish that fundamental mechanisms of sensitization, a simple form of learning, have changed across evolution. In this research the consequences of evolution will be examined with respect to a more complex form of learning, classical conditioning. The hypothesis to be examined is whether loss or reduction in the simple mechanisms of sensitization reduces the persistence of the more complex neural changes underlying classical conditioning. This study will lead to a deeper understanding of the neural mechanisms of learning and memory.