In general, sensitization refers to the process by which animals learn to associate one stumbles with another because it is important to do so.  Pavlov originally outlined the concepts habituation, in which an animal learns to ignore a stimulus because it is unimportant, and sensitization. (3).  Over the years, both sensitization and habituation have been crucial in Kandel's research in <em>Aplysia</em> on long- vs. short-term memory.  In many of Kandel's experiments, repeated strong tail-shocks lead to a sensitized gill-withdrawal reflex that eventually emerges as a result of a light touch to the siphon.  Here, continued administration of serotonin or cAMP produces a stronger response over time as sensitization occurs.