The missing link between neurobiology and behavior in Aplysia conditioning

Over the past decades, a wealth of findings has led to a substantial change in the assumed complexity of classical conditioning. The combined evidence indicates that temporal pairing is neither necessary nor sufficient for the formation of an associative connection. At the same time, studies of model invertebrate nervous systems have allowed us to ask a series of questions about the molecular basis of associative conditioning. The discovery of a pairing-sensitive mechanism in the gill-withdrawal circuitry of Aplysia is regarded as the hallmark of the reductionist approach. This review outlines the insights gathered from behavioral and neurobiological studies. Furthermore, the conceptual frameworks guiding research at the ‘what’ and ‘how’ levels of analysis are compared and contrasted. I argue that a rich cognitive view of conditioning has emerged at the ‘what’ level, whereas the traditional notion of temporal pairing still drives research at the ‘how’ level. A complete account of classical conditioning has to await the resolving of this discordance.