In memory of consolidation

How an animal learns, remembers, and uses information to guide adaptive behavior remains one of the most challenging questions in science today. Much progress was made in the twentieth century, and new tools available to neurobiological investigators have accelerated progress in the new century. Nevertheless, the road has been rocky and progress sometimes impeded by periodic polemic debates at a conceptual level. Retrospective examination of the nature of the divisive issues and how they were (or were not) resolved could help steer a new generation of investigators away from similar pitfalls and impasses. The same applies to scientists from other disciplines, recently joining in the “search for the engram,” who might not be aware of the vast literature generated, mainly by psychologists, in the middle decades of the last century. Our purpose here is not to furnish a complete review of this literature, but to provide a historical perspective for some of the unresolved issues that continue to be discussed within the context of the field of neurobiology of memory. For more general reviews, refer to McGaugh (2000) and Dudai (2004). Scientific investigation of memory processes was initiated at the end of the 19th century by psychologists in Germany, Ebbinghaus (1885) and then Mueller and Pilzecker (1900). Their studies of verbal learning and retention in human subjects led them to conclude that a memory trace was formed gradually over time after acquisition and they coined the term consolidation. Contemporary with this were the very influential clinical observations and theoretical elaborations of the French psychiatrist, Ribot (1882). From his studies of amnesic patients, he formulated “La loi de regression,” which simply notes that, as memories age, they become more resistant to trauma-induced amnesia.