Lost forever or temporarily misplaced? The long debate about the nature of memory impairment

Studies of memory impairment in humans and experimental animals have been fundamental to learning about the organization of memory and its cellular and molecular substrates. When memory impairment occurs, especially after perturbations of the nervous system, the question inevitably arises whether the impairment reflects impaired information storage or impaired accessibility. This topic has been the subject of considerable commentary and experimental work over the years. In this reappraisal, I first consider four broad areas of behavioral study from the 1970s and 1980s that led to a dominant and compelling view of memory impairment as a deficit of information storage. Second, I identify some ambiguities that arise about how the terms "storage" and "retrieval" are applied, especially when the evidence is somewhat indirect and based on a behavioral-psychological level of analysis. I then review neurobiological findings that have been largely overlooked in these discussions. The relevant studies are ones where it has been possible to monitor neurons and synapses in direct relation to behavioral memory, for example, in animals with simple nervous systems and in single cell recordings from behaving monkeys. This work provides a straightforward and illuminating perspective on the question and confirms the view that first emerged from less direct evidence.