The anatomy of a memory: convergence of results across a diversity of tests

Abstract The notion of an anatomical basis for learning has its roots in ancient Greece and has been restated and refined in modern terminology by numerous contributors 1–4 . A fundamental tenet of these early structural theories was that the basic wiring diagram of the brain could be modified throughout life and that learning was capable of producing continuous structural changes, typically including the growth of novel connections. Prior to the last two decades, evidence for this type of neuronal growth was restricted to embryonic development and regeneration of peripheral tissues. Later studies demonstrated that injury, environmental pressures, and internal body state could also produce structural changes in the adult CNS 5 . More recently it has become clear that the arrangement of synaptic connections in the mature nervous system can undergo striking changes even during normal functioning 6 . As the diversity of species and plastic processes subjected to morphological scrutiny has increased, convergence upon a set of structurally detectable phenomena has begun to emerge. Although several aspects of synaptic structure appear to change with experience, the most consistent potential substrate for memory storage during behavioral modification is an alteration in the number and/or pattern of synaptic connections.