THROUGHOUT THIS BOOK WE HAVE EMPHASIZED that all behavior is a function of the brain and that malfunctions of the brain produce characteristic disturbances of behavior. Behavior is also shaped by experience. How does experience act on the neural circuits of the brain to change behavior? How is new information acquired by the brain, and once acquired, how is it stored, retrieved, and remembered?
In the previous chapter, we saw that memory is not a single process but has at least two major forms. Implicit memory operates unconsciously and automatically, as in the memory for conditioned responses, habits, and perceptual and motor skills, whereas explicit memory operates consciously, as in the memory for people, places, and objects. The circuitry for long-term memory storage differs between explicit and implicit memory. Long-term storage of explicit memory begins in the hippocampus and the medial temporal lobe of the neocortex, whereas long-term storage of different types of implicit memory requires a family of neural structures: the neocortex for priming, the striatum for skills and habits, the amygdala for Pavlovian threat conditioning (also known as fear conditioning), the cerebellum for learned motor skills, and certain reflex pathways for nonassociative learning such as habituation and sensitization (Figure 53–1).
Two forms of long-term memory involve different brain systems. Implicit memory involves the neocortex, striatum, amygdala, cerebellum, and, in the simplest cases, the reflex pathways themselves. Explicit memory requires the medial temporal lobe and the hippocampus, as well as certain areas of neocortex (not shown).
Over time, explicit memories are transferred to different regions of the neocortex. In addition, many cognitive, motor, and perceptual skills that we initially store as explicit memory ultimately become so ingrained with practice that they become stored as implicit memory. The transference from explicit to implicit memory and the difference between them is dramatically demonstrated in the case of the English musician and conductor Clive Waring, who in 1985 sustained a viral infection of his brain (herpes encephalitis) that affected the hippocampus and temporal cortex. Waring was left with a devastating loss of memory for events or people he had encountered even a minute or two earlier, but his ability to read music, play the piano, or conduct a chorale was unaffected. Once a performance was completed, however, he could not remember a thing about it.
Similarly, the abstract expressionist painter William de Kooning developed severe disturbances of explicit memory as a result of Alzheimer disease. As the disease progressed and his memory for people, places, and objects deteriorated, he nevertheless continued to produce important and interesting paintings. This aspect of his creative personality was relatively untouched.
In this chapter, we examine the cellular and molecular mechanisms that underlie implicit memory storage in invertebrate and vertebrate animals. We focus on learning about threats (sometimes ...