In discussing long-term memory, scientists have found it useful to distinguish between several kinds of memory that rely on different brain systems. The major distinction is between declarative memory, which refers to the conscious memory of facts and events, and nondeclarative memory, which refers to nonconscious memory of skills, habits, or other modes of learning that proceed beneath the surface of conscious awareness. Declarative memory is what most people call memory. It depends on the integrity of the hippocampus and related structures of the medial temporal lobe. Nondeclarative memory affects our behavior without our explicit knowledge. It is a heterogeneous collection of nonconscious memory abilities that depend on various other structures within the brain (Squire et al., 1993; see Figure 1).

One well-studied component of nondeclarative memory is procedural memory. The difference between declarative memory and procedural memory is the difference between "knowing that" and "knowing how." Procedural learning describes...