Memory is composed of several different
abilities that depend on different brain systems
(1). A fundamental distinction is between
the capacity for conscious recollection
of facts and events (declarative or explicit
memory) and various nondeclarative
(implicit) forms of memory that are expressed
in skills, habits, and simple forms of
conditioning. This distinction is dramatically
evident in amnesic patients, who have
bilateral damage to the hippocampal formation
or related midline diencephalic brain
structures. These patients have severely impaired
declarative memory and are profoundly
forgetful. Yet these same patients
have a fully intact capacity for nondeclarative
memory (2). Indeed, a large body of
literature involving both humans and experimental
animals can now be understood
by recognizing that memory tasks requiring
declarative memory depend on the integrity
of the hippocampal formation and related
structures, whereas tasks requiring nondeclarative
memory can be performed normally
after damage to these structures and
are supported by other brain systems. Declarative
memory is what is meant by the
term “memory” in ordinary language. It is
involved in modeling the external world,
and its contents can be brought to consciousness
as a verbal proposition or as a
mental image. By contrast, nondeclarative
memory is expressed through performance
without affording access to any conscious
memory content or even awareness that
memory is being used. This form of memory
permits cumulative changes in perceptual
and response systems and allows for the
gradual development of new skills and
habits.