Memory is the explicit or implicit recall of information
encoded in the recent or distant past. Current
conceptualizations of memory, however, do not view
the construct as a unitary system but rather divide it
into hierarchical taxonomic modules based on duration
of retention and the type of information that
is being retrieved. Among the more fully elucidated
conceptualizations of memory systems is one characterized
by Larry Squire and colleagues, in which
long-term memory is divided into declarative and
nondeclarative subcomponents. Declarative, or explicit,
memory refers to the ability to consciously recall
facts (semantic memory), events (episodic memory),
or perceptual information (perceptual memory).
Nondeclarative memory requires the implicit recall
of information and is usually divided into procedural,
priming, or simple conditioning paradigms. Information
that is retained on the order of seconds or minutes
is usually referred to as short-term memory and
is thought to represent a memory system distinct from
long-term memory. Working memory, which comprises
short-term memory, refers to the short-term
store required to perform certain mental operations
during retention. The following sections examine the
impact of normal aging on different types of memory,
as well as some of the potential moderators and mediators
of cognitive aging. The information presented
is organized hierarchically, following the memory
systems just outlined.
Increased age puts an individual at risk for the
development of neurodegenerative disorders, such as
Alzheimer’s disease (AD). Central to the dementia syndrome
that characterizes AD is the gradual and progressive
loss of long-term memory functions. Although
the vast majority of older adults do not develop
dementia, most experience some degree of cognitive
change. Following the elucidation of memory systems
and their component parts in the cognitive and cognitive
neuroscience literature, there has been a recent
interest in the impact of age on the different memory
systems, independent of the devastating effects of
dementia. Among well-screened individuals who do
not meet diagnostic criteria for dementia, both crosssectional
and longitudinal studies demonstrate that
the different memory component systems do not
uniformly age; rather they show differential vulnerability
to aging effects.
Long-Term Memory
Declarative Memory
Semantic memory As noted, semantic memory refers
the recall of general or factual knowledge. Older adults
commonly complain of subjective semantic memory
problems when, for example, they report difficulty
recalling the names of common objects or other welllearned
information. Yet, despite these subjective complaints,
semantic memory is among the more stable
memory systems across the adult life span. The construct
can be operationally defined by requiring subjects
to define words or provide the answers to factual
questions (e.g., the capital of a certain country), such
as on the Vocabulary and Information subtests of the
Wechsler Adult Intelligence Scales. Semantic memory
is often included as part of the definition of ‘crystallized
intelligence,’ which reflects an accumulation of
information acquired over time and that is relatively
impermeable to the effects of normal aging or mild
brain disease.
It is well established that semantic memory shows
very little decline in normal aging. In fact, semantic
knowledge accumulation and memory increase into
the sixth and seventh decades of life and may show
only a gradual decline afterward. Much of our understanding
of the impact of age on semantic memory has
come from large-scale longitudinal and cross-sectional
studies of normal aging. For example, longitudinal
data from the Canberra Study , which followed a random
sample of adults over the age of 70 years, demonstrated
that crystallized abilities remained stable over
approximately 8 years. This pattern of stability was
apparent when age-associated differences (i.e., crosssectional
analysis) were considered as well. Similarly,
Denise Park and her colleagues measured knowledgebased
verbal ability, including three semantic memory
tasks tapping word knowledge, in a large sample of
healthy adults ranging in age from 20 to 92 years, and
found a gradual increase in performance across the age
groups. This finding is again consistent with the idea
that semantic knowledge accumulates across the life
span with little or no deleterious effects of normal aging.
Although there is little cross-sectional or longitudinal
evidence to suggest that semantic memory changes significantly
with normal aging, why are subjective complaints
of semantic recall so common among older
adults? One phenomenon, termed ‘tip-of-the-tongue’
(TOT), may explain this occurrence. TOT refers to