464 Semantic Memory and Priming
Effects of Delay. Measures of implicit and explicit
memory also show differential effects of delay. Several stud-
ies manipulating retention interval have found that repetition
priming is less affected by delay than explicit memory. For
instance, Jacoby and Dallas (1981), Tulving, Schacter, and
Stark (1982), and Mitchell and Brown (1988) found that rep-
etition priming persisted with little change across delays of
days and weeks (using perceptual identification, word frag-
ment completion, and picture naming tasks, respectively),
whereas recognition performance declined sharply across the
same delays. Researchers have since demonstrated intact rep-
etition priming for pictures at delays up to 48 weeks (Cave,
1997) and for words at delays up to 16 months (Sloman,
Hayman, Ohta, Law, & Tulving, 1988). These findings sug-
gest that repetition priming can be a relatively permanent
form of long-term memory rather than a temporary facilita-
tion due to a recent encounter with a stimulus.
Developmental Differences. Research on developmen-
tal differences between implicit and explicit memory has
been another source of observed dissociations. Many studies
have been performed examining developmental dissocia-
tions between repetition priming and explicit memory in pop-
ulations as young as 3 years (e.g., Drummey & Newcombe,
1995; Greenbaum & Graf, 1989), 4 years (e.g., Hayes &
Hennessy, 1996), and 5 years (e.g., Carroll et al., 1985). In
studies comparing repetition-priming performance of chil-
dren and adults, equivalent levels of repetition priming were
detected across all tested age levels. In contrast, explicit
memory performance continued to show developmental im-
provements up to at least age 12 (e.g., Carroll et al., 1985).
Similarly, different developmental trends between explicit
and implicit memory have been detected in the aged, with el-
derly participants typically showing decreases in explicit
memory performance relative to younger adults, despite
showing equivalent levels of repetition-priming performance
(e.g., Graf & Ryan, 1990; Mitchell, 1989); for a discussion of
specific implicit memory impairments in elderly participants
with Alzheimer’s disease, see Gabrieli et al. (1999).
Multiple Systems Versus Processing Theories
Multiple Systems Theories. One way to account for disso-
ciations between implicit and explicit memory has been to
postulate separate memory systems in the brain for different
types of memory (e.g., N. J. Cohen & Squire, 1980; Schacter,
1990; Squire, 1986; Tulving & Schacter, 1990). Researchers
postulating distinct memory systems derive support for this
notion in large part from studies with amnesic patients. Ac-
cording to this view, the brain damage in amnesia selectively
affects the memory system for conscious recollection, leav-
ing the system (or systems) responsible for other forms of
memory intact. Evidence from studies involving amnesic
patients suggests that different neural structures underlie per-
formance on tests that rely on different kinds of memory.
Because these memory systems operate largely indepen-
dently of each other, dissociations between performance on
tasks that utilize different systems are to be expected. For ex-
ample, the hippocampus seems to play a crucial role in ex-
plicit memory, yet implicit memory performance seems to be
unaffected by damage to the hippocampus. The hippocam-
pus, then, must be part of an explicit memory system. Thus,
dissociations between different measures of memory are ex-
plained by appealing to different memory systems. Based on
this view, a taxonomy of memory can be established to clas-
sify measures based on the neural mechanisms with which
they are associated.
It is important to note, however, that a single (or one-way)
dissociation between memory phenomena, such as that ob-
served in amnesics, does not necessarily imply separate
memory systems. The data from studies with amnesics could
be explained within a single-system framework, for instance,
by arguing that explicit memory tasks are more demanding of
the neurological resources of a single memory system than
implicit tasks. Thus, damage to the memory system, as oc-
curs with amnesia, may leave the system too injured to meet
the demands of an explicit memory task, yet not so injured
as to affect performance on a less demanding implicit mem-
ory task. Many such functional hierarchies might be imag-
ined that incorporate implicit and explicit memory into a
single system. Any such structure, however, can only predict
the one-way dissociation of intact implicit memory with im-
paired explicit memory. In the previous example, for in-
stance, implicit memory is more resilient because it is less
demanding on the memory system. Thus, if damage to the
system were to occur such that implicit memory were im-
paired, explicit memory would necessarily be impaired
because, according to this formulation, the demands on the
system are greater for explicit memory tasks than for implicit
memory tasks. However, evidence for a doubledissociation
between implicit and explicit memory has been reported
by Gabrieli and colleagues (Gabrieli, Fleischman, Keane,
Reminger, & Morrell, 1995; Keane, Gabrieli, Mapstone,
Johnson, & Corkin, 1995), who have studied patients with
occipital lobe lesions. These patients demonstrated impaired
repetition-priming performance in a perceptual identification
task despite intact explicit memory performance. The results
of these studies have been used to support the notion that
implicit memory phenomena such as repetition priming are
mediated by brain systems separate from those mediating