462 Semantic Memory and Priming
amnesic patients exhibit intact performance on measures of
repetition priming, such as word-stem completion, picture-
fragment completion, and picture naming (e.g., Cave &
Squire, 1992; N. J. Cohen & Squire, 1980; Graf, Squire, &
Mandler, 1984; Shimamura, 1986, 1993; Squire, 1987;
Warrington & Weiskrantz, 1968).
In a now-classic study examining repetition priming in
amnesic patients, Graf et al. (1984) used a word-stem com-
pletion paradigm, in which participants see partially com-
pleted words (e.g., ele_____) at test and are asked to fill
in the blanks to form the first word that comes to mind.
Although no reference is made to an earlier list of words,
participants are more likely to complete the stringelephant
if they sawelephantduring an earlier study session than
if they did not. Graf et al. found that normal and amnesic
participants showed equal levels of repetition priming
on this task. Explicit memory instructions, however,
changed the pattern of results. When asked to complete the
word stems with study list words, normal participants im-
proved dramatically in their ability to produce study words,
whereas amnesic participants did not improve at all. This
study and others have shown that robust repetition priming
can occur in the absence of explicit memory (for a review,
see Shimamura, 1986), and have not only identified areas of
the brain crucial to explicit memory but also contributed to
the notion that repetition priming may be a form of implicit
memory subserved by regions of the brain other than those
damaged in amnesia (Cave & Squire, 1992; Schacter, 1990;
Squire, 1992; Squire et al., 1993). However, as shall be
seen, the notion that repetition priming represents a distinct
memory system has been at the center of much controversy.
Indeed, not all researchers even acknowledge that repetition
priming reflects an aspect of memory.
Models of Repetition Priming
Theories of implicit memory have typically not been
concerned with specific processing assumptions, and few
research studies have attempted to provide detailed descrip-
tions of the processes underlying repetition priming. As a
consequence, model development is not as advanced as in
other areas of memory research.
Logogen Model
According to Morton’s (1969) model of word recognition,
words are mentally represented by feature counters, called
logogens. An incoming word stimulus causes information to
accumulate in the counters for all words that share properties
with that stimulus. A word is recognized when the amount of
information accumulated in a logogen exceeds the threshold
value for that logogen. Repetition priming can be explained
as the lowering of the threshold for a previously encoun-
tered word (or, equivalently, as the raising of the logogen’s
resting activation level).Counter ModelThe counter model (Ratcliff & McKoon, 1997) is a variant
of the logogen model and was developed to explain repeti-
tion priming in perceptual identification. According to the
model, each word is represented by a counter, and a decision
is made based upon the accumulation of counts. Counts can
correspond to perceptual features or to noise (null counts).
Null counts are needed to allow the system to respond when
there is little or no perceptual information coming from the
stimulus. The characteristic of the model that allows it to ex-
plain repetition priming is that counters can become attrac-
tors of counts. The counter for a previously studied word
can steal counts from the counters of similar words. This
mechanism produces a pattern of bias in repetition priming
because theft of counts is based on similarity and occurs re-
gardless of whether or not the repeated word is the target.
Consider, for example, forced-choice perceptual identifica-
tion in which a target word (e.g.,lied) is briefly flashed and
the subject must then choose between two similar options
(e.g.,liedvs.died). If the flashed target islied,prior study
ofliedcauses an increase in performance; but if the flashed
target isdied,prior study ofliedcauses a decrement in per-
formance (because its counter steals counts fromdied’s
counter). Put another way, people are biased to see the word
that was studied previously, even when it is not the target. A
potential limitation of the counter model is that it only applies
to perceptual identification, which is just one of many tasks in
which repetition priming is observed. Of course, this need not
present a problem if one takes the view that repetition priming
is merely the by-product of the task in which the effect ap-
pears (Ratcliff & McKoon, 1997). In this view, repetition
priming in perceptual identification may be caused by entirely
different mechanisms from those responsible for repetition
priming in another task, such as fragment completion. An-
other problem for the counter model is that there is evidence
that prior study can produce increased sensitivity in addition
to bias (e.g., Bowers, 1999; Wagenmakers, Zeelenberg, &
Raaijmakers, 2000). Although the counter model can be
modified to account for these findings (Ratcliff & McKoon,
2000), this change represents a major conceptual shift in the
model.