Download CHARMed, But Not Convinced: Comment on Metcalfe (1990)

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Journal of Experimental Psychology: General
1991, Vol. 120, No. I, 101-105
CHARMed, But Not Convinced: Comment on Metcalfe (1990)
D. Stephen Lindsay
Williams College
Metcalfe (1990) proposed her Composite Holographic Associative Recall Model (CHARM) as a
model of eyewitness suggestibility. CHARM has many appealing properties and performed well
in simulations of suggestibility. Most important, CHARM provides a formal mechanism by
which suggestions impair memory without affecting performance on McCloskey and Zaragoza's
(1985a) Modified Test. Nonetheless, a number of shortcomings limit CHARM'S usefulness as a
model of suggestibility: (a) in the simulations, control terms differ from those in human studies;
(b) the model makes a counterintuitive prediction about performance on a recognition pair
composed of an event detail and its control term; (c) CHARM models association, not remembering; and (d) most of the intelligence in the simulations lies in the programmer rather than in
CHARM itself, which limits the model's constraint on memory blending.
Loftus and her colleagues (e.g., Loftus, Miller, & Burns,
1978) have demonstrated that misleading suggestions about
details in an event can impair performance on tests of memory
for those details. For example, subjects might view a slide
sequence that includes a photo of a man holding a hammer
(the event detail) and later receive either a misleading suggestion to the effect that the man had been holding a wrench
(the suggested detail) or neutral information to the effect that
the man had been holding a tool (the control). On a subsequent forced-choice recognition test of memory for details
seen in the slides, subjects more often err on misled than on
control items by selecting the suggested detail (e.g., wrench)
rather than the event detail (e.g., hammer).
Loftus (e.g., Loftus, 1981, 1989; Loftus & Loftus, 1980)
interpreted these findings as evidence that misleading suggestions impair memory for event details. McCloskey and Zaragoza (I985a) argued that there are reasons to expect poorer
misled than control performance on Loftus's "Standard Test"
even if misleading suggestions do not impair ability to remember event details. Their arguments were sharpened by the
results of six experiments in which the critical test pair consisted of the event detail and a novel distractor (e.g., hammer
vs. screwdriver). McCloskey and Zaragoza argued that if
suggestions impair memory for event details, then subjects
should do more poorly on misled than control items on this
"Modified Test." No such effect was obtained.
In a response to McCloskey and Zaragoza's (1985a) critique, Loftus, Schooler, and Wagenaar(I985) argued that the
Work on this comment was supported by Williams College and
the Bronfman Science Center.
1 thank Janet Metcalfe for engaging in a very productive dialogue
concerning an earlier draft of this article. 1 also thank David Levine
for help on matrix algebra; William Batchelder, Colleen Kelley, and
an anonymous reviewer for helpful and insightful comments on
earlier drafts; Jonathan Schooler for bringing Ray Pike's article to my
attention; and Marcia Johnson for general support and advice.
Correspondence concerning this article should be addressed to D.
Stephen Lindsay, Department of Psychology, Williams College,
Bronfman Science Center, Williamstown, Massachusetts 01267.
101
Modified Test is insensitive to subtle "blending" effects, in
which the suggested detail distorts the memory trace of the
event detail. McCloskey and Zaragoza (1985b) rejected this
defense and questioned the unexplained selectivity and intelligence of the blending process described by Loftus et al.
(1985): "Why, when a stop sign and a yellow yield sign are
blended, is the result a red yield sign as opposed to a yellow
stop sign, or an orange sign with 5.5 sides and the word
YSEOD on it?" (p. 386). They concluded that "the memory
blend notion is not sufficiently well-developed to merit serious
consideration" (p. 386).
Metcalfe (1990) recently presented a well-developed theory
of memory blends in the form of her Composite Holographic
Associative Recall Model (CHARM). Like suggestibility studies involving human subjects, the CHARM simulations consisted of three phases (see Table 1). First, two vectors were
convolved (e.g., a vector called "man" was convolved with a
vector called "hammer"). Second, one of the previously stored
vectors was convolved with a third vector (e.g., the vector
called "man" was convolved with a vector called "wrench"
[the misleading suggestion] or with a vector called "tool" [the
control term]), creating a composite memory trace including
information about both associations. At test, the vector common to both associations ("man") was correlated with the
composite memory trace and the resultant vector (a blend of
both "hammer" and either "wrench" or "tool") was compared
with the vectors in a recognition pair. The Standard Test was
simulated by using the event and suggested vectors as the
recognition pair, whereas the Modified Test consisted of the
event vector and a new vector (e.g., "screwdriver").
The results of these simulations corresponded closely to
McCloskey and Zaragoza's (1985a) findings (see Table 1).
The correct vector was selected less often when both of the
vectors in the recognition test pair had been convolved with
the cue (i.e., Misled/Standard Test) than when only the correct vector in the test pair had been convolved with the cue
(i.e., Control/Standard Test and Misled/Modified Test and
Control/Modified Test). Results of other simulations nicely
mirrored Belli's (1989) and Tversky and Tuchin's (1989)
findings with yes-no tests; Chandler's (1989) studies in which
control terms were not presented in the post-event informa-
102
COMMENTS
Table 1
Phases in CHARM Simulations of Suggestibility
Phase
Event
Misled condition
Control condition
[MAN'HAMMER]
[MAN'HAMMER]
Narrative
[MAN'HAMMER +
MAN»WRENCH]
[MAN'HAMMER +
MANTOOL]
Test cue
MAN#[MAN*HAMMER
+ MAN*WRENCH] =
MAN#[MAN*HAMMER
+ MAN'TOOL] =
HXOMER
34% correct
64% correct
66% correct
65% correct
nwfcBwes
Standard test
(HAMMER/WRENCH)
Modified test
(HAMMER/SCREWDRIVER)
Note. Percentages are from Metcalfe (1990, Simulation la). The contents of the composite memory
trace (which are not themselves accessible to consciousness) are shown in brackets. The symbol *
indicates convolved with and the symbol # indicates correlated with. The superimposed words to the
right of the equal signs represent the single "blended" vectors produced when the composite trace is
cued.
tion; Zaragoza, McCloskey, and Jamis's (1987) recall data;
and Loftus's (1977) and Belli's (1988) color shifts.
Metcalfe's research is valuable for a number of reasons.
CHARM'S interactive nature, use of composite storage, and
success at simulating a variety of memory phenomena (Eich,
1982, 1985; Metcalfe, 1990) make it an appealing model. At
a more specific level, this work helps to integrate the suggestibility literature into the broader context of contemporary
theoretical approaches to memory and cognition. This is
important because discussions of suggestibility have sometimes been theoretically naive. Most important, CHARM
demonstrates a formal mechanism by which suggestions could
impair ability to remember event details without affecting
performance on the Modified Test. Noninteractive, unidimensional trace strength models of memory would hold that
virtually any memory-impairing effect of suggestions would
affect performance on the Modified Test (Zaragoza &
McCloskey, 1989). CHARM provides a formal demonstration
of a system in which the effects of interpolated material on
ability to remember event details depends, in part, on how
memory is tested.
Despite these virtues, CHARM suffers from several shortcomings that limit its usefulness as a model of eyewitness
suggestibility.' First, the control terms used in the simulations
differ in an important way from those used in studies of
human suggestibility. Second, the model makes a counterintuitive prediction about performance on a forced-choice test
pair composed of an event detail and its control term. Third,
in its current form, CHARM is a model of association, not a
model of remembering. Finally, CHARM itself does little to
constrain the nature or extent of memory blending because
most of the intelligence involved in the simulations lies in the
human who runs them rather than in CHARM itself.
Prototype Versus Superordinate Control Terms
In most of Metcalfe's (1990) simulations, the vectors representing the event detail, suggested detail, and novel detail
were constructed to be similar to the control term. This was
done by making randomly selected subsets of the features of
each of the specific terms ("hammer," "screwdriver," and
"wrench") have the same values as they did in the vector that
represented the control term ("tool"). Consistent with other
researchers' terminology (e.g., Posner & Keele, 1968), Metcalfe referred to these control terms as prototypes. In studies
of human suggestibility the controls have been superordinate
terms (e.g., "tool"), not prototypes (e.g., ?). Metcalfe identified
her control terms as both prototypes and superordinates (e.g.,
"Item 32 was the prototype 'tool'"; p. 152). CHARM can
handle prototypicality, but is is much less clear that it can
represent the relationship between basic level and superordinate terms. This is a complex issue, but I would argue that a
superordinate term does not reduce to a blend of its subordinates; for example, "tool" does not refer to an object with
features typical of hammers, screwdrivers, wrenches, etc.2 The
difference between category terms and prototypes is important
for many reasons. In the current context, one important
difference is that the control terms used in studies of human
suggestibility are true and accurate references to the event
details (the man was holding a tool, and he was not holding a
"hamscrewren").
1
Metcalfe (1990) suggested that the real-world implications of her
model are restricted to cases in which there is a real-world object that
closely matches the blend: "There are no real world objects that
comprise a blend between a stop sign and a yield sign... [so] a literal
blend would be ruled out immediately, even if such were retrieved
from memory (as the CHARM model says it is)" (p. 158). This need
not limit the real-world implications of the model, because the
response selection processes could be made to return a "no response"
decision if the retrieved vector does not adequately match a lexical
vector. Thus, blending could impair response selection even if the
blend does not match any vector in the lexicon.
Mn natural categories, a prototype is the most representative
exemplar of a category. Rosch and Mervis (1975) found that the
items judged to be most prototypical of a category shared the greatest
number of features with other category members, and Rosch, Mervis,
Gray, Johnson, and Boyes-Braem (1976) found that members of
basic level categories have shapes and motor movements in common.
These ideas and findings do not suggest, however, that superordinates
reduce to prototypes.
103
COMMENTS
Counterintuitive Prediction
CHARM'S Intelligence
Even if CHARM could use superordinate terms analogous
to those used in studies with human subjects, its responses to
such control terms would likely differ from humans'. Like
people, CHARM more often errs on the Standard Test (event
detail "hammer" vs. suggested detail "wrench") when the
suggested detail had been added to the composite trace than
when the control term ("tool") had been added to it. Consider
what would happen if the test pair consisted of the event
detail and the control term (e.g., "hammer" vs. "tool"). Given
such a test pair, CHARM would perform more poorly in the
control condition (in which "tool" had been added to the
composite trace) than in the misled condition (in which
"wrench" had been added to the trace). For CHARM, there
is little difference between misleading suggestions and control
terms. Indeed, in the simulations in which within-category
structure was not represented, the sole difference between
control terms and misleading suggestions was that only the
latter appeared on the test. In those simulations, "hammer,"
"wrench," and "tool" were arbitrary labels assigned to unrelated vectors. Therefore simply switching the labels "Misled/
Standard Test" and "Control/Standard Test" on Metcalfe's
data tables provides a good prediction of how CHARM would
perform on a "hammer" versus "tool" test pair when no
within-category structure was represented in the items: The
"impairment" observed in the control condition given such a
test pair would be the same as the impairment observed in
the misled condition given the Standard Test.3 It is not
immediately obvious what human subjects would do if tested
with a "hammer" versus "tool" pair, but it would be odd
indeed if misled performance exceeded control performance
on such a test pair to the same (or nearly the same) extent as
control performance exceeded misled performance on the
Standard Test.4
As was mentioned above, McCloskey andZaragoza (1985b)
decried the unexplained intelligence Loftus et al. (1985) attributed to blending, noting that no account had been offered
for why blending results in the particular sensible errors used
as lures on the test. At first glance, CHARM appears to answer
this critique: The nature of blending is specified by the ordered
quality of vectors, the extent of blending distortion is determined by the similarity of the vectors stored in the composite
trace (and by weights, etc.), and only sensible blends appear
as output because the responses are selected by matching the
blended output vector to a lexicon of stored vectors.
On closer examination, however, it appears that CHARM
itself accounts for little of this intelligence. As Craik and
Lockhart (1986) pointed out in a comment on Metcalfe's
(Eich's) 1985 article, the model assumes that the features of
an item are inherent in the stimulus itself, and this assumption
is difficult to justify: "While stimulus properties place constraints on possible processing operations, there is little purpose in attempting to list features of a stimulus apart from
cognitive operations... [because] without some point of view
or conceptualization of a stimulus event, the number of such
features is unbounded" (p. 361). Lewandowsky and Hockley
(1987) provided empirical support for this criticism in the
form of two studies demonstrating effects of orienting task
when stimulus materials were held constant.
The likelihood and the nature of memory blends in
CHARM depend on how objects are represented and on how
to-be-associated pairs of items are selected. CHARM tells us
neither how stimuli are transformed into ordered and
weighted sets of "features" representing discrete "items" nor
how pairs of items are selected to be associated. This limitation
is not at all unique to CHARM; most if not all formal models
of memory sidestep issues of encoding and categorization. In
the absence of any account of these processes, however,
CHARM itself does relatively little to answer McCloskey and
Zaragoza's (1985b) criticism of the unexplained selectivity
and intelligence of blending.
Association Versus Remembering
In its present form, CHARM is a model of association, not
a model of remembering. Remembering involves an attribution of aspects of current mental experience to prior experience (cf. Anderson & Bower, 1972; Jacoby, Kelley, & Dywan,
1989; Johnson, 1989;Kintsch, 1974; Mandler, 1972;Tulving,
1984). In the case of eyewitness suggestibility, what is to be
modeled is a memory error in which subjects allegedly remember something that was suggested to them as something that
they witnessed during a particular past event (Lindsay &
Johnson, 1989). People do not recollect only abstract, decontextualized lexical "items," but rather aspects of specific past
events. Furthermore, people generate many associations during attempts to recall that they do not emit as recalled items
(e.g., Anderson & Bower, 1972). CHARM has no basis for
attributing a retrieved vector to a particular past event, because no information about occurrence (source, context, conditions of encounter) is stored. Thus, although association is
undoubtedly a very important contributor to and aspect of
remembering, CHARM would be a much better model of
memory if it were modified to use information about conditions of encounter (source).
3
This effect might be somewhat smaller than a reversal if withincategory structure was represented (i.e., "tool" used as a prototype
and "hammer" and "wrench" derived from it). Because of the similarity relations, both the control term and the misleading suggestion
would produce less distortion of the event detail in this case, and
control terms would produce slightly less distortion of event details
than would misleading suggestions. On the other hand, within-category similarity would also make the "tool" versus "hammer" discrimination more difficult than the Standard Test ("hammer" vs.
"wrench") discrimination. In any case, as long as "hammer,"
"wrench," and "tool" were discriminable, CHARM would more often
err on a "hammer" versus "tool" test pair if "tool" had been added
to the composite trace (control condition) than if "wrench" had been
added to it (misled condition).
4
CHARM's prediction that distortion of memory of the event
detail increases with the dissimilarity between the event detail and
the suggested detail also seems counterintuitive. Would a misleading
suggestion to the efTect that the calculator had been hidden under a
cloth really impair memory for "hammer" to a greater extent than a
suggestion to the effect that the calculator was hidden under a wrench?
104
COMMENTS
General Critique of Convolution-Correlation Models
At a more general level, vector convolution-correlation
models such as CHARM seem problematic for a number of
reasons. First, it is difficult to imagine an efficient system in
which all items are represented by the same ordered set of
features (cf. Craik & Lockhart, 1986; Pike, 1984). To represent
ideas as diverse as "hammer," "tool," and "thief with the
same ordered set of features, the vectors would have to be
very large (with values of zero on most features). Convolution
and correlation with large vectors are very time-consuming
processes (indeed, even encoding items into such large vectors
might take a lot of time). Second, people do not restrict
association formation to single pairs of items. In Metcalfe's
(1990) studies, memory of a slide that shows a man in an
office hiding a stolen calculator under a hammer in his tool
box was simulated by convolving vectorial representations of
"man" and "hammer." If people associate "man" with "hammer," surely they also associate "man" with "calculator," "tool
box," "office," "thief," "slide sequence," and so on. Pair-wise
association between particular objects seems an odd way to
simulate human memory for a naturalistic scene—if only
because a very large (possibly unbounded) number of associations would have to be performed in very little time. Third,
given a processor that could perform a large number of
convolutions involving large vectors in very little time, the
composite trace would quickly become very messy. With the
associations mentioned above, for example, the retrieval cue
"man" would produce a single vector composed of a superimposed melange of features from "hammer," "calculator,"
"tool box," "office," "thief," and "slide sequence."
Summary
In summary, the data produced by the simulations reported
in Metcalfe (1990) very closely match the pattern of results
obtained with human subjects, and CHARM has several
appealing properties. Most important, CHARM'S interactive
nature provides a formal mechanism by which misleading
suggestions could impair memory for event details without
affecting performance on the McCloskey and Zaragoza
(1985a) Modified Test. Nonetheless, in its present form the
model does little to expand our understanding of eyewitness
memory. The control terms used in the simulations differ in
an important way from those used in studies with humans,
and the model makes a counterintuitive prediction about
responses to control terms presented in recognition test pairs.
In its present form, CHARM is a model of association, not
of remembering, because no information about conditions of
encounter is stored. Moreoever, CHARM itself tells us little
about the nature or likelihood of memory blends, because
most of the intelligence involved in the simulations lies in the
human who runs them rather than in the model itself. Finally,
the model's key assumptions appear to be problematic. Although these shortcomings might be reduced in modified
versions of CHARM or in other formal models, it may be
more profitable to make use of some of the general principles
that underlie CHARM (e.g., the interactive relation between
encoding and retrieval) in research conducted with human
subjects.
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Received May 1, 1990
Revision received July 20, 1990
Accepted August 1, 1990 •
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