Link to recent papers on unconscious mental life.
Conscious and Unconscious Memory
John F. Kihlstrom
University of California, Berkeley
State University of New York
College at Old Westbury
Note: This is a longer, more fully referenced version of an article which wil appear in S. Schneider & M. Velmans (Eds.) Blackwell Companion to Consciousness.
,, 2e. : Blackwell Oxford, U.K.
Conscious recollection appears to be governed by seven principles: elaboration, organization, time-dependency, cue-dependency, encoding specificity, schematic processing, and reconstruction. However, these same principles may not apply to unconscious, or implicit, memory. Implicit memory is most commonly reflected in priming effects which occur in the absence of conscious recollection. Dissociations between explicit and implicit memory have been observed in patients suffering various sorts of brain damage, in other forms of amnesia, in behavioral performance of neurologically intact subjects, and in brain-imaging studies of memory. The most popular interpretation of these dissociations holds that explicit and implicit memory are mediated by separate and independent brain systems. However, there are also compelling interpretations in terms of dual processes operating on the contents of a single memory system.
Keywords: amnesia; declarative knowledge; explicit memory; implicit memory; priming; procedural knowledge; source amnesia
Conscious and Unconscious Memory
In the earliest years of scientific psychology, research focused on immediate conscious experience, in the form of sensations and percepts analyzed first by psychophysicists like Weber and Fechner, physiological psychologists like Helmholtz, and structuralists like Wundt and Titchener. Wundt believed that “higher” mental processes, such as memory, were not amenable to experimental study. But Hermann von Ebbinghaus proved him wrong in 1885: by counting repetitions and calculating savings in relearning, Ebbinghaus invented the verbal-learning paradigm that has dominated the scientific study of memory ever since (Anderson, 1985; Gorfein & Hoffman, 1987; Tulving & Craik, 2000).
Knowledge and Memory
We speak of “memory” but in fact there appear to be many memories. The “modal” multistore model of memory (Atkinson & Shiffrin, 1968; Waugh & Norman, 1965) postulates a number of distinct storage structures (or systems) in the mind. These include modality-specific sensory registers (e.g., iconic and echoic memory); primary, short-term, or working memory; and secondary or long-term memory. The distinction between short- and long-term memory is more a matter of distraction than retention interval. Short-term memory applies while our attention is focused on something; long-term memory applies after our attention has turned elsewhere. The term “working” memory emphasizes the fact that short-term memory is not merely a passive repository of stimulus information; it is where memories are actively used in ongoing cognitive processing (A. Baddeley, 1992, 2012; A. D. Baddeley & Hitch, 1974).
For most people,
long-term memory. As
James put it in the
proper... is the
knowledge of a former state of mind after it has already once
consciousness; or rather it is the knowledge of an event, or
which meantime we have not been thinking, with the
that we have thought or experienced it before.” At the same time,
short-term or working
memory is often identified with consciousness.
Primary memory is sometimes viewed as a separate memory
secondary memory; in other theories, primary memory is
identified with those
representations stored in long-term memory which are currently
in a state of
The knowledge stored in long-term memory comes in two broad forms (Anderson, 1976). Declarative knowledge constitutes our fund of factual knowledge, and can be represented by sentence-like propositions. Procedural knowledge consists of our cognitive repertoire of rules and skills, and can be represented by “if-then” structures known as productions. Within the domain of declarative knowledge, we can distinguish episodic memory, or autobiographical memory for events that have occurred in our personal past, and semantic memory, a sort of impersonal mental dictionary. Procedural knowledge can be further classified into motoric and perceptual-cognitive skills. The declarative-procedural distinction has its immediate origins in computer science and artificial intelligence (Winograd, 1975), but can be traced back to Ryle’s (Ryle, 1949) distinction between “knowing that” and “knowing how”, and Bergson’s (Bergson, 1911) assertion that “the past survives in two forms” -- as recollections and as habits. Again, episodic memory is what most people mean by “memory”, as opposed to “knowledge”. Conceptually, an episodic memory trace contains a description of some event, the unique spatiotemporal context in which that event occurred, and reference to the self as the agent or patient, stimulus or experiencer, of that event (Kihlstrom, 1995, 1997a).
A popular framework for memory research is stage analysis, according to which memories are analogous to books in a library, or the information contained within them (Crowder, 1976; Melton, 1963). Mental representations of events are encoded as memory traces, which are retained in memory storage; these traces are then subject to retrieval. Stage analysis gives rise to the distinction is between availability and accessibility (Tulving & Pearlstone, 1966): Encoded memories, available in memory storage, may not be accessible when retrieval is attempted. On the other hand, they may operate unconsciously, even when conscious remembering fails. Hence the distinction between explicit memory, or conscious recollection, and implicit memory, or the influence of a past event on the person’s experience, thought, or action, in the absence of, or independent of, conscious recollection of that event (Schacter, 1987).
Seven (Plus or Minus Two) Principles of Conscious Recollection
For most of the century following Ebbinghaus, the psychology of memory was concerned with conscious recollection – with our ability to recall or recognize events that had occurred in the past. From this research emerged a small set of principles that largely govern how human memory operates:
1. Elaboration: Memory is a function of the degree to which an event is related to pre-existing knowledge at the time of encoding (F.I.M. Craik & Lockhart, 1972).
2. Organization: Memory is also a function of the degree to which individual events are related to each other (Bower, 1970; G. Mandler, 1979).
3. Time-Dependency: Memory generally fades with time, due to decay, displacement, consolidation failure, or interference among competing memory traces (Ballard, 1913). On the other hand, memory consolidation itself takes time (Lecner, Squire, & Byrne, 1999; J. L. McGaugh, 1966; J.L. McGaugh, 2000).
4. Cue-Dependency: Successful remembering is a function of the informational value of the cues provided at the time of retrieval (Tulving, 1974).
5. Encoding Specificity: Remembering also depends on a match between the cues present at the time of retrieval match those processed at the time of encoding (Tulving & Thomson, 1973).
6. Schematic Processing: Events that are relevant to currently active beliefs, expectations, and attitudes are remembered better than those that are irrelevant; among schema-relevant events, those which are incongruent with these mental schemata are remembered better than those which are congruent (Hastie, 1981).
7. Reconstruction: Memory reflects a mix of information contained in the memory trace and knowledge derived from other sources (Bartlett, 1932). In the final analysis, memories are beliefs, and remembering an event is more like writing a story from fragmentary notes than reading it from a book.
Different classes of memory may operate according to somewhat different principles. Although elaborative rehearsal seems to be necessary for encoding in long-term memory, for example, rote maintenance rehearsal will suffice to keep material active in short-term memory (F. I. M. Craik & Watkins, 1973). Elaboration is critical for explicit memory, but less important for implicit memory (L.L. Jacoby & Dallas, 1981). Forgetting from the sensory registers and short-term memory is produced by decay and displacement, which affect availability; forgetting in long-term memory appears to be a problem of proactive and retroactive interference, which affect accessibility (Anderson, 1974; Anderson & Reder, 1999).
Dissociating Explicit and Implicit Memory
For most of
its history, the scientific study of episodic memory was
concerned mostly with
conscious recollection, to the extent that it was concerned with
at all, and the notion of unconscious memory was relegated
mostly to the
Freudian fantasyland. But
the 1960s, research began to suggest that the notion of
was valid after all – if not in the Freudian form. Of particular interest
were studies of
patients with the amnesic
caused by damage to the hippocampus and related structures in
temporal lobe, or to the mammillary bodies and related
structures in the
diencephalon. In a
This is a priming effect, in which the processing of one
item (e.g., at the
time of study) influences the processing of another item (e.g.,
at the time of
test). In positive
priming, the prime
facilitates processing of the target; in negative priming, the
processing of the target. In
instance, the priming effect indicates that the studied items
were encoded in
memory, retained in storage, and influenced performance on the
test. The fact that
equivalent levels of
priming occurred in neurologically intact subjects, who
remembered the priming
episode normally, and amnesic patients, who had very poor
that priming can be dissociated from conscious recollection.
A later experiment
by Graf et al.
established a standard method for exploring dissociations
between explicit and
implicit memory (Graf, Squire,
1984). Tests of recall and recognition differ from the
employed by Warrington and Weiskrantz on a number of dimensions,
of which the
most important is the informational value of the cues presented
subjects. For their
explicit task, Graf
et al. asked subjects to complete word-stems with items from the
for the implicit task, subjects were asked to complete the stems
with the first
words that came to mind. Amnesic
performed poorly on stem-cued recall, but normally on
conforming the dissociation between explicit and implicit
On the basis of
evidence like this,
Schacter distinguished between two expressions of episodic
memory: explicit and
implicit (Schacter, 1987; see
also Roediger, 1990). Explicit memory refers
recollection of a past event, as exemplified by performance on
recognition tests. By
memory refers to any effect of an event on subsequent
experience, thought, or
action. Priming is,
of course, just such
an effect. The
priming and recall in amnesic patients indicates implicit memory
can persist in
the absence of explicit memory.
Subsequent research identified a number of different
explicit and implicit memory (for a more comprehensive review,
see Reder, Park, &
Kieffaber, 2009, who also
note important exceptions to these outcomes).
Probably the most
convincing of these involve the amnesic syndrome and other forms
definition, these syndromes
are marked by an impairment of explicit memory; but they also
priming and other manifestations of implicit memory. These conditions
include anterograde amnesia
associated with bilateral damage to the hippocampus and other
structures in the
medial temporal lobe (e.g., Schacter,
Squire, 1992); anterograde (Squire, Shimamura, & Graf, 1985)
retrograde (Dorfman, Kihlstrom,
Misiaszek, 1995) amnesia occurring as a consequence of
therapy (ECT) for depression; general anesthesia administered to
patients (Kihlstrom, Schacter,
Cork, & Hurt,
1990 see Chapter 51); conscious sedation in outpatient
surgery (Cork, Heaton, &
Kihlstrom, 1996; see also
Chapter 51); dementia, including Alzheimer’s disease (Salmon, Shimamura, Butters, & Smith, 1988);
amnesia (Kihlstrom, 1980, 2007);
“functional” or “psychogenic” amnesias encountered in genuine
cases of dissociative
disorder (Aarts & Custers,
& Schacter, 2000), including dissociative amnesia,
fugue, and the interpersonality amnesia of dissociative identity
known as multiple personality disorder).
Normal age-related declines in memory primarily affect
leaving implicit memory intact (Light
Voie, 1993; Light & Singh, 1987).
And neurologically intact subjects show significant
relearning for items that, because of the long retention
they can neither recall nor recognize (Nelson,
One of the most
these dissociations is observed in source amnesia, where
patients and subjects
acquire new declarative and procedural knowledge, but have no
recollection of the learning episode (Brown
& Murphy, 1989; Evans & Thorne, 1966; Schacter,
McClachlan, 1984; Shimamura & Squire, 1987).
In the cases noted above, implicit memory can occur in the absence of explicit memory. Conceptually similar “functional” dissociations can even be observed in individuals with normal memory, where implicit memory is in some sense independent of explicit memory (for a review, see Reder et al., 2009; Roediger & McDermott, 1993). For example, a number of experimental manipulations have substantial effects on explicit memory, but little or no effect on implicit memory. These include depth of processing at the time of encoding (Jacoby & Dallas, 1981); the generation effect (Jacoby, 1983); repetitions (Parkin, Reid, & Russo, 1990) and exposure duration (Jacoby & Dallas, 1981). Implicit memory is less vulnerable to variations in retention interval (Jacoby & Dallas, 1981), and to interference effects (Graf & Schacter, 1987). Still other manipulations affect implicit memory, but not explicit memory. These include: a modality shift between study and test (Graf, Shimamura, & Squire, 1985); changing the font of a visual stimulus (Roediger & Blaxton, 1987); and changing the voice of an auditory stimulus (Church & Schacter, 1994).
indicate that performance of explicit and implicit memory tasks
with different patterns of brain activity (Schacter
& Buckner, 1998).
example, Schott et al. found that visual word priming was
decreased activity in the fusiform area, frontal, and
extrastriate cortex; cued
recall was associated with increased activity in posterior
precuneus, and inferior parietal lobe (Schott et
al., 2005; Schott, Richardson-Klavehn, Heinze, & Duzel,
meta-analysis of fMRI studies showed clear
evidence of medial-temporal/hippocampal activation during
episodic encoding and
retrieval (Spaniol et al., 2009). Of course, the
dissociation observed in
amnesic patients also suggests that performance on explicit
memory tasks relies
on an intact hippocampus, and other structures in the medial
while priming is dependent on the cerebral cortex – a point to
which we will
There are exceptions to these findings, which complicate the picture somewhat (Reder et al., 2009). Consider, for example, the functional dissociation involving depth of processing, which is so well accepted that it has almost become part of the operational definition of implicit memory (you know that priming is an expression of implicit memory, as opposed to something else, if it is independent of depth of processing). Two comprehensive meta-analyses of this literature, aggregating the studies, found that there was, indeed, an effect of depth of processing on priming, even if individual studies lacked the power to detect it (Brown & Mitchell, 1994; Challis & Brodbeck, 1992). The effect was smaller on implicit than on explicit memory, but it was there nonetheless. So, in this respect at least, implicit memory may not operate on different principles than explicit memory.
between explicit and implicit memory come in several forms (Dunn & Kirsner, 1988;
Gardiner, & Java, 1996; Teuber, 1955). Most often, they take
the form of single
dissociations, in which a single
independent variable – amnesia, experimental manipulation, or
brain region --
affects one expression of memory, explicit or implicit, but not
the other. Statistically,
however, single dissociations
take the form of statistical interactions (i.e., between some
variable and dependent variables measuring explicit and implicit
are often difficult to interpret.
ago, for example, Chapman and Chapman pointed out that spurious
can emerge as artifacts of differential item difficulty (Chapman & Chapman, 1973, 1978). Similarly, Loftus
noted that non-crossover
interactions (which is what single dissociations are) may be
“removable” if the
dependent variables are placed on the same scale of measurement
(Loftus, 1978; Wagenmakers,
Krypotos, Criss, &
Iverson, 2012). More
Graf et al. pointed out that differences between explicit and
are only meaningful if the cues presented at the time of test
are matched for
their informational value (Graf,
Mandler, 1984). Comparing
free recall to implicit word-stem completion, for example,
the explicit-implicit dimension with cue information: word-stems
informational value than free-recall tests.
A more appropriate comparison would pit stem-completion
For these and other reasons, double dissociations are the “Holy Grail” of cognitive neuropsychology and cognitive neuroscience. Double dissociations take the form of a crossover interaction, in which a single independent variable has opposite effects on two dependent variables, and they allow firm conclusions that performance of the two tasks is mediated by distinct processes, cognitive modules, or brain systems. Unfortunately, double dissociations are also exceedingly rare. However, many ostensible double dissociations are not genuine crossover dissociations. Rather, they are more like twin dissociations, in which one independent variable affects explicit but not implicit memory, while another affects implicit but not explicit memory. As such, they are vulnerable to removability and the other problems discussed earlier.
Implicit memory is usually tested with a priming task, but priming comes in a number of different forms (Cofer, 1967; Roediger, 1990). Most research on implicit memory has focused on repetition priming, in which the target of the priming test is a recapitulation or token, in whole or in part, of the prime itself. For example, subjects might study a word like doctor and then be asked to complete the stem doc- or the fragment d-c-o- with the first word that comes to mind, to identify the word doctor when presented against a noisy background (perceptual identification), or to decide whether the letter string doctor is a legal word (lexical decision). But semantic priming effects, also known as indirect priming, can also be observed when subjects who have studied a word like doctor are asked to give free associations to cues like nurse, or to generate instances of categories like occupations (McNamara, 2005). Repetition priming can be mediated by a perception-based representation that is limited to the physical attributes of the prime and its configuration in space and time, but semantic priming requires a meaning-based representation that includes information about the semantic and conceptual features of the prime. Semantic priming can be studied with the same tasks normally used to measure repetition priming, such as perceptual identification and lexical decision, provided that the target and prime are linked by meaning rather than physical similarity. But it is more commonly studied with tasks such as free-association and category generation.
and implicit memory are sometimes referred to as “declarative”
memory (Cohen & Squire,
“declarative” and “nondeclarative” memory (Squire,
Knowlton, & Musen, 1993), respectively. The
declarative-procedural usage was initially
based on the view that preserved learning in amnesia was limited
knowledge such as cognitive and motor skills, and an
interpretation of priming
and conditioning (some forms of which are also preserved in
procedural in nature. While
implicit expressions of memory may be mediated by procedural or
knowledge, the declarative-nondeclarative distinction risks
interpretation of explicit memories as representations that can
“declared” with the propositional format in which declarative
acquire new declarative knowledge as well, provided that they do
not have to
remember the circumstances in which they learned it (as in
amnesia can also
spare semantic priming, which is mediated by semantic memory --
which in turn
is an aspect of declarative knowledge (Arndt,
Passannante, & Hirshman, 2004; Barnier, Bryant, &
Briscoe, 2001; David,
Brown, Pojoga, & David, 2000; Gardner, Boller, Moreines,
1973; Keane et al., 1997; Kihlstrom, 1980; Levy, Stark, &
Squire, 2004). Unfortunately,
semantic priming have been much less studied -- with
consequences for theories
of implicit memory, as noted below.
Tests of explicit and implicit memory are sometimes referred to as “direct” and “indirect”, or “intentional” and “incidental”, respectively (Johnson & Hasher, 1987; Richardson-Klavehn & Bjork, 1988). That is to say, recall and recognition test memory directly, while savings or priming tests memory indirectly. This can cause confusion, as Cofer distinguished between “direct” (repetition) and “indirect” (semantic) forms of priming, raising the risk that repetition priming could be labeled as “direct-indirect” memory, and semantic priming as “indirect-indirect” memory (Cofer, 1967). Put another way, subjects deliberately intend to consciously recall past events, while priming occurs incidentally when the subject is engaged with some other kind of task. It should be understood, though, that the direct-indirect distinction applies to memory tests and not to expressions of memory. In principle, priming could be used to assess consciously accessible memories that the subject declines to report, much as psychophysiological measures are used in forensic lie-detection. Similarly, a conscious memory could emerge spontaneously in the course of a priming test – a situation that has been referred to as “involuntary explicit memory”. Moreover, conscious recollection can occur incidentally, as in flashbacks of traumatic memory when some aspect of a task reminds the subject of a past event (Berntsen, 1996, 2009). In the final analysis, both the “direct-indirect” and “intentional-incidental” dichotomies fail to capture the essence of the explicit-implicit distinction – which is that explicit memory is conscious recollection, and implicit memory is unconscious memory, of the past.
Theories of Explicit and Implicit Memory
That explicit and implicit memory are dissociable – by various forms of amnesia, by experimental manipulations, and in neuroimaging studies – is now widely accepted. A variety of theoretical accounts have been offered to explain these dissociations, which boil down to two basic categories: multiple memory systems and multiple memory processes.
Multiple Memory Systems
Based on the
popular in cognitive neuroscience (Fodor, 1983),
a number of theorists have suggested that explicit and implicit
the performance of separate memory systems in the brain (Schacter & Tulving, 1994b; Schacter & Wagner,
Wagner, & Buckner, 2000; Squire, 2004; Squire &
For example, Squire has identified the neural
substrate of the “declarative” memory system with the medial
including the hippocampus and related structures (Squire & Zola-Morgan, 1991), and the
to the declarative system will impair
explicit memory for facts and events but spare implicit memory,
mediated by other, “nondeclarative”, memory systems. There are at least
five of these, mediating:
procedural memory for skills and habits, located in the
striatum; priming and
perceptual learning, located in the neocortex; simple classical
located in the amygdala (for emotional responses) and cerebellum
responses) and nonassociative learning, located in various
Schacter and his
that the encoding and retrieval of explicit episodic memories is
the medial temporal lobe system, while various forms of implicit
mediated by systems located in the cerebral cortex. Repetition priming is
mediated by a set of perceptual
that store representations of the physical structure of the
prime, but not its
meaning (Tulving & Schacter,
priming, as well as the explicit
retrieval of semantic knowledge, is mediated by a separate
system tied to the prefrontal cortex; procedural knowledge is
also cortical in
nature (Schacter et al., 2000). They also identify
working memory with
different cortical centers supporting each of its various
components (Baddeley, 2012).
The relational memory theory of Eichenbaum and Cohen (Cohen & Eichenbaum, 1993;
Eichenbaum & Cohen, 2001) is an elaboration of
Cohen and Squire’s
original distinction between declarative and procedural memory (Cohen & Squire, 1980). On this account,
declarative memory is not
exactly synonymous with explicit memory; rather, the hippocampal
critical for all forms of memory that relate arbitrary or
between the constituent features of an event – regardless of
hippocampus-independent procedural system supports the tuning or
of processing modules engaged during initial learning. These processing
modules are not dedicated to
memory per se, but rather reflect plasticity within modules
perceptual, motor, and other functions.
Yet another take on
multiple-systems view is provided by Bowers and Marsolek (Bowers & Marsolek, 2003). Instead of invoking
multiple memory systems,
they propose that implicit memory is a byproduct of brain
systems that are
devoted to perceptual pattern recognition, conceptual
processing, and motor
behavior, rather than memory per se. On their view,
implicit memory is a byproduct
of the learning capability of these systems.
These systems have individual memories, in that they are
encoding and recognizing information, but they are not memory
Although Bowers and
approach is based on contemporary theories of object
psycholinguistics, and concept formation, it has its deeper
roots in a proposal
by Ewald Hering, the 19th-century sensory
physiologist, that memory
is “a universal function of all organized matter” (Hering, 1870/1880, p. 63).
Hering’s ideas, in turn, were promoted by Samuel Butler,
author of Erewhon and
The Way of All Flesh, in a ground-breaking book on
predated Ebbinghaus (Butler,
memory, on Hering’s and
The multiple memory systems view has been very attractive, not least because something like the doctrine of modularity lies at the heart of contemporary cognitive neuroscience. Dissociations, whether between explicit and implicit memory or any other measures, are readily explained by postulating separate brain modules underlying performance on each task. The downside, however, is that it can be tempting to invoke a new brain system whenever we encounter a new dissociation. Based on studies of verbal repetition priming, for example, some theorists postulated the existence of a visual word form system, associated with the extrastriate cortex, which mediates visual stem-completion, and an auditory word-form system mediating auditory perceptual identification (Schacter & Tulving, 1994a). However, it is extremely unlikely that a word-form system actually exists in the brain, for the simple reason that writing is only about 5,000 years old – not enough time for the brain to have evolved such a system. More likely, the perceptual processing of words is mediated by a more generic system which mediates the identification and classification of familiar visual stimuli of all sorts – not just reading (Changeux & Dehaene, 1989; Dehaene & Cohen, 2011). For this reason, the challenge for multiple-systems theories is to develop a set of principles that would tell us when to stop making such inferences.
Unitary System Theories
By contrast with the
multiple-systems view, other theories hold that explicit and
of memory are the products of a single memory system. Perhaps the most
intuitively appealing of
these is the activation
has its roots in generic associative network models of memory. For example, Rozin
suggested that, once
encoded, a node in a network retained some residual activation
for a period of
time – an amount sufficient to support priming, if not conscious
recollection (Rozin, 1976).
A more elaborate version was proposed by Mandler, who
priming in all its forms is mediated by the automatic activation
integration, at the time of encoding, of pre-existing knowledge
corresponding to the prime; explicit memory, by contrast,
elaboration to establish new relations among activated
structures (Graf & Mandler,
1984; Mandler, 1980).
But activation, integration, and elaboration
all take place within a single memory system.
Roediger’s transfer-appropriate processing view (Roediger & McDermott, 1993) holds that most
tasks, such as repetition priming, are “perceptually driven”, in
require access only to surface features of an object; by
memory tasks are typically “conceptually driven”, in that they
to semantic or contextual information associated with the
studied item. In
this view, dissociations occur because
explicit memory depends on “top-down” or “symbolic” processing,
memory depends on “bottom-up” or “data-driven” processing.
Yet a third single-systems view invokes Jacoby’s process dissociation framework (Jacoby, 1991). In this view, explicit memory is largely a product of conscious, controlled, effortful, deliberate processing, while implicit memory is largely a product of unconscious, automatic, effortless, involuntary processing. Jacoby has further introduced a method, the process dissociation procedure, which measures the relative contributions of automatic and controlled processing to any task by pitting them against each other in the “method of opposition”. A typical result of the PDP is to confirm that the performance of normal subjects on a memory task is mediated by a mix of controlled and automatic processes, while the performance of amnesic patients is largely supported by automatic processes. One implication of Jacoby’s theory is that explicit memory is a product of controlled processing while implicit memory is a product of automatic processing; but both processes operate within the same memory system.
In some sense, it might seem that implicit memories are simply “weak” memories – too weak to be consciously remembered, but strong enough to give rise to priming effects. Shanks and his colleagues have offered two different variations on this theme. The first was based on a connectionist model of memory -- itself perhaps the epitome of a unitary memory model (Kinder & Shanks, 2001; Kinder & Shanks, 2003). Simulating amnesia by imposing a slower rate of learning, the model predicts that amnesia will affect recognition more than priming. An alternative computational model, based on signal-detection theory, holds that the same memory underlies both explicit and implicit performance, but that the two tasks differ in terms of the distribution of noise in which the signal is embedded – greater for priming than for recognition (Berry, Shanks, & Henson, 2008). The model predicts that conditions that affect overall memory strength (like amnesia or deep processing) will be more likely to affect recognition than priming. Both models, then, yielded exactly the sort of dissociation that gave rise to the distinction between explicit and implicit memory in the first place – without assuming multiple memory systems.
Reder and her colleagues have also argued that explicit and implicit memory draw on the same stored representation (Reder et al., 2009). In their view, explicit memory requires the formation of new associations, such as between a representation of the item and a representation of the episodic context in which it was presented (see also Kihlstrom, 1997a). In most priming tasks, however, there is no such new association – the individual item, such as the word doctor, simply stands alone. Reder’s argument is especially powerful because it is implemented in a general-purpose computational model of memory and cognition which predicts not only a wide variety of explicit-implicit dissociations, involving amnesia, experimental manipulations, and brain-imaging, but also predicts the circumstances under which such dissociations will not be observed. So, for example, the model predicts that hippocampal amnesia will impair not only explicit recollection but implicit memory for new associations as well as semantic priming, because all require the formation of associations between prime and target; however, hippocampal amnesia will not affect repetition priming, which involves only individual items (Chun, 2005; Wang, Lazzara, Ranganath, Knight, & Yonelinas, 2010). The theory has the extra advantage of being congruent with recent reinterpretations of the functional role of the hippocampus itself, which is to support processing of the relations among elements in a memory (Eichenbaum, 2003, 2008; Eichenbaum & Cohen, 2001). In Reder’s view, the difference between explicit and implicit memory is not that one is conscious and the other unconscious, but that one is relational and the other one typically is not.
antithesis, and synthesis: naturally, some theorists have
proposed models that
attempt to combine the virtues of the multiple- and
easiest way to reconcile
these two viewpoints is to propose that different expressions of
different processes, rather than different memory systems, but
processes are themselves, mediated by different brain systems. For example, Henke has
proposed that separate
brain systems support three different processing modes (Henke, 2010).
rapid encoding of flexible associations (involving the
neocortex), slow encoding of rigid associations (basal ganglia,
neocortex), and rapid encoding of single or unitized items
and neocortex). Like
memory theory that inspired it, Henke argues that hippocampal
not involve conscious awareness.
Although none of these systems is expressly identified
memory, damage to the hippocampus but not to the parahippocampal
neocortex will impair episodic memory but spare repetition
priming – the very
dissociation classically observed in the amnesic syndrome.
By contrast, Moscovitch and his colleagues have suggested that memory is governed by a very large number of processing components, each associated with a different brain region (Cabeza & Moscovitch, 2013; Moscovitch, 1992). For example, they argue that the hippocampus supports flexible relational processing involved in both conscious recollection and priming, so long as the priming involves semantic or other relations between representations. In a similar vein, the ventral parietal cortex supports bottom-up attention involved in episodic memory retrieval. With such a proliferation of processing components, the component-process framework can account for virtually any pattern of task associations and dissociations that research might discover – an asset that is also a liability. Cabeza and Moscovitch agree that this approach lacks the appearance of parsimony, but also argue that it makes predictions that are both strong and falsifiable.
Testing the Theories
Each of these views has its strengths and weaknesses, not least because they evolved in different research contexts. Multiple-systems theories are based largely on work with neurological patients, while single-system theories emerged mostly from work on neurologically intact subjects. The multiple-systems views bask in the reflected glory of cognitive neuroscience, but are bedeviled by the temptation to invoke a new memory system to explain every new dissociation revealed by research. The activation view gives a plausible account of priming results, but finds it difficult to explain how activation could persist for days or months – as it is sometimes observed to do (Squire, Shimamura, & Graf, 1987; Tulving, Hayman, & Macdonald, 1991). The transfer-appropriate processing view can explain dissociations not only between explicit and implicit memory, but also those that occur between two explicit or two implicit memory tasks (e.g., one perceptual, the other conceptual in nature; Blaxton, 1989), but has some difficulty explaining dissociations between semantic priming and explicit memory, both of which are, in its terms, conceptually driven. A further question is whether it is appropriate to term explicit memory as conceptually driven in the first place.
The PDP view, for
its part, offers
a way to reconcile single-system and multiple-system views: on
that automatic and controlled processes are mediated by separate
modules that operate on a single memory store.
At the very least, it has provided an increasingly
popular technique for
measuring the contributions of automatic and controlled
processes to task
the hybrid theories
just described, it offers a way to reconcile single-system and
views. However, the
mathematics of the
PDP requires the troubling assumption that these processes are
each other. An
alternative view, also
consistent with a single-system view of memory, describes
as embedded in, and thus redundant with, controlled ones (Joordens & Merikle, 1993; Curran
1995; Joordens & Merikle, 1993).
One area where the
make competing predictions is with respect to implicit memory
unfamiliar information. Activation
would seem to suggest that this is not possible, because there
is – by
definition – no pre-existing knowledge structure stored in
memory to be
activated, or modified, by perceptual input.
By contrast, the multiple-systems views are, at least in
to the acquisition of new information.
In fact, there is considerable evidence for priming of
items such as dot patterns and novel objects (e.g.,
Musen & Squire, 1992; Schacter, Cooper, & Treadwel,
though not, apparently, for line drawings of “impossible”
objects that cannot
exist in three-dimensional space (much like the drawings of the
M.C. Escher; Schacter, 1990). Although
interpretation of these findings
remains somewhat controversial (Ratcliff
McKoon, 1996), priming for novel stimuli would appear
to support the
multiple-systems view that repetition priming is the product of
representation system that encodes and preserves structural
stimulus events. Priming
does not occur
for impossible objects because the perceptual representation
system cannot form
a structural description of objects that cannot exist in
The situation with respect to priming for verbal materials, such as words, is more complicated. Early results, which showed priming for words like candy and number (which have pre-existing representations in semantic memory) but not for pseudowords like canber and numdy (which do not) are, of course consistent with the activation view of implicit memory (Diamond & Rozin, 1984). Bowers found priming for words (like kite), nonwords that followed the rules of English orthography (like kers) and for illegal nonwords (like xyks), again contradicting the activation view (Bowers, 1994). However, as Bowers himself noted, the priming he obtained for illegal nonwords may have been contaminated by explicit memory, which softens the blow somewhat. On the other hand, Dorfman found priming for pseudowords made up of familiar morphemes (like genvive) and familiar syllables (like fasney), but not for pseudosyllabic pseudowords (like erktofe) made up of elements that are neither morphemes nor syllables in English (Dorfman, 1994, 1999). These results are consistent with the view that priming of novel (and familiar) words results from the activation and integration of pre-existing sublexical components stored in memory: priming cannot not occur where there are no such components to be activated.
The failure to find
an effect for
impossible objects may suggest that activation of prior
to priming of a novel stimulus.
in the verbal domain, Stark and McClelland (2000)
found a strong repetition effect for words (e.g., bond) and pseudowords that followed the rules of
orthography (e.g., corm);
nonwords composed of random consonants (e.g., bdxj) yielded much weaker priming.
This study is noteworthy because the paradigm permitted
priming in the absence of conscious recognition -- ruling out
by explicit memory that has compromised interpretation of prior
nonwword and novel object priming.
Further adjudication between activation and acquisition
come from neuroimaging studies, as a decrease in neural
activity: a decrease in
activation during retrieval may reflect activation of
whereas an increase would seem to implicate the acquisition of
new knowledge (Henson, 2003).
The theoretical debate continues back-and-forth, but theoretical development is hampered by the fact that experimental research on implicit memory is almost exclusively focused on a single experimental paradigm – namely, repetition priming. Roediger and McDermott once estimated that some 80% of implicit memory tests are perceptual in nature, involving variants on repetition priming, about 10% conceptual, and the remaining 10% procedural, and the situation has not changed since then (Roediger & McDermott, 1993). Viewed in this light, it is not surprising to find theorists proposing that implicit memory is the product of a perceptual representation systems, or of perceptually based processing. But if implicit memory extends to semantic priming, as indeed it does (Barnier et al., 2001; David et al., 2000; Kihlstrom, 1980; Levy et al., 2004), such theories are too limited to account for the phenomenon. Repetition priming may be independent of depth of processing -- though a more accurate statement would be that it is only relatively independent; Brown & Mitchell, 1994; Challis & Brodbeck, 1992); but this is unlikely to be the case for semantic priming. Repetition priming may be modality specific – though not hyperspecific (Rajaram & Roediger, 1993); but again, this is unlikely to be the case for semantic priming. Research on implicit memory must move beyond repetition priming if we are ever to determine its true nature.
Interactions between Explicit and Implicit Memory
largely to the hegemony of cognitive neuroscience, the most
popular theory of
implicit memory remains some version of the multiple-systems
view. Even so,
claims for a strict separation of
these memory systems should not be made too strongly. If these various
memory modules were truly
independent of each other, we would expect to see neurological
explicit memory is spared and implicit memory impaired. The reverse, of
course, is what is commonly
observed in amnesia. In
fact, only one
such case has been reported (Gabrieli,
Keane, Reminger, & Morrell, 1995; M.M. Keane, Gabrieli,
Mapstone, Johnson, & Corkin, 1995; Wagner, Stebbins,
& Gabrieli, 1998) and its status is uncertain. The patient in
question, known as M.S., had a
scotoma secondary to brain surgery. He
performed normally on a recognition test but poorly on a visual
repetition priming. However,
normal performance on a test of conceptual priming. His poor visual
priming performance may
reflect an extensive lesion in occipital cortex, but on the
basis of the
conceptual priming results it can hardly be said that he lacked
their underlying basis, the interaction between explicit and
can also be observed in other ways.
Subjects who consciously recognize that the items on a
perceptual-identification test (for example) come from a
wordlist may develop a mental set that actually enhances their
performance – which is why researchers in this area take care to
awareness” in their subjects (Bowers
Schacter, 1990), and why Jacoby’s “process
dissociation” procedure has
become so popular (Yonelinas
& Jacoby, 2012).
Densely amnesic patients are not able to take
advantage of explicit memory, of course, but that does not mean
recollection cannot influence priming in other circumstances.
Moreover, there is considerable evidence that subjects can take strategic advantage of implicit memory to enhance their performance on tests of explicit memory. Although free recall epitomizes conscious recollection, both Mandler (1980) and Jacoby (1991) have argued that recognition judgments can be mediated by either conscious recollection of the test item, or by a feeling of familiarity that might be based on priming. If so, then when implicit memory is spared, subjects can strategically capitalize on the priming-based feeling of familiarity to enhance their performance on recognition tests (Mandler, Hamson, & Dorfman, 1990; Yonelinas, Aly, Wang, & Koen, 2010). We know that, as a rule, recognition is superior to recall in normal subjects and this is also true for neurological patients with the amnesic syndrome (Sullivan & Verfaellie, 1999), depressed patients receiving ECT (Dorfman et al., 1995), demented patients suffering from Alzheimer’s disease (Snodgrass & Corwin, 1988), and normal subjects with posthypnotic amnesia (Kihlstrom, 1997b). In addition, studies of recollective experience indicate that amnesic recognition is typically accompanied by intuitive feelings of familiarity, rather than full-fledged remembering (Knowlton & Squire, 1995; Verfaellie, Giovanello, & Keane, 2001).
Accordingly, it seems reasonable to suggest that successful recognition in amnesia can be mediated by spared implicit memory, in the form of repetition or semantic priming. This claim has been vigorously debated by Squire and his colleagues, who insist that priming is inaccessible to conscious awareness, and so cannot serve as a basis for recognition (e.g., Levy et al., 2004). Despite methodological issues cutting this way and that, studies employing the process-dissociation procedure clearly indicate that, even among amnesic patients, recognition can be mediated by a priming-based feeling of familiarity (Yonelinas, 2001; Yonelinas, Kroll, Dobbins, Lazzara, & Knight, 1998) – as theory suggests they might be, and as the subjects themselves say they are. It may be that recollection and familiarity are governed by separate memory systems (Aggleton & Brown, 1999); but against a further proliferation of memory systems, it may be more parsimonious to conclude that explicit and implicit memory interact after all (Dew & Cabeza, 2011).
The Phenomenal Experience of Remembering
The role of
familiarity in recognition brings us full circle, to the
of remembering. In
a seminal paper,
Tulving distinguished between two different recollective
remembering, or one’s concrete awareness of oneself in the past,
one’s abstract knowledge of the past (Tulving,
remember-know distinction maps onto his earlier distinction
and semantic memory (Tulving,
remember-know distinction was further
developed by Gardiner, who showed that “remember” judgments were
affected by depth of processing, while “know” judgments were not
(Gardiner, 1988). This is not
necessarily the functional
dissociation we would expect between explicit episodic and
both of which are the product of deep processing; but it is just
the sort of
functional dissociation we would expect to find between explicit
priming memory. For
the remember-know distinction maps onto Mandler’s distinction
and familiarity – closer, that is, to the distinction between
implicit memory. “Remembering”
the conscious retrieval of an episode, including a
representation of the event,
its spatiotemporal context, and the role of the self as agent or
stimulus or experiencer (Brown
& Fish, 1983;
Fillmore, 1968). Viewed
“knowing” reflects one’s abstract, impersonal knowledge of the
The remember-know distinction has proved quite valuable in research on memory and amnesia (Yonelinas, 2001; Yonelinas et al., 2010). However, it now seems that we should make at least a tripartite distinction among three varieties of recollective experience: “remembering”, or conscious retrieval from episodic memory; “knowing”, or conscious retrieval from semantic memory; and “feeling”, or an inference based on a priming-based feeling of familiarity. There may even be a fourth variety: “believing”, or an inference concerning the past based on other world-knowledge. All of these may be dissociable, differential impairment in various forms of amnesia, different effects of experimental manipulations, and different patterns of neural activity. If so, amnesic patients and others with severely impaired autobiographical memory (Palombo, Alain, Soderlund, Khuu, & Levine, 2015) may be able to use these alternative routes to recollect the past.
The Implicit and the Unconscious
Together with the concept of automaticity, research on implicit memory constituted our first steps toward a revival of interest in unconscious mental life (Kihlstrom, 1987). Although the psychological unconscious suffered much in the 20th century from taint by Freudian psychoanalysis – one reason why theorists choose to speak of “implicit” memory rather than “unconscious” memory – the concepts and methods employed to study implicit learning and memory have now been extended to other domains, such as perception (Kihlstrom, Barnhardt, & Tataryn, 1992) and even thinking (J. Dorfman, Shames, & Kihlstrom, 1996) – and beyond cognition to emotion (Kihlstrom, Mulvaney, Tobias, & Tobis, 2000) and motivation (Kihlstrom, 2015; McClelland, Koestner, & Weinberger, 1989). In this way, the study of implicit learning and memory offer a new, non-Freudian perspective on unconscious mental life – and, in turn, on consciousness itself.
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