Psychology Implicit versus Explicit Memory
by
Reza Habib, Alicia Olechowski
  • LAST MODIFIED: 24 May 2017
  • DOI: 10.1093/obo/9780199828340-0190

Introduction

One of the most significant discoveries in the scientific study of human memory is that memory is not unitary. This was noted by William James over a hundred years ago when he spoke of primary and secondary memory. In the 21st century, primary and secondary memory are referred to as short-term or working memory and long-term memory, respectively. More recently, within the past fifty years, important discoveries have led to the fractionation of long-term memory into various systems, processes, and states of awareness. The focus of the present review is the distinction between implicit and explicit memory. When most people think about long-term memory, they think about explicit memory: memory that requires effort to retrieve and is retrieved consciously (e.g., “What did you do this past weekend?”). Implicit memory, on the other hand, reflects transfer from past experiences to the present moment in the absence of either effortful retrieval or conscious awareness; often the experiences prime present performance automatically and without awareness, such as when a musician plays a well-practiced piece of music.

General Overviews

Many books have been written on the topics of implicit and explicit memory. Schacter 1996 and Schacter 2001 are two popular nonfiction titles on implicit and explicit memory. Undergraduate texts by Neath and Surprenant 2003 and Eichenbaum 2008 introduce various aspects of memory, including implicit and explicit memory. The edited volumes by Lewandowsky, et al. 1989 and Marsolek and Bowers 2003 focus on implicit memory, whereas the edited volumes by Roediger and Craik 1989 and Tulving and Craik 2000 cover memory broadly. Finally, Tulving 1983 describes in detail the theory of episodic memory, the memory system hypothesized to underlie explicit memory. Together, these volumes present a comprehensive overview of implicit and explicit memory.

  • Eichenbaum, H. 2008. Learning & memory. New York: W. W. Norton.

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    This textbook presents an interdisciplinary introduction to conscious (episodic/semantic) and unconscious (implicit/procedural) forms of memory based on contemporary human and non-human animal research.

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    • Lewandowsky, S., J. C. Dunn, and K. Kirsner. 1989. Implicit memory: Theoretical issues. Hillsdale, NJ: Lawrence Erlbaum.

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      This edited volume, based on proceedings of a conference on implicit memory, contains articles that consider the characteristics of implicit memories, the nature of their underlying representations, and theories and models to account for implicit memories.

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      • Marsolek, C. J., and J. S. Bowers. 2003. Rethinking implicit memory. Oxford: Oxford Univ. Press.

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        This edited volume presents a more quantitative-theoretical perspective on implicit memory, focusing on priming within the context of word and object recognition and explicitly contrasting the memory systems and processing views.

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        • Neath, I., and A. e. M. Surprenant. 2003. Human memory: An introduction to research, data, and theory. Belmont, CA: Thomson/Wadsworth.

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          This textbook presents a comprehensive overview of topics in human memory with a focus toward processing theories of memory.

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          • Roediger, H. L., III, and F. I. M. Craik. 1989. Varieties of memory and consciousness: Essays in honour of Endel Tulving. Hillsdale, NJ: Lawrence Erlbaum.

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            This Festschrift broadly covers topics in explicit and implicit memory, including several important chapters discussing the relation and dissociations between these two forms of memory.

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            • Schacter, D. L. 1996. Searching for memory: The brain, the mind, and the past. New York: Basic Books.

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              A popular book by one of the leading memory researchers. Presents an accessible introduction to to the topic of explicit and implicit memory.

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              • Schacter, D. L. 2001. The seven sins of memory: How the mind forgets and remembers. Boston: Houghton Mifflin.

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                Another popular book on human memory targeting a general audience. Presents research on memory from the perspective of seven “sins”—or failings—of everyday memory.

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                • Tulving, E. 1983. Elements of episodic memory. New York: Oxford Univ. Press.

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                  This book is by one of the preeminent experts on human memory. Tulving discusses the distinction between episodic and semantic memory and presents research on encoding and retrieval of memories.

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                  • Tulving, E., and F. I. M. Craik. 2000. The Oxford handbook of memory. Oxford and New York: Oxford Univ. Press.

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                    The chapters in this handbook of memory present a comprehensive overview of memory as studied in the laboratory as well as in naturalistic environment. Several chapters also discuss the neurobiology of memory as well as theories to account for different types of memory.

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                    Journals

                    Original research on implicit and explicit memory appears in many general psychology, cognitive psychology, and neuroscience journals. Journal of Experimental Psychology: Learning, Memory, and Cognition, Memory and Cognition, and the Journal of Memory and Language are the most common sources for original experimental psychology research on human memory. Several journals focus on publishing original research on the neuroscience of memory including Learning and Memory and the Neurobiology of Learning and Memory.

                    Review Papers

                    The papers cited in this section present overviews of the empirical research on implicit and explicit memory and the relation between the two. Schacter 1987; Schacter, et al. 1993; and Roediger 1990 focus mainly on implicit memory and dissociations of implicit memory from explicit memory. The scope of Richardson-Klavehn and Bjork 1988 is broader, comparing parallel effects and dissociations within implicit, within explicit memory, and between implicit and explicit memory. Wiggs and Martin 1998 focus mainly on the neural mechanisms underlying implicit memory while Schacter and Buckner 1998 and Henson 2003 review a growing body of neuroimaging studies of implicit memory.

                    • Henson, R. N. A. 2003. Neuroimaging studies of priming. Progress in Neurobiology 70.1: 53–81.

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                      Henson’s review focuses on positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) studies of repetition priming. Henson observes that the most common neural correlate of repetition priming in functional neuroimaging research is decreased neural activity for repeated (primed) versus novel (unprimed) stimuli, with the site of the deactivation being determined by the nature of the behavioral task under study.

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                      • Richardson-Klavehn, A., and R. A. Bjork. 1988. Measures of memory. Annual Review of Psychology 39.1: 475–543.

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                        Richardson-Klavehn and Bjork focus on what they call task-comparison methodology: examining the interrelation amongst different measures of memory (e.g., free recall, recognition, word stem completion, etc.). This approach naturally lends itself to the review of dissociations, as well as parallel effects between explicit, between implicit, and between explicit and implicit measures of memory. Also considers theoretical approaches that could account for parallel effects and dissociations between measures of memory.

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                        • Roediger, H. L., III. 1990. Implicit memory: Retention without remembering. American Psychologist 45.9: 1043.

                          DOI: 10.1037/0003-066X.45.9.1043Save Citation »Export Citation »E-mail Citation »

                          Roediger provides historical context for implicit memory and then proceeds to review the reasons for the renewed interest in implicit memory, namely the finding that this form of memory tends to be preserved in patients with amnesia. The remainder of the review considers theoretical perspectives on the implicit/explicit memory distinction stemming from findings of dissociations.

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                          • Schacter, D. L. 1987. Implicit memory: History and current status. Journal of Experimental Psychology: Learning, Memory, and Cognition 13.3: 501–518.

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                            Schacter provides a historical analysis of implicit memory that covers contemporary research findings and theoretical approaches. The findings behind dissociations between implicit and explicit memory are explored.

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                            • Schacter, D. L., and R. L. Buckner. 1998. Priming and the brain. Neuron 20.2: 185–195.

                              DOI: 10.1016/S0896-6273(00)80448-1Save Citation »Export Citation »E-mail Citation »

                              Schacter and Buckner review priming, a form of implicit memory, from a neurological perspective. That is, they review research that highlights the impact of various neurological disorders such as amnesia, dementia, and focal brain lesions on implicit memory and priming. They conclude with a review of functional neuroimaging research of implicit memory and perceptual (and conceptual) forms of priming.

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                              • Schacter, D. L., C. Y. Chiu, and K. N. Ochsner. 1993. Implicit memory: A selective review. Annual Review Neuroscience 16:159–182.

                                DOI: 10.1146/annurev.ne.16.030193.001111Save Citation »Export Citation »E-mail Citation »

                                In this review article, Schacter, Chiu, and Ochsner provide an overview of advancements in implicit memory research focusing on studies from 1988 until 1993. That authors discuss studies related to priming and skill learning in both amnesic patients and controls with the goal of demonstrating that human memory involves multiple systems and subsystems.

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                                • Wiggs, C. L., and A. Martin. 1998. Properties and mechanisms of perceptual priming. Current Opinion in Neurobiology 8.2: 227–233.

                                  DOI: 10.1016/S0959-4388(98)80144-XSave Citation »Export Citation »E-mail Citation »

                                  Perceptual priming is a form of implicit memory in which repeated performance is enhanced relative to original performance as a function of prior exposure or practice. Wiggs and Martin review the general properties of perceptual priming and consider its developmental trajectory. Then, based on electrophysiology and functional neuroimaging research, they propose a neurobiological mechanism (i.e., repetition suppression) that may underlie perceptual priming.

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                                  Behavioral Dissociations between Implicit Memory and Explicit Memory

                                  Repetition priming is a form of implicit memory in which an earlier stimulus item enhances behavior (primes) when it is repeated at a later time. For example, repetition priming occurs when a participant decides more quickly that a character string (e.g., BANANA) is a word (a lexical decision task) when that string had been previously encountered in comparison to making the same decision on a newly encountered character string (one that has not been encountered previously and that is being seen for the first time). The difference in reaction time for the lexical decision between the repeated string and the new string is a measure of repetition priming and is an implicit measure of memory: the participant was not asked to “remember” the string from an earlier presentation; he or she was simply asked to make a lexical decision as quickly as possible. The faster processing is due to priming from the earlier encounter with the item. A dissociation occurs when an independent variable affects two (or more) dependent variables differently. After the initial discovery of implicit memory as a form of long-term memory different from what then became known as explicit memory, the experimental strategy often adopted by researchers was to compare the effects of different experimental manipulations on both implicit and explicit memory measures in a single experiment or across a series of interconnected experiments. Patterns of parallel findings, where an independent variable affected the implicit and explicit memory measure in a similar way, and patterns of dissociations, where an independent variable affected the implicit and explicit memory measures differently, were used to develop theoretical models explaining the nature of long-term implicit and explicit memories.

                                  Perceptual Implicit Memory

                                  There are different forms of repetition priming—perceptual in which there is a perceptual/physical relationship between the initial and repeated presentation of a stimulus (e.g., the word BANANA is presented in uppercase for study and later again presented in uppercase at test—here there is a match in perceptual form of the stimulus; alternatively if the word BANANA is presented in uppercase for study and later presented in lowercase (banana) at test, there is a perceptual mismatch in form), and conceptual, in which there is a conceptual (i.e., meaning) relationship between the initial and repeated presentation of a stimulus. The papers in this section describe empirical studies of perceptual repetition priming and their comparison to explicit tests of memory. Jacoby and Dallas 1981, Tulving, et al. 1982, and Jacoby 1983 examined the effect of various experimental manipulations on implicit and explicit memory. Musen and Treisman 1990; Schacter, et al. 1990; and Schacter and Church 1992 extend the findings on implicit memory to auditory and nonverbal tasks. Marsolek, et al. 1992 provides an innovative neurobiological account of visual priming effects.

                                  • Jacoby, L. L. 1983. Perceptual enhancement: Persistent effects of an experience. Journal of Experimental Psychology: Learning, Memory, and Cognition 9.1: 21–38.

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                                    Jacoby suggests that cue type determines whether perceptual priming and recognition memory display similar or different patterns of behavior. Recognition memory, he argues, relies both on meaning and the perceptual characteristics of the test cue, while priming relies solely on the perceptual characteristics of the test cue. Jacoby supports his argument with evidence that both types of memory are similarly affected by manipulations of context, retention interval, and proactive interference.

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                                    • Jacoby, L. L., and M. Dallas. 1981. On the relationship between autobiographical memory and perceptual learning. Journal of Experimental Psychology: General 110.3: 306–340.

                                      DOI: 10.1037/0096-3445.110.3.306Save Citation »Export Citation »E-mail Citation »

                                      Provides an extensive account of the effects of various variables and manipulations on recognition memory (explicit memory) and perceptual identification (implicit memory). Observed significant effects of level of processing and task difficulty on recognition memory but no effect of the same manipulation on perceptual identification. These results were meaningful because of the extensive evidence that human memory does not operate as a unitary system.

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                                      • Marsolek, C. J., S. M. Kosslyn, and L. R. Squire. 1992. Form-specific visual priming in the right cerebral hemisphere. Journal of Experimental Psychology: Learning, Memory, and Cognition 18.3: 492–508.

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                                        Examines perceptual priming and explicit memory by rapidly presenting test stimuli to the left or right visual field. Greater perceptual priming was observed when stimuli were first presented to the left visual field (and hence, right hemisphere). A similar pattern was not observed for explicit memory. Argues that the brain possesses a specialized system for processing form-specific visual information located in the right hemisphere.

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                                        • Musen, G., and A. Treisman. 1990. Implicit and explicit memory for visual patterns. Journal of Experimental Psychology: Learning, Memory, and Cognition 16.1: 127–137.

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                                          Musen and Treisman examine implicit and explicit memory for novel visual pattern stimuli. This study was a welcome addition to the extant literature that had mainly utilized verbal stimuli. The article demonstrated statistical independence between the implicit and explicit memory tasks, as had been shown previously; however, the study extended to a new category of stimuli, namely nonverbal visual patterns. Musen and Treisman interpreted their findings within a multiple memory system account of long-term memory.

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                                          • Schacter, D. L., and B. A. Church. 1992. Auditory priming: Implicit and explicit memory for words and voices. Journal of Experimental Psychology: Learning, Memory, and Cognition 18.5: 915–930.

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                                            Similar to the previous work, Schacter and Church explored implicit and explicit memory for stimuli other than visually presented words, namely auditory stimuli in identification and completion tasks. Schacter and Church observed dissociations between implicit and explicit memory that paralleled those observed for visually presented verbal material. They argued that a pre-semantic perceptual representation system could underlie the auditory priming effects they observed.

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                                            • Schacter, D. L., L. A. Cooper, and S. M. Delaney. 1990. Implicit memory for unfamiliar objects depends on access to structural descriptions. Journal of Experimental Psychology: General 119.1: 5–24.

                                              DOI: 10.1037/0096-3445.119.1.5Save Citation »Export Citation »E-mail Citation »

                                              Examines implicit memory for novel three-dimensional objects. Used two types of three-dimensional objects: possible objects and impossible objects. Impossible objects contained a side or angle that would make its existence impossible in the real world. In addition to finding dissociations between implicit and explicit memory for these novel three-dimensional objects, the article noted that significant repetition priming was only observed for possible three-dimensional objects.

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                                              • Tulving, E., D. L. Schacter, and H. A. Stark. 1982. Priming effects in word-fragment completion are independent of recognition memory. Journal of Experimental Psychology: Learning, Memory, and Cognition 8.4: 336.

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                                                This article demonstrated independence between recognition memory (explicit memory) and word fragment completion (implicit memory). These results were among the first to demonstrate that the dissociation (different pattern of behavior) observed between two different tasks (an explicit and implicit task) in amnesic populations was also observable in non-amnesic adults.

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                                                Conceptual Implicit Memory

                                                The papers in this section describe empirical studies of conceptual repetition priming in which there is a conceptual or semantic relationship between the stimuli. Typical paradigms for conceptual repetition priming include category-exemplar generation as demonstrated in Graf, et al. 1985 and Hamann 1990, word association in Shimamura and Squire 1984, verb generation in Seger, et al. 1999, and semantic classification as in Vriezen, et al. 1995. Another paradigm involves the creation of new associations between unrelated word pairs (e.g., mother/calendar). This paradigm, as used in Schacter and Graf 1986, has properties common to both perceptual and conceptual priming and can thus be considered a hybrid. Blaxton 1989 used implicit and explicit perceptual and conceptual tasks to ask whether a processing account or a memory systems account better explain dissociations between implicit and explicit memory.

                                                • Blaxton, T. A. 1989. Investigating dissociations among memory measures: Support for a transfer-appropriate processing framework. Journal of Experimental Psychology: Learning, Memory, and Cognition 15.4: 657.

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                                                  Blaxton examined whether a memory systems account (episodic versus semantic/implicit versus explicit) or a processing account (data driven versus conceptually driven) better accounted for dissociations between implicit and explicit memory. Four tests were devised: (1) an explicit data-driven task, (2) an explicit conceptually driven task, (3) an implicit data-driven task, and (4) an implicit conceptually driven task. Blaxton concluded that the data aligned better with the processing view than with the systems view.

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                                                  • Graf, P., A. P. Shimamura, and L. R. Squire. 1985. Priming across modalities and priming across category levels: Extending the domain of preserved function in amnesia. Journal of Experimental Psychology: Learning, Memory, and Cognition 11.2: 386–396.

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                                                    Examines conceptual priming on a category-exemplar generation task in amnesic and control subjects. Participants were shown items belonging to several different categories for study and later received the category labels and were asked to generate the first exemplars that came to mind. Amnesic and control participants performed similarly on this task, demonstrating preserved conceptual priming in amnesia.

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                                                    • Hamann, S. B. 1990. Level-of-processing effects in conceptually driven implicit tasks. Journal of Experimental Psychology: Learning, Memory, and Cognition 16.6: 970.

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                                                      Hamann explored the role of elaborative processing on two conceptually driven implicit memory tasks: general knowledge retrieval and category exemplar generation. On both tasks, as with explicit memory measures such as recall and recognition, Hamann found that elaborative processing did increase the magnitude of priming, thus extending the domain of the effects of elaborative processing to two new and different conceptual repetition priming tasks.

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                                                      • Schacter, D. L., and P. Graf. 1986. Effects of elaborative processing on implicit and explicit memory for new associations. Journal of Experimental Psychology: Learning, Memory, and Cognition 12.3: 432.

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                                                        Schacter and Graf show in this article that elaborative processing (i.e., semantic encoding) is necessary for the formation of new associations and conceptual priming, demonstrating parallel effects to explicit memory, and different from findings on perceptual priming tasks. They further demonstrate, however, that the degree and type of elaboration while affecting explicitly memory, does not influence conceptual implicit memory for new associations.

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                                                        • Seger, C. A., L. A. Rabin, J. E. Desmond, and J. D. Gabrieli. 1999. Verb generation priming involves conceptual implicit memory. Brain and Cognition 41.2: 150–177.

                                                          DOI: 10.1006/brcg.1999.1116Save Citation »Export Citation »E-mail Citation »

                                                          Seger and colleagues used verb generation to examine conceptual priming. Participants were presented nouns (e.g., ladder) and were required to generate corresponding verbs (e.g., climb). A conceptual (meaning) relationship exists between the nouns and the verbs. Findings demonstrated that conceptual priming was due to retrieval of the verb and could be externally (reading the verb) or internally (generating the verb) driven, covert or overt, and occur across different languages.

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                                                          • Shimamura, A. P., and L. R. Squire. 1984. Paired-associate learning and priming effects in amnesia: A neuropsychological study. Journal of Experimental Psychology: General 113.4: 556–570.

                                                            DOI: 10.1037/0096-3445.113.4.556Save Citation »Export Citation »E-mail Citation »

                                                            Shimamura and Squire used an implicit word-association task to compare conceptual priming between amnesic and non-amnesic subjects. Subjects studied highly related word pairs (TABLE-CHAIR) and were later tested on a word-association task in which they responded with the first word that came to mind for each cue word (TABLE-?). Across two studies, comparable performance was found between amnesic and control subjects, indicating that conceptual priming is preserved in amnesia.

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                                                            • Vriezen, E. R., M. Moscovitch, and S. A. Bellos. 1995. Priming effects in semantic classification tasks. Journal of Experimental Psychology: Learning, Memory, and Cognition 21.4: 933–946.

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                                                              Examines conceptual priming on a semantic classification task. Subjects were shown words and pictures and made semantic decision (e.g., Is it man made? Is it larger than a breadbox?). Later the same words and pictures were shown again and subjects were required to make the same semantic classification or a different semantic classification. Demonstrates different aspects of conceptual priming on the semantic classification task.

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                                                              Developmental Dissociations between Implicit Memory and Explicit Memory

                                                              Explicit memory has been shown to gradually increase in capacity from childhood into early adulthood and then decrease from middle age into old age. Given the resistance of implicit memory to disruption in amnesia, researchers began to wonder whether implicit memory was also resistant to developmental changes over the lifespan. The following articles explore dissociations between implicit memory in children, younger, and older adults. Light and Singh 1987, Chiarello and Hoyer 1988, and Mitchell, et al. 1990 compare implicit to explicit memory in young and elderly adults. Parkin and Streete 1988, Naito 1990, and Perez, et al. 1998, on the other hand, consider the other end of the developmental spectrum by comparing implicit memory to explicit memory in young children and adults. Murphy, et al. 2003 offers an alternative account of developmental dissociations in implicit memory.

                                                              • Chiarello, C., and W. J. Hoyer. 1988. Adult age differences in implicit and explicit memory: Time course and encoding effects. Psychology and Aging 3.4: 358–366.

                                                                DOI: 10.1037/0882-7974.3.4.358Save Citation »Export Citation »E-mail Citation »

                                                                In contrast to the findings of Light and Singh 1987, Chiarello and Hoyer reported that young subjects outperformed elderly subjects on both an implicit word stem completion task and an explicit word stem cued recall task. These findings were used to argue that for at least some forms of implicit memory, the developmental trajectory is the same as for explicit memory.

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                                                                • Light, L. L., and A. Singh. 1987. Implicit and explicit memory in young and older adults. Journal of Experimental Psychology: Learning, Memory, and Cognition 13.4: 531–541.

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                                                                  Compares perceptual priming, as measured by word completion and perceptual identification tasks, to explicit memory—measured with free recall, cued recall, and recognition—in young and elderly adults. They observed a decrease across the lifespan in explicit memory but no similar decrease on the perceptual measures of implicit memory, thus demonstrating a developmental dissociation between implicit and explicit memory.

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                                                                  • Mitchell, D. B., A. S. Brown, and D. R. Murphy. 1990. Dissociations between procedural and episodic memory: Effects of time and aging. Psychology and Aging 5.2: 264–276.

                                                                    DOI: 10.1037/0882-7974.5.2.264Save Citation »Export Citation »E-mail Citation »

                                                                    Compares priming (picture naming) and recognition memory across multiple delays in young and elderly adults. Explicit memory was significantly poorer in elderly than in younger adults and also declined with increasing retention interval. In contrast, there were no age differences on the picture naming implicit memory task, and while priming did decline initially, it remained stable as a function of retention interval.

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                                                                    • Murphy, K., E. McKone, and J. Slee. 2003. Dissociations between implicit and explicit memory in children: The role of strategic processing and the knowledge base. Journal of Experimental Child Psychology 84.2: 124–165.

                                                                      DOI: 10.1016/S0022-0965(03)00002-XSave Citation »Export Citation »E-mail Citation »

                                                                      Offers an alternative explanation of developmental dissociations between implicit and explicit memory: developmental changes in implicit memory are correlated with gains in underlying knowledge; when there are developmental gains in underlying knowledge, then a concomitant developmental change in priming will be observed. When priming relies on developmentally stable knowledge, however, then implicit memory will be relatively stable developmentally.

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                                                                      • Naito, M. 1990. Repetition priming in children and adults: Age-related dissociation between implicit and explicit memory. Journal of Experimental Child Psychology 50.3: 462–484.

                                                                        DOI: 10.1016/0022-0965(90)90081-ISave Citation »Export Citation »E-mail Citation »

                                                                        Similar to the findings in Parkin and Streete 1988, a developmental dissociation is found to exist between an implicit verbal memory test (word-fragment completion) and explicit recall and recognition tasks. Specifically, Naito found that whereas priming was stable between seven-year-olds, twelve-year-olds, and adults, explicit recall performance improved with age.

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                                                                        • Parkin, A. J., and S. Streete. 1988. Implicit and explicit memory in young children and adults. British Journal of Psychology 79.3: 361–369.

                                                                          DOI: 10.1111/j.2044-8295.1988.tb02295.xSave Citation »Export Citation »E-mail Citation »

                                                                          Parkin and Streete examined priming (picture naming) and recognition memory in young children and in adults. No developmental differences were observed in priming. Alternatively, performance on picture recognition test improved with age. This developmental dissociation between implicit and explicit memory and between young children and adults complements the same dissociation observed between adults and the elderly.

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                                                                          • Perez, L. A., Z. F. Peynircioglu, and T. A. Blaxton. 1998. Developmental differences in implicit and explicit memory performance. Journal of Experimental Child Psychology 70.3: 167–185.

                                                                            DOI: 10.1006/jecp.1998.2449Save Citation »Export Citation »E-mail Citation »

                                                                            Examines perceptual and conceptual implicit and explicit memory between preschool, elementary-school, and college students. While performance on the conceptual explicit memory task improved with age, priming on the perceptual and conceptual implicit memory tasks and performance on the perceptual explicit memory did not differ developmentally. This study extends developmental dissociations between implicit and explicit memory to include conceptual priming.

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                                                                            Neurological Dissociations between Implicit Memory and Explicit Memory

                                                                            Research on implicit memory began when it was noticed that patients with amnesia who failed at recall and recognition showed normal or near-normal performance on what later came to be known as implicit memory and repetition priming. Since that time, much research has examined the memory abilities of patients with amnesia, as well as other neurological disorders such as Parkinson’s disease, Huntington’s disease, various forms of focal brain injury, and Alzheimer’s disease. The early studies by Warrington and Weiskrantz 1968 and Corkin 1968 reveal that amnesic patients, including the soon-to-be-famous “H.M.,” could demonstrate successful memory if tested under specific conditions—what later came to be known as implicit or non-declarative memory, despite impairments in other types of memory. Gabrieli, et al. 1990 tests patient H.M.’s memory more than two decades later, once again demonstrating impaired explicit and preserved implicit memory. Tulving, et al. 1991 demonstrates a similar pattern in another famous amnesic, patient “K.C.,” revealing preserved implicit memory over a year’s time. Graf, et al. 1984 examines dissociations between implicit and explicit memory in three types of amnesic patients, whereas Shimamura, et al. 1987 extends dissociations between implicit and explicit memory to different types of dementia. Schacter, et al. 1991 examines non-verbal implicit memory in patients with amnesia. Finally, Shimamura 1986 presents a review of research on implicit memory in amnesia.

                                                                            • Corkin, S. 1968. Acquisition of motor skill after bilateral medial temporal-lobe excision. Neuropsychologia 6.3: 255–265.

                                                                              DOI: 10.1016/0028-3932(68)90024-9Save Citation »Export Citation »E-mail Citation »

                                                                              In response to the groundbreaking finding that H.M. could learn to perform a mirror-drawing task despite severe amnesia, Corkin demonstrated that he also showed long-term improvement at performing additional motor skill tasks. These findings added important support to the proposal that memory may in fact involve dissociable systems that depend on different structures in the brain.

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                                                                              • Gabrieli, J. D., W. Milberg, M. M. Keane, and S. Corkin. 1990. Intact priming of patterns despite impaired memory. Neuropsychologia 28.5: 417–427.

                                                                                DOI: 10.1016/0028-3932(90)90069-ZSave Citation »Export Citation »E-mail Citation »

                                                                                Gabrieli and colleagues compared famed amnesic patient H.M.’s memory for novel, nonverbal dot patterns against that of normal control subjects. Despite impaired recognition (explicit memory) for the dot patterns, H.M. demonstrated intact priming equivalent to that exhibited by the control participants, supporting the dissociation between implicit and explicit memory in amnesia.

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                                                                                • Graf, P., L. R. Squire, and G. Mandler. 1984. The information that amnesic patients do not forget. Journal of Experimental Psychology: Learning, Memory, and Cognition 10.1: 164–178.

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                                                                                  Demonstrates that while implicit (word stem completion) memory is preserved in patients with amnesia, explicit (free recall, cued recall, and recognition) memory is significantly impaired, showing evidence for a dissociation between implicit and explicit memory as a function of brain injury.

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                                                                                  • Shimamura, A. P. 1986. Priming effects in amnesia: Evidence for a dissociable memory function. Quarterly Journal of Experimental Psychology Section A 38.4: 619–644.

                                                                                    DOI: 10.1080/14640748608401617Save Citation »Export Citation »E-mail Citation »

                                                                                    Provides a review of research on implicit memory and repetition priming in amnesia. The paper considers the history of implicit memory in amnesia starting with the work of Warrington and Weiskrantz, the boundary conditions for observing preserved priming in amnesia, the relationship between priming and explicit memory, and theoretical accounts for preserved priming in amnesia.

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                                                                                    • Schacter, D. L., L. A. Cooper, M. Tharan, and A. B. Rubens. 1991. Preserved priming of novel objects in patients with memory disorders. Journal of Cognitive Neuroscience 3.2: 117–130.

                                                                                      DOI: 10.1162/jocn.1991.3.2.117Save Citation »Export Citation »E-mail Citation »

                                                                                      Extends their findings for priming of novel three-dimensional objects to patients with amnesia. In this paper, they report that amnesic patients show robust priming for novel three-dimensional objects equivalent to levels shown by control participants in the absence of explicit recognition for those objects. These findings extend dissociations between explicit and implicit memory in amnesia to include novel three-dimensional nonverbal stimuli.

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                                                                                      • Shimamura, A. P., D. P. Salmon, L. R. Squire, and N. Butters. 1987. Memory dysfunction and word priming in dementia and amnesia. Behavioral Neuroscience 101.3: 347–351.

                                                                                        DOI: 10.1037/0735-7044.101.3.347Save Citation »Export Citation »E-mail Citation »

                                                                                        In this study, explicit and implicit memory performance was compared among patients with Alzheimer’s disease, Korsakoff syndrome, and Huntington’s disease. Verbal explicit memory performance in all three groups was impaired relative to a control group. Verbal implicit memory, however, was only impaired in patients with Alzheimer’s disease. Concludes that brain regions implicated in Alzheimer’s disease must extend beyond those structures necessary for explicit memory.

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                                                                                        • Tulving, E., C. A. Hayman, and C. A. Macdonald. 1991. Long-lasting perceptual priming and semantic learning in amnesia: A case experiment. Journal of Experimental Psychology: Learning, Memory, and Cognition 17.4: 595–617.

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                                                                                          Tulving and colleagues tested implicit memory in another famous amnesic, patient K.C. The authors taught K.C. three word sentences and then tested his memory for those sentences up to one year later. They found that despite a severe deficit in explicit memory, K.C. demonstrated preserved implicit memory one year later. This demonstration of preserved implicit memory after one year in a densely amnesic patient strengthens the argument that the brain injuries that impair explicit memories do not affect implicit memories.

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                                                                                          • Warrington, E. K., and L. Weiskrantz. 1968. New method of testing long-term retention with special reference to amnesic patients. Nature 217.5132: 972–974.

                                                                                            DOI: 10.1038/217972a0Save Citation »Export Citation »E-mail Citation »

                                                                                            Utilizes the implicit picture completion task to test implicit memory in amnesic patients and control participants. They observed that amnesic patients learned to identify pictures more accurately within a training session and across multiple days of training. This contrasted with poor recall and recognition. The observation that amnesic patients could retain some type of information alerted researchers to the possibility that different forms of long-term memory exist.

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                                                                                            Neuroimaging Dissociations between Implicit Memory and Explicit Memory

                                                                                            Historically, to study the relation between brain and behavior, researchers would recruit patients with specific forms of brain damage and then examine those aspects of behavior that were impaired as a result of the brain injury. Functional neuroimaging—Event-Related Potentials, Positron Emission Tomography (PET), and functional Magnetic Resonance Imaging (fMRI)—offers the opportunity to examine brain-behavior relations in healthy children and adults without brain injury. Modern PET and fMRI research into the neural correlates of memory and cognition began in earnest in the mid- to late-1980s, whereas research relying on event-related potentials began several decades earlier. Squire, et al. 1992 were the first to demonstrate that implicit memory might be associated with reduced rather than increased neural activity. This pattern was extended to conceptual priming by Wagner, et al. 2000b. Wagner, et al. 2000a and Schott, et al. 2005 examine the relation between implicit and explicit memory, and Henson, et al. 2000 examines the effect of stimulus familiarity on the neural response to implicit memory. Maccotta and Buckner 2004 demonstrates a direct link between the magnitude of behavioral priming and neural activity in priming related brain regions. Finally, Paller and Kutas 1992 and Rugg, et al. 1998 employ event-related potentials (ERPs) to examine the temporal and neural correlates of implicit and explicit memories.

                                                                                            • Henson, R., T. Shallice, and R. Dolan. 2000. Neuroimaging evidence for dissociable forms of repetition priming. Science 287.5456: 1269–1272.

                                                                                              DOI: 10.1126/science.287.5456.1269Save Citation »Export Citation »E-mail Citation »

                                                                                              Examines behavioral and neural priming for novel and familiar nonverbal stimuli (faces and symbols). When stimuli were familiar, they observed priming-related neural deactivations. However, for unfamiliar stimuli, increased activity was observed as a function of stimulus repetition: that is, for unfamiliar stimuli, repetition priming was associated with an increase rather than a decrease in neural activity.

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                                                                                              • Maccotta, L., and R. L. Buckner. 2004. Evidence for neural effects of repetition that directly correlate with behavioral priming. Journal of Cognitive Neuroscience 16.9: 1625–1632.

                                                                                                DOI: 10.1162/0898929042568451Save Citation »Export Citation »E-mail Citation »

                                                                                                Where previous brain imaging studies had associated a decrease in neural activity with repetition priming, Maccotta and Buckner’s study demonstrates a quantitative relationship between the two by observing that participants who showed greater reductions in neural activity also demonstrated the greatest magnitude of priming. That is, the magnitude of neural priming (neural deactivations) was directly correlated with the magnitude of the behavioral priming effect (decrease in reaction times to repeated stimuli).

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                                                                                                • Paller, K., and M. Kutas. 1992. Brain potentials during memory retrieval provide neurophysiological support for the distinction between conscious recollection and priming. Journal of Cognitive Neuroscience 4.4: 375–392.

                                                                                                  DOI: 10.1162/jocn.1992.4.4.375Save Citation »Export Citation »E-mail Citation »

                                                                                                  Paller and Kutas examined ERPs to implicit word identification and explicit recall and recognition. The study found a later brain potential associated with explicit memory but an earlier potential associated with implicit memory. These data suggest that implicit memory has a different and earlier time course than explicit memory.

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                                                                                                  • Rugg, M. D., R. E. Mark, P. Walla, A. M. Schloerscheidt, C. S. Birch, and K. Allan. 1998. Dissociation of the neural correlates of implicit and explicit memory. Nature 392.6676: 595–598.

                                                                                                    DOI: 10.1038/33396Save Citation »Export Citation »E-mail Citation »

                                                                                                    Compares ERPs for old words that were misclassified as new with new words. The direct comparison of these two types of stimuli revealed ERPs that were different from ERPs associated with old words that were correctly recognized as old, both in their temporal properties and in their spatial topography. This study suggests that the time course and functional neuroanatomy of implicit memories differs from explicit memories.

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                                                                                                    • Schott, B. H., R. N. Henson, A. Richardson-Klavehn, C. Becker, V. Thoma, H. -J. Heinze, and E. Düzel. 2005. Redefining implicit and explicit memory: The functional neuroanatomy of priming, remembering, and control of retrieval. Proceedings of the National Academy of Sciences of the United States of America 102.4: 1257–1262.

                                                                                                      DOI: 10.1073/pnas.0409070102Save Citation »Export Citation »E-mail Citation »

                                                                                                      Subjects were instructed to use provided retrieval cues to remember words from a prior study (explicit instructions), but if they could not, to then report the first word that came to mind (implicit instructions). Schott and colleagues separated neural activity for these two classes of stimuli. Consistent with prior research, implicit retrieval (priming) was associated with deactivations whereas explicit retrieval was associated with increased neural activity.

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                                                                                                      • Squire, L. R., J. G. Ojemann, F. M. Miezin, S. E. Petersen, T. O. Videen, and M. E. Raichle. 1992. Activation of the hippocampus in normal humans: A functional anatomical study of memory. Proceedings of the National Academy of Sciences of the United States of America 89.5: 1837–1841.

                                                                                                        DOI: 10.1073/pnas.89.5.1837Save Citation »Export Citation »E-mail Citation »

                                                                                                        In one of the earliest studies of the neural mechanisms of explicit and implicit memory, Squire and colleagues reported that explicit verbal cued recall depended on the right hippocampus. Verbal repetition priming, however, was associated with deactivation in posterior brain structures—primed stimuli required less neural activity than novel stimuli. This observation contributed to many theories of the neurophysiological mechanism underlying implicit memory and repetition priming.

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                                                                                                        • Wagner, A. D., W. Koutstaal, A. Maril, D. L. Schacter, and R. L. Buckner. 2000b. Task-specific repetition priming in left inferior prefrontal cortex. Cerebral Cortex 10.12: 1176–1184.

                                                                                                          DOI: 10.1093/cercor/10.12.1176Save Citation »Export Citation »E-mail Citation »

                                                                                                          Demonstrates that repetition priming, when involving semantic processing, resulted in deactivations in the left prefrontal cortex. This study, along with the study by Squire and colleagues and many similar studies, began to shape the understanding that repeated stimuli are processed more efficiently, both behaviorally (faster reaction times, more accurate responses, etc.) and physiologically (less neural activity). The site of neural priming (physiological deactivations) appears to be task-specific.

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                                                                                                          • Wagner, A. D., A. Maril, and D. L. Schacter. 2000a. Interactions between forms of memory: When priming hinders new episodic learning. Journal of Cognitive Neuroscience 12.Suppl 2: 52–60.

                                                                                                            DOI: 10.1162/089892900564064Save Citation »Export Citation »E-mail Citation »

                                                                                                            Examines whether the amount of behavioral priming exhibited by participants predicted their performance on an explicit memory test, both behaviorally and physiologically. Somewhat paradoxically, they noted that the greater the amount of behavioral and neural priming that a subject exhibited, the poorer their explicit memory tended to be. Wagner and colleagues suggested that more efficient neural processing could hinder explicit remembering.

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                                                                                                            Theoretical Accounts of Implicit and Explicit Memory

                                                                                                            As the wealth of evidence in support of dissociations (or parallel effects) between implicit and explicit memory accumulated, researchers turned to developing theories to account for the pattern of behavioral, developmental, and neurological dissociations between these two forms of memory. In general, two classes of theories were advanced: those that proposed that the dissociations between implicit and explicit memory reflected the operations of different memory systems, as discussed in Tulving and Schacter 1990 and Squire, et al. 1993, and those that proposed that the dissociations reflected the operations of different types of memory processes, as discussed in Jacoby 1991 and Roediger, et al. 1989. The most popular memory systems view proposed a distinction between a non-declarative or perceptual representation system(s) that supported implicit memory and a declarative system consisting of episodic and semantic memory that supported explicit memory. Most processing views made a distinction between an automatic or data-driven process that supported retrieval on implicit memory tasks and a conceptual-driven or controlled process that supported retrieval on explicit memory tasks. Gabrieli, et al. 1999 presents a third distinction to account for implicit/explicit memory dissociations, whereas Ratcliff and McKoon 1997 introduces a mathematical model to account for a range of findings on the implicit perceptual identification task.

                                                                                                            • Gabrieli, J. D., C. J. Vaidya, and M. Stone, et al. 1999. Convergent behavioral and neuropsychological evidence for a distinction between identification and production forms of repetition priming. Journal of Experimental Psychology: General 128.4: 479–498.

                                                                                                              DOI: 10.1037/0096-3445.128.4.479Save Citation »Export Citation »E-mail Citation »

                                                                                                              Neither the systems nor the processing views can account for a dissociation between two different perceptual tests of implicit memory, word-identification and word-stem completion, in patients with Alzheimer’s disease. Gabrieli and colleagues propose a novel distinction between implicit tasks that require identification and implicit tasks that require production of stimuli, arguing that this represents a third distinction in implicit memory, complementing the systems and processing views.

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                                                                                                              • Jacoby, L. L. 1991. A process dissociation framework: Separating automatic from intentional uses of memory. Journal of Memory and Language 30.5: 513–541.

                                                                                                                DOI: 10.1016/0749-596X(91)90025-FSave Citation »Export Citation »E-mail Citation »

                                                                                                                Jacoby introduces the process dissociation procedure (PDP) as an experimental approach to deriving estimates for the contributions of automatic and intentional processes to performance on implicit and explicit tests of memory. Although no implicit or explicit test of memory is process-pure, most implicit tests rely to a greater extent on automatic processes, whereas most explicit tests rely to a greater extent on intentional or controlled memory processes.

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                                                                                                                • Ratcliff, R., and G. McKoon. 1997. A counter model for implicit priming in perceptual word identification. Psychological Review 104.2: 319–343.

                                                                                                                  DOI: 10.1037/0033-295X.104.2.319Save Citation »Export Citation »E-mail Citation »

                                                                                                                  Presents the counter model to explain a range of priming phenomena on the implicit perceptual identification task. Rather than focusing on different processes or systems to account for the data, the counter model adapts a model of word identification, focusing on quantitatively describing the mechanisms that compute information, produce responses, and hold information in the perceptual identification task.

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                                                                                                                  • Roediger, H. L., III, M. S. Weldon, and B. H. Challis. 1989. Explaining dissociations between implicit and explicit measures of retention: A processing account. In Varieties of memory and consciousness: Essays in honour of Endel Tulving. Edited by H. L. Roediger III and F. I. M. Craik, 3–42. Hillsdale, NJ: Lawrence Erlbaum.

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                                                                                                                    Using an alternative processing account to Jacoby’s PDP model, this study adapted the dual processing transfer appropriate processing (TAP) model to account for the dissociations observed between implicit and explicit memory. The central tenet of TAP, like PDP, is that two processes, one data-driven and the other conceptually driven, underlie the differences observed on implicit and explicit memory tasks, with implicit memory tasks relying more on data-driven processes and explicit memory tasks relying more on conceptually driven processes.

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                                                                                                                    • Squire, L. R., B. Knowlton, and G. Musen. 1993. The structure and organization of memory. Annual Review of Psychology 44.1: 453–495.

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                                                                                                                      Proposes that explicit memory tasks depend on a declarative memory system whereas implicit memory tasks depend on a non-declarative memory system. Declarative memory is hippocampally based and has different characteristics than non-declarative memory that supports repetition priming. Preserved priming in amnesia is due to the sparing of the non-hippocampally based non-declarative memory system, whereas impaired explicit memory results from damage to the hippocampally based declarative memory system.

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                                                                                                                      • Tulving, E., and D. L. Schacter. 1990. Priming and human memory systems. Science 247.4940: 301–306.

                                                                                                                        DOI: 10.1126/science.2296719Save Citation »Export Citation »E-mail Citation »

                                                                                                                        Proposes that implicit memory is supported by modality-specific perceptual representation systems (PRS) whereas explicit memory is supported by episodic memory. PRS and episodic memory are hypothesized to have different operating characteristics, different underlying neural systems, and different developmental trajectories. Preserved priming in amnesia is explained by arguing that whereas the episodic memory system, mediated by the hippocampus in the medial temporal lobes, is damaged, the modality-specific cortically based PRS remain intact and support priming.

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                                                                                                                        Current Developments in Implicit Memory

                                                                                                                        Although the pace of research on implicit memory has slowed in the early 21st century, research does continue on understanding the properties of implicit memory and its relation to explicit memory. Now that the implicit memory phenomenon has been well established and its dissociations from explicit memory well characterized, the most recent research has focused more on expanding the theoretical understanding of implicit memory. Wang and Yonelinas 2012 examines whether the familiarity/recollection distinction could account for individual differences on conceptual priming and explicit memory, whereas Topolinski 2012 looks to see whether a sensorimotor hypothesis could underlie the benefits of prior exposure. Finally, Berry, et al. 2012 contrasts processing and systems theories in accounting for implicit and explicit memory data.

                                                                                                                        • Berry, C. J., D. R. Shanks, M. Speekenbrink, and R. N. Henson. 2012. Models of recognition, repetition priming, and fluency: Exploring a new framework. Psychological Review 119.1: 40–79.

                                                                                                                          DOI: 10.1037/a0025464Save Citation »Export Citation »E-mail Citation »

                                                                                                                          Uses a formal modeling procedure to examine four different mathematical models of performance on repetition priming, perceptual fluency, and recognition memory, both in healthy individuals and memory impaired patients. Using the continuous-identification-with-recognition paradigm, this study shows that the simplest of their four models—a single memory system strength-signal based on signal detection theory—can best account for the experimental data they reported.

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                                                                                                                          • Topolinski, S. 2012. The sensorimotor contributions to implicit memory, familiarity, and recollection. Journal of Experimental Psychology: General 141.2: 260–281.

                                                                                                                            DOI: 10.1037/a0025658Save Citation »Export Citation »E-mail Citation »

                                                                                                                            Topolinski examines whether stimulus-specific sensorimotor simulations—or mental reenactments of stimulus encoding—underlie priming and familiarity through processing fluency but not explicit recollection. Across seven studies, Topolinski demonstrated that domain-specific sensorimotor inhibition decreased priming (mere exposure effect, word fragment completion), familiarity (measured on the Remember/Know and ROC paradigms), and autonomic skin conductance responses but left recollection and free recall intact.

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                                                                                                                            • Wang, W. C., and A. P. Yonelinas. 2012. Familiarity is related to conceptual implicit memory: An examination of individual differences. Psychonomic Bulletin and Review 19.6: 1154–1164.

                                                                                                                              DOI: 10.3758/s13423-012-0298-7Save Citation »Export Citation »E-mail Citation »

                                                                                                                              Examines the relation between measures of recollection and familiarity as derived on a recognition test with conceptual priming (free-association test) and explicit associative cued recall performance. The findings showed that measures of familiarity correlated with amount of conceptual priming, whereas measures of recollection correlated with explicit cued recall performance. The findings argue against models that propose that implicit and explicit memory depend on different types of processes.

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