A role for the hippocampal formation in implicit memory: a 3-D PET study

The apparent preservation of word priming effects in amnesia has been interpreted as supporting the view that implicit memory depends on brain systems that are independent of mesial temporal lobe structures which are in part responsible for explicit memory disorders. Nevertheless, a number of studies have demonstrated word priming deficits in amnesic patients relatively to normal subjects, suggesting that such structures may also be involved in implicit memory. To determine whether one such structure, the hippocampal formation, is a component of the brain system subserving word priming, a 3-D PET study was carried out in 13 normal individuals. Encoding was carried out using the brief multiple presentation technique, a procedure that allows one to effectively circumvent contamination of implicit memory tasks by explicit memory strategies. Results revealed that word priming was indeed associated with an activation of the right hippocampal formation. This finding of an hippocampal involvement in word priming calls into question the notion of absolute dissociability between the brain systems underlying performance on various explicit and implicit memory tasks.     

Preserved priming across study-test picture transformations in patients with Alzheimer's disease

Picture-naming priming was examined across different study-test transformations to explore the nature of memory representations of objects supporting implicit memory processes in patients with Alzheimer's disease (AD). Although severely impaired in explicit memory for pictures and words, AD patients demonstrated normal priming across perceptual transformations in picture orientation (Experiment 1) and picture size (Experiment 2) and across symbolic transformations from words to pictures (Experiment 3). In addition, the priming across alterations in picture size was invariant. This demonstrates that AD patients have preserved implicit memory for high-level, abstract representations of objects.     

A test of the frontal lobe functioning hypothesis of age deficits in production priming.

Older adults have been hypothesized to show reduced priming relative to younger adults on implicit memory tests that require production of a response because these tasks place high demands on attentional processes associated with frontal lobe function, which are often reduced with age (see D. A. Fleischman & J. D. E. Gabrieli, 1998). The current study directly tested this frontal lobe hypothesis of age effects in production priming. Younger adults and older adults who differed in their attentional abilities as measured by a battery of neuropsychological tests were given two production priming tasks, word stem completion and category production, followed by explicit free recall tests. Results showed that explicit memory performance was reduced by age and older adults' frontal functioning. Age and frontal functioning influenced category production priming but not word stem completion priming. Results failed to support the frontal account of age reductions in production priming. Instead, results implicate the influence of other processes often involved in production priming tasks, such as explicit memory strategies and response competition, as critical for understanding age effects in implicit memory performance. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sublexical components in implicit memory for novel words.

Five experiments with 280 university students investigated the role of sublexical components in implicit memory for novel words. Priming on an implicit word judgment task occurred consistently for nonwords formed out of familiar linguistic components (morphemes and syllables) but minimally for nonwords formed out of unfamiliar pseudosyllabic components. This effect was dissociable from explicit memory and insensitive to changes in the surface features of the stimuli. Moreover, it depended on unitization of stimulus components as opposed to priming of individual components. Results are interpreted in terms of the activation and integration of prior linguistic knowledge and as evidence against the role of new (perceptual or episodic) representations in implicit memory for new information. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Stochastic independence between two implicit memory tasks.

Two experiments tested the statistical relation between performances on two implicit, priming tests of memory: word-fragment completion and perceptual identification. If performance on implicit priming tests is mediated by a single memory system, then stochastic dependence between them should be found. Contrary to this prediction, stochastic independence was obtained between performances on the two tests in all but one condition. For that condition (Experiment 1) the results suggest that subjects may have knowingly used information derived from the first test to complete the second. When perceived similarity was eliminated by altering contextual cues (Experiment 2), stochastic independence was found in this condition as well. The results are not consistent with current multiple memory system interpretations that assign all implicit, priming tests to one system and explicit tests to another. An alternative interpretation is that the degree of dependence between performances on memory tests is determined by the similarity of the component processes that the tests engage and of the information they use. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Does implicit memory extend to legal and illegal nonwords?

In 4 experiments, implicit and explicit memory for words and nonwords were compared. In Exps 1–2 memory for words and legal nonwords (e.g., kers) was assessed with an identification (implicit) and a recognition (explicit) memory task: Robust priming was obtained for both words and nonwords, and the priming effects dissociated from explicit memory following a levels-of-processing manipulation (Exp 1) and following a study-test modality shift (Exp 2). In Exp 3, priming for legal and illegal nonwords (e.g., xyks) was observed on an identification task, and the effects dissociated from explicit memory following a levels-of-processing manipulation. Finally, in Exp 4, significant inhibitory priming for legal nonwords was observed when a lexical-decision task was used. Results suggest that implicit memory can extend to legal and illegal nonwords. Implications for theories of implicit memory are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Midazolam amnesia and conceptual processing in implicit memory.

Prominent theories of implicit memory (D. Schacter, B. Church, & J. Treadwell, 1994) emphasize the dominant role of perceptual processing in mediating priming on perceptual implicit memory tests. Examinations of the effects of conceptual processing on perceptual implicit memory tests have produced ambiguous results. Although a number of investigations (e.g., J. Toth & R. Hunt, 1990) have demonstrated that variations in conceptual processing affect priming on perceptual implicit memory tests, these effects may arise because of the contaminating effects of explicit memory. The current experiment examined this controversy using midazolam, a benzodiazepine that produces a dense, albeit temporary, anterograde amnesia when injected prior to study. The experiment examined whether the effects of generation found on the implicit memory test of perceptual identification were affected by a midazolam injection prior to study. Results demonstrated that midazolam substantially diminished generation effects in free and cued recall, as well as overall performance on these tests, but had no detectable effect on the generation effect in perceptual identification. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of elaborative and perceptual integrative processes in perceptual-motor performance.

Three experiments address the dependence of both explicit and implicit memory performance on elaborative processes for a perceptual-motor task, pursuit rotor. Explicit memory performance was reflected by recognition of previously encountered pursuit rotor stimuli. Implicit memory performance (priming) was identified in Exp 1 as an advantage in pursuit rotor performance for old stimuli that Ss failed to explicitly recognize. In Exps 2 and 3, the types of strategies that Ss engaged in during training and test phases were manipulated. Results indicated that explicit memory performance depended on elaborative processes that emphasized which specific stimuli were encountered, whereas reliable implicit memory performance appeared only under a control no-instruction condition. Discussion focuses on attention to perceptual-integrative processes for priming. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Associative priming in word fragment completion: a dissociation between explicit and implicit retrieval processes

Three experiments investigated associative priming in word fragment completion. In associative priming, the study word that acts as a prime is semantically related in some way to the response word that the subject must produce or respond to at test. For example, a prime might be semantically related to the solution to its paired word fragment (e.g. study "VANILLA", solve fragment "-H-C--A-E" at test, solution is "CHOCOLATE"). Associative priming therefore differs from both repetition and conceptual priming, in which the studied primes are themselves the words that must be produced or responded to at test. In Experiment 1, associative primes were found to influence word fragment completion performance on an explicit test, but not on an implicit test. Experiment 2 demonstrated that the effects of associative primes on explicitly instructed fragment completion cannot be attributed to the specific information about cue-prime relationships that is included in the explicit instructions. Experiment 3 demonstrated that a manipulation of modality, a variable known to disrupt implicit retrieval processes, disrupts repetition priming on an explicit test, but not associative priming. The results of these three experiments suggest that whereas repetition primes are retrieved from memory by both explicit and implicit retrieval processes, associative primes are retrieved by only explicit processes. These data suggest that implicit retrieval processes are cue-dependent processes which automatically retrieve memory information that provides a good match to retrieval cues. Explicit retrieval processes are cue-independent, functioning as an intentional retrieval set to access particular categories or types of memory information.     

Implicit memory: Retention without remembering.

Explicit measures of human memory, such as recall or recognition, reflect conscious recollection of the past. Implicit tests of retention measure transfer (or priming) from past experience on tasks that do not require conscious recollection of recent experiences for their performance. The article reviews research on the relation between explicit and implicit memory. The evidence points to substantial differences between standard explicit and implicit tests, because many variables create dissociations between these tests. For example, although pictures are remembered better than words on explicit tests, words produce more priming than do pictures on several implicit tests. These dissociations may implicate different memory systems that subserve distinct memorial functions, but the present argument is that many dissociations can be understood by appealing to general principles that apply to both explicit and implicit tests. Phenomena studied under the rubric of implicit memory may have important implications in many other fields, including social cognition, problem solving, and cognitive development. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Explicit memory and repetition priming in dementia: evidence for a common basic mechanism underlying conscious and unconscious retrieval deficits

The present study was designed to assess performance of Alzheimer's (AD) and Multi-infarct (MID) demented patients on the explicit and implicit versions of two memory tasks, namely Word-Stem Completion and Word-Pair Learning. Consistently with previous studies, the AD patients were deficient on the explicit and implicit versions of both tasks. In MID patients, a dissociation emerged between normal implicit and deficient explicit Word-Stem Completion. Two multiple regression analyses were performed to evaluate patients' ability on measures of lexical-semantic competence, explicit memory, and global intellective efficiency in predicting level of repetition priming. The results demonstrate a close association between explicit and implicit memory performance in AD patients but no relation between repetition priming level and measures of lexical-semantic competence or general intelligence. Overall, the results of the present study do not support previous conceptualizations suggesting that a breakdown in the structure of semantic memory is at the root of deficient priming in demented patients. Alternative interpretations of the deficient repetition priming effect in dementia, based on a common mechanism underlying conscious and unconscious retrieval deficits, are discussed.     

Age differences in implicit memory: Fragmented object identification and category exemplar generation.

In a cross-sectional study of 164 participants aged 21 to 91, the authors examined age differences on two implicit tests, fragmented object identification (FOI) and category exemplar generation (CEG), and on tests of explicit memory, attention, and verbal fluency. FOI results revealed impaired perceptual skill learning in those over 60 and a decrease in perceptual priming across young, middle-aged, and older groups. CEG priming was impaired in those over 80. Regression analysis revealed explicit contamination of priming on both the FOI and CEG tests. Across the three implicit measures, age accounted for 4 to 13% of the variance when explicit memory was controlled. Semantic fluency predicted CEG priming, suggesting possible frontal lobe involvement on the test. Altogether, results indicate that age has a small but reliable influence on implicit memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Implicit and explicit memory retrieval within and across the disconnected cerebral hemispheres.

The authors report that the divided right and left cerebral hemispheres are both adept at priming, a type of implicit memory, and that subcortical routes can mediate implicit memory retrieval across the hemispheres. Two individuals with complete section of all forebrain commissures, L.B. and N.G., were given tests of recall and recognition (explicit memory) and stem-completion priming (implicit memory). The right and left hemispheres of N.G. and the left hemisphere of L.B. exhibited significant intrahemispheric priming. Interhemispheric priming was similar in extent to intrahemispheric priming. Under several intra- and interhemispheric conditions, significant implicit retrieval occurred in the face of abnormal explicit retrieval. Because all forebrain commissures had been severed in L.B. and N.G., interhemispheric priming was necessarily supported by subcortical structures. The observed dissociation between explicit and implicit memory performance indicates that the routes of interhemispheric transfer vary with type of memory retrieval. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conceptual priming in perceptual identification for patients with Alzheimer's disease and a patient with right occipital lobectomy.

Two experiments examined explicit recognition memory and perceptual and conceptual contributions to implicit perceptual-identification repetition priming for patients with Alzheimer's disease (AD) and Patient M.S. with right-occipital lobectomy. Participants read words (perceptual encoding) and generated words (conceptual encoding) from a definition and letter cue (e.g., "a vehicle for moving the injured—a"). AD patients demonstrated impaired explicit and intact implicit memory for both perceptually and conceptually encoded words. M.S. demonstrated the opposite pattern: intact explicit and impaired implicit memory in both encoding conditions. The double dissociation between AD and M.S. on implicit and explicit memory tasks is discussed in terms of a putative visual memory mechanism in the right-occipital cortex that interacts with lexical mechanisms to yield perceptual-identification priming after perceptual and conceptual encoding. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Intact and impaired conceptual memory processes in amnesia.

To examine the status of conceptual memory processes in amnesia, a conceptual memory task with implicit or explicit task instructions was given to amnesic and control groups. After studying a list of category exemplars, participants saw category labels and were asked to generate as many exemplars as possible (an implicit memory task) or to generate exemplars that had been in the prior study list (an explicit memory task). After incidental deep or shallow encoding of exemplars, amnesic patients showed normal implicit memory performance (priming), a normal levels-of-processing effect on priming, and impaired explicit memory performance. After intentional encoding of exemplars, amnesic patients showed impaired implicit and explicit memory performance. Results suggest that although amnesic patients can show impairments on implicit and explicit conceptual memory tasks, their deficit does not generalize to all conceptual memory tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Repetition Priming and Recognition Memory in Younger and Older Persons: Temporal Stability and Performance.

The dissociation between intact priming and reduced explicit memory in aging has been called into question on the basis of limited information that suggests that priming measures are less reliable than explicit memory measures. No aging study to date, however, has offered a comprehensive examination of the reliability of these measures. Age-related performance and test-retest stability coefficients for multiple priming and explicit memory tasks were compared. Age effects were found on some tasks but not others, and stability was not related to task performance. Stability was similar for implicit versus explicit task instructions, younger versus older persons, and studied versus unstudied items. Results indicate that dissociations between priming and explicit memory performances cannot be accounted for by differential reliability of the measures. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Age Differences in Implicit Memory: Conceptual, Perceptual, or Methodological?

The authors examined age differences in conceptual and perceptual implicit memory via word-fragment completion, word-stem completion, category exemplar generation, picture-fragment identification, and picture naming. Young, middle-aged, and older participants (N=60) named pictures and words at study. Limited test exposure minimized explicit memory contamination, yielding no reliable age differences and equivalent cross-format effects. In contrast, explicit memory and neuropsychological measures produced significant age differences. In a follow-up experiment, 24 young adults were informed a priori about implicit testing. Their priming was equivalent to the main experiment, showing that test trial time restrictions limit explicit memory strategies. The authors concluded that most implicit memory processes remain stable across adulthood and suggest that explicit contamination be rigorously monitored in aging studies. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociative effect of massed repetition on implicit and explicit measures of memory.

Ss saw or heard words presented once, or repeated 4 or 16 times in massed fashion, and then received an implicit or explicit memory test. Massed repetition did not increase priming on word fragment completion beyond that obtained from a single presentation but did enhance performance on various explicit tests (free recall, recognition, question cued recall, and word fragment cued recall) and an implicit general knowledge test. Modality of presentation affected implicit and explicit word fragment cued tests but did not affect performance on any of the other tests. Levels of processing affected performance on implicit and explicit question cued tests. These results are consistent with a transfer appropriate processing account of dissociations among memory measures and imply that massed repetition promotes conceptual processing but does not entail a repetition of perceptual-based processes responsible for priming on word fragment completion. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory systems do not divide on consciousness: Reinterpreting memory in terms of activation and binding.

There is a popular hypothesis that performance on implicit and explicit memory tasks reflects 2 distinct memory systems. Explicit memory is said to store those experiences that can be consciously recollected, and implicit memory is said to store experiences and affect subsequent behavior but to be unavailable to conscious awareness. Although this division based on awareness is a useful taxonomy for memory tasks, the authors review the evidence that the unconscious character of implicit memory does not necessitate that it be treated as a separate system of human memory. They also argue that some implicit and explicit memory tasks share the same memory representations and that the important distinction is whether the task (implicit or explicit) requires the formation of a new association. The authors review and critique dissociations from the behavioral, amnesia, and neuroimaging literatures that have been advanced in support of separate explicit and implicit memory systems by highlighting contradictory evidence and by illustrating how the data can be accounted for using a simple computational memory model that assumes the same memory representation for those disparate tasks. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Transfer-appropriate processing for implicit and explicit memory.

To examine how feature-specific pattern-analyzing processes affect implicit and explicit memory test performance, words were displayed for study and testing in 2 visually distinct formats: upside down vs normal for Exp 1, upside down vs backward for Exp 2, and in Applesoft pudgy vs shadow typeface for Exp 3. Implicit and explicit memory performance were assessed with word identification and recognition tests, respectively. The results showed larger priming effects when the study and test formats were the same rather than different, but only in some experimental conditions. The discussion focuses on how skill and processing strategies contribute to format-specific effects on implicit and explicit memory test performance, and it outlines a theoretical account based on the idea of transfer-appropriate processing. (PsycINFO Database Record (c) 2011 APA, all rights reserved)