Impaired implicit memory for gist information in amnesia

The influence of recollection and familiarity in the formation and updating of associative representations

Prior representations affect future learning. Little is known, however, about the effects of recollective or familiarity-based representations on such learning. We investigate the ability to reuse or reassociate elements from recollection- and familiarity-based associations to form new associations. Past neuropsychological research suggests that hippocampal, and presumably recollective, representations are more flexible than extra-hippocampal, presumably familiarity-based, representations. We therefore hypothesize that the elements of recollective associations, as opposed to familiarity-based representations, may be more easily manipulated and decoupled from each other, and facilitate the formation of new associations. To investigate this hypothesis we used the AB/AC learning paradigm. Across two recall studies we observed an advantage in learning AC word pairs if AB word pairs were initially recollected. Furthermore, AB word pairs were more likely to intrude during a final AC test if those AB word pairs were initially familiarity-based. A third experiment using a recognition version of the AB/AC paradigm ruled out the possibility that our findings were due to memory strength. Our results support the idea that elements in recollective associative traces may be more discretely coded, leading to their flexible use, whereas elements in familiarity-based associative traces are less flexible.

Age-related increases in false recognition: the role of perceptual and conceptual similarity

Older adults (OAs) are more likely to falsely recognize novel events than young adults, and recent behavioral and neuroimaging evidence points to a reduced ability to distinguish overlapping information due to decline in hippocampal pattern separation. However, other data suggest a critical role for semantic similarity. Koutstaal et al. [(2003) false recognition of abstract vs. common objects in older and younger adults: testing the semantic categorization account, J. Exp. Psychol. Learn. 29, 499-510] reported that OAs were only vulnerable to false recognition of items with pre-existing semantic representations. We replicated Koutstaal et al.'s (2003) second experiment and examined the influence of independently rated perceptual and conceptual similarity between stimuli and lures. At study, young and OAs judged the pleasantness of pictures of abstract (unfamiliar) and concrete (familiar) items, followed by a surprise recognition test including studied items, similar lures, and novel unrelated items. Experiment 1 used dichotomous "old/new" responses at test, while in Experiment 2 participants were also asked to judge lures as "similar," to increase explicit demands on pattern separation. In both experiments, OAs showed a greater increase in false recognition for concrete than abstract items relative to the young, replicating Koutstaal et al.'s (2003) findings. However, unlike in the earlier study, there was also an age-related increase in false recognition of abstract lures when multiple similar images had been studied. In line with pattern separation accounts of false recognition, OAs were more likely to misclassify concrete lures with high and moderate, but not low degrees of rated similarity to studied items. Results are consistent with the view that OAs are particularly susceptible to semantic interference in recognition memory, and with the possibility that this reflects age-related decline in pattern separation.

False memories to emotional stimuli are not equally affected in right- and left-brain-damaged stroke patients

Previous research has attributed to the right hemisphere (RH) a key role in eliciting false memories to visual emotional stimuli. These results have been explained in terms of two right-hemisphere properties: (i) that emotional stimuli are preferentially processed in the RH and (ii) that visual stimuli are represented more coarsely in the RH. According to this account, false emotional memories are preferentially produced in the RH because emotional stimuli are both more strongly and more diffusely activated during encoding, leaving a memory trace that can be erroneously reactivated by similar but unstudied emotional items at test. If this right-hemisphere hypothesis is correct, then RH damage should result in a reduction in false memories to emotional stimuli relative to left-hemisphere lesions. To investigate this possibility, groups of right-brain-damaged (RBD, N=15), left-brain-damaged (LBD, N=15) and healthy (HC, N=30) participants took part in a recognition memory experiment with emotional (negative and positive) and non-emotional pictures. False memories were operationalized as incorrect responses to unstudied pictures that were similar to studied ones. Both RBD and LBD participants showed similar reductions in false memories for negative pictures relative to controls. For positive pictures, however, false memories were reduced only in RBD patients. The results provide only partial support for the right-hemisphere hypothesis and suggest that inter-hemispheric cooperation models may be necessary to fully account for false emotional memories.

Lexical Priming in Alzheimer's Disease and Aphasia

Lexical priming was examined in patients with Alzheimer's disease and in aphasic patients. Control participants were divided into young and elderly [cf. Arroyo-Anlló et al.: Eur J Cogn Psychol 2004;16:535-553]. For lexical priming, a word-stem completion task was used. Normal elderly participants had lexical priming scores that were significantly lower than those of young individuals. Analysis of covariance with age and educational level as covariates showed that the control participants, aphasic and Alzheimer patients did not differ significantly on the lexical priming task. Our results suggest that performance in the lexical priming task diminishes with physiological aging, but is not significantly affected by mild or moderate Alzheimer's disease or by fluent or non-fluent aphasia.

Context effects in auditory implicit memory

The context effect in implicit memory is the finding that presentation of words in meaningful context reduces or eliminates repetition priming compared to words presented in isolation. Virtually all of the research on the context effect has been conducted in the visual modality but preliminary results raise the question of whether context effects are less likely in auditory priming. Context effects in the auditory modality were systematically examined in five experiments using the auditory implicit tests of word-fragment and word-stem completion. The first three experiments revealed the classical context effect in auditory priming: Words heard in isolation produced substantial priming, whereas there was little priming for the words heard in meaningful passages. Experiments 4 and 5 revealed that a meaningful context is not required for the context effect to be obtained: Words presented in an unrelated audio stream produced less priming than words presented individually and no more priming than words presented in meaningful passages. Although context effects are often explained in terms of the transfer-appropriate processing (TAP) framework, the present results are better explained by Masson and MacLeod's (2000) reduced-individuation hypothesis.

An fMRI study of verbal episodic memory encoding in amnestic mild cognitive impairment

Amnestic mild cognitive impairment (aMCI) is a high-risk and often prodromal state for the development of Alzheimer's disease (AD) and is characterised by isolated episodic memory impairment. Functional neuroimaging studies in healthy subjects consistently report left prefrontal cortex (PFC) activation during verbal episodic memory encoding. The PFC activation at encoding is related to semantic processing which enhances memory. The purpose of this study was to ascertain whether impaired verbal episodic memory in aMCI is related to PFC dysfunction. Using functional magnetic resonance imaging (fMRI) we compared 10 aMCI patients with 10 elderly controls during verbal encoding. The encoding task was sensitive to the effects of semantic processing. Subsequent recognition was tested to measure encoding success. Behavioural results revealed impaired recognition and a lower false recognition rate for semantically related distracters (lures) in aMCI, which suggest impaired semantic processing at encoding. Both groups activated left hemispheric PFC, insula, premotor cortex and cerebellum, but group comparisons revealed decreased activation in left ventrolateral PFC in the aMCI group. The magnitude of activation in left ventrolateral PFC during encoding was positively correlated with recognition accuracy in the control group but not in the aMCI group. We propose that verbal episodic memory impairment in aMCI is related to PFC dysfunction which affects semantic processing at encoding.

Semantic and repetition priming effects for Deese/Roediger—McDermott (DRM) critical items and associates produced by DRM and unrelated study lists

Two lexical decision experiments investigated priming for a critical item (CI, sleep) and its related yoked associate (YA, blanket) when one had been studied in a related Deese/Roediger-McDermott (DRM) list (Experiments 1 & 2) or a list of totally unrelated words (Experiment 2) and the other had been nonstudied. Semantic priming from the related DRM list occurred for nonstudied CIs (but not YAs) regardless of whether the CI received within-test priming from its studied related YA during the lexical decision task, though the effect in the absence of within-test priming averaged across experiments was only significant by a one-tailed test. Also averaged across experiments, repetition priming occurred for both studied CIs and YAs when they had been studied in related DRM lists whether or not there was also within-test priming from a nonstudied related yoked pairmate, though individual effects within the two experiments were sometimes not significant. Repetition priming boosted semantic priming from related DRM lists less for CIs than for YAs, similar to the finding that memory discriminability is poorer for CIs than for YAs in episodic recognition. This smaller repetition priming boost for CIs than for YAs occurred to the same degree when the CIs or YAs were studied in an unrelated list. When nonstudied CIs and YAs were totally unrelated to all previously studied items and separated by 3-7 items in the lexical decision task, aYA produced a small 16 msec priming effect for its CI, averaged across both experiments. The implications of these results for the activation account of the DRM false-memory effect and for single-prime versus multiple-prime long-term semantic priming effects are discussed. The online addendum may be downloaded from www.psychonomic.org/archive.

The cognitive neuroscience of constructive memory: remembering the past and imagining the future

Episodic memory is widely conceived as a fundamentally constructive, rather than reproductive, process that is prone to various kinds of errors and illusions. With a view towards examining the functions served by a constructive episodic memory system, we consider recent neuropsychological and neuroimaging studies indicating that some types of memory distortions reflect the operation of adaptive processes. An important function of a constructive episodic memory is to allow individuals to simulate or imagine future episodes, happenings and scenarios. Since the future is not an exact repetition of the past, simulation of future episodes requires a system that can draw on the past in a manner that flexibly extracts and recombines elements of previous experiences. Consistent with this constructive episodic simulation hypothesis, we consider cognitive, neuropsychological and neuroimaging evidence showing that there is considerable overlap in the psychological and neural processes involved in remembering the past and imagining the future.