Implicit and explicit memory: a functional dissociation in persons with Down syndrome

Encoding and retrieval are two important phases of memory.Encoding produces memory engraved and retrieval reactivates previously encoded information.The approach to the relationship between encoding and retrieval is meaningful for the better understanding of the nature of memory.As for explicit memory,the relation of encoding and retrieval has been investigated by many researchers who have found out that the performance of a secondary task during encoding reduced the later memory performance,but the division of the attention in the same way during retrieval had virtually no effect on memory performance,which indicated an asymmetry between encoding and retrieval processes.With respect to implicit memory,an asymmetry was also found in one of our previous studies(Meng Guo,2006),but the asymmetry is somewhat different from that in explicit memory,that is,the performance of a concurrent task during encoding had no effect on later task performance,but interference during retrieval disrupted priming,and reduced task performance.In order to clarify the issues we conducted an ERP experiment.The experiment was conducted with 16 undergraduate students as subjects and Chinese characters as stimuli.The experiment adopted a study-to-test paradigm,in which participants performed a "shallow"(color) study task or a "deep"(pleasant) study task,followed by either a lexical decision(implicit) test(section 1) or a recognition(explicit) test(section 2).In interference task participants were asked to account the total number of "+" in a regulated orientation which appeared with a word,and was performed concurrently with either the encoding or the retrieval phase of the memory task for encoding interference condition or retrieval interference condition.The results showed that:(1) the effects of interference to encoding in implicit memory test were different from that to retrieval.Interference during encoding had virtually no effect on N400 or P600 old/new ERP components,but interference during retrieval significantly reduced N400 old/new and P600 old/new components,which abolished behavior priming.(2) The effects of interference to encoding on ERP components in explicit memory test were different from that to retrieval.Interference during encoding had significant influence on later P600 old/new effect,but interference during retrieval did not produce such effect.The results confirmed the finding of Meng Guo(2006),that is,interference during encoding had effect on explicit memory,but left implicit memory intact.Interference during retrieval affected implicit memory,but had little effect on explicit memory.So the relationship between encoding and retrieval was different between implicit and explicit memory,thus providing further evidence on the dissociation between implicit and explicit memory.

Background and objectives: Implicit (unconscious) and explicit (conscious) memory associations with drugs have been examined primarily using verbal cues. However, drug seeking, drug use behaviors, and relapse in chronic cocaine and other drug users are frequently triggered by viewing substance-related visual cues in the environment. We thus examined implicit and explicit memory for drug picture cues to understand the relative extent to which conscious and unconscious memory facilitation of visual drug cues occurs during cocaine dependence. Methods: Memory for drug-related and neutral picture cues was assessed in 14 inpatient cocaine-dependent polydrug users and a comparison group of 21 young adults with limited drug experience (n = 35). Participants completed picture cue exposure, free recall and recognition tasks to assess explicit memory, and a repetition priming task to assess implicit memory. Results: Drug cues, compared to neutral cues, were better explicitly recalled and implicitly primed, and especially so in the cocaine group. In contrast, neutral cues were better explicitly recognized, and especially in the control group. Conclusion: Certain forms of explicit and implicit memory for drug cues were enhanced in cocaine users compared to controls when memory was tested a short time following cue exposure. Enhanced unconscious memory processing of drug cues in chronic cocaine users may be a behavioral manifestation of heightened drug cue salience that supports drug seeking and taking. There may be value in expanding intervention techniques to utilize cocaine users’ implicit memory system.

Neural Correlates of Perceptual/Semantic Encoding and Implicit/Explicit Retrieval: An fMRI Study T. Park (tpark@chonnam.ac.kr) Department of Psychology, Chonnam National University Kwangju 500-757, Republic of Korea The important distinction between implicit and explicit memory tests is based on intentional effort and conscious recollection experience during retrieval process (Schacter, 1987), which have been suggested to be associated with increased activity in prefrontal and medial temporal regions, respectively (Schacter & Buckner, 1998). The present study investigated brain areas activated during tasks involving implicit and explicit memory retrieval, and examined neural correlates of conscious recollection and intentional effort during memory retrieval. Methods Whole-brain functional MRI was used to examine 8 subjects during retrieval in a block-designed fMRI experiment (Fig. 1). Two incidental study conditions were manipulated: Semantic and perceptual word encoding conditions. This manipulation of level of processing (LoP) was expected to yield two retrieval conditions that differed with regard to intentional retrieval effort and successful conscious recollection: Semantic encoding yielding low level of retrieval effort with high level of retrieval success and perceptual encoding yielding high level of effort with low level of retrieval success. After studying, word fragment completion (WFC) task was presented and then cued recall (CR) task was presented with word fragment cues. PER 30S SEM 30S 30S PER 30S 30S UNS PER SEM 30S SEM 30S 30S 30S UNS PER SEM 30S 30S 30S 30S 30S 30S 30S 30S 30S Fig. 1. A schematic illustration of study (above) and retrieval (below) paradigm. Critical study blocks (Semantic Encoding and Perceptual Encoding) were 30-s long separated by 30-s blocks of fixation (+). Critical retrieval blocks (Unstudied, Perceptually studied, Semantically studied) were also 30-s long separated by 30-s blocks of fixation in each of WFC and CR tasks. Results During explicit retrieval (CR) of the semantically encoded words, right inferior frontal regions (Brodman Areas 45, 47) were activated but right anterior frontal regions (BA 10) were deactivated. These results suggest different roles of different prefrontal regions during explicit (episodic) memory retrieval: BA 45/47 involved in conscious recollection and BA 10 in intentional effort (McIntosh et al., 1997). Also, parahippocampal gyrus was activated during explicit retrieval of the semantically encoded words, and this result supports the idea that medial temporal lobe is a neural correlate of conscious retrieval success. During implicit retrieval (WFC), occipital lobe (BA 17, 18 including fusiform gyrus) showed reduced activation when word fragments were primed, and this result supports the view that posterior areas are neural correlates of perceptual priming. Unexpectedly, right parahippocampal gyrus showed increased activation during implicit retrieval of the semantically encoded words. This result suggests that LoP effects which were often observed in studies of implicit memory retrieval could be the result of involuntary recollection (explicit contamination). Acknowledgements This research was supported as a Brain Neuroinformatics Research Program sponsored by Korean Ministry of Science and Technology. References McIntosh, A. R., Nyberg, L., Bookstein, F. L., & Tulving, E. (1997). Differential functional connectivity of prefrontal and medial temporal cortices during episodic memory retrieval. Human Brain Mapping, 5, 323-327. Schacter, D. L. (1987). Implicit memory: History and current status. Journal of Experimental Psychology: Learning, Memory, and Cognition, 13, 501-518. Schacter, D. L., & Buckner, R. L. (1998). On the relations among priming, conscious recollection, and intentional retrieval: Evidence from neuroimaging research. Neurobiology of Learning and Memory, 70, 284-303.

High doses of opioid associated with low doses of hypnotic is a popular anaesthetic technique since the use of remifentanil has become widespread. This type of anaesthesia could result in a higher incidence of implicit memory. Ten patients were anaesthetised with a target-controlled infusion of remifentanil (target concentration of 8 ng mL(-1)) combined with a target-controlled infusion of propofol with progressive stepwise increases until loss of consciousness was reached. A tape containing 20 words was then played to the patients. Bispectral index (BIS, Aspect Medical Systems, Newton, MA, USA) was continuously monitored during the whole study period. Implicit and explicit memories were tested between 2 and 4 h after recovery. Loss of consciousness was obtained with a mean calculated propofol plasma concentration of 1.3 +/- 0.4 microg mL(-1). At this low hypnotic concentration no implicit or explicit memory was found in the three postoperative memory tests. Median (range) BIS value during word presentation was 93 (80-98). In our group of young American Society of Anesthesiologists (ASA) I/II patients, no explicit or implicit memory was found when the calculated concentration of propofol combined with a high concentration of remifentanil was maintained at the level associated with loss of consciousness with high BIS values.

Perceptual identification task points to continuity between implicit memory and recall

The author introduces implicit (versus explicit) memory to examine advertising effects on brand choice. Whereas explicit memory is demonstrated by the conscious recollection of an event, implicit memory is inferred by an improvement in some task performance as the result of having experienced the event. This research shows that memory-based choice benefits from advertising that enhances conceptually driven implicit memory whereas stimulus-based choice benefits from advertising that enhances perceptually driven implicit memory. The findings provide evidence that the two types of implicit memory and explicit memory are distinct constructs of memory; the findings also suggest that implicit memory measures may be more useful indicators of advertising effectiveness than explicit memory measures.

Four models were compared on repeated explicit memory (fragment cued recall) or implicit memory (fragment completion) tasks (Hayman & Tulving, 1989a). In the experiments, when given explicit instructions to complete fragments with words from a just-studied list-the explicit condition-people showed a dependence relation between the first and the second fragment targeted at the same word. However, when subjects were just told to complete the (primed) fragments-the implicit condition-stochastic independence between the two fragments resulted. Three distributed models-CHARM, a competitive-learning model, and a back-propagation model produced dependence, as in the explicit memory test. In contrast, a separate-trace model, MINERVA, showed independence, as in the implicit task. It was concluded that explicit memory is based on a highly interactive network that glues or binds together the features within the items, as do the first three models. The binding accounts for the dependence relation. Implicit memory appears to be based, instead, on separate non interacting traces.

The aim of this study was to investigate the effect of emotional load of words (positive, negative and neutral) on explicit and implicit memory performances of participants with low and high alexithymia levels. The sample of the study consisted of 381 participants who were undergraduate students studying at Aydın Adnan Menderes University (M = 20.48, S = 1.73), with the age range between 18 and 26, and the second stage was consisted of 95 participants (M = 21,07, S = 2.21) with the age range between 18 and 26. Demographic Information Form and Toronto Alexithymia Scale were applied to the participants at the first stage. Participants who obtained one standard deviation below and above from Toronto Alexithymia Scale were invited to the second stage. Then the word completion task (implicit memory task) and free recall task (explicit task) were applied. The experimental design was 2 (Alexithymia Lev-el: low and high) x 3 (Emotional Use of Words: positive, negative, neutral) mixed ANOVA. The results showed significant difference between low and high alexithymia on explicit memory task. Participants with high levels of alexithymia remembered fewer words than those with low levels. There was no significant difference in neutral words on the main effect of emotional load according to alexithymia level. However, the group with high alexithymia remembered positive and negative words less than neutral words as compared to low alexithymia group. There was no significant difference in implicit memory performance according to interaction effect of alexithymia level and emotional load. Findings showed that high alexithymia level caused a deterioration on explicit memory but did not cause an impairment on implicit memory. The effect of alexithymia level on performance of remembering emotional words in explicit memory and implicit memory tasks was discussed in the light of emotional memory enhancement effect and related literature.

In 4 experiments, implicit and explicit memory for words and nonwords were compared. In Experiments 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 (Experiment 1) and following a study-test modality shift (Experiment 2). In Experiment 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 Experiment 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.

Differences of relevance in implicit and explicit memory tests: An ERP study

It is well documented that explicit (declarative, conscious) memory declines in normal aging. Studies have shown a progressive reduction in this form of memory with age, and healthy older adults (typically aged 65+ years) usually perform worse than younger adults (typically aged 18–30 years) on laboratory tests of explicit memory such as recall and recognition. In contrast, it is less clear whether implicit (procedural, unconscious) memory declines or remains stable in normal aging. Implicit memory is evident when previous experiences affect (e.g., facilitate) performance on tasks that do not require conscious recollection of those experiences. This can manifest in rehearsed motor skills, such as playing a musical instrument, but is typically indexed in the laboratory by the greater ease with which previously studied information is processed relative to non-studied information (e.g., repetition priming). While a vast amount of research has accumulated to suggest that implicit memory remains relatively stable over the adult lifespan, and is similar in samples of young and older adults, other studies have in contrast revealed that implicit memory is subject to age-related decline. Improving methods for determining whether implicit memory declines or remains stable with age is an important goal for future research, as the issue not only has significant implications for an aging society regarding interventions likely to ameliorate the effects of age-related explicit memory decline, but can also inform our theoretical understanding of human memory systems.

False memories represent distorted memories which are remembered incorrectly or didn’t happen at all in reality. In the present study, the period of time between encoding and testing has been manipulated, in order to explore the effects of delay on implicit and explicit false memories. Two groups of subjects were tested, using the DRM paradigm, for false implicit and explicit memories. One group had no delay period between encoding and test phases, the other group was tested at one week delay after the encoding phase. Results showed no significant differences between implicit and explicit false memories. The delay had significant effect only for true memories, which were significantly lower after one-week delay. Conclusions and future directions of research are discussed. Keywords: False memories, implicit false memories, explicit false memories, true memories, delay

There is abundant evidence that memory impairment in dementia in patients with Alzheimer’s disease (AD) is related to explicit, conscious forms of memory, whereas implicit, unconscious forms of memory function remain relatively intact or are less severely affected. Only a few studies have been performed on spatial memory function in AD, showing that AD patients’ explicit spatial memory is impaired, possibly related to hippocampal dysfunction. However, studies on implicit spatial memory in AD are lacking. The current study set out to investigate implicit and explicit spatial memory in AD patients (n = 18) using an ecologically valid computer task, in which participants had to remember the locations of various objects in common rooms. The contribution of implicit and explicit memory functions was estimated by means of the process dissociation procedure. The results show that explicit spatial memory is impaired in AD patients compared with a control group (n = 21). However, no group difference was found on implicit spatial function. This indicates that spared implicit memory in AD extends to the spatial domain, while the explicit spatial memory function deteriorates. Clinically, this finding might be relevant, in that an intact implicit memory function might be helpful in overcoming problems in explicit processing.

Explicit memory declines with age, but age effects on implicit memory are debated. This issue is important because if implicit memory is age invariant, it may support effective interventions in individuals experiencing memory decline. In this study, we overcame several methodological issues in past research to clarify age effects on implicit memory (priming) and their relationship to explicit memory (recognition, source memory). We (a) recruited a large life-span sample of participants (N = 1,072) during a residency at the Science Museum in London, (b) employed an implicit task that was unaffected by explicit contamination, and (c) systematically manipulated attention and depth of processing to assess their contribution to age effects. Participants witnessed a succession of overlapping colored objects, attending to one color stream and ignoring the other, and identified masked objects at test before judging whether they were previously attended, unattended, or new. Age significantly predicted decline in both explicit and implicit memory for attended items.

Category accessibility as implicit memory