Explicit and implicit memory for music in healthy older adults and patients with mild Alzheimer’s disease

To the Editor:-We read with interest the article by Ripart et al. 1 that provides clinical evidence to conclude that single-injection, high-volume medial canthus episcleral anesthesia is an efficient and safer alternative to peribulbar anesthesia. The salient features of their method are the injection of a relatively high volume (8 -10 ml) of anesthetic solution and the use of adjuncts, such as clonidine premed-

The present study investigated (a) whether the pattern of performance on implicit and explicit memory of patients with type 2 diabetes mellitus (DM2) is more similar to those of patients with Alzheimer's disease (AD) or to cognitively normal older adults and (b) whether glycosylated hemoglobin levels (a measure of glucose regulation) are related to performance on the two memory tasks, implicit word-stem completion and "old-new" recognition. The procedures of both memory tasks included encoding and memory test phases separated by a short delay. Three groups of participants (healthy older adults, DM2 patients and AD patients) completed medical and psychological assessments and performed both memory tasks on a computer. The results of the word-stem completion task showed similar implicit memory in the three groups. By contrast, explicit recognition of the three groups differed. Implicit memory was not affected by either normal or pathological aging, but explicit memory deteriorated in the two groups of patients, especially in AD patients, showing a severe impairment compared to the cognitively healthy older adults. Importantly, glycosylated hemoglobin levels were not related to performance on either implicit or explicit memory tasks. These findings revealed a clear dissociation between explicit and implicit memory tasks in normal and pathological aging. Neuropsychologists and clinicians working with TM2 patients should be aware that the decline of voluntary, long-term explicit memory could have a negative impact on their treatment management. By contrast, the intact implicit memory of the two clinical groups could be used in rehabilitation.

Attending to Remember and Remembering to Attend

Consistent evidence from human and experimental animals studies indicates that memory is organized into two relatively independent systems with different functions and brain mechanisms. The explicit memory system, dependent on the hippocampus and adjacent medial temporal lobe structures, refers to conscious knowledge acquisition and intentional recollection of previous experiences. The implicit memory system, dependent on the striatum, refers to learning of complex information without awareness or intention. The functioning of implicit memory can be observed in progressive, gradual improvement across many trials in performance on implicit learning tasks. The influence of explicit memory on implicit memory has not been precisely identified yet. According to data from some studies, explicit memory seems to exhibit no influence on implicit memory,whereas the other studies indicate that explicit memory may inhibit or facilitate implicit memory. The analysis of performance on implicit learning tasks in patients with different severity of explicit memory impairment due to Alzheimer's disease allows one to identify the potential influence of the explicit memory system on the implicit memory system. 51 patients with explicit memory impairment due to Alzheimer's disease (AD) and 36 healthy controls were tested. Explicit memory was examined by means of a battery of neuropsychological tests. Implicit habit learning was examined on probabilistic classification task (weather prediction task). Patients with moderate explicit memory impairment performed the implicit task significantly better than those with mild AD and controls. Results of our study support the hypothesis of competition between the implicit and explicit memory systems in humans.

The distinction between explicit and implicit memory is fundamental to current memory research. Explicit memory involves conscious remembering of prior episodes, often by means of intentional retrieval of those episodes, whereas implicit memory involves influences of prior episodes on current behaviour without intentional retrieval, and sometimes without conscious remembering of those prior episodes. Much evidence confirms that explicit memory and implicit memory have different neural bases at the retrieval stage, but what about the encoding stage? Little evidence is provided owing to methodological ambiguities in prior studies which often compared incidental tests with intentional tests. In fact, brain activity in one test can reflect not only implicit (memory) but also explicit memory. Addressing these ambiguities has awaited a theoretical approach that distinguishes implicit (memory) and explicit memory for specific episodes in one test. To explore this question, a forced-choice recognition was conducted to produce priming without awareness of memory retrieval. We suggest that recognition mechanisms allied with explicit memory are different from recognition mechanisms allied with implicit memory. An ERP experiment was conducted with a study-to-test paradigm, in which participants performed a color study task, followed by a forced-choice recognition. There are two stages during recognition. Two words (one old and one new) were presented in a forced-choice recognition, and subjects were asked to choose the old one. If subjects could not choose a studied word, they were encouraged to guess. After choosing, subjects would report whether the word was from the study stage or not. Neural activities during the study phase were recorded. The Dm for explicit memory was identified by contrasting ERPs to words for which the studied word was selected and endorsed it as an old word versus ERPs to words for which the studied word was unselected; The Dm for implicit memory was identified by contrasting ERPs to words for which the studied word was selected but failed to endorse it as an old word versus ERPs to words for which the studied word was unselected. The results showed that implicit and explicit memory share a 200~300ms frontal-central negative-going Dm effect, which maybe reflect attention at encoding, so that these words can be retrieved implicitly or explicitly. Implicit memory involved a temporal negative-going Dm effect from 200ms after stimulus onset, which maybe reflect encoding into the perceptual representation system. Explicit memory involved an earlier (400-600ms) right prefrontal, positive-going Dm effect, as well as a late (600-1200ms) parietal negative-going Dm effect. These effects maybe reflect elaborated processing and encoding into the episodic memory system. The results suggested that implicit and explicit memory are not completely independent of each other. The truth is that they have both independent and shared components at encoding.

Despite the fact that directed-forgetting effects have been attributed to either retrieval inhibition or selective encoding, there has been no compelling evidence to suggest that either mechanism regulates performance in both implicit and explicit memory. Therefore, in two experiments we sought (a) to determine whether directed forgetting influences tests of implicit (lexical decision) and explicit (recognition) memory and (b) to examine the relative contributions of the encoding and retrieval mechanisms thought to mediate directed forgetting by having participants perform an external interference task (i.e., sequential finger tapping) at either encoding or retrieval. In Experiment 1, directed-forgetting effects were demonstrated by better performance on remember-cued than on forget-cued words for both lexical decision and recognition. In Experiment 2, external interference disrupted directed forgetting in lexical decision when it occurred at retrieval and in recognition at encoding. These results demonstrate that although directed forgetting occurs on both implicit and explicit tests, it may be independently regulated by differential retrieval on the former and selective encoding on the latter. The discussion focuses on the differential excitation of remember- and forget-cued word representations, as inhibitory processing seemed not to account for directed-forgetting effects in either implicit or explicit memory.

According to the traditional memory-systems view, the hippocampus is critical during explicit (conscious) long-term memory, whereas other brain regions support implicit (nonconscious) memory. In the last two decades, some fMRI studies have reported hippocampal activity during implicit memory tasks. The aim of the present discussion paper was to identify whether any implicit memory fMRI studies have provided convincing evidence that the hippocampus is associated with nonconscious processes without being confounded by conscious processes. Experimental protocol and analysis parameters included the stimulus type(s), task(s), measures of subjective awareness, explicit memory accuracy, the relevant fMRI contrast(s) or analysis, and confound(s). A systematic review was conducted to identify implicit memory studies that reported fMRI activity in the hippocampus. After applying exclusion criteria, 13 articles remained for analysis. We found that there were no implicit memory fMRI studies where subjective awareness was absent, explicit memory performance was at chance, and there were no confounds that could have driven the observed hippocampal activity. The confounds included explicit memory (including false memory), imbalanced attentional states between conditions (yielding activation of the default-mode network), imbalanced stimuli between conditions, and differential novelty. As such, not a single fMRI study provided convincing evidence that implicit memory was associated with the hippocampus. Neuropsychological evidence was also considered, and implicit memory deficits were caused by factors known to disrupt brain regions beyond the hippocampus, such that the behavioral effects could not be attributed to this region. The present results indicate that implicit memory is not associated with the hippocampus.

Normal aging affects explicit memory while leaving implicit memory relatively spared. Normal aging also modifies how emotions are processed and experienced, with increasing evidence that older adults (OAs) focus more on positive information than younger adults (YAs). The aim of the present study was to investigate how age-related changes in emotion processing influence explicit and implicit memory. We used emotional melodies that differed in terms of valence (positive or negative) and arousal (high or low). Implicit memory was assessed with a preference task exploiting exposure effects, and explicit memory with a recognition task. Results indicated that effects of valence and arousal interacted to modulate both implicit and explicit memory in YAs. In OAs, recognition was poorer than in YAs; however, recognition of positive and high-arousal (happy) studied melodies was comparable. Insofar as socioemotional selectivity theory (SST) predicts a preservation of the recognition of positive information, our findings are not fully consistent with the extension of this theory to positive melodies since recognition of low-arousal (peaceful) studied melodies was poorer in OAs. In the preference task, YAs showed stronger exposure effects than OAs, suggesting an age-related decline of implicit memory. This impairment is smaller than the one observed for explicit memory (recognition), extending to the musical domain the dissociation between explicit memory decline and implicit memory relative preservation in aging. Finally, the disproportionate preference for positive material seen in OAs did not translate into stronger exposure effects for positive material suggesting no age-related emotional bias in implicit memory. (PsycINFO Database Record

Forgotten but not gone: FMRI evidence of implicit memory for negative stimuli 24 hours after the initial study episode

Three critical facts about explicit and implicit memory test performance were discovered in the early 1980s, and they provided the impetus for the current widespread research interest on this topic. First, there was the finding that patients with amnesia can show entirely normal performance on implicit memory tests, such as word stem completion, despite being severely impaired on traditional explicit memory tests, such as free and cued recall. Second, findings from healthy young adults showed that a variety of experimental manipulations have different effects on performance of explicit and implicit memory tests. Third, experiments on life - span development revealed that whereas explicit memory test performance increases in childhood and declines in late adulthood, implicit memory performance remains the same across this period.These basic facts about implicit and explicit memory led us (e.g., Graf, 1991; Graf & Mandler, 1984; Graf & Schacter, 1989) to propose a view of memory based on the notion of transfer appropriate processing (TAP) of Morris, Bransford, and Franks (1977). We make three specific assumptions to explain the disassociation between implicit and explicit memory. First, along with Mandler (1980), we distinguish between two memory organizing processes -- integration and elaboration. Integration is the process by which the various elements that make up an item are related to each other so as to form a single unit, and elaboration is the process by which such units are related to each other. Second, we assume that every kind of study task engages a combination of integrative and elaborative processing, but that some tasks focus primarily on integrative processing, and others focus more heavily on elaborative processing. Third, we also assume that every kind of test engages a combination of integrative and elaborative processing, but that implicit memory tests depend primarily on integrative processing, whereas explicit tests focus more heavily on elaborative processing. By these assumptions and the idea of TAP, it follows that implicit memory test performance is mediated by study/test overlaps in integrative processing, whereas performance of explicit tests is mediated by study/test overlaps in elaborative processing.A review of the literature shows that research has revealed a great deal about elaborative processing, and that we have much less insight intothe factors that control/guide integrative processing. One widespread assumption is that integrative processing is automatic, and is guided primarily by the data -- the sensory/perceptual properties of the stimuli that are presented for study and test. We have examined this assumption in several recent studies (e.g., Graf & Ryan, 1990; Ryan & Graf, 1992) and were surprised by the findings. They showed that just like elaborative processing, integrative processing is complex and dependent on many factors, including the strategies that are subject initiated and guided (thus, integrative processing not automatic) and the specific cues and requirements that define each study/test condition.We are also conducting investigations into what specific attributes of to - be - remembered targets and target contexts are effective as cues for implicit and explicit memory test performance. …

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.

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.

Transfer‐appropriate processing theory (Roediger, Weldon, & Challis, 1989) proposes that dissociations between performance on explicit and implicit memory tests arise because these tests often rely on different types of information processing (e.g., perceptual processing vs conceptual processing). This perspective predicts that implicit and explicit memory tasks that rely primarily on conceptual processing should show comparable results, not dissociations. Numerous studies have demonstrated such similarities. It is, however, possible that these results arise from explicit memory contamination of performance on implicit memory tasks. To address this issue, an experiment was conducted in which participants were administered the sedative midazolam prior to study. Midazolam is known to create a temporary, but dense, period of anterograde amnesia. The effects of blocking stimulus materials by semantic category at study and generation at study were investigated on category exemplar production and category‐cued recall. The results of this study demonstrated a dissociation of the effects of midazolam on category exemplar production and category‐cued recall. Specifically, midazolam reduced the effect of blocking stimulus materials in category‐cued recall, but not in category exemplar production. The differential effect of midazolam on explicit and implicit memory is at odds with transfer‐appropriate processing theory and suggests that theories of memory must distinguish the roles of different types of conceptual processing on implicit and explicit memory tests.

How our memory is affected as we age has been given considerable attention over recent decades as we strive to understand the cognitive processes involved. Memory types have been identified as either explicit (declarative - related to episodes or semantics) or implicit (non-declarative – related to procedures, habits, or earlier priming). Studies have identified likely age-related decline in explicit but not implicit memory though there are opposing results suggested from other studies. It is thought cognitive reserve capacities might explain any non-decline as aging individuals use alternative or additional pathways to ‘remember’. This theory might be supported indirectly if older members remember material accurately but take longer to supply answers. In our current study we re-examined whether age-related differences in accuracy and speed of access in memory are present in both implicit and explicit memory processes and we increased the number of experimental age groups (from 2 to 3) - most previous studies have compared just two groups (young, and old). With three groups (young, middle-old, and older aged groups) we can identify trends across the age range towards deterioration or preservation of memory. We examined sixty-six participants (49 females; 17 males) aged 18 to 86 years (M = 50.27, SD = 21.06) from South-Eastern Queensland and divided these into younger (18 to 46 years of age), middle old (50 to 64) and older aged (65+) cohorts. Participants were administered tasks assessing implicit and explicit memory using computer presentations. Consistent with most prior research, no age differences were identified on accuracy in the implicit memory tasks (verbal and non-verbal, including priming), suggesting that memory for implicit material remains preserved. However, on the explicit memory tasks, older adults performed less accurately than the younger adults, indicative of decline in explicit memory as we age. The finding of a decline in explicit memory but no significant decline in implicit memory confirms most earlier research and is consistent with a view of modular decline rather than overall decline in memory with increasing age. In addition, differences found in speed of response in otherwise accurate implicit memory with older respondents significantly slower, suggests possible support for the cognitive reserve hypothesis.

Primary objective: To examine implicit and explicit memory outcome in children who had sustained severe traumatic brain injury (TBI) through childhood. Opposite patterns of impairments were expected: (i) impaired implicit memory in children with early TBI (TBI-EC, < 6 years) and (ii) impaired explicit memory in children with late TBI (TBI-LC, ≥ 6 years).Research design: Children who had sustained severe TBI more then 1 year ago were assessed.Methods and procedure: Fourteen children who had sustained severe TBI (TBI-EC, n = 10 and TBI-LC, n = 4) between 8 months and 13 years 7 months of age and 13 non-injured control subjects (NC) participated. Implicit (repetition priming and skill learning) and explicit verbal memory were examined.Results: The TBI group performed worse on implicit (repetition priming) and explicit memory tasks compared to the NC group. Moreover, impairments were found in implicit and explicit memory in the TBI-EC, but not in the TBI-LC group.Conclusions: This study has shown, for the first time, that severe childhood TBI may compromise not only explicit, but also implicit memory. Nevertheless, instead of a selective implicit memory impairment, it was found that children who sustained injuries in early childhood present with impairments in both memory systems.