Implicit, explicit, and semantic memory functions in Alzheimer's disease and Huntington's disease.

A priming task involving a word-stem completion paradigm was administered to patients with Alzheimer's disease (AD), patients with Huntington's disease (HD), and normal control subjects. The task was done under conditions of both implicit and explicit recall. Explicit and implicit recall were positively correlated in all three groups. After controlling for explicit recall ability through ANCOVA, AD patients were found to be normally susceptible to the effects of priming on implicit recall. HD patients, however, exhibited significantly increased susceptibility to priming, suggesting that they may have carried out the implicit task in a manner different from that of normals and AD patients. In a second experiment, AD patients were found to supply words of significantly lower association strength than the other two groups in a "free association" task using words from a published list of word association norms. This apparent degradation of semantic memory was found to be strongly correlated with explicit recall performance, suggesting that explicit, implicit, and semantic memory functions decline in parallel in AD. Results are discussed with respect to the difficulties inherent in attempts to demonstrate selective impairments of conceptually distinct forms of memory.     

Implicit and explicit memory in Alzheimer's disease and Parkinson's disease

Abstract

Several tasks examined implicit and explicit memory in demographically matched samples of Alzheimer's (AD) and Parkinson's disease (PD) patients, and healthy elderly subjects. A fragmented pictures test, word stem-completion repetition priming, and a pursuit-rotor tracking task, followed by explicit memory tests, were given. AD patients were impaired on all explicit tests and on word stem-completion priming, but were intact on pursuit-rotor tracking and the skill learning (SL) component of the fragmented pictures test. PD patients were significantly better than AD patients on all explicit memory tests, but were selectively impaired on the SL component of the fragmented pictures test. Finally, a mirror-reading test was given to the PD patients and control subjects, with no significant differences found in performances between the two groups. Results are discussed in terms of hypothetical cognitive processes and brain circuits underlying different implicit and explicit memory domains.     

Differential patterns of memory loss among patients with Alzheimer's disease, Huntington's disease, and alcoholic Korsakoff's syndrome

Patients with Huntington's disease (HD), alcoholic Korsakoff's syndrome (KS), and Alzheimer's disease (AD) were compared with normal control subjects on a task designed to assess recognition memory for different classes of stimuli: spatial, verbal, color, pattern, and facial. In addition, recall of verbal stimuli was assessed at two delay intervals. On recognition testing, AD and KS patients were impaired on each of the five stimulus conditions. However, HD patients, though impaired on four of the recognition conditions, were unimpaired when verbal stimuli were used. On recall testing, the AD, HD, and KS groups were equally impaired at the shorter delay (15 s). However, at the longer delay (two minutes), the KS and HD patients, though still impaired relative to the normal control group, performed significantly better than the AD group.     

Severity of explicit memory impairment due to Alzheimer’s disease improves effectiveness of implicit learning

Background 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. Objectives 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. Patients and methods 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). Results Patients with moderate explicit memory impairment performed the implicit task significantly better than those with mild AD and controls. Conclusion Results of our study support the hypothesis of competition between the implicit and explicit memory systems in humans.     

Intact haptic priming in normal aging and Alzheimer's disease: evidence for dissociable memory systems

This study is the first to report complete priming in Alzheimer's disease (AD) patients and older control subjects for objects presented haptically. To investigate possible dissociations between implicit and explicit objects representations, young adults, Alzheimer's patients, and older controls performed a speeded object naming task followed by a recognition task. Similar haptic priming was exhibited by the three groups, although young adults responded faster than the two older groups. Furthermore, there was no difference in performance between the two healthy groups. On the other hand, younger and older healthy adults did not differ on explicit recognition while, as expected, AD patients were highly impaired. The double dissociation suggests that different memory systems mediate both types of memory tasks. The preservation of intact haptic priming in AD provides strong support to the idea that object implicit memory is mediated by a memory system that is different from the medial-temporal diencephalic system underlying explicit memory, which is impaired early in AD. Recent imaging and behavioral studies suggest that the implicit memory system may depend on extrastriate areas of the occipital cortex although somatosensory cortical mechanisms may also be involved.     

Preserved priming but insensitivity to perceptual fluency on recognition judgments in Alzheimer's disease

Background: Previous research on Alzheimer's disease (AD) has not yielded a consensus regarding the preservation of automatic memory processes, although there is a consensus that conscious recollection processes are impaired in AD. Methods: In the present study, we examined perceptual specificity effects (PSEs) in word recognition judgments (explicit memory task; Experiment 1) and word fragment completion (implicit memory task; Experiment 2) performed by individuals with mild AD and elderly adults without dementia (controls). Results: In recognition judgments, control subjects, but not individuals with AD, demonstrated PSEs (Experiment 1). In contrast, neither group showed PSEs on word fragment completion and their priming magnitudes were comparable (Experiment 2). Conclusions: The findings suggest that perceptually automatic processes in explicit memory judgments and implicit memory processes are different and that the former are specifically impaired in AD.     

Implicit memory in patients with schizophrenia and normal controls: Effects of task demands on susceptibility to priming

Abstract

Two experiments employing a stem-completion priming paradigm are reported. The first of these compared patients with schizophrenia (SC) to normal controls and demonstrated impaired implicit memory in the SC patients under task conditions identical to those used previously with other patient groups. The second experiment was designed to examine the effects of implicit task demands and stimulus selection upon susceptibility to priming, with a second group of SC patients and normal controls. Results indicated that the ability to carry out the task demands of the implicit condition (i.e., generate completions for word-stems) was inversely related to susceptibility to priming in both the SC patients and controls. In addition, the baseline probability of specific completions was found to be correlated with the ease of priming those completions, suggesting a possible mechanism for producing statistical dissociations between implicit and explicit retrieval conditions with this paradigm. These findings suggest that certain implicit tasks may be susceptible to nonmemory psychological influences that have not been adequately investigated to date; these may be responsible for “normalizing” the performance of amnesic patients as well as producing statistical dissociations from explicit memory tasks.     

Implicit and explicit memory for spatial information in Alzheimer's disease

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.     

Memory impairment differs in frontotemporal dementia and Alzheimer's disease

The aim of this study was to assess short-term and long-term explicit memory and implicit memory in frontotemporal dementia (FTD; frontal variant) and to compare FTD and Alzheimer's disease (AD) patients with similar severity of dementia. Fifteen FTD patients [mean age: 68 years; Mini-Mental State (MMS): 24], 30 probable AD patients (mean age: 72 years; MMS: 23) and 12 healthy subjects participated in the study. The three groups were comparable in terms of gender and educational level. Short-term memory was assessed with the digit span and Corsi block-tapping tests. Explicit verbal memory was assessed with the Grober and Buschke test, and implicit memory with a verbal priming task and a fragmented picture test. FTD patients demonstrated a genuine memory deficit with impaired digit span, encoding deficit and retrieval strategy difficulties, but preserved implicit verbal and visual priming. Memory patterns differed in AD and FTD: short-term memory and free recall were similarly decreased in FTD and AD but cues provided more benefit to FTD than to AD; encoding was more impaired and the forgetting rate was faster in AD than in FTD; priming was lower in AD than in FTD. AD patients with clinical and imaging frontal lobe dysfunction tended to have lower memory performance and to differ even more from FTD patients than AD patients without frontal lobe dysfunction.     

Intact memory for implicit contextual information in Korsakoff's amnesia

Implicit contextual learning is the ability to acquire contextual information from our surroundings without conscious awareness. Such contextual information facilitates the localization of objects in space. In a typical implicit contextual learning paradigm, subjects need to find a target among a number of distractors during visual search. Some of the configurations of stimuli are repeated during the experiment resulting in faster responses than for novel configurations, without subjects being aware of their repetition. Patients with Korsakoff's syndrome (KS) have been found to show devastating explicit spatial amnesia. Less is know about their implicit spatial memory abilities. The aim of the present research was to examine whether implicit contextual learning is intact in KS. Therefore, eighteen KS patients and twenty-two age-IQ- and education-matched controls performed the Implicit Contextual Learning task and a paradigm intended to assess explicit, spatial working memory, i.e. the Box task. Intact implicit contextual learning was observed in both the control group and the KS patients. In turn KS patients did have markedly lower explicit spatial working memory scores. The implicit learning effect was not related to the spatial working memory scores. Together these results clearly suggest that implicit and explicit spatial memory have a different neurocognitive basis.     

Assessment of nondeclarative learning in severe Alzheimer dementia: the Implicit Memory Test (IMT)

Although patients with Alzheimer dementia (AD) have impaired explicit memory, more automatic, implicit aspects of learning and memory may be relatively preserved. However, neuropsychological tests for the assessment of implicit memory are lacking. This study examines a newly developed test, the Implicit Memory Test, in 28 patients with severe AD (mini-mental state examination 5 to 12) and 22 cognitively unimpaired matched controls (mini-mental state examination 25 to 29). The Implicit Memory Test consists of visually presented word (stem-completion) and picture (fragmented picture identification) subtests, each comprising 3 learning trials and a delayed test. Explicit memory was also assessed, using the verbal paired-associate learning subtest from the Wechsler Memory Scale and the Visual Association Test. Patients with AD obtained a floor performance on both explicit memory tests, whereas a significant learning curve was found for both the stem-completion and the fragmented pictures subtests of the Implicit Memory Test. Delayed testing on the fragmented pictures subtest showed a preserved performance that may have been mediated by implicit learning. Delayed performance on the stem-completion subtest, however, showed clear memory decay that suggests contamination by explicit memory function, at least in the controls. These findings extend the earlier results on word-stem completion and fragmented picture identification in patients with mild-to-moderate AD and indicate that residual learning capacity can be assessed in severe AD.     

Word-list generation deficits in dementia

Abstract

To evaluate the specificity of the semantic fluency deficit in Alzheimer's disease (AD), we compared the performances of patients with AD, Huntington's disease (HD), vascular dementia (VD), and healthy control subjects on tasks of category (i.e., semantic) and first-letter (i.e., phonemic/lexical) word list generation. As compared to age-appropriate controls, all three patient groups demonstrated relatively more impaired semantic than phonemic fluency. Dementia severity did not affect this relationship. Thus, the greater vulnerability of semantically guided fluency is not specific to AD but occurs in other dementias as well. Deficits in both the organization of semantic memory and retrieval from long-term storage appear to contribute to the relatively poorer performance on semantic than phonemic fluency tasks observed in patients with AD, VD, and HD.     

Are Alzheimer's disease patients able to learn visual prototypes?

Recently, controversial results emerged regarding visual prototype learning in Alzheimer's disease (AD). The aim of this study was to elucidate this issue in a larger population of AD patients. The AD patients (N=72) and age-matched healthy control subjects (N=25) learned to recognize and to categorize visual dot patterns. In comparison with the control subjects, the AD patients as a group showed dysfunctions in the recognition task, whereas categorization was relatively spared in their case. Recognition was impaired in patients with mild AD (Mini-Mental score: 18-23) and moderate AD (Mini-Mental score<18), whereas categorization was impaired only in patients with moderate AD. These results suggest that while the medio-temporal/diencephalic explicit memory system is markedly affected even in early AD, the sensory neocortical areas mediating implicit category learning display a sufficient degree of functional capacity until later stages of the disease.     

“Below average” self-assessed school performance and Alzheimer's disease in the Aging,Demographics, and Memory Study

BackgroundA low level of formal education is becoming accepted as a risk factor for Alzheimer's disease (AD). Although increasing attention has been paid to differences in educational quality, no previous studies addressed participants' own characterizations of their overall performance in school. We examined whether self-assessed school performance is associated with AD beyond the effects of educational level alone.MethodsParticipants were drawn from the population-representative Aging, Demographics, and Memory Study (ADAMS, 2000-2002). The ADAMS participants were asked about their performance in school. Possible response options included “above average,” “average,” or “below average.” The ADAMS participants also underwent a full neuropsychological battery, and received a research diagnosis of possible or probable AD.ResultsThe 725 participants (mean age, 81.8 years; 59% female; 16% African-American) varied in self-assessed educational performance: 29% reported “above average,” 64% reported “average,” and 7% reported “below average” school performance. Participants with a lower self-assessed school performance had higher proportions of AD: 11% of participants with “above average” self-assessed performance had AD, as opposed to 12% of participants with “average” performance and 26% of participants with “below average” performance (P < 0.001). After controlling for subjects' years in school, a literacy test score (Wide-Range Achievement Test), age, sex, race/ethnicity, apolipoprotein E-?4 status, socioeconomic status, and self-reported comorbidities, respondents with “below average” self-assessed school performance were four times more likely to have AD compared with those of “average” performance (odds ratio, 4.0; 95% confidence interval, 1.2–14). “Above average” and “average” self-assessed school performance did not increase or decrease the odds of having AD (odds ratio, 0.9; 95% confidence interval, 0.5–1.7).ConclusionsWe suggest an association between “below average” self-assessed school performance and AD beyond the known association with formal education. Efforts to increase cognitive reserve through better school performance, in addition to increasing the number of years of formal education in early life, may be important in reducing vulnerability throughout the life course.     

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

Introduction: Previous studies have found that music paired with lyrics at encoding may improve the memory performance of patients with mild Alzheimer’s disease (AD). To further explore memory for different types of musical stimuli, the current study examined both implicit and explicit memory for music with and without lyrics compared to spoken lyrics.

Method: In this mixed design, patients with probable mild AD (n = 15) and healthy older adults (n = 13) listened to auditory clips (song, instrumental, or spoken lyrics varied across three sessions) and then had their memory tested. Implicit memory was measured by the mere exposure effect. Explicit recognition memory was measured using a confidence-judgment receiver operating characteristic (ROC) paradigm, which allowed examination of the separate contributions made by familiarity and recollection.

Results: A significant implicit memory mere exposure effect was found for both groups in the instrumental and song but not the spoken condition. Both groups had the best explicit memory performance in the spoken condition, followed by song, and then instrumental conditions. Healthy older adults demonstrated more recollection than patients with AD in the song and spoken conditions, but both groups performed similarly in the instrumental condition. Patients with AD demonstrated more familiarity in the instrumental and song conditions than in the spoken condition.

Conclusions: The results have implications for memory interventions for patients with mild AD. The implicit memory findings suggest that patients with AD may still show a preference for information familiar to them. The explicit memory results support prior findings that patients with AD rely heavily on familiarity, but also suggest that there may be limitations on the benefits that music can provide to recognition memory performance.     

Is verbal recognition memory really different in Huntington's and Alzheimer's disease.

Verbal recall and recognition were examined in Huntington's disease (HD) and Alzheimer's disease (AD) patients. Subgroups of HD and AD patients were matched for overall severity of dementia. Subjects were administered the Hopkins Verbal Learning Test, a list-learning task with three free-recall trials followed immediately by one yes/no recognition trial with semantically related and unrelated distractors. The matched AD and HD groups did not differ in the number of words recalled, although the HD patients showed slightly greater improvement over trials. Recognition performance was evaluated with measures of accuracy and response bias that are independent of each other. The matched groups did not differ in overall recognition accuracy, but the AD patients tended to have a more liberal ("yea-saying") response bias than did the HD patients. In addition, only the AD patients were differentially enticed to false-positive responding by semantically related distractors. The results suggest that the rule for making decisions when uncertain, rather than memory strength per se, distinguishes the recognition memory performance of AD and HD patients.     

Successive memory test performance and priming in Alzheimer's disease: evidence from the word-fragment completion task

This study assessed the performance of patients with Alzheimer's disease and healthy controls in a successive memory test paradigm. Subjects studied lists of words. Following study, tests of recognition (an explicit memory task) and primed word fragment completion (an implicit memory task) were administered. Since the same words were used in the two tasks, we were able to calculate the degree of dependence between recognition performance and primed word fragment completion. AD patients evidenced impaired recognition memory. In contrast, priming was intact. The pattern of correlation between the two tasks was similar in healthy controls and in AD. Independence between recognition and fragment completion was obtained when recognition preceded the fragment completion task, but not when fragment completion preceded recognition.     

Neuropsychological evidence for multiple implicit memory systems: a comparison of Alzheimer's, Huntington's, and Parkinson's disease patients

The performances of patients with dementia of the Alzheimer type (DAT), patients with Huntington's disease (HD), and demented and nondemented patients with Parkinson's disease (PD) were compared on 2 tests of implicit memory that do not require the conscious recollection of prior study episodes: (1) a pursuit-rotor motor learning task and (2) a lexical priming test. The HD patients were found to be impaired on the motor learning but not the lexical priming task, whereas the DAT patients evidenced the opposite relationship on these tasks. The demented, but not the nondemented, PD patients were found to be impaired on both tests of implicit memory. For both the HD and PD patients, deficits on the motor learning task correlated significantly with severity of dementia but not with level of primary motor dysfunction. The noted double dissociation between HD and DAT patients indicates that different forms of implicit memory, all of which are intact in amnesia, are dependent upon distinct neuroanatomic systems. Motor skill learning may be mediated by a corticostriatal system, whereas verbal priming may depend upon the integrity of the neocortical association areas involved in the storage of semantic knowledge. The results for the PD patients suggest that the demented PD patients have endured damage to the neurologic systems subserving both motor learning and lexical priming.     

Implicit and explicit memory in Alzheimer's disease and Parkinson's disease

Several tasks examined implicit and explicit memory in demographically matched samples of Alzheimer's (AD) and Parkinson's disease (PD) patients, and healthy elderly subjects. A fragmented pictures test, word stem-completion repetition priming, and a pursuit-rotor tracking task, followed by explicit memory tests, were given. AD patients were impaired on all explicit tests and on word stem-completion priming, but were intact on pursuit-rotor tracking and the skill learning (SL) component of the fragmented pictures test. PD patients were significantly better than AD patients on all explicit memory tests, but were selectively impaired on the SL component of the fragmented pictures test. Finally, a mirror-reading test was given to the PD patients and control subjects, with no significant differences found in performances between the two groups. Results are discussed in terms of hypothetical cognitive processes and brain circuits underlying different implicit and explicit memory domains.     

Normal mere exposure effect with impaired recognition in Alzheimer's disease

We investigated the mere exposure effect and the explicit memory in Alzheimer's disease (AD) patients and elderly control subjects, using unfamiliar faces. During the exposure phase, the subjects estimated the age of briefly flashed faces. The mere exposure effect was examined by presenting pairs of faces (old and new) and asking participants to select the face they liked. The participants were then presented with a forced-choice explicit recognition task. Controls subjects exhibited above-chance preference and recognition scores for old faces. The AD patients also showed the mere exposure effect but no explicit recognition. These results suggest that the processes involved in the mere exposure effect are preserved in AD patients despite their impaired explicit recognition. The results are discussed in terms of Seamon et al.'s (1995) proposal that processes involved in the mere exposure effect are equivalent to those subserving perceptual priming. These processes would depend on extrastriate areas which are relatively preserved in AD patients.