The contribution of implicit and explicit memory to the effects of errorless learning: a comparison between young and older adults.

There is evidence that the prevention of errors during learning might be helpful in improving an impaired memory performance, both in amnesia as well as in normal age-related memory decline. Although errorless learning is a promising technique for use in rehabilitation practice, the underlying mechanisms are unclear. That is, it has been suggested that the beneficial effects of errorless learning operate through implicit memory, whereas others implicate that it is explicit memory that is responsible for the enhanced memory performance after errorless learning. The current study examined the contribution of implicit and explicit memory function to the memory performance after errorless and errorful learning using the process-dissociation procedure. A group of young adults (N=40) was compared to a matched group of older individuals (N=40) on a spatial memory task (i.e., learning the locations of everyday objects in a room). The results clearly show age-related decline in explicit spatial memory, while implicit spatial memory was unaffected. Furthermore, the young group benefited from errorless learning compared to errorful learning, while the older group did not show a difference between the two learning conditions. Also, it was found that the effects of errorful learning were related to explicit memory function, and not implicit processing, corroborating and extending recent findings.     

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.     

The underlying mechanisms of semantic memory loss in Alzheimer's disease and semantic dementia

Patients with Alzheimer's disease (AD) and patients with semantic dementia (SD) both exhibit deficits on explicit tasks of semantic memory such as picture naming and category fluency. These deficits have been attributed to a degradation of the stored semantic network. An alternative explanation attributes the semantic deficit in AD to an impaired ability to consciously retrieve items from the semantic network. The present study used an implicit lexical-decision priming task to examine the integrity of the underlying semantic network in AD and SD patients matched for degree of impairment on explicit semantic memory tasks. The AD (n=11) and SD (n=11) patient groups were matched for age, education, level of dementia and impairment on four explicit semantic memory tasks. Healthy elderly participants (n=22) were matched for age and education. Semantic priming effects were evaluated for three types of semantic relationships (attributes, category coordinates, and category superordinates) and compared to lexical associative priming. Healthy controls showed significant priming across all conditions. In contrast, AD patients showed normal superordinate priming, and significant (although somewhat reduced) coordinate priming, but no attribute priming. SD patients showed no priming effect for any semantic relationship. All groups showed significant associative priming. The results indicate that SD patients do indeed have substantial degradation of semantic memory, while AD patients have a partially intact network, accounting for priming in superordinate and coordinate conditions. These findings suggest that AD patients' impairment on explicit semantic tasks is the product of deficient explicit retrieval in combination with a partially degraded semantic network.     

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.     

The mere exposure effect in patients with schizophrenia

The mere exposure effect refers to the development of an emotional preference for previously unfamiliar material because of frequent exposure to that material. This study compared schizophrenia subjects (n = 20) to normal controls (n = 21) to determine whether implicit memory, as demonstrated by the mere exposure effect, was intact. Patients with schizophrenia demonstrated a normal preference for both verbal and visual materials seen earlier relative to novel materials, despite impaired performance on a recognition task for explicit memory using similar materials. Previous studies of schizophrenia subjects have shown a dissociation between implicit and explicit memory on verbal tasks. We found a similar dissociation demonstrated by normal functioning on an implicit memory task and impaired functioning on an explicit memory task. Potential implications of these findings are discussed with regard to treatment and rehabilitation.     

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.     

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.     

Priming performance in Alzheimer's disease: the role of task sensitivity.

Studies examining implicit memory performance in Alzheimer's disease (AD) have yielded inconsistent findings, with these patients demonstrating impaired performance within some priming studies and intact performance within others. The present study examined the role of task sensitivity in detecting impaired priming in memory-impaired patients. Twelve healthy older adults and 12 AD patients were administered a picture fragment identification test. Task sensitivity was increased by employing stimulus cues expected to produce larger and more variable priming effects than obtained in previous studies. A simple comparison of priming scores revealed that the AD patients demonstrated significantly impaired priming relative to normal control participants. However, further analysis of priming in relation to certain stimulus characteristics revealed that AD patients often demonstrated impaired priming when overall priming effects were large but relatively intact priming when priming effects were small. These findings suggest that the prevention of ceiling effects in control participants may aid in the detection of impaired priming in patient populations.     

Neural correlates of episodic memory in behavioral variant frontotemporal dementia

Impaired episodic memory is currently an exclusion criterion for behavioral variant frontotemporal dementia (bv-FTD), although prior studies have shown that neuropsychological memory performance varies from very impaired to intact in such patients. Our study investigated i) whether this variability might be due to the admixture of true bv-FTD and phenocopy syndrome patients and ii) the neural correlates of episodic memory deficits in bvFTD. Groups of patients with true bvFTD (n = 14), phenocopy syndrome (n = 6), Alzheimer's disease (AD) (n = 14), and healthy controls (n = 15) underwent memory testing and had MRI scanning with ratings of regional brain atrophy. Phenocopy patients did not differ to controls on memory scores or atrophy ratings. By contrast, bvFTD and AD patients were impaired on both measures in comparison to controls and more importantly, bvFTD and AD did not differ on memory scores. Atrophy patterns differed, with AD showing typical medial temporal lobe atrophy, while bvFTD patients had predominantly prefrontal cortex atrophy. In bvFTD neuropsychological memory performance correlated with frontal atrophy ratings while in AD significant correlations were found between memory and both medial temporal lobe and frontal atrophy ratings. Taken together, out data shows that bvFTD patients can show a similar degree of episodic memory impairment on neuropsychological tests to AD patients, however, the neural correlates differ. The previously variable reported memory performance in bvFTD is likely due to the inclusion of phenocopy patients, who are mostly undistinguishable from controls. These findings have implications for the diagnosis of bvFTD.     

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.     

A dissociation between implicit and explicit verbal memory in left temporal lobe epilepsy

PURPOSE: Temporal lobe epilepsy patients are well known to present deficits on explicit verbal memory procedures (e.g., recall, recognition). The integrity of implicit memory procedures in these patients is not established. Previous studies in this area used implicit memory measures contaminated by the effects of explicit memory. METHODS: We examined the integrity of verbal implicit and explicit memory in left temporal lobe epilepsy (LTLE) patients and hypothesized that a clear dissociation in performance would be found with a relative preservation of implicit memory. TLE patients (n = 15) and age- and education-matched healthy normal patients (n = 15) were shown a 40-word study list, followed by a test phase requiring completion of word stems based on the study words or new/unseen words. Experimental conditions involved instructions to provide either the old (study) words or novel/nonlist words when completing the stem. Measures of automaticity and recollection provided uncontaminated indices of implicit and explicit memory, respectively. RESULTS: The data showed a significant difference (p < 0.001) between the patients (Recollection, 0.12; SD, 0.18) and controls (0.50, SD, 0.15) on the measure of explicit memory. In contrast, the patients (Automaticity, 0.51; SD, 0.11) and controls (0.45, SD, 0.18) performed similarly on the implicit memory measure, with patient scores clearly at normative levels based on other Process Dissociation Procedure data. CONCLUSIONS: The data demonstrate the integrity of implicit memory in LTLE patients. Finding a dissociation between the two forms of verbal memory in LTLE patients provides evidence that they rely on different neuroanatomic systems.     

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.     

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.     

INS Workshop program Antwerp,Belguium July 5, 1989

Abstract

While patients with Huntington's disease (HD) and Korsakoff's syndrome (KS) demonstrate “preserved” performance on implicit word-stem completion tasks, patients with Alzheimer's disease (AD) are typically found to be impaired. It has been hypothesized that AD patients do poorly as a result of degenerative changes in posterior cortoil association areas thought to mediate performance on this type of implicit memory task, and that the relative sparing of these areas in KS and HD results in their “preserved” performance. The present study was undertaken to examine the implicit memory performance of AD patients on this task after equating their explicit performance to that of normal controls by manipulating the number of encoding exposures. When this was accomplished, the implicit memory performance of AD patients was equivalent to that of controls. The results are discussed within the context of a critical review of the evidence for the existence of separable neural systems in mediating implicit and explicit memory.     

Perception, attention, and working memory are disproportionately impaired in dementia with Lewy bodies compared with Alzheimer's disease

OBJECTIVE: To test the hypotheses that visuoperceptual and attentional ability are disproportionately impaired in patients having dementia with Lewy Bodies (DLB) compared with Alzheimer's disease (AD). METHODS: A comprehensive battery of neuropsychological tasks designed to assess working, episodic, and semantic memory, and visuoperceptual and attentional functions was given to groups of patients with DLB (n=10) and AD (n=9), matched for age, education, and mini mental state examination (MMSE), and to normal controls (n=17). RESULTS: Both patient groups performed equally poorly on tests of episodic and semantic memory with the exception of immediate and delayed story recall, which was worse in the AD group. Digit span was by contrast spared in AD. The most striking differences were on tests of visuoperceptual/spatial ability and attention. Whereas patients with AD performed normally on several subtests of the visual object and space perception battery, the DLB group showed substantial impairments. In keeping with previous studies, the AD group showed deficits in selective attention and set shifting, but patients with DLB were more impaired on virtually every test of attention with deficits in sustained, selective, and divided attention. CONCLUSIONS: Patients with DLB have substantially greater impairment of attention, working memory, and visuoperceptual ability than patients with AD matched for overall dementia severity. Semantic memory seems to be equally affected in DLB and AD, unlike episodic memory, which is worse in AD. These findings may have relevance for our understanding of the genesis of visual hallucinations, and the differential diagnosis of AD and DLB.     

Allocentric memory impaired and egocentric memory intact as assessed by virtual reality in recent-onset schizophrenia

Present evidence suggests that schizophrenia is associated with explicit memory deficits, whereas implicit memory seems to be largely preserved. Virtual reality studies on declarative allocentric memory in schizophrenia are rare, and studies on implicit egocentric memory in schizophrenia are lacking. However, virtual realities have a major advantage for the assessment of spatial navigation and memory formation, as computer-simulated first-person environments can simulate navigation in a large-scale space. Twenty-five subjects with recent-onset schizophrenia were compared with 25 healthy matched control subjects on two virtual reality tasks affording the navigation and learning of a virtual park (allocentric memory) and a virtual maze (egocentric memory). Compared with control subjects, schizophrenia subjects were significantly impaired in learning the virtual park. However, schizophrenia subjects were as able as control subjects to learn the virtual maze. Stronger disorganized symptoms of schizophrenia subjects were significantly related to more errors on the virtual maze. It is concluded that egocentric spatial learning adds to the many other implicit cognitive skills being largely preserved in schizophrenia. Possibly, the more global neural network supporting egocentric spatial learning is less affected than the declarative hippocampal memory system in early stages of schizophrenia and may offer opportunities for compensation in the presence of focal deficits.     

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.     

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.     

Semantic activation and implicit memory in Alzheimer disease

Patients with Alzheimer disease (AD) invariably display pronounced deficits in verbal memory when retention is tested explicitly. The present study examined the possibility that tasks which require memory only implicitly would be performed normally. Moderately demented patients with probable AD were severely impaired in free recall of a word list. On a subsequent word association test, the AD patients were less likely than normals to give items from the recall list as their word associations. The results suggest that implicit verbal memory, as well as explicit memory, is impaired in AD. While the magnitude of the activation effect was significantly reduced in AD patients, it was uncorrelated with recall performance or a measure of global cognitive functioning. Memory activation may thus depend on neural circuitry outside the traditional (i.e. temporo-limbic) memory system.