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

This study aimed at investigating implicit and explicit long-term memory functioning in subjects with Down syndrome (DS) compared to Mental-Age (MA) matched normal children. For this purpose, tests of verbal and visuo-perceptual explicit memory, verbal and visual repetition priming and procedural learning tasks were administered to 14 DS and 20 MA subjects. Our results document comparable implicit memory abilities in the two groups. In contrast, regarding explicit memory, normal children performed better than DS individuals. These results reveal a functional dissociation between implicit and explicit memory in subjects with DS. Theoretical and rehabilitative implications are discussed.     

Intact implicit learning in schizophrenia

OBJECTIVE: Schizophrenia impairs performance on explicit, but not implicit, memory tasks, indicating that conscious awareness at retrieval is a critical determinant of impaired memory. The authors investigated implicit learning, i.e., knowledge acquisition in the absence of conscious awareness, in patients with schizophrenia. METHOD: An artificial grammar learning task was used to assess implicit learning in 48 patients with schizophrenia and 24 healthy comparison subjects. The subjects were first presented with letter strings that were generated according to the rules of a finite-state grammar paradigm. They were then required to indicate whether new letter strings were "grammatical," depending on whether or not the strings corresponded to the rules. IQ, working memory, explicit memory, verbal fluency, and speed of processing were also assessed. RESULTS: Patients performed significantly worse than the comparison subjects on cognitive tasks that assessed episodic memory, verbal fluency, working memory, and speed of processing. In contrast, patients classified as being correct more grammatical than nongrammatical letter strings, and the magnitude of the difference was similar to that observed in healthy comparison subjects. CONCLUSIONS: Implicit learning, as assessed with an artificial grammar learning task, is intact in patients with schizophrenia. Conscious awareness might be a critical determinant of memory impairment both at encoding and at retrieval.     

Verbal memory processes in schizophrenia patients and biological relatives of schizophrenia patients: intact implicit memory, impaired explicit recollection

Verbal memory deficits are arguably the most common cognitive abnormalities in biological relatives of schizophrenia patients. Because verbal memory is a complex cognitive function, it is necessary to differentiate its intact and compromised aspects in order to reveal aberrant neural systems that reflect genetic risk in relatives of schizophrenia patients. Using an experimental verbal memory task, we examined encoding, free-recall, repetition priming, and recognition of verbal material in 22 schizophrenia patients, 22 first-degree biological relatives of schizophrenia patients, and 23 nonpsychiatric control participants. Schizophrenia patients exhibited intact repetition priming, but worse size judgment task performance (encoding), recall, and recognition than the control participants. Biological relatives of schizophrenia patients exhibited intact size judgment task performance, repetition priming, and recognition, but a free-recall deficit. Although size judgment task performance at encoding was associated with recall of verbal material in schizophrenia and control groups, in the relative group encoding performance was associated with the degree of repetition priming. Findings are consistent with impaired explicit recollection of verbal material, but intact implicit verbal memory in schizophrenia patients and biological relatives of schizophrenia patients.     

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.     

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.     

Neurocognitive function in clinically stable men with bipolar I disorder or schizophrenia and normal control subjects.

BACKGROUND: Patients with bipolar disorder and schizophrenia have been shown to have neurocognitive deficits when compared with control subjects. The degree and pattern of impairment between psychiatric groups have rarely been compared, especially when subjects are psychiatrically stable. METHODS: Using a standard neurocognitive battery, we compared euthymic outpatients with bipolar disorder (n = 40), stable patients with schizophrenia (n = 20), and subjects with no psychiatric disorder (n = 22). The neurocognitive domains assessed included executive functioning, verbal memory, visual memory, procedural learning, visuoconstructive ability, and language functions. Effect sizes were calculated for each cognitive domain across groups. RESULTS: Stable schizophrenic subjects demonstrated a generalized cognitive impairment across most domains compared with control subjects, with average effect sizes of .9. Euthymic bipolar subjects were significantly impaired compared with control subjects only in executive functioning (Wisconsin Card Sorting Task) and verbal memory (California Verbal Learning Test) domains (effect sizes in the .8-.9 range). Performance on the executive function measures was bimodal among bipolar subjects, suggesting two subgroups: one with relatively normal and one with impaired executive functioning. No significant differences between the bipolar patient group and control subjects were observed in visuoconstructive ability, procedural learning, or language function. CONCLUSIONS: Both euthymic bipolar subjects and relatively stable schizophrenic subjects differed from control subjects in neurocognitive function. Among schizophrenic subjects, a generalized cognitive impairment was observed, and the degree of impairment was greater in the schizophrenic compared with the bipolar subjects. Subjects with bipolar disorder were impaired in two specific domains (verbal memory and executive function). Furthermore, within the bipolar group there was a subset with relatively normal executive functioning and a subset with significant impairment. Possible reasons for the persistence of these neurocognitive deficits in some subjects with bipolar disorder during periods of euthymia are reviewed.     

Memory impairment and auditory evoked potential gating deficit in schizophrenia

Impaired sensory gating and memory function were reported in a study of 10 schizophrenic patients and 10 age- and sex-matched normal subjects. The P50 component of the auditory evoked potential was used as an index of gating. Explicit memory was tested with the Wechsler Memory Scale and implicit memory by artificial grammar learning. The schizophrenic patients showed deficits in both verbal paired associate and visual reproduction tasks. They demonstrated impaired implicit learning in color patterns but not letter strings. They also showed impaired P50 sensory gating. Three-dimensional brain mapping revealed a differential distribution of brain potentials in the processing of S1 and S2 at either P50 or N100 in both groups. However, the group difference was not statistically confirmed. In the controls, both implicit letter-string learning and explicit verbal paired associates were positively correlated with N100 gating, suggesting an association of the early attentive component with lexicons. In the schizophrenic patients, color-pattern implicit learning was positively correlated with P50 gating. The modality-specific impairment of implicit learning in schizophrenia may reflect a failure of adaptive filtering on the flooding input from color patterns.     

Does cognition predict community function only in schizophrenia?: a study of schizophrenia patients, bipolar affective disorder patients, and community control subjects

Cognitive deficits predict functioning in schizophrenia; however, little is known as to whether the association is present in other mental disorders. If specific cognitive deficits uniquely predict functional impairment in schizophrenia the association of select aspects of brain dysfunction with daily living would suggest an intervention target and perhaps a means by which to improve the functioning of schizophrenia patients. The relationship of cognition and functioning was investigated in schizophrenia (n=39), bipolar affective disorder (n=27), and nonpsychiatric control (n=38) participants to determine whether the associations varied across groups. We examined verbal memory, verbal learning, verbal fluency, vigilance, executive functioning, symptomatology, and generalized cognitive functioning for associations with social function. Correlational analyses revealed particular cognitive domains (e.g., verbal memory) to be associated with social functioning in schizophrenia, bipolar, and control subjects; however generalized cognitive function and symptomatology were also associated with social functioning in patients. Multiple regression analyses revealed that in schizophrenia poor verbal memory predicted worse social functioning even after the effects of generalized cognitive dysfunction were considered. Verbal memory indices failed to account for variance in social function in bipolar patients and control subjects after consideration of generalized cognitive function. Bipolar patients with worse planning and problem solving tended to have worse social functioning. Therefore, unlike schizophrenia patients who may fail to process verbally mediated material, bipolar patients' difficulty with logical approaches to problems in daily living may have the greatest impact on their community function.     

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.     

Schizophrenia patients demonstrate a dissociation on declarative and non-declarative memory tests

Declarative memory refers to the recall and recognition of factual information. In contrast, non-declarative memory entails a facilitation of memory based on prior exposure and is typically assessed with priming and perceptual-motor sequencing tasks. In this study, schizophrenia patients were compared to normal comparison subjects on two computerized memory tasks: the Word-stem Priming Test (n=30) and the Pattern Sequence Learning Test (n=20). Word-stem Priming includes recall, recognition (declarative) and priming (non-declarative) components of memory. The schizophrenia patients demonstrated an impaired performance on recall of words with relative improvement during the recognition portion of the test. Furthermore, they performed normally on the priming portion of the test. Thus, on tests of declarative memory, the patients had retrieval deficits with intact performance on the non-declarative memory component. The Pattern Sequence Learning Test utilizes a serial reaction time paradigm to assess non-declarative memory. The schizophrenia patients' serial reaction time was significantly slower than that of comparison subjects. However, the patients' rate of acquisition was not different from the normal comparison group. The data suggest that patients with schizophrenia process more slowly than normal, but have an intact non-declarative memory. The schizophrenia patients' dissociation on declarative vs. non-declarative memory tests is discussed in terms of possible underlying structural impairment.     

Intellectual functioning and memory deficits in schizophrenia

BACKGROUND: There is converging evidence about the existence of different subgroups of patients with schizophrenia in relation to intellectual ability (intelligence quotient [IQ]). Studying cognitive deficits in such patients in relation to IQ, and more specifically to memory, could help determine the patterns of preserved and impaired functioning in cognitive abilities in association with patterns of preserved and compromised intellect. This information could serve to delimit the possibilities of treatment and rehabilitation in those patients. METHODS: A total of 44 patients with schizophrenia completed a cognitive battery that included executive functioning, attention, speed of information processing, working memory, explicit memory, implicit memory, and everyday memory. Their IQ was also measured to identify 2 subgroups with an IQ of 85 as the cutoff point. Then, differences between the groups in the neurocognitive measures were studied. RESULTS: Performance in executive functioning, attention, working memory, and everyday memory, but not that in speed of information processing, explicit memory, and implicit memory, was associated with intellectual functioning. Patients performed at the same level in perceptual implicit memory but at a lower level in conceptual implicit memory as did healthy control subjects. DISCUSSION: Cognitive deficits in schizophrenia are associated with intellectual functioning. Implicit memory should not be considered as a unique entity. It is suggested that conceptual implicit memory deficit may be a core feature of schizophrenia.     

Verbal memory and verbal fluency in adolescents with schizophrenia spectrum disorders

Aim: Although impaired verbal memory and verbal fluency are frequently found in adults with schizophrenia, there has been a paucity of studies investigating adolescents with schizophrenia. Thus, the aim of the present study was to investigate the main subcomponents of verbal memory and verbal fluency in adolescents with schizophrenia spectrum disorders. Methods: Verbal learning and memory and verbal fluency was assessed in 21 adolescents with schizophrenia spectrum disorders (mean age, 15.4 years) compared with 28 healthy adolescents (mean age, 15.1 years). Results: The patient group performed significantly below healthy controls on measures of learning, delayed recall and on a frequency estimation task. No differences between the groups were found for measures of recognition, retention, implicit memory, or susceptibility to interference. Although they had impaired delayed recall the patients remembered most of what they actually learned. The patient group was impaired on phonological and semantic fluency, but there were no differences between the groups with respect to clustering or switching on the fluency tasks, when controlling for total output. There was no disproportionate impairment in semantic, as compared to phonological fluency, in the patient group. Conclusions: Adolescents with schizophrenia spectrum disorders exhibit impairments in verbal learning and verbal fluency, which might have an impact on the individual's everyday functioning.     

Selective speech perception alterations in schizophrenic patients reporting hallucinated "voices"

OBJECTIVE: The authors tested a model of hallucinated "voices" based on a neural network computer simulation of disordered speech perception. METHOD: Twenty-four patients with schizophrenia spectrum disorders who reported hallucinated voices were compared with 21 patients with schizophrenia spectrum disorders who did not report voices and 26 normal subjects. Narrative speech perception was assessed through use of a masked speech tracking task with three levels of superimposed phonetic noise. A sentence repetition task was used to assess grammar-dependent verbal working memory, and an auditory continuous performance task was used to assess nonlanguage attention. RESULTS: Masked speech tracking task and sentence repetition performance by hallucinating patients was impaired relative to both nonhallucinating patients and normal subjects. Although both hallucinating and nonhallucinating patients demonstrated auditory attention impairments when compared to normal subjects, the two patient groups did not differ with respect to these variables. CONCLUSIONS: Results support the hypothesis that hallucinated voices in schizophrenia arise from disrupted speech perception and verbal working memory systems rather than from nonlanguage cognitive or attentional deficits.     

Attentional and memory bias for emotional information in crime victims with acute posttraumatic stress disorder (PTSD)

A combined emotional Stroop, implicit memory (tachistoscopic identification) and explicit memory (free recall) task with three types of words (trauma-related, positive, and neutral) and two exposure conditions (subliminal and supraliminal) was administered to 39 crime victims with acute posttraumatic stress disorder (PTSD) and 39 age- and sex-matched controls. PTSD subjects showed supraliminal Stroop interference for trauma-related words and a similar effect on positive words. A specific explicit memory bias was found for trauma-related words among the PTSD subjects, but no preattentive bias on the subliminally presented words, nor any implicit memory bias. Findings suggest that acute PTSD subjects have an attentional and memory bias for threat-related material. Methodological limitations of the study are reviewed, and it is proposed that further studies are needed in order to elucidate whether acute PTSD Ss display a preattentive and implicit memory bias for trauma-related material.     

精神分裂症记忆与阳性、阴性症状的研究

目的探讨精神分裂症患者记忆特点及与阳性、阴性症状的关系。方法采用修正的加工分离记忆实验程序测试精神分裂症患者记忆变化情况,用PANSS评定精神分裂症患者阳性、阴性症状分。结果精神分裂症外显记忆与对照组比较明显受损(P0.05),其文字概念和图像概念实验类型驱动的内隐记忆成绩与对照组比较也受损(P0.05);阳性症状为主的患者外显记忆成绩均高于阴性症状为主的患者组(P0.05);阳性症状为主的患者内隐记忆成绩与以阴性症状为主的患者组之间的差异无统计学意义(P0.05);阳性症状与外显记忆无显著相关关系(P0.05),阴性症状与外显记忆呈显著负相关关系(P0.01);阳性症状、阴性症状与内隐记忆均无显著相关关系(P0.05)。结论精神分裂症外显记忆严重受损,而内隐记忆不同程度的受损;外显记忆与阳性症状无相关性,与阴性症状有显著相关;内隐记忆与阳性、阴性症状均无明显相关性。     

Neuropsychological functioning in early onset psychotic disorders

This paper examines whether neuropsychological profiles of youth with early onset psychotic disorders predicted diagnostic or clinical status. Youth with schizophrenia (n=27), bipolar disorder (n=22), and psychosis NOS (n=20) were included. Subjects received an extensive neuropsychological evaluation, including measures of general cognition, attention, memory, and executive functioning. Medication status was not controlled. No statistically significant neurocognitive differences across diagnostic groups were found. Compared to standardized norms, youth with schizophrenia demonstrated deficits in general cognition, verbal learning, recall, sustained effort, and social knowledge. Subjects with bipolar disorder and psychosis NOS exhibited deficits on measures of verbal learning, recall, and sustained effort similar to those of youth with schizophrenia. Neurocognitive deficits in memory and attention appeared to be common among youth with psychotic illnesses, regardless of diagnosis. Those with schizophrenia may have greater global cognitive deficits and problems with social knowledge. Across diagnoses, subjects demonstrated relative strengths on tests that provided them with immediate feedback, and performed most poorly on tests requiring delayed recall.     

Contrasts in memory functions between adolescents with schizophrenia or ADHD

Previous research on memory and schizophrenia has relied on a limited number of global memory measures instead of a comprehensive assessment of various memory components. In addition, little effort has been directed at examining memory functioning in patients with early-onset schizophrenia. Published research often lacks a relevant neuropsychiatric comparison group to control for attention difficulties. Patients with Attention Deficit Hyperactivity Disorder (ADHD) were included in the present study for this purpose. To our knowledge, a direct comparison of the two patient groups on memory functions has never been made. In the present study, both adolescents with schizophrenia and adolescents with ADHD were compared on a comprehensive memory test battery. Nineteen adolescents with schizophrenia were compared to 20 ADHD adolescents and 30 normally functioning adolescents on measures of working memory and long-term episodic memory, including tests of verbal and visual memory, free recall and recognition memory. The performance of the adolescents with schizophrenia was impaired as compared to the normal group on most of the memory measures. They performed significantly more poorly than the adolescents with ADHD on the visual memory tests. The ADHD group scored more impaired than the schizophrenia group on working memory tests with focus on distractibility. The findings suggest a general memory deficit among adolescents with schizophrenia related to both verbal and visual material. Impairment on the measures of visual memory is specific to schizophrenia and does not characterise the ADHD subjects.     

An analysis of Korsakoff patients recall following varying types of distractor activity.

Ten alcoholic Korsakoff patients and ten alcoholic control subjects were given several short-term memory, distractor type tasks in which consonant trigrams (verbal) and random shapes (nonverbal) were employed as the to-be-retained materials. The Korsakoff patients demonstrated a severe verbal memory deficit when the retention interval was filled with verbal distractor activity, but their retention of the same verbal material was intact when a nonverbal distractor task was used. In contrast, the Korsakoff patients' retention of nonverbal material was impaired regardless of the nature of the distractor activity. In fact, their retention of nonverbal material was impaired even when an unfilled retention interval was used.     

Fronto-hippocampal function during temporal context monitoring in schizophrenia.

BACKGROUND: Patients with schizophrenia have difficulty using contextual information to recall the source of information. Given the importance of the hippocampus and prefrontal cortex (PFC) in this type of memory, we hypothesized that this cognitive deficit stemmed from aberrant fronto-hippocampal activation during memory retrieval. METHODS: Patients with schizophrenia (n = 16) and age-matched comparison subjects (n = 16) underwent functional magnetic resonance imaging while performing a verbal memory task that requires intact use of temporal context. Blood oxygen-level dependent (BOLD) signal during correct memory decisions was compared between the two groups with statistical parametric mapping. RESULTS: Contrary to our hypotheses, patients with schizophrenia demonstrated nearly identical memory performance to that of the comparison subjects. Despite this, there were significant between-group BOLD signal differences, including a pattern of task-dependent hypofrontality or hyperfrontality. In addition, whereas the highest-performing subset of the comparison group demonstrated robust modulation of hippocampal activity, this pattern was not seen in the highest-performing patients with schizophrenia. CONCLUSIONS: Despite memory performance similar to that of comparison subjects, patients with schizophrenia activated different neural pathways to achieve this success. This might reflect underlying neuropathology in fronto-hippocampal circuitry, the use of an alternate cognitive strategy to accomplish task performance, or both.     

Declarative memory and WCST-64 performance in subjects with schizophrenia and healthy controls

The Wisconsin Card Sorting Test (WCST) is a set-switching task used extensively to study impaired executive functioning in schizophrenia. Declarative memory deficits have also been associated with schizophrenia and may affect WCST performance because continued correct responding depends on remembering the outcome of previous responses. This study examined whether performance in visual and verbal declarative memory tasks were associated with WCST performance. Subjects comprised 30 patients with schizophrenia or schizoaffective disorder (SCZ) and 30 demographically matched healthy controls (CON) who were tested on the WCST, the Benton Visual Retention Test (BVRT), the California Verbal Learning Test (CVLT), and the Continuous Performance Test (CPT). SCZ subjects showed significant correlations between visual and verbal declarative memory and performance on the WCST-64 that were in the hypothesized direction such that worse memory performance was associated with worse performance on the WCST. CON subjects did not show a significant relationship between visual or verbal memory and WCST-64 performance. Fisher's r to z transformations indicated that the associations between declarative memory and WCST-64 performance in the SCZ subjects differed significantly from those of CON subjects. The findings suggest that interpretations of WCST-64 scores for subjects with schizophrenia should be considered in light of their declarative memory functioning.