Effects of ApoE genotype and mild cognitive impairment on implicit learning

The past, present, and future of selection history

The last ten years of attention research have witnessed a revolution, replacing a theoretical dichotomy (top-down vs. bottom-up control) with a trichotomy (biased by current goals, physical salience, and selection history). This third new mechanism of attentional control, selection history, is multifaceted. Some aspects of selection history must be learned over time whereas others reflect much more transient influences. A variety of different learning experiences can shape the attention system, including reward, aversive outcomes, past experience searching for a target, target‒non-target relations, and more. In this review, we provide an overview of the historical forces that led to the proposal of selection history as a distinct mechanism of attentional control. We then propose a formal definition of selection history, with concrete criteria, and identify different components of experience-driven attention that fit within this definition. The bulk of the review is devoted to exploring how these different components relate to one another. We conclude by proposing an integrative account of selection history centered on underlying themes that emerge from our review.

Apolipoprotein E ɛ4-related effects on cognition are limited to the Alzheimer's disease spectrum

Whether the deleterious effects of APOE4 are restricted to the Alzheimer’s disease (AD) spectrum or cause cognitive impairment irrespectively of the development of AD is still a matter of debate, and the focus of this study. Our analyses included APOE4 genotype, neuropsychological variables, amyloid-βeta (Aβ) and Tau markers, FDG-PET values, and hippocampal volumetry data derived from the healthy controls sample of the ADNI database. We formed 4 groups of equal size (n = 30) based on APOE4 carriage and amyloid-PET status. Baseline and follow-up (i.e., 48 months post-baseline) results indicated that Aβ-positivity was the most important factor to explain poorer cognitive performance, while APOE4 only exerted a significant effect in Aβ-positive subjects. Additionally, multiple regression analyses evidenced that, within the Aβ-positive sample, hippocampal volumetry explained most of the variability in cognitive performance for APOE4 carriers. These findings represent a strong support for the so-called preclinical/prodromal hypothesis, which states that the reported differences in cognitive performance between healthy carriers and non-carriers are mainly due to the APOE4’s capability to increase the risk of AD. Moreover, our results reinforce the notion that a synergistic interaction of Aβ and APOE4 elicits a neurodegenerative process in the hippocampus that might be the main cause of impaired cognitive performance.

Serial Reaction Time Task Performance in Older Adults with Neuropsychologically Defined Mild Cognitive Impairment

BACKGROUND

Studies have found that individuals with mild cognitive impairment (MCI) exhibit a range of deficits outside the realm of primary explicit memory, yet the role of response speed and implicit learning in older adults with MCI have not been established.

OBJECTIVE

The current study aims to explore and document response speed and implicit learning in older adults with neuropsychologically defined MCI using a simple serial reaction (SRT) task. In addition, the study aims to explore the feasibility of a novel utilization of the simple cognitive task using machine learning procedures as a proof of concept.

METHOD

Participants were 22 cognitively healthy older adults and 20 older adults with MCI confirmed through comprehensive neuropsychological evaluation. Two-sample t-test, multivariate regression, and mixed-effect models were used to investigate group difference in response speed and implicit learning on the SRT task. We also explored the potential utility of SRT feature analysis through random forest classification.

RESULTS

With demographic variables controlled, the MCI group showed overall slower reaction time and higher error rate compared to the cognitively healthy volunteers. Both groups showed significant simple motor learning and implicit learning. The learning patterns were not statistically different between the two groups. Random forest classification achieved overall accuracy of 80.9%.

CONCLUSIONS

Individuals with MCI demonstrated slower reaction time and higher error rate compared to cognitively healthy volunteers but demonstrated largely preserved motor learning and implicit sequence learning. Preliminary results from random forest classification using features from SRT performance supported further research in this area.

Neural dynamics supporting auditory long-term memory effects on target detection

Auditory long-term memory has been shown to facilitate signal detection. However, the nature and timing of the cognitive processes supporting such benefits remain equivocal. We measured neuroelectric brain activity while young adults were presented with a contextual memory cue designed to assist with the detection of a faint pure tone target embedded in an audio clip of an everyday environmental scene (e.g., the soundtrack of a restaurant). During an initial familiarization task, participants heard such audio clips, half of which included a target sound (memory cue trials) at a specific time and location (left or right ear), as well as audio clips without a target (neutral trials). Following a 1-h or 24-h retention interval, the same audio clips were presented, but now all included a target. Participants were asked to press a button as soon as they heard the pure tone target. Overall, participants were faster and more accurate during memory than neutral cue trials. The auditory contextual memory effects on performance coincided with three temporally and spatially distinct neural modulations, which encompassed changes in the amplitude of event-related potential as well as changes in theta, alpha, beta and gamma power. Brain electrical source analyses revealed greater source activity in memory than neutral cue trials in the right superior temporal gyrus and left parietal cortex. Conversely, neutral trials were associated with greater source activity than memory cue trials in the left posterior medial temporal lobe. Target detection was associated with increased negativity (N2), and a late positive (P3b) wave at frontal and parietal sites, respectively. The effect of auditory contextual memory on brain activity preceding target onset showed little lateralization. Together, these results are consistent with contextual memory facilitating retrieval of target-context associations and deployment and management of auditory attentional resources to when the target occurred. The results also suggest that the auditory cortices, parietal cortex, and medial temporal lobe may be parts of a neural network enabling memory-guided attention during auditory scene analysis.

Is there more room to improve? The lifespan trajectory of procedural learning and its relationship to the between- and within-group differences in average response times

Characterizing the developmental trajectories of cognitive functions such as learning, memory and decision making across the lifespan faces fundamental challenges. Cognitive functions typically encompass several processes that can be differentially affected by age. Methodological issues also arise when comparisons are made across age groups that differ in basic performance measures, such as in average response times (RTs). Here we focus on procedural learning–a fundamental cognitive function that underlies the acquisition of cognitive, social, and motor skills–and demonstrate how disentangling subprocesses of learning and controlling for differences in average RTs can reveal different developmental trajectories across the human lifespan. Two hundred-seventy participants aged between 7 and 85 years performed a probabilistic sequence learning task that enabled us to separately measure two processes of procedural learning, namely general skill learning and statistical learning. Using raw RT measures, in between-group comparisons, we found a U-shaped trajectory with children and older adults exhibiting greater general skill learning compared to adolescents and younger adults. However, when we controlled for differences in average RTs (either by using ratio scores or focusing on a subsample of participants with similar average speed), only children (but not older adults) demonstrated superior general skill learning consistently across analyses. Testing the relationship between average RTs and general skill learning within age groups shed light on further age-related differences, suggesting that general skill learning measures are more affected by average speed in some age groups. Consistent with previous studies of learning probabilistic regularities, statistical learning showed a gradual decline across the lifespan, and learning performance seemed to be independent of average speed, regardless of the age group. Overall, our results suggest that children are superior learners in various aspects of procedural learning, including both general skill and statistical learning. Our study also highlights the importance to test, and control for, the effect of average speed on other RT measures of cognitive functions, which can fundamentally affect the interpretation of group differences in developmental, aging and clinical psychology and neuroscience studies.

Impaired memory-guided attention in asymptomatic APOE4 carriers

Attention and memory may be impaired in individuals at-risk for Alzheimer's disease (AD), though standard cognitive assessments typically study the two in parallel. In reality, attention and memory interact to facilitate information processing, and thus a more integrative approach is required. Here, we used a novel auditory paradigm to assess how long-term memory for auditory scenes facilitates detection of an auditory target in asymptomatic carriers of Apolipoprotein E4 (APOE4), the principle risk gene for late-onset AD. We tested 60 healthy middle-aged adults with varying doses of APOE4 - 20 APOE4 homozygotes (E4/E4), 20 heterozygotes (E3/E4) and 20 non-carriers (E3/E3) - to determine effect on memory-guided attention. While explicit memory was unaffected by genotype, APOE4 dose significantly impaired memory-guided attention. A relationship between explicit memory and memory-guided attention was observed in non-carriers, but this correlation was not significant in E3/E4 and E4/E4 carriers, suggesting that APOE4 carriers rely less on explicit memory to facilitate attention. Since memory-guided attention declined with age in APOE4 homozygotes, this impairment may reflect early disease rather than being a life-long trait. In sum, asymptomatic individuals at increased genetic risk of AD show an age-dependent decline in attention-memory interaction when memory alone is not impaired.

Apolipoprotein ε4 is associated with better cognitive control allocation in healthy young adults

Many gene variants may impair our health and cognitive abilities at old age, but some of them paradoxically improve the same or similar functions at much younger age (antagonistic pleiotropy hypothesis). Such a diametric pattern may also hold true for the ancestral Apolipoprotein E (APOE) ε4 allele, which increases the risk for Alzheimer's disease and cognitive decline in old age, but may benefit (pre)frontal (executive) functions in young carriers. We therefore investigated potential cognitive benefits of the risk allele on cognitive control capacities and top-down control allocation ("metacontrol") in n = 190 healthy young adults. On a behavioral level, we found young APOE ε4 carriers to better adapt to different degrees of cognitive control requirements, with superior performance in case of high control demands. On a neurophysiological level, these group differences were reflected by modulations of the N450 component, which were rooted in activation differences of the superior frontal gyrus (SFG, BA8). Taken together, our results suggest that young ε4 carriers are more efficient than non-carriers at allocating cognitive control resources based on the actual task requirements (i.e. metacontrol), as they seem to experience less conflict/exert less effort and recruit fewer additional prefrontal areas when task set complexity increases. We further found that ε2 carriers processed implicit spatial stimulus features to a stronger degree than ε3 and ε4 carriers, but failed to benefit from this, as the additional information likely increased response selection conflicts. This finding should however be treated with ample caution as the group of ε2 carriers was comparatively small.

APOE ε4 Genotype and Cognitive Reserve Effects on the Cognitive Functioning of Healthy Elders

AIM

To test the association between cognitive performance and APOE genotype, and to assess potential modifications of this association by sociodemographic and neuroanatomical factors in a sample of 74 healthy elders.

METHODS

Firstly, we explored the isolated role of the APOE ɛ4 genotype (i.e., APOE4) in different neuropsychological tests, and then the effects of its interaction with sociodemographic (i.e., age, gender, and educational level) and neuroanatomical (i.e., hippocampal volumes) variables. Subsequently, we performed the same analyses after dividing the sample into two subgroups according to their Mini-Mental State Examination scores (control-high group ≥29 and control-low group < 29).

RESULTS

In the whole group, APOE4 carriers exhibited a significantly poorer execution in several cognitive domains including global cognitive functioning, episodic memory, verbal fluency, and naming. This effect was more noticeable in older and less educated subjects. The separated analyses revealed that APOE4 carriers in the control-low group exhibited lower scores in global cognitive functioning and episodic memory, while no effects were observed in the control-high group. Neither gender nor hippocampal volumes showed a significant interaction effect with APOE genotype.

CONCLUSIONS

Current results point out that APOE4 genotype influences healthy aged cognition, although factors such age or educational attainment seem to modulate its effects.

A Speech Recognition-based Solution for the Automatic Detection of Mild Cognitive Impairment from Spontaneous Speech

BACKGROUND

Even today the reliable diagnosis of the prodromal stages of Alzheimer's disease (AD) remains a great challenge. Our research focuses on the earliest detectable indicators of cognitive decline in mild cognitive impairment (MCI). Since the presence of language impairment has been reported even in the mild stage of AD, the aim of this study is to develop a sensitive neuropsychological screening method which is based on the analysis of spontaneous speech production during performing a memory task. In the future, this can form the basis of an Internet-based interactive screening software for the recognition of MCI.

METHODS

Participants were 38 healthy controls and 48 clinically diagnosed MCI patients. The provoked spontaneous speech by asking the patients to recall the content of 2 short black and white films (one direct, one delayed), and by answering one question. Acoustic parameters (hesitation ratio, speech tempo, length and number of silent and filled pauses, length of utterance) were extracted from the recorded speech signals, first manually (using the Praat software), and then automatically, with an automatic speech recognition (ASR) based tool. First, the extracted parameters were statistically analyzed. Then we applied machine learning algorithms to see whether the MCI and the control group can be discriminated automatically based on the acoustic features.

RESULTS

The statistical analysis showed significant differences for most of the acoustic parameters (speech tempo, articulation rate, silent pause, hesitation ratio, length of utterance, pause-per-utterance ratio). The most significant differences between the two groups were found in the speech tempo in the delayed recall task, and in the number of pauses for the question-answering task. The fully automated version of the analysis process - that is, using the ASR-based features in combination with machine learning - was able to separate the two classes with an F1-score of 78.8%.

CONCLUSION

The temporal analysis of spontaneous speech can be exploited in implementing a new, automatic detection-based tool for screening MCI for the community.

Underpowered samples, false negatives, and unconscious learning

The scientific community has witnessed growing concern about the high rate of false positives and unreliable results within the psychological literature, but the harmful impact of false negatives has been largely ignored. False negatives are particularly concerning in research areas where demonstrating the absence of an effect is crucial, such as studies of unconscious or implicit processing. Research on implicit processes seeks evidence of above-chance performance on some implicit behavioral measure at the same time as chance-level performance (that is, a null result) on an explicit measure of awareness. A systematic review of 73 studies of contextual cuing, a popular implicit learning paradigm, involving 181 statistical analyses of awareness tests, reveals how underpowered studies can lead to failure to reject a false null hypothesis. Among the studies that reported sufficient information, the meta-analytic effect size across awareness tests was d_z = 0.31 (95 % CI 0.24–0.37), showing that participants’ learning in these experiments was conscious. The unusually large number of positive results in this literature cannot be explained by selective publication. Instead, our analyses demonstrate that these tests are typically insensitive and underpowered to detect medium to small, but true, effects in awareness tests. These findings challenge a widespread and theoretically important claim about the extent of unconscious human cognition.

Apolipoprotein ɛ4 breaks the association between declarative long-term memory and memory-based orienting of spatial attention in middle-aged individuals

Apolipoprotein (APOE) ɛ4 genotype has been identified as a risk factor for late-onset Alzheimer disease (AD). The memory system is mostly involved in AD, and memory deficits represent its key feature. A growing body of studies has focused on the earlier identification of cognitive dysfunctions in younger and older APOE ɛ4 carriers, but investigation on middle-aged individuals remains rare. Here we sought to investigate if the APOE ɛ4 genotype modulates declarative memory and its influences on perception in the middle of the life span. We tested 60 middle-aged individuals recruited according to their APOE allele variants (ɛ3/ɛ3, ɛ3/ɛ4, ɛ4/ɛ4) on a long-term memory-based orienting of attention task. Results showed that the APOE ɛ4 genotype impaired neither explicit memory nor memory-based orienting of spatial attention. Interestingly, however, we found that the possession of the ɛ4 allele broke the relationship between declarative long-term memory and memory-guided orienting of visuo-spatial attention, suggesting an earlier modulation exerted by pure genetic characteristics on cognition. These findings are discussed in light of possible accelerated brain ageing in middle-aged ɛ4-carriers, and earlier structural changes in the brain occurring at this stage of the lifespan.

Learning to predict is spared in mild cognitive impairment due to Alzheimer's disease

Learning the statistics of the environment is critical for predicting upcoming events. However, little is known about how we translate previous knowledge about scene regularities to sensory predictions. Here, we ask whether patients with mild cognitive impairment due to Alzheimer's disease (MCI-AD) that are known to have spared implicit but impaired explicit recognition memory are able to learn temporal regularities and predict upcoming events. We tested the ability of MCI-AD patients and age-matched controls to predict the orientation of a test stimulus following exposure to sequences of leftwards or rightwards oriented gratings. Our results demonstrate that exposure to temporal sequences without feedback facilitates the ability to predict an upcoming stimulus in both MCI-AD patients and controls. Further, we show that executive cognitive control may account for individual variability in predictive learning. That is, we observed significant positive correlations of performance in attentional and working memory tasks with post-training performance in the prediction task. Taken together, these results suggest a mediating role of circuits involved in cognitive control (i.e. frontal circuits) that may support the ability for predictive learning in MCI-AD.

Learning temporal statistics for sensory predictions in mild cognitive impairment

Training is known to improve performance in a variety of perceptual and cognitive skills. However, there is accumulating evidence that mere exposure (i.e. without supervised training) to regularities (i.e. patterns that co-occur in the environment) facilitates our ability to learn contingencies that allow us to interpret the current scene and make predictions about future events. Recent neuroimaging studies have implicated fronto-striatal and medial temporal lobe brain regions in the learning of spatial and temporal statistics. Here, we ask whether patients with mild cognitive impairment due to Alzheimer's disease (MCI-AD) that are characterized by hippocampal dysfunction are able to learn temporal regularities and predict upcoming events. We tested the ability of MCI-AD patients and age-matched controls to predict the orientation of a test stimulus following exposure to sequences of leftwards or rightwards orientated gratings. Our results demonstrate that exposure to temporal sequences without feedback facilitates the ability to predict an upcoming stimulus in both MCI-AD patients and controls. However, our fMRI results demonstrate that MCI-AD patients recruit an alternate circuit to hippocampus to succeed in learning of predictive structures. In particular, we observed stronger learning-dependent activations for structured sequences in frontal, subcortical and cerebellar regions for patients compared to age-matched controls. Thus, our findings suggest a cortico-striatal-cerebellar network that may mediate the ability for predictive learning despite hippocampal dysfunction in MCI-AD.

Relationship of contextual cueing and hippocampal volume in amnestic mild cognitive impairment patients and cognitively normal older adults

There is currently some debate as to whether hippocampus mediates contextual cueing. In the present study, we examined contextual cueing in patients diagnosed with mild cognitive impairment (MCI) and healthy older adults, with the main goal of investigating the role of hippocampus in this form of learning. Amnestic MCI (aMCI) patients and healthy controls completed the contextual cueing task, in which they were asked to search for a target (a horizontal T) in an array of distractors (rotated L's). Unbeknownst to them, the spatial arrangement of elements on some displays was repeated thus making the configuration a contextual cue to the location of the target. In contrast, the configuration for novel displays was generated randomly on each trial. The difference in response times between repeated and novel configurations served as a measure of contextual learning. aMCI patients, as a group, were able to learn spatial contextual cues as well as healthy older adults. However, better learning on this task was associated with higher hippocampal volume, particularly in right hemisphere. Furthermore, contextual cueing performance was significantly associated with hippocampal volume, even after controlling for age and MCI status. These findings support the role of the hippocampus in learning of spatial contexts, and also suggest that the contextual cueing paradigm can be useful in detecting neuropathological changes associated with the hippocampus.

Apolipoprotein E alleles in Cuban patients with mild cognitive impairment

BACKGROUND

Apolipoprotein E (ApoE) ε4 genotype is the most clearly documented risk factor for Alzheimer's disease (AD). Epidemiological studies demonstrate an accelerated rate of progression to dementia and AD in patients with mild cognitive impairment (MCI). We assessed the ApoE allele and genotypes frequencies in Cuban patients with MCI.

METHODS

We performed ApoE genotyping of 74 Cuban patients more than 65 years old. Cognitive assessments included the Mini-Mental State Examination (MMSE) and a cognitive battery for evaluating memory, attention, perception, and executive function.

RESULTS

Cognitive impairments were characterized by amnesia and executive deficits in patients with MCI. The Apo ε4 allele frequency was 0.196 in patients with MCI, 10-fold higher than that in the controls. Patients carrying the ε4 allele exhibited poorer performance in MMSE and tests assessing executive function and short-term memory than noncarriers.

CONCLUSIONS

The patients exhibited amnestic MCI multiple domains. Cognitive performance was worse in patients who carried the ApoE ε4 allele.

Contextual cueing effects across the lifespan

The authors evaluated age-related variations in contextual cueing, which reflects the extent to which visuospatial regularities can facilitate search for a target. Previous research produced inconsistent results regarding contextual cueing effects in young children and in older adults, and no study has investigated the phenomenon across the life span. Three groups (6, 20, and 70 years old) were compared. Participants located a designated target stimulus embedded in a context of distractor stimuli. During exposure, the location of the target could be predicted from the location of the distracters in each display. During test, these predictable displays were intermixed with new displays that did not predict the target location. Response times to locating predictable relative to unpredictable targets were compared. All groups exhibited facilitation effects greater than 0 (95% CIs [.02, .11], d = .4; [.01, .12], d = .4; and [.01, .10], d = .4, for the children, young adults, and older adults, respectively) indicating that contextual cueing is robust across a wide age range. The relative magnitude of contextual cueing effects was essentially identical across the age range tested, F(2, 103) = 1.71, eta rho2 = .02. The authors argue that a mechanism that uses environmental covariation is available to all age ranges, but the expression of the contextual cueing may depend on the way it is measured.

Probabilistic sequence learning in mild cognitive impairment

Mild Cognitive Impairment (MCI) causes slight but noticeable disruption in cognitive systems, primarily executive and memory functions. However, it is not clear if the development of sequence learning is affected by an impaired cognitive system and, if so, how. The goal of our study was to investigate the development of probabilistic sequence learning, from the initial acquisition to consolidation, in MCI and healthy elderly control groups. We used the Alternating Serial Reaction Time task (ASRT) to measure probabilistic sequence learning. Individuals with MCI showed weaker learning performance than the healthy elderly group. However, using the reaction times only from the second half of each learning block—after the reactivation phase—we found intact learning in MCI. Based on the assumption that the first part of each learning block is related to reactivation/recall processes, we suggest that these processes are affected in MCI. The 24-h offline period showed no effect on sequence-specific learning in either group but did on general skill learning: the healthy elderly group showed offline improvement in general reaction times while individuals with MCI did not. Our findings deepen our understanding regarding the underlying mechanisms and time course of sequence acquisition and consolidation.

Lack of reliable evidence for a distinctive ε4-related cognitive phenotype that is independent from clinical diagnostic status: findings from the Australian Imaging, Biomarkers and Lifestyle Study

Individuals who carry the apolipoprotein E ε4 polymorphism have an increased risk of late-onset Alzheimer's disease. However, because possession of the ε4 allele confers an increased risk for the diagnosis of dementia, it has proven problematic in older individuals to dissociate the influence of ε4 on cognitive capacity per se as distinct from its influence on clinical diagnostic status. We report a statistical approach that attempts to partial out the influence of diagnostic group membership (Alzheimer's disease, mild cognitive impairment, healthy control) from the influence of apolipoprotein ε4 genetic status on cognitive functioning. The cognitive phenotype hypothesis predicts that ε4-positive individuals will show cognitive deficits (relative to matched ε4-negative individuals) independent of the development of Alzheimer's disease. By contrast, the prodromal/preclinical Alzheimer's disease hypothesis proposes that the effect of apolipoprotein E status on cognitive performance is a function of the increased risk of dementia in individuals with the ε4 allele. We evaluated these hypotheses in the Australian Imaging, Biomarkers and Lifestyle cohort (n = 1112). We first determined whether previously reported findings concerning ε4 status and age-related neuropsychological performance could be explained by the inadvertent recruitment of people with mild cognitive impairment into the healthy control group. We then tested each diagnostic group in isolation to identify any neuropsychological patterns that may be attributed to the ε4 allele. Finally, as interactions between the ε4 allele and age have previously been reported in cognitive functioning within healthy elderly populations, we attempted to determine whether the inclusion of mild cognitively impaired individuals in the sample may drive this relationship. An extensive battery of standardized, well-validated neuropsychological tasks was administered to a final sample of 764 healthy control subjects, 131 individuals with mild cognitive impairment and 168 individuals with Alzheimer's disease. The effect of the ε4 allele on cognitive performance was assessed using a statistical mediation analysis and supplemented with Bayesian methods to address a number of the limitations associated with Fisherian/Neyman-Pearsonian significance testing. Our findings support the prodromal/preclinical Alzheimer's disease hypothesis and do not support the concept of a distinctive ε4-related cognitive phenotype.

Amnestic mild cognitive impairment: topological reorganization of the default-mode network

PURPOSE

To investigate the topologic reorganization of the default-mode network (DMN) in patients with mild cognitive impairment (MCI) and whether, relative to healthy control subjects, patients with MCI would be more likely to show disrupted functional connectivity and altered topological configuration of the DMN during the memory task compared with that observed during the resting state.

MATERIALS AND METHODS

This study was approved by the institutional review board of Beijing Normal University Imaging Center for Brain Research. Written informed consent was obtained from each participant. Healthy control subjects (n = 26) and patients with amnestic MCI (aMCI) (n = 25) performed an episodic memory task and also rested while undergoing functional magnetic resonance imaging. Task-induced deactivations were identified and parcellated into different regions associated with the DMN. Functional connectivity across all pairs of regions was computed to construct the DMN architecture. Graph theoretical approaches were used to characterize topological properties of this network.

RESULTS

Patients with aMCI showed similar deactivation in the DMN to that observed in healthy control subjects (P > .05) but showed significantly decreased anterior-to-posterior functional connectivity only during the task (P < .05). Significant increases in local efficiency (P < .05), but not in global efficiency (P > .05), were observed in aMCI only during the task. Decreased functional connectivity was predictive of increased local efficiency (r = -0.35, P = .015). Significant correlations between these network measures and cognitive performance (P < .05) indicated their potential use as early markers to assess the risk of Alzheimer disease (AD).

CONCLUSION

This study suggests the early onset functional reorganization of the DMN toward a nonoptimized regularity configuration in aMCI and expands the understanding of dynamic functional reorganization in brain networks along the continuum from normal aging to AD dementia.

Subtypes of mild cognitive impairment in older postmenopausal women: the Women's Health Initiative Memory Study

Mild cognitive impairment (MCI) is a transitional state between normal cognitive functioning and dementia. A proposed MCI typology classifies individuals by the type and extent of cognitive impairment, yet few studies have characterized or compared these subtypes. Four hundred forty-seven women 65 years of age and older from the Women's Health Initiative Memory Study were classified into the 4 MCI subgroups and a "no impairment" group and compared on clinical, sociodemographic, and health variables. A cognitive deficit in at least 1 domain was present in 82.1% of participants, with most (74.3%) having deficits in multiple cognitive domains. Only 4.3% had an isolated memory deficit, whereas 21.3% had an isolated nonmemory deficit. Of the 112 women who met all MCI criteria examined, the most common subtype was amnestic multidomain MCI (42.8%), followed by nonamnestic multiple domain MCI (26.7%), nonamnestic single domain (24.1%), and amnestic single domain MCI (6.3%). Subtypes were similar with respect to education, health status, smoking, depression, and prestudy and onstudy use of hormone therapy. Despite the attention it receives in the literature, amnestic MCI is the least common type highlighting the importance of identifying and characterizing other nonamnestic and multidomain subtypes. Further research is needed on the epidemiology of MCI subtypes, clinical and biologic differences between them, and rates for conversion to dementia.

Dopamine transporter genotype predicts implicit sequence learning

Implicit learning, the non-conscious acquisition of sequential and spatial environmental regularities, underlies skills such as language, social intuition, or detecting a target in a complex scene. We examined relationships between a variation of the dopamine transporter (DAT1) gene (SLC6A3), which influences dopamine transporter expression in the striatum, and two forms of implicit learning that differ in the regularity to be learned and in striatal involvement. Participants, grouped as 9-repeat carriers or 10/10 homozygotes, completed the triplets learning task (TLT) and the spatial contextual cueing task (SCCT). The TLT assesses sequence learning, recruiting the striatal system, particularly as training continues. In contrast, the SCCT assesses spatial context learning, recruiting medial temporal brain networks. For both tasks, participants demonstrated learning in faster and/or more accurate responses to repeating patterns or spatial arrays. As predicted, TLT learning was greater for the 9-repeat carriers than the 10/10 group (despite equal overall accuracy and response speed) whereas there were no significant group differences in SCCT. Thus, presence of the DAT1 9-repeat allele was beneficial only for implicit sequence learning, indicating the influence of DAT1 genotype on one form of implicit learning and supporting evidence that implicit learning of sequential dependencies and spatial layouts recruit different neural systems.

Effect of ApoE epsilon4 allele on visual evoked potentials and resultant flow coupling in patients with Alzheimer

The apolipoprotein E epsilon4 (ApoE epsilon4) allele is a strong susceptibility factor for Alzheimer disease, which promotes neurodegeneration and cerebrovascular dysfunction. To address this issue in more detail, we simultaneously obtained visual evoked potentials and resultant hemodynamic responses in newly diagnosed Alzheimer patients without signs of vascular lesions on a cerebral magnetic resonance imaging (MRI) scan. Patients were grouped according to ApoE genotype (n = 19 ApoE epsilon4 carrier and n = 12 noncarrier). ApoE epsilon4 carrier had significantly longer peak latencies and a trend to higher interpeak latencies of late potential components. Potential amplitudes and hemodynamic responses were similar in both groups. At the incidental stage of disease process, it appears that the ApoE epsilon4 allele mainly promotes neuronal dysfunction rather than aggravates neurovascular dysfunction. Studies with larger patient samples are warranted to corroborate the first findings.

Intact implicit learning in autism spectrum conditions

Individuals with autism spectrum condition (ASC) have diagnostic impairments in skills that are associated with an implicit acquisition; however, it is not clear whether ASC individuals show specific implicit learning deficits. We compared ASC and typically developing (TD) individuals matched for IQ on five learning tasks: four implicit learning tasks--contextual cueing, serial reaction time, artificial grammar learning, and probabilistic classification learning tasks--that used procedures expressly designed to minimize the use of explicit strategies, and one comparison explicit learning task, paired associates learning. We found implicit learning to be intact in ASC. Beyond no evidence of differences, there was evidence of statistical equivalence between the groups on all the implicit learning tasks. This was not a consequence of compensation by explicit learning ability or IQ. Furthermore, there was no evidence to relate implicit learning to ASC symptomatology. We conclude that implicit mechanisms are preserved in ASC and propose that it is disruption by other atypical processes that impact negatively on the development of skills associated with an implicit acquisition.

Anterior prefrontal involvement in implicit contextual change detection

Anterior prefrontal cortex is usually associated with high level executive functions. Here, we show that the frontal pole, specifically left lateral frontopolar cortex, is involved in signaling change in implicitly learned spatial contexts, in the absence of conscious change detection. In a variant of the contextual cueing paradigm, participants first learned contingencies between distractor contexts and target locations implicitly. After learning, repeated distractor contexts were paired with new target locations. Left lateral frontopolar [Brodmann area (BA) 10] and superior frontal (BA9) cortices showed selective signal increase for this target location change in repeated displays in an event-related fMRI experiment, which was most pronounced in participants with high contextual facilitation before the change. The data support the view that left lateral frontopolar cortex is involved in signaling contextual change to posterior brain areas as a precondition for adaptive changes of attentional resource allocation. This signaling occurs in the absence of awareness of learned contingencies or contextual change.

Oxidatively modified proteins in Alzheimer's disease (AD), mild cognitive impairment and animal models of AD: role of Abeta in pathogenesis

Oxidative stress has been implicated in the pathogenesis of a number of diseases including Alzheimer's disease (AD). The oxidative stress hypothesis of AD pathogenesis, in part, is based on beta-amyloid peptide (Abeta)-induced oxidative stress in both in vitro and in vivo studies. Oxidative modification of the protein may induce structural changes in a protein that might lead to its functional impairment. A number of oxidatively modified brain proteins were identified using redox proteomics in AD, mild cognitive impairment (MCI) and Abeta models of AD, which support a role of Abeta in the alteration of a number of biochemical and cellular processes such as energy metabolism, protein degradation, synaptic function, neuritic growth, neurotransmission, cellular defense system, long term potentiation involved in formation of memory, etc. All the redox proteomics-identified brain proteins fit well with the appearance of the three histopathological hallmarks of AD, i.e., synapse loss, amyloid plaque formation and neurofibrillary tangle formation and suggest a direct or indirect association of the identified proteins with the pathological and/or biochemical alterations in AD. Further, Abeta models of AD strongly support the notion that oxidative stress induced by Abeta may be a driving force in AD pathogenesis. Studies conducted on arguably the earliest stage of AD, MCI, may elucidate the mechanism(s) leading to AD pathogenesis by identifying early markers of the disease, and to develop therapeutic strategies to slow or prevent the progression of AD. In this review, we summarized our findings of redox proteomics identified oxidatively modified proteins in AD, MCI and AD models.

An abbreviated implicit spatial context learning task that yields greater learning

The spatial contextual cuing task (SCCT) (Chun & Jiang, 1998) is an implicit learning task that appears to depend on the medial temporal lobes. This unusual combination has been of interest in functional imaging studies and research with clinical populations, where testing time is at a premium. However, the original version of the SCCT is time-consuming. In this study, 29 young adults (age range, 18-22 years) completed the SCCT, in which participants respond to the orientation of a target in arrays containing 11 distractors. Either 12 (original version) or 6 (abbreviated version) arrays repeated across the experiment, with the remaining novel arrays being generated randomly. Results revealed that the magnitude of learning (faster responses to repeated versus novel arrays) was larger when there were fewer repeated arrays, with no explicit awareness in most participants. Thus, the abbreviated version remained implicit, with the additional benefit of increasing the magnitude of learning.

Depression, cognition, apolipoprotein e genotype: latent class approach to identifying subtype

OBJECTIVES

Possessing the epsilon4 allele of apolipoprotein E (APOE-epsilon4) genotype is associated with cognitive impairment in nondemented older adults. The authors hypothesized that they might find a subtype of depression related to impaired cognitive performance associated with the APOE-epsilon4 allele.

DESIGN

A survey conducted between 2001 and 2003 with APOE genotyping.

SETTING

Primary care offices in the Baltimore area.

PARTICIPANTS

The study sample consisted of 305 adults aged 65 or older with complete information on APOE genotyping and covariates.

MEASUREMENTS

The authors used the latent class model to classify respondents according to symptom criteria of American Psychiatric Association's Diagnostic and Statistical Manual as assessed in the Composite International Diagnostic Interview and the following four measures of cognitive function: the Mini-Mental State Exam, Hopkins Verbal Learning Test, Controlled Oral Word Association Test, and the Brief Test of Attention. The authors examined the relationship between class membership and APOE genotype.

RESULTS

The latent class model yielded three classes: a nondepressed class, a class with depressive symptoms and average cognitive functioning, and a class with depressive symptoms (particularly thoughts of death and suicide) and impaired cognitive functioning. Possessing at least one APOE-epsilon4 allele was not predictive of class membership.

CONCLUSION

A subgroup of elderly patients with depressive symptoms, cognitive impairment, and a high likelihood of experiencing thoughts of death or suicide may exist that may not be related to APOE-epsilon4. Subgroups of older patients with depressive symptoms may be important to identify because of the association with thoughts of death or suicide and cognitive impairment.

Cognitive discrepancies versus APOE genotype as predictors of cognitive decline in normal-functioning elderly individuals: a longitudinal study

OBJECTIVES

Cognitive-discrepancy analysis has been shown to be a useful technique for detecting subtle cognitive deficits in normal-functioning elderly individuals who are genetically at-risk for Alzheimer disease (AD). However, studies that have used cognitive-discrepancy measures to date have used retrospective or cross-sectional designs, and the utility of this approach to predict cognitive decline has not been examined in a prospective investigation.

DESIGN

Longitudinal study.

SETTING

San Diego, CA, Veterans Administration Hospital.

PARTICIPANTS

Twenty-four normal-functioning elderly individuals participated in the study, with 16 subjects exhibiting no change in their Dementia Rating Scale (DRS) scores over an 1-year period (Stable Group), and 8 subjects exhibiting a decline in DRS scores over the 1-year period (Decline group).

MEASUREMENTS

A cognitive-discrepancy measure isolating cognitive switching was computed that contrasted performance on a new higher-level task of executive functioning (a Stroop/Switching measure) relative to a composite measure of lower-level Stroop conditions.

RESULTS

a) In the year before their cognitive changes, the Decline group exhibited a significantly larger cognitive-discrepancy (Stroop/Switching versus lower-level Stroop conditions) score compared with a control (Stable) group; and b) the cognitive-discrepancy measure was superior to APOE genotype in predicting DRS decline.

CONCLUSION

Cognitive-discrepancy analysis isolating a component executive function ability not only seems to be a useful tool for identifying individuals at risk for cognitive deficits, but also shows promise in predicting individuals who may show subtle cognitive decline over time.

Implicit probabilistic sequence learning is independent of explicit awareness

Studies into interactions between explicit and implicit motor sequence learning have yielded mixed results. Some of these discrepancies have been attributed to difficulties in isolating implicit learning. In the present study, the effect of explicit knowledge on implicit learning was investigated using a modified version of the Alternating Serial Response Time (ASRT) task, a probabilistic sequence learning paradigm that yields continuous and relatively pure measures of implicit learning. Results revealed that implicit learning occurred to the same extent, whether or not subjects had explicit knowledge. Some evidence, however, indicated that explicit knowledge could interfere with the expression of implicit learning early in training. In addition, there were dissociations between learning measures, in that reaction time and accuracy were differentially affected by explicit knowledge. These findings indicate that implicit sequence learning occurs independently of explicit knowledge, and help to explain previous discrepant findings.