Neocortical disconnectivity disrupts sensory integration in Alzheimer's disease

Sensory processing deficits and related cortical pathological changes in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder primarily affecting cognitive functions. However, sensory deficits in AD start to draw attention due to their high prevalence and early onsets which suggest that they could potentially serve as diagnostic biomarkers and even contribute to the disease progression. This literature review examines the sensory deficits and cortical pathological changes observed in visual, auditory, olfactory, and somatosensory systems in AD patients, as well as in various AD animal models. Sensory deficits may emerge at the early stages of AD, or even precede the cognitive decline, which is accompanied by cortical pathological changes including amyloid-beta deposition, tauopathy, gliosis, and alterations in neuronal excitability, synaptic inputs, and functional plasticity. Notably, these changes are more pronounced in sensory association areas and superficial cortical layers, which may explain the relative preservation of basic sensory functions but early display of deficits of higher sensory functions. We propose that sensory impairment and the progression of AD may establish a cyclical relationship that mutually perpetuates each condition. This review highlights the significance of sensory deficits with or without cortical pathological changes in AD and emphasizes the need for further research to develop reliable early detection and intervention through sensory systems.

Folate-Dependent Cognitive Impairment Associated With Specific Gene Networks in the Adult Mouse Hippocampus

Short-term folate deficiency has been linked to cognitive defects. Given folate's role in regulating nucleotide synthesis and DNA and histone methylation, these changes are often linked to altered gene expression and might be controlled by specific regulatory networks. In our study we examined the effects of folic acid (FA) deficient or replete diets in mice, containing either no source of folate or normal FA intake, beginning post-weaning and persisting through the end of adult life at 18 months. Our goal was to assess levels of cognition in these mice using the novel object test and then connect the cognitive results to genetic changes. FA deficient mice showed significant memory impairment compared to control counterparts beginning at 5 months and persisting through 17 months, as determined by the novel object test. These deficits were associated with 363 significantly downregulated and 101 significantly upregulated genes in the deficient condition compared to the control condition in microarray analysis of hippocampal tissue. Many of these gene expression changes were determined to be specific to the hippocampus. Significant ontological categories for differential genes included nucleotide regulation, ion channel activity, and MAPK signaling; while some of these categories contain genes previously mapped to cognitive decline, other genes have not previously been associated with cognition. To determine proteins possibly involved in regulation of these genes, we performed bioinformatics analysis and found enriched motifs of for MafB and Zfp410 binding sites. These genes and enriched motifs may represent targets for treatment or investigation of memory-related diseases.

Review of Gait, Cognition, and Fall Risks with Implications for Fall Prevention in Older Adults with Dementia

BACKGROUND

Older people with cognitive impairment are at increased risk of falls; however, fall prevention strategies have limited success in this population. The aim of this paper is to review the literature to inform a theoretical framework for fall prevention in older adults with dementia.

SUMMARY

A narrative review was conducted on fall risk factors in people with cognitive impairment, the relationship between cognition and gait, and their joint impact on the risk of falls. This was used to develop a theoretical framework for fall prevention for people with dementia. Executive function and motor function are closely related as they share neuroanatomy. This close relationship has been confirmed by observational studies including neuroimaging and intervention studies. Executive function is the cognitive domain most commonly associated with gait dysfunction. Attention, sensory integration, and motor planning are the sub-domains of executive function associated with risk of falls through gait dysfunction, whereas cognitive flexibility, judgement, and inhibitory control affect risk of falls through risk-taking behaviour. Key Messages: Gait, cognition, and falls are closely related. The comorbidity and interaction between gait abnormality and cognitive impairment may underpin the high prevalence of falls in older adults with dementia. Gait assessment and cognitive assessment, particularly executive function, should be integrated in fall risk screening. Assessment results should be interpreted and utilised using a multidisciplinary approach; specific strategies such as customised gait training and behavioural modulation should be considered as part of falls prevention for people with dementia.

What underlies visual selective attention development? Evidence that age-related improvements in visual feature integration influence visual selective attention performance

Visual selective attention (VSA) improves across childhood. Conjunction search tasks require integrating multiple visual features in order to find a target among distractors and are often used to measure VSA. Motivated by the visual system's architecture and developmental changes in neural connectivity, we predicted that feature integration across separate visual pathways (e.g., color and motion) should develop later than feature integration within the same visual pathways (e.g., luminance and motion). A total of 89 4- to 10-year-old children completed a visual search task that manipulated whether feature integration was between separate parallel visual pathways or within the same visual pathway. We first examined whether color-motion integration was associated with a performance cost relative to luminance-motion integration across childhood. We found that color-motion integration was worse than luminance-motion integration in early childhood but that this difference decreased with age. We also examined whether luminance-motion and color-motion visual search performance developed differently across childhood. Reaction time (RT) visual search slopes for the luminance-motion condition were both stable across childhood and steeper overall than those for the color-motion condition. In contrast, RT search slopes for the color-motion condition became steeperincrease across childhood. Finally, we found that age-related improvements in color-motion integration, relative to luminance-motion integration, were associated with longer color-motion search rates across childhood. These data suggest that age-related improvements in color-motion feature integration may increase competition between color-motion targets and distractors, thereby increasing the amount of time needed to process distractors as nontargets during the selection process.

Fear of falling: A manifestation of executive dysfunction?

OBJECTIVE

Fear of falling (FoF) may be an early marker of decline in global cognitive functioning, but associations with specific domains of cognitive functioning are unclear. The aim was to examine associations between FoF and 4-year decline in memory, processing speed, and executive functioning in adults aged 50 years and older.

METHODS

Data were from 5174 participants (mean age = 62.6 ± 8.9 years, range = 50-91, 54.5% female) in The Irish Longitudinal Study on Ageing, a population-based study.

MEASUREMENTS

FoF was self-reported in 2009 to 2011. Immediate and delayed recall, Colour Trails 1 and 2, choice reaction time, sustained attention to response task, and verbal fluency were measured in 2009 to 2011 and 2014 to 2015. Prospective associations between FoF and domains of cognitive functioning were examined using linear mixed modelling. Adjustment was made for demographic and health factors. Interactions with age were examined.

RESULTS

In 2009 to 2011, 20.6% of participants reported FoF. No statistically significant interaction of FoF with age was found for any of the associations (P ≥ .06). Participants with FoF had greater decline on delayed recall (B = -0.19; 95% CI, -0.32 to -0.06), verbal fluency (B = -0.52; 95% CI, -0.88 to -0.18); and the ln-transformed scores for the Colour Trails 1 test (B = -0.04; 95% CI, -0.07 to -0.01) and the Colour Trails 2 test (B = -0.04; 95% CI, -0.06 to -0.02) than participants without FoF. No statistically significant associations were found for any of the other outcomes.

CONCLUSIONS

FoF may be an indicator of decline in domains of cognitive functioning, particularly those related to executive function and processing speed. However, studies with longer follow-up and/or higher average age are required to confirm this.

Motion-induced position mis-localization predicts the severity of Alzheimer's disease

Patients with Alzheimer's disease (AD) often exhibit motion processing deficits. It is unclear whether the localization of moving objects - a perceptual process tightly linked to motion - is impaired or intact in AD. In this study, we used the phenomenon of illusory shift of position induced by motion as a behavioural paradigm to probe how the spatial representation differs between AD patients and healthy elderly controls. We measured the magnitudes of motion-induced position shift in a group of AD participants (N = 24) and age-matched elderly observers (N = 24). We found that AD patients showed weakened position mis-localization, but only for motion stimuli of slow speeds. For fast motion, the position mis-localization did not differ significantly between groups. Furthermore, we showed that the magnitudes of position mis-localization can predict the severity of AD; that is, patients with more severe symptoms had less preserved position mis-localization. Our results suggest that AD pathology impacts not only motion processing per se, but also the perceptual process related to motion such as the localization of moving objects.

Motion-induced position shift in early Alzheimer's disease

The localization of object position in space is one of the most important visual abilities in humans. Motion-induced position shift is a perceptual illusion in which the position of a moving object is perceived to be shifted in the direction of motion. In this study, we wanted to explore whether and how Alzheimer’s disease (AD) affects this illusion. We recruited a group of patients with early AD and a group of age-matched healthy controls. In our experiments, two drifting Gabor patches moving in opposite directions were presented and participants were asked to report whether the upper Gabor appeared rightwards or leftwards of the lower one. We measured the psychometric functions, of which the point of subjective alignment was taken as the magnitude of motion-induced position shift. We compared the position shift across the two groups at three different retinal eccentricities. We found that position shifts were systematically smaller in the AD group as comparing to the elderly control group. Our data demonstrated that AD patients were less prone to motion-induced position shift. The results add to the existing knowledge of perceptual deficits in AD patients. We suggest that motion induced position shift may be effective as a new behavioral indicator for AD identification.

Differential Contributions of Selective Attention and Sensory Integration to Driving Performance in Healthy Aging and Alzheimer's Disease

OBJECTIVES

Patients with Alzheimer's disease (AD) demonstrate deficits in cross-cortical feature binding distinct from age-related changes in selective attention. This may have consequences for driving performance given its demands on multisensory integration. We examined the relationship of visuospatial search and binding to driving in patients with early AD and elderly controls (EC).

METHODS

Participants (42 AD; 37 EC) completed search tasks requiring either luminance-motion (L-M) or color-motion (C-M) binding, analogs of within and across visual processing stream binding, respectively. Standardized road test (RIRT) and naturalistic driving data (CDAS) were collected alongside clinical screening measures.

RESULTS

Patients performed worse than controls on most cognitive and driving indices. Visual search and clinical measures were differentially related to driving behavior across groups. L-M search and Trail Making Test (TMT-B) were associated with RIRT performance in controls, while C-M binding, TMT-B errors, and Clock Drawing correlated with CDAS performance in patients. After controlling for demographic and clinical predictors, L-M reaction time significantly predicted RIRT performance in controls. In patients, C-M binding made significant contributions to CDAS above and beyond demographic and clinical predictors. RIRT and C-M binding measures accounted for 51% of variance in CDAS performance in patients.

CONCLUSIONS

Whereas selective attention is associated with driving behavior in EC, cross-cortical binding appears most sensitive to driving in AD. This latter relationship may emerge only in naturalistic settings, which better reflect patients' driving behavior. Visual integration may offer distinct insights into driving behavior, and thus has important implications for assessing driving competency in early AD. (JINS, 2018, 24, 486-497).

Faulty Adaptation to Repeated Face-Name Associative Pairs in Mild Cognitive Impairment is Predictive of Cognitive Decline

OBJECTIVE

We examined BOLD (Blood-Oxygen-Level Dependent) activity reduction upon stimuli repetition of face-name pairs in older adults with amnestic (aMCI) and non-amnestic (naMCI) mild cognitive impairment diagnosed using a comprehensive actuarial method, and relationships between activity reduction and behavioral indices.

METHOD

Twenty-nine cognitively healthy older adults (CHs) and 20 with MCI (n = 12 aMCI; n = 8 naMCI) underwent functional MRI event-related imaging, a comprehensive neuropsychological battery, and 1-year follow-up exam. During scanning, participants were shown face-name pairs 1-3 times and administered a post-scan recognition task.

RESULTS

The MCI group demonstrated less activity reduction upon repetition of face-name pairs within the MTL and other regions compared to CHs. Less activity reduction was associated with poorer Time 1 neuropsychological performance for the CH group and poorer post-scan recognition performance for the MCI group. Less activity reduction was related to poorer neuropsychological performance at Time 2 in the MCI group. Within MCIs, those with aMCI demonstrated less activity reduction upon repetition of face-name pairs than those with naMCI.

CONCLUSIONS

Distinct patterns of brain activity were identified in the MCI group compared to CHs, and aMCI compared to naMCI. Activated regions were not restricted to traditional memory circuitry, implicating a wider network of regions involved in the encoding of associative tasks. Findings add support to the hypothesis that lack of reduced BOLD activity reflects "faulty adaptation" to repeated stimuli and that reduction in activity represents successful encoding processes. They also provide further support for use of the face-name paradigm as a marker of prodromal Alzheimer's disease, and method to distinguish between MCI subtypes.

Impaired Center-Surround Suppression in Patients with Alzheimer's Disease

Alzheimer's disease (AD) is often associated with declined visual processing abilities. Here we tested whether the functions of center-surround suppression- a hallmark property in the visual system- are altered by AD. To this end, we recruited three groups of participants (AD, elderly, and young) in a motion direction discrimination task, in which we measured the temporal duration threshold of a  drifting Gabor with varying stimulus sizes. We first replicated the phenomena of center-surround suppression that the required duration for discriminating a high contrast grating decreases with increasing stimulus size. We then showed that the magnitudes of suppression varied among the three groups. There was progressive reduction of suppression in the elderly and AD groups compared with the young group. Interestingly, we found that the levels of suppression can predict the severity of dementia in the AD group. Our results suggest that AD is associated with impaired center-surround functions in the visual motion processing pathway.

Long-range cortical dynamics: a perspective from the mouse sensorimotor whisker system

In the mammalian neocortex, the capacity to dynamically route and coordinate the exchange of information between areas is a critical feature of cognitive function, enabling processes such as higher-level sensory processing and sensorimotor integration. Despite the importance attributed to long-range connections between cortical areas, their exact operations and role in cortical function remain an open question. In recent years, progress has been made in understanding long-range cortical circuits through work focused on the mouse sensorimotor whisker system. In this review, we examine recent studies dissecting long-range circuits involved in whisker sensorimotor processing as an entry point for understanding the rules that govern long-range cortical circuit function.

A SEMantic and EPisodic Memory Test (SEMEP) Developed within the Embodied Cognition Framework: Application to Normal Aging, Alzheimer's Disease and Semantic Dementia

Embodiment has highlighted the importance of sensory-motor components in cognition. Perception and memory are thus very tightly bound together, and episodic and semantic memories should rely on the same grounded memory traces. Reduced perception should then directly reduce the ability to encode and retrieve an episodic memory, as in normal aging. Multimodal integration deficits, as in Alzheimer's disease, should lead to more severe episodic memory impairment. The present study introduces a new memory test developed to take into account these assumptions. The SEMEP (SEMantic-Episodic) memory test proposes to assess conjointly semantic and episodic knowledge across multiple tasks: semantic matching, naming, free recall, and recognition. The performance of young adults is compared to healthy elderly adults (HE), patients with Alzheimer's disease (AD), and patients with semantic dementia (SD). The results show specific patterns of performance between the groups. HE commit memory errors only for presented but not to be remembered items. AD patients present the worst episodic memory performance associated with intrusion errors (recall or recognition of items never presented). They were the only group to not benefit from a visual isolation (addition of a yellow background), a method known to increase the distinctiveness of the memory traces. Finally, SD patients suffer from the most severe semantic impairment. To conclude, confusion errors are common across all the elderly groups, whereas AD was the only group to exhibit regular intrusion errors and SD patients to show severe semantic impairment.

A Caenorhabditis elegans Nutritional-status Based Copper Aversion Assay

To ensure survival, organisms must be capable of avoiding unfavorable habitats while ensuring a consistent food source. Caenorhabditis elegans alter their locomotory patterns upon detection of diverse environmental stimuli and can modulate their suite of behavioral responses in response to starvation conditions. Nematodes typically exhibit a decreased aversive response when removed from a food source for over 30 min. Observation of behavioral changes in response to a changing nutritional status can provide insight into the mechanisms that regulate the transition from a well-fed to starved state. We have developed an assay that measures a nematode's ability to cross an aversive barrier (i.e. copper) then reach a food source over a prolonged period of time. This protocol builds upon previous work by integrating multiple variables in a manner that allows for continued data collection as the organisms shift towards an increasingly starved condition. Moreover, this assay permits an increased sample size so that larger populations of nematodes can be simultaneously evaluated. Organisms defective for the ability to detect or respond to copper immediately cross the chemical barrier, while wild type nematodes are initially repelled. As wild type worms are increasingly starved, they begin to cross the barrier and reach the food source. We designed this assay to evaluate a mutant that is incapable of responding to diverse environmental cues, including food sensation or detection of aversive chemicals. When evaluated via this protocol, the defective organisms immediately crossed the barrier, but were also incapable of detecting a food source. Hence, these mutants repeatedly cross the chemical barrier despite temporarily reaching a food source. This assay can straightforwardly test populations of worms to evaluate potential pathway defects related to aversion and starvation.

Disrupted pursuit compensation during self-motion perception in early Alzheimer's disease

Our perception of the world is remarkably stable despite of distorted retinal input due to frequent eye movements. It is considered that the brain uses corollary discharge, efference copies of signals sent from motor to visual regions, to compensate for distortions and stabilize visual perception. In this study, we tested whether patients with Alzheimer’s disease (AD) have impaired corollary discharge functions as evidenced by reduced compensation during the perception of optic flow that mimics self-motion in the environment. We asked a group of early-stage AD patients and age-matched healthy controls to indicate the perceived direction of self-motion based on optic flow while tracking a moving target with smooth pursuit eye movement, or keeping eye fixation at a stationary target. We first replicated the previous findings that healthy participants were able to compensate for distorted optic flow in the presence of eye movements, as indicated by similar performance of self-motion perception between pursuit and fixation conditions. In stark contrast, AD patients showed impaired self-motion perception when the optic flow was distorted by eye movements. Our results suggest that early-stage AD pathology is associated with disrupted eye movement compensation during self-motion perception.

Altered motion repulsion in Alzheimer's disease

Recent research in Alzheimer’s disease (AD) indicates that perceptual impairments may occur before the onset of cognitive declines, and can thus serve as an early noninvasive indicator for AD. In this study, we focused on visual motion processing and explored whether AD induces changes in the properties of direction repulsion between two competing motions. We used random dot kinematograms (RDKs) and measured the magnitudes of direction repulsion between two overlapping RDKs moving different directions in three groups of participants: an AD group, an age-matched old control group, and a young control group. We showed that motion direction repulsion was significantly weaker in AD patients as comparing to both healthy controls. More importantly, we found that the magnitude of motion repulsion was predictive of the assessment of clinical severity in the AD group. Our results implicate that AD pathology is associated with altered neural functions in visual cortical areas and that motion repulsion deficit is a behavioral biomarker for the tracking of AD development.

Visual Perceptual Organization Ability in Autopsy-Verified Dementia with Lewy Bodies and Alzheimer's Disease

OBJECTIVES

Prominent impairment of visuospatial processing is a feature of dementia with Lewy bodies (DLB), and diagnosis of this impairment may help clinically distinguish DLB from Alzheimer's disease (AD). The current study compared autopsy-confirmed DLB and AD patients on the Hooper Visual Organization Test (VOT), a test that requires perceptual and mental reorganization of parts of an object into an identifiable whole. The VOT may be particularly sensitive to DLB since it involves integration of visual information processed in separate dorsal and ventral visual "streams".

METHODS

Demographically similar DLB (n=28), AD (n=115), and normal control (NC; n=85) participants were compared on the VOT and additional neuropsychological tests. Patient groups did not differ in dementia severity at time of VOT testing. High and Low AD-Braak stage DLB subgroups were compared to examine the influence of concomitant AD pathology on VOT performance.

RESULTS

Both patient groups were impaired compared to NC participants. VOT scores of DLB patients were significantly lower than those of AD patients. The diagnostic sensitivity and specificity of the VOT for patients versus controls was good, but marginal for DLB versus AD. High-Braak and low-Braak DLB patients did not differ on the VOT, but High-Braak DLB performed worse than Low-Braak DLB on tests of episodic memory and language.

CONCLUSIONS

Visual perceptual organization ability is more impaired in DLB than AD but not strongly diagnostic. The disproportionate severity of this visual perceptual deficit in DLB is not related to degree of concomitant AD pathology, which suggests that it might primarily reflect Lewy body pathology. (JINS, 2016, 22, 609-619).

Motion discrimination in dementia with Lewy bodies and Alzheimer disease

OBJECTIVE

Visual processing abilities of patients with dementia with Lewy bodies (DLB) or Alzheimer disease (AD) dementia were assessed psychophysically using a simple horizontal motion discrimination task that engages the dorsal visual processing stream.

METHODS

Participants included patients with mild dementia with DLB, AD dementia or Parkinson disease (PD) with dementia (PDD), without dementia with PD, and normal controls. Participants indicated the left or right direction of coherently moving dots that were embedded within dynamic visual noise provided by randomly moving dots. The proportion of coherently moving dots was increased or decreased across trials to determine a threshold at which participants could correctly indicate their direction with greater than 80% accuracy.

RESULTS

Motion discrimination thresholds of patients with DLB and PDD were comparable and significantly higher (i.e., worse) than those of patients with AD dementia. The thresholds of patients with AD dementia and patients with PD were normal. These results were confirmed in subgroups of patients with DLB/PDD and AD dementia with autopsy-confirmed disease. A motion discrimination threshold greater than 0.23 distinguished between DLB/PDD and AD dementia with 67% sensitivity and 85% specificity.

CONCLUSIONS

Differential deficits in detecting direction of simple horizontal motion suggest that dorsal processing stream dysfunction is greater in DLB and PDD than in AD dementia. Therefore, impaired performance on simple visual motion discrimination tasks that specifically engage occipitoparietal brain regions suggests the presence of Lewy body pathology.

Visual search in Dementia with Lewy Bodies and Alzheimer's disease

Visual search is an aspect of visual cognition that may be more impaired in Dementia with Lewy Bodies (DLB) than Alzheimer's disease (AD). To assess this possibility, the present study compared patients with DLB (n = 17), AD (n = 30), or Parkinson's disease with dementia (PDD; n = 10) to non-demented patients with PD (n = 18) and normal control (NC) participants (n = 13) on single-feature and feature-conjunction visual search tasks. In the single-feature task participants had to determine if a target stimulus (i.e., a black dot) was present among 3, 6, or 12 distractor stimuli (i.e., white dots) that differed in one salient feature. In the feature-conjunction task participants had to determine if a target stimulus (i.e., a black circle) was present among 3, 6, or 12 distractor stimuli (i.e., white dots and black squares) that shared either of the target's salient features. Results showed that target detection time in the single-feature task was not influenced by the number of distractors (i.e., "pop-out" effect) for any of the groups. In contrast, target detection time increased as the number of distractors increased in the feature-conjunction task for all groups, but more so for patients with AD or DLB than for any of the other groups. These results suggest that the single-feature search "pop-out" effect is preserved in DLB and AD patients, whereas ability to perform the feature-conjunction search is impaired. This pattern of preserved single-feature search with impaired feature-conjunction search is consistent with a deficit in feature binding that may be mediated by abnormalities in networks involving the dorsal occipito-parietal cortex.

Embodied cognition of aging

Embodiment is revolutionizing the way we consider cognition by incorporating the influence of our body and of the current context within cognitive processing. A growing number of studies which support this view of cognition in young adults stands in stark contrast with the lack of evidence in favor of this view in the field of normal aging and neurocognitive disorders. Nonetheless, the validation of embodiment assumptions on the whole spectrum of cognition is a mandatory step in order for embodied cognition theories to become theories of human cognition. More pragmatically, aging populations represent a perfect target to test embodied cognition theories due to concomitant changes in sensory, motor and cognitive functioning that occur in aging, since these theories predict direct interactions between them. Finally, the new perspectives on cognition provided by these theories might also open new research avenues and new clinical applications in the field of aging. The present article aims at showing the value and interest to explore embodiment in normal and abnormal aging as well as introducing some potential theoretical and clinical applications.

Repetition priming and cortical arousal in healthy aging and Alzheimer's disease

Repetition priming refers to a form of implicit memory in which prior exposure to a stimulus facilitates the subsequent processing of the same or a related stimulus. One frequently used repetition priming task is word-stem completion priming. In this task, participants complete a series of beginning word stems with the first word that comes to mind after having viewed, in an unrelated context, words that can complete some of the stems. Patients with Alzheimer's disease (AD) exhibit a significant deficit in word-stem completion priming, but the neural mechanisms underlying this deficit have yet to be identified. The present study examined the possibility that the word-stem completion priming deficit in AD is due to disruption of ascending neuromodulatory systems that mediate cortical arousal by comparing word-stem completion priming and behavioral measures of spatial orienting and phasic alerting. Results showed that in healthy elderly controls higher levels of phasic alerting were associated with a sharpening of the temporal dynamics of priming across two delay intervals: those with higher levels of alerting showed more immediate priming but less delayed priming than those with lesser levels of alerting. In patients with AD, priming was impaired despite intact levels of phasic alerting and spatial orienting, and group status rather than individual levels of alerting or orienting predicted the magnitude of their stem-completion priming. Furthermore, the change in priming across delays they displayed was not related to level of alerting or orienting. These findings support the role of the noradrenergic projection system in modulating the level of steady-state cortical activation (or "cortical tonus") underlying both phasic alerting and the temporal dynamics of repetition priming. However, impaired priming in patients with AD does not appear to be due to disruption of this neuromodulatory system.

Visuo-cognitive skill deficits in Alzheimer's disease and Lewy body disease: A comparative analysis

Dementia is a chronic neurodegenerative disorder characterized by progressive cognitive loss. Alzheimer's disease (AD) and the Lewy body disease are the two most common causes of age-related degenerative dementia. Visuo-cognitive skills are a combination of very different cognitive functions being performed by the visual system. These skills are impaired in both AD and dementia with Lewy bodies (DLB). The aim of this review is to evaluate various studies for these visuo-cognitive skills. An exhaustive internet search of all relevant medical databases was carried out using a series of key-word applications, including The Cochrane Library, MEDLINE, PSYCHINFO, EMBASE, CINAHL, AMED, SportDiscus, Science Citation Index, Index to Theses, ZETOC, PEDro and occupational therapy (OT) seeker and OT search. We reviewed all the articles until March 2013 with key words of: Visual skills visual cognition dementia AD, but the direct neurobiological etiology is difficult to establish., Dementia of Lewy body disease. Although most studies have used different tests for studying these abilities, in general, these tests evaluated the individual's ability of (1) visual recognition, (2) visual discrimination, (3) visual attention and (4) visuo-perceptive integration. Performance on various tests has been evaluated for assessing these skills. Most studies assessing such skills show that these skills are impaired in DLB as compared with AD. Visuo-cognitive skills are impaired more in DLB as compared with AD. These impairments have evident neuropathological correlations, but the direct neurobiological etiology is difficult to establish.

Prototype learning and dissociable categorization systems in Alzheimer's disease

Recent neuroimaging studies suggest that prototype learning may be mediated by at least two dissociable memory systems depending on the mode of acquisition, with A/Not-A prototype learning dependent upon a perceptual representation system located within posterior visual cortex and A/B prototype learning dependent upon a declarative memory system associated with medial temporal and frontal regions. The degree to which patients with Alzheimer's disease (AD) can acquire new categorical information may therefore critically depend upon the mode of acquisition. The present study examined A/Not-A and A/B prototype learning in AD patients using procedures that allowed direct comparison of learning across tasks. Despite impaired explicit recall of category features in all tasks, patients showed differential patterns of category acquisition across tasks. First, AD patients demonstrated impaired prototype induction along with intact exemplar classification under incidental A/Not-A conditions, suggesting that the loss of functional connectivity within visual cortical areas disrupted the integration processes supporting prototype induction within the perceptual representation system. Second, AD patients demonstrated intact prototype induction but impaired exemplar classification during A/B learning under observational conditions, suggesting that this form of prototype learning is dependent on a declarative memory system that is disrupted in AD. Third, the surprisingly intact classification of both prototypes and exemplars during A/B learning under trial-and-error feedback conditions suggests that AD patients shifted control from their deficient declarative memory system to a feedback-dependent procedural memory system when training conditions allowed. Taken together, these findings serve to not only increase our understanding of category learning in AD, but to also provide new insights into the ways in which different memory systems interact to support the acquisition of categorical knowledge.

The profile of executive functioning in amnestic mild cognitive impairment: disproportionate deficits in inhibitory control

Amnestic mild cognitive impairment (aMCI) represents a group of individuals who are highly likely to develop Alzheimer's disease (AD). Although aMCI is typically conceptualized as involving predominantly deficits in episodic memory, recent studies have demonstrated that deficits in executive functioning may also be present, and thorough categorization of cognitive functioning in MCI may improve early diagnosis and treatment of AD. We first provide an extensive review of neuropsychology studies that examined executive functioning in MCI. We then present data on executive functioning across multiple sub-domains (divided attention, working memory, inhibitory control, verbal fluency, and planning) in 40 aMCI patients (single or multiple domain) and 32 normal elderly controls (NECs). MCI patients performed significantly worse than NECs in all 5 sub-domains, and there was impairment (>1.0 SD below the mean of NECs) in all sub-domains. Impairment on each test was frequent, with 100% of MCI patients exhibiting a deficit in at least one sub-domain of executive functioning. Inhibitory control was the most frequently and severely impaired. These results indicate that executive dysfunction in multiple sub-domains is common in aMCI and highlights the importance of a comprehensive neuropsychological evaluation for fully characterizing the nature and extent of cognitive deficits in MCI.

The neuropsychological profile of Alzheimer disease

Neuropsychological assessment has featured prominently over the past 30 years in the characterization of dementia associated with Alzheimer disease (AD). Clinical neuropsychological methods have identified the earliest, most definitive cognitive and behavioral symptoms of illness, contributing to the identification, staging, and tracking of disease. With increasing public awareness of dementia, disease detection has moved to earlier stages of illness, at a time when deficits are both behaviorally and pathologically selective. For reasons that are not well understood, early AD pathology frequently targets large-scale neuroanatomical networks for episodic memory before other networks that subserve language, attention, executive functions, and visuospatial abilities. This chapter reviews the pathognomonic neuropsychological features of AD dementia and how these differ from "normal," age-related cognitive decline and from other neurodegenerative diseases that cause dementia, including cortical Lewy body disease, frontotemporal lobar degeneration, and cerebrovascular disease.

Visual spatial cognition in neurodegenerative disease

Visual spatial impairment is often an early symptom of neurodegenerative disease; however, this multi-faceted domain of cognition is not well-assessed by most typical dementia evaluations. Neurodegenerative diseases cause circumscribed atrophy in distinct neural networks, and accordingly, they impact visual spatial cognition in different and characteristic ways. Anatomically-focused visual spatial assessment can assist the clinician in making an early and accurate diagnosis. This article will review the literature on visual spatial cognition in neurodegenerative disease clinical syndromes, and where research is available, by neuropathologic diagnoses. Visual spatial cognition will be organized primarily according to the following schemes: bottom-up/top-down processing, dorsal/ventral stream processing, and egocentric/allocentric frames of reference.

Neuropsychological assessment of dementia

Neuropsychological studies show that cognitive deficits associated with Alzheimer's disease (AD) are distinct from age-associated cognitive decline. Quantitative and qualitative differences are apparent across many cognitive domains, but are especially obvious in episodic memory (particularly delayed recall), semantic knowledge, and some aspects of executive functions. The qualitatively distinct pattern of deficits is less salient in very old AD patients than in younger AD patients. Although decline in episodic memory is usually the earliest cognitive change that occurs prior to the development of the AD dementia syndrome, asymmetry in cognitive abilities may also occur in this "preclinical" phase of the disease and predict imminent dementia. Discrete patterns of cognitive deficits occur in AD and several neuropathologically distinct age-associated neurodegenerative disorders. Knowledge of these differences helps to clinically distinguish among various causes of dementia and provides useful models for understanding brain-behavior relationships that mediate cognitive abilities affected in various neurodegenerative diseases.

The use of chromatic information for motion segmentation: differences between psychophysical and eye-movement measures

Previous psychophysical studies have shown that chromatic (red/green) information can be used as a segmentation cue for motion integration. We investigated the mechanisms mediating this phenomenon by comparing chromatic effects (and, for comparison, luminance effects) on motion integration between two measures: (i) directional eye movements with the notion that these responses are mediated mainly by low-level motion mechanisms, and (ii) psychophysical reports, with the notion that subjects' reports should employ higher-level (attention-based) mechanisms if available. To quantify chromatic (and luminance) effects on motion integration, coherent motion thresholds were obtained for two conditions, one in which the signal and noise dots were the same 'red' or 'green' chromaticity (or the same 'bright' or 'dark' luminance), referred to as homogeneous, and the other in which the signal and noise dots were of different chromaticities (or luminances), referred to as heterogeneous. 'Benefit ratios' (theta(HOM)/theta(HET)) were then computed, with values significantly greater than 1.0 indicating that chromatic (or luminance) information serves as a segmentation cue for motion integration. The results revealed a high and significant chromatic benefit ratio when the measure was based on psychophysical report, but not when it was based on an eye-movement measure. By contrast, luminance benefit ratios were roughly the same (and significant) for both measures. For comparison to adults, eye-movement data were also obtained from 3-month-old infants. Infants showed marginally significant benefit ratios in the luminance, but not in the chromatic, condition. In total, these results suggest that the use of chromatic information as a segmentation cue for motion integration relies on higher-level mechanisms, whereas luminance information works mainly through low-level motion mechanisms.

Visual perception in prediagnostic and early stage Huntington's disease

Disturbances of visual perception frequently accompany neurodegenerative disorders but have been little studied in Huntington's disease (HD) gene carriers. We used psychophysical tests to assess visual perception among individuals in the prediagnostic and early stages of HD. The sample comprised four groups, which included 201 nongene carriers (NG), 32 prediagnostic gene carriers with minimal neurological abnormalities (PD1); 20 prediagnostic gene carriers with moderate neurological abnormalities (PD2), and 36 gene carriers with diagnosed HD. Contrast sensitivity for stationary and moving sinusoidal gratings, and tests of form and motion discrimination, were used to probe different visual pathways. Patients with HD showed impaired contrast sensitivity for moving gratings. For one of the three contrast sensitivity tests, the prediagnostic gene carriers with greater neurological abnormality (PD2) also had impaired performance as compared with NG. These findings suggest that early stage HD disrupts visual functions associated with the magnocellular pathway. However, these changes are only observed in individuals diagnosed with HD or who are in the more symptomatic stages of prediagnostic HD.

Abnormal-induced theta activity supports early directed-attention network deficits in progressive MCI

The electroencephalography (EEG) theta frequency band reacts to memory and selective attention paradigms. Global theta oscillatory activity includes a posterior phase-locked component related to stimulus processing and a frontal-induced component modulated by directed attention. To investigate the presence of early deficits in the directed attention-related network in elderly individuals with mild cognitive impairment (MCI), time-frequency analysis at baseline was used to assess global and induced theta oscillatory activity (4-6Hz) during n-back working memory tasks in 29 individuals with MCI and 24 elderly controls (EC). At 1-year follow-up, 13 MCI patients were still stable and 16 had progressed. Baseline task performance was similar in stable and progressive MCI cases. Induced theta activity at baseline was significantly reduced in progressive MCI as compared to EC and stable MCI in all n-back tasks, which were similar in terms of directed attention requirements. While performance is maintained, the decrease of induced theta activity suggests early deficits in the directed-attention network in progressive MCI, whereas this network is functionally preserved in stable MCI.

Use of functional magnetic resonance imaging in the early identification of Alzheimer's disease

A growing body of evidence suggests that a preclinical phase of Alzheimer's disease (AD) exists several years or more prior to the overt manifestation of clinical symptoms and is characterized by subtle neuropsychological and brain changes. Identification of individuals prior to the development of significant clinical symptoms is imperative in order to have the greatest treatment impact by maintaining cognitive abilities and preserving quality of life. Functional magnetic resonance imaging (fMRI) offers considerable promise as a non-invasive tool for detecting early functional brain changes in asymptomatic adults. In fact, evidence to date indicates that functional brain decline precedes structural decline in preclinical samples. Therefore, fMRI may offer the unique ability to capture the dynamic state of change in the degenerating brain. This review examines the clinical utility of blood oxygen level dependent (BOLD) fMRI in those at risk for AD as well as in early AD. We provide an overview of fMRI findings in at-risk groups by virtue of genetic susceptibility or mild cognitive decline followed by an appraisal of the methodological issues concerning the diagnostic usefulness of fMRI in early AD. We conclude with a discussion of future directions and propose that BOLD-fMRI in combination with cerebral blood flow or diffusion techniques will provide a more complete accounting of the neurovascular changes that occur in preclinical AD and thus improve our ability to reliably detect early brain changes prior to disease onset.

Is Alzheimer's disease a disconnection syndrome?

In Alzheimer's disease (AD), loss of connectivity in the patient's brain has been evidenced by a range of electrophysiological and neuroimaging studies. However, few neuropsychological research projects have sought to interpret the cognitive modifications following the appearance of AD in terms of a disconnection syndrome. In this paper, we sought to investigate brain connectivity in AD via the study of a crossmodal effect. More precisely, we examined the integration of auditory and visual speech information (the McGurk effect) in AD patients and matched control subjects. Our results revealed impaired crossmodal integration during speech perception in AD, which was not associated with disturbances in the separate processing of auditory and visual speech stimuli. In conclusion, our data suggest the occurrence of a specific, audio-visual integration deficit in AD, which might be the consequence of a connectivity breakdown and corroborate the observation from other studies of crossmodal deficits between the auditory and visual modalities in this population.