Visual short-term memory deficits associated with GBA mutation and Parkinson's disease

Cognition and Activity of Daily Living Function in people with Parkinson's disease

The ability to perform activities of daily living (ADL) function is a multifaceted construct that reflects functionality in different daily life situations. The loss of ADL function due to cognitive impairment is the core feature for the diagnosis of Parkinson's disease dementia (PDD). In contrast to Alzheimer's disease, ADL impairment in PD can be compromised by various factors, including motor and non-motor aspects. This narrative review summarizes the current state of knowledge on the association of cognition and ADL function in people with PD and introduces the concept of "cognitive ADL" impairment for those problems in everyday life that are associated with cognitive deterioration as their primary cause. Assessment of cognitive ADL impairment is challenging because self-ratings, informant-ratings, and performance-based assessments seldomly differentiate between "cognitive" and "motor" aspects of ADL. ADL function in PD is related to multiple cognitive domains, with attention, executive function, and memory being particularly relevant. Cognitive ADL impairment is characterized by behavioral anomalies such as trial-and-error behavior or task step omissions, and is associated with lower engagement in everyday behaviors, as suggested by physical activity levels and prolonged sedentary behavior. First evidence shows that physical and multi-domain interventions may improve ADL function, in general, but the evidence is confounded by motor aspects. Large multicenter randomized controlled trials with cognitive ADL function as primary outcome are needed to investigate which pharmacological and non-pharmacological interventions can effectively prevent or delay deterioration of cognitive ADL function, and ultimately the progression and conversion to PDD.

Forward and backward spatial recall in Parkinson's disease and matched controls: A 1-year follow-up study

Patients with Parkinson's disease (PD) exhibit a domain-general visuospatial dysfunction; however, no previous study has examined changes over time in forward and backward spatial recall in PD against controls. To evaluate changes in short-term (STM) and working memory (WM) dysfunction in PD, the current study assessed performance on a computer-modified version of the Corsi Block-Tapping Test (forward and backward recall) at two-time points 1 year apart, while simultaneously exploring associations with potentially relevant demographic and clinical variables. We enrolled 38 patients with PD and 38 controls matched for age, sex, and Montreal Cognitive Assessment (MoCA) total scores. Linear mixed-effects models analyzed the primary measured variables (forward and backward scores). At baseline, the dysfunction effect sizes were as follows: forward recall (-0.45, 95% CI [-0.90, 0.01]) and backward recall (-0.26, 95% CI [-0.71, 0.19]). At follow-up, patients exhibited substantially greater difficulties in backward recall (-0.65, 95% CI [-1.18, -0.13]) compared to the baseline assessment, whereas the forward dysfunction effect size remained almost the same (-0.43, 95% CI [-0.94, 0.09]). Age (p = .005, f = 0.35) and total scores on MoCA (p = .017, f = 0.18), irrespective of group and recall condition, were significant predictors of spatial block scores. The pattern of dysfunction effect sizes indicates that, in contrast to forward recall, backward recall dysfunction in PD worsened 1-year after the baseline assessment, presumably reflecting the progression of PD-related visuospatial WM dysfunction.

NIH Toolbox performance of persons with Parkinson's disease according to GBA1 and STN-DBS status

OBJECTIVE

Mutations in the glucocerebrosidase (GBA1) gene and subthalamic nucleus deep brain stimulation (STN-DBS) are independently associated with cognitive dysfunction in persons with Parkinson's disease (PwP). We hypothesized that PwP with both GBA1 mutations and STN-DBS are at greater risk of cognitive dysfunction than PwP with only GBA1 mutations or STN-DBS, or neither. In this study, we determined the pattern of cognitive dysfunction in PwP based on GBA1 mutation status and STN-DBS treatment.

METHODS

PwP who are GBA1 mutation carriers with or without DBS (GBA1+DBS+, GBA1+DBS-), and noncarriers with or without DBS (GBA1-DBS+, GBA1-DBS-) were included. Using the NIH Toolbox, cross-sectional differences in response inhibition, processing speed, and episodic memory were compared using analysis of variance with adjustment for relevant covariates.

RESULTS

Data were available for 9 GBA1+DBS+, 14 GBA1+DBS-, 17 GBA1-DBS+, and 26 GBA1-DBS- PwP. In this cross-sectional study, after adjusting for covariates, we found that performance on the Flanker test (measure of response inhibition) was lower in GBA1+DBS+ PwP compared with GBA1-DBS+ PwP (P = 0.030).

INTERPRETATION

PwP who carry GBA1 mutations and have STN-DBS have greater impaired response inhibition compared with PwP with STN-DBS but without GBA1 mutations. Longitudinal data, including preoperative scores, are required to definitively determine whether GBA1 mutation carriers respond differently to STN-DBS, particularly in the domain of response inhibition.

Demands on perceptual and mnemonic fidelity are a key determinant of age-related cognitive decline throughout the lifespan

Aging results in less detailed memories, reflecting reduced fidelity of remembered compared to real-world representations. We tested whether poorer representational fidelity across perception, short-term memory (STM), and long-term memory (LTM) are among the earliest signs of cognitive aging. Our paradigm probed target-lure object mnemonic discrimination and precision of object-location binding. Across the lifespan, cognitive deficits were observed in midlife when detailed stimulus representations were required for perceptual and short/long-term forced choice mnemonic discrimination. A continuous metric of object-location source memory combined with computational modeling demonstrated that errors in STM and LTM in middle-aged adults were largely driven by a loss of precision for retrieved memories, not necessarily by forgetting. On a trial-by-trial basis, fidelity of item and spatial information was more tightly bound in LTM compared to STM with this association being unaffected by age. Standard neuropsychological tests without demands on memory quality (digit span, verbal learning) were less sensitive to age effects than STM and LTM precision. Perceptual discrimination predicted mnemonic discrimination. Neuropsychological proxies for prefrontal executive functions correlated with STM, but not LTM fidelity. Conversely, neuropsychological indicators of hippocampal integrity correlated with mnemonic discrimination and precision of both STM and LTM, suggesting partially dissociable mechanisms of interindividual variability in STM and LTM fidelity. These findings suggest that reduced representational fidelity is a hallmark of cognitive aging across perception, STM, and LTM and can be observed from midlife onward. Continuous memory precision tasks may be promising for the early detection of subtle age-related cognitive decline. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Visual Dysfunction in Parkinson's Disease

Non-motor symptoms in Parkinson's disease (PD) include ocular, visuoperceptive, and visuospatial impairments, which can occur as a result of the underlying neurodegenerative process. Ocular impairments can affect various aspects of vision and eye movement. Thus, patients can show dry eyes, blepharospasm, reduced blink rate, saccadic eye movement abnormalities, smooth pursuit deficits, and impaired voluntary and reflexive eye movements. Furthermore, visuoperceptive impairments affect the ability to perceive and recognize visual stimuli accurately, including impaired contrast sensitivity and reduced visual acuity, color discrimination, and object recognition. Visuospatial impairments are also remarkable, including difficulties perceiving and interpreting spatial relationships between objects and difficulties judging distances or navigating through the environment. Moreover, PD patients can present visuospatial attention problems, with difficulties attending to visual stimuli in a spatially organized manner. Moreover, PD patients also show perceptual disturbances affecting their ability to interpret and determine meaning from visual stimuli. And, for instance, visual hallucinations are common in PD patients. Nevertheless, the neurobiological bases of visual-related disorders in PD are complex and not fully understood. This review intends to provide a comprehensive description of visual disturbances in PD, from sensory to perceptual alterations, addressing their neuroanatomical, functional, and neurochemical correlates. Structural changes, particularly in posterior cortical regions, are described, as well as functional alterations, both in cortical and subcortical regions, which are shown in relation to specific neuropsychological results. Similarly, although the involvement of different neurotransmitter systems is controversial, data about neurochemical alterations related to visual impairments are presented, especially dopaminergic, cholinergic, and serotoninergic systems.

Genetic Evidence for Endolysosomal Dysfunction in Parkinson’s Disease: A Critical Overview

Parkinson’s disease (PD) is the second most common neurodegenerative disorder in the aging population, and no disease-modifying therapy has been approved to date. The pathogenesis of PD has been related to many dysfunctional cellular mechanisms, however, most of its monogenic forms are caused by pathogenic variants in genes involved in endolysosomal function (LRRK2, VPS35, VPS13C, and ATP13A2) and synaptic vesicle trafficking (SNCA, RAB39B, SYNJ1, and DNAJC6). Moreover, an extensive search for PD risk variants revealed strong risk variants in several lysosomal genes (e.g., GBA1, SMPD1, TMEM175, and SCARB2) highlighting the key role of lysosomal dysfunction in PD pathogenesis. Furthermore, large genetic studies revealed that PD status is associated with the overall “lysosomal genetic burden”, namely the cumulative effect of strong and weak risk variants affecting lysosomal genes. In this context, understanding the complex mechanisms of impaired vesicular trafficking and dysfunctional endolysosomes in dopaminergic neurons of PD patients is a fundamental step to identifying precise therapeutic targets and developing effective drugs to modify the neurodegenerative process in PD.

Neuronopathic GBA1L444P Mutation Accelerates Glucosylsphingosine Levels and Formation of Hippocampal Alpha-Synuclein Inclusions

The most common genetic risk factor for Parkinson's disease (PD) is heterozygous mutations GBA1, which encodes for the lysosomal enzyme, glucocerebrosidase. Reduced glucocerebrosidase activity associates with an accumulation of abnormal α-synuclein (α-syn) called Lewy pathology, which characterizes PD. PD patients heterozygous for the neuronotypic GBA1L444P mutation (GBA1^+/L444P) have a 5.6-fold increased risk of cognitive impairments. In this study, we used GBA1^+/L444P mice of either sex to determine its effects on lipid metabolism, expression of synaptic proteins, behavior, and α-syn inclusion formation. At 3 months of age, GBA1^+/L444P mice demonstrated impaired contextual fear conditioning, and increased motor activity. Hippocampal levels of vGLUT1 were selectively reduced in GBA1^+/L444P mice. We show, using mass spectrometry, that GBA1L444P expression increased levels of glucosylsphingosine, but not glucosylceramide, in the brains and serum of GBA1^+/L444P mice. Templated induction of α-syn pathology in mice showed an increase in α-syn inclusion formation in the hippocampus of GBA1^+/L444P mice compared with GBA1^+/+ mice, but not in the cortex, or substantia nigra pars compacta. Pathologic α-syn reduced SNc dopamine neurons by 50% in both GBA1^+/+ and GBA1^+/L444P mice. Treatment with a GlcCer synthase inhibitor did not affect abundance of α-syn inclusions in the hippocampus or rescue dopamine neuron loss. Overall, these data suggest the importance of evaluating the contribution of elevated glucosylsphingosine to PD phenotypes. Further, our data suggest that expression of neuronotypic GBA1L444P may cause defects in the hippocampus, which may be a mechanism by which cognitive decline is more prevalent in individuals with GBA1-PD.SIGNIFICANCE STATEMENT Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are both pathologically characterized by abnormal α-synuclein (α-syn). Mutant GBA1 is a risk factor for both PD and DLB. Our data show the expression of neuronotypic GBA1L444P impairs behaviors related to hippocampal function, reduces expression of a hippocampal excitatory synaptic protein, and that the hippocampus is more susceptible to α-syn inclusion formation. Further, our data strengthen support for the importance of evaluating the contribution of glucosylsphingosine to PD phenotypes. These outcomes suggest potential mechanisms by which GBA1L444P contributes to the cognitive symptoms clinically observed in PD and DLB. Our findings also highlight the importance of glucosylsphingosine as a relevant biomarker for future therapeutics.

Mechanisms underlying corruption of working memory in Parkinson's disease

Working memory (WM) impairments are reported to occur in patients with Parkinson's disease (PD). However, the mechanisms are unclear. Here, we investigate several putative factors that might drive poor performance, by examining the precision of recall, the order in which items are recalled and whether memories are corrupted by random guessing (attentional lapses). We used two separate tasks that examined the quality of WM recall under different loads and retention periods, as well as a traditional digit span test. Firstly, on a simple measure of WM recall, where patients were asked to reproduce the orientation of a centrally presented arrow, overall recall was not significantly impaired. However, there was some evidence for increased guessing (attentional lapses). On a new analogue version of the Corsi-span task, where participants had to reproduce on a touchscreen the exact spatial pattern of presented stimuli in the order and locations in which they appeared, there was a reduction in the precision of spatial WM at higher loads. This deficit was due to misremembering item order. At the highest load, there was reduced recall precision, whereas increased guessing was only observed at intermediate set sizes. Finally, PD patients had impaired backward, but not forward, digit spans. Overall, these results reveal the task- and load-dependent nature of WM deficits in PD. On simple low-load tasks, attentional lapses predominate, whereas at higher loads, in the spatial domain, the corruption of mnemonic information-both order item and precision-emerge as the main driver of impairment.

Oculomotor rehearsal in visuospatial working memory

The neural and cognitive mechanisms of spatial working memory are tightly coupled with the systems that control eye movements, but the precise nature of this coupling is not well understood. It has been argued that the oculomotor system is selectively involved in rehearsal of spatial but not visual material in visuospatial working memory. However, few studies have directly compared the effect of saccadic interference on visual and spatial memory, and there is little consensus on how the underlying working memory representation is affected by saccadic interference. In this study we aimed to examine how working memory for visual and spatial features is affected by overt and covert attentional interference across two experiments. Participants were shown a memory array, then asked to either maintain fixation or to overtly or covertly shift attention in a detection task during the delay period. Using the continuous report task we directly examined the precision of visual and spatial working memory representations and fit psychophysical functions to investigate the sources of recall error associated with different types of interference. These data were interpreted in terms of embodied theories of attention and memory and provide new insights into the nature of the interactions between cognitive and motor systems.

Glucocerebrosidase mutations and Parkinson disease

The discovery of glucocerebrosidase (GBA1) mutations as the greatest numerical genetic risk factor for the development of Parkinson disease (PD) resulted in a paradigm shift within the research landscape. Efforts to elucidate the mechanisms behind GBA1-associated PD have highlighted shared pathways in idiopathic PD including the loss and gain-of-function hypotheses, endoplasmic reticulum stress, lipid metabolism, neuroinflammation, mitochondrial dysfunction and altered autophagy-lysosomal pathway responsible for degradation of aggregated and misfolded a-synuclein. GBA1-associated PD exhibits subtle differences in phenotype and disease progression compared to idiopathic counterparts notably an earlier age of onset, faster motor decline and greater frequency of non-motor symptoms (which also constitute a significant aspect of the prodromal phase of the disease). GBA1-targeted therapies have been developed and are being investigated in clinical trials. The most notable are Ambroxol, a small molecule chaperone, and Venglustat, a blood-brain-barrier-penetrant substrate reduction therapy agent. It is imperative that further studies clarify the aetiology of GBA1-associated PD, enabling the development of a greater abundance of targeted therapies in this new era of precision medicine.

Intact high-resolution working memory binding in a patient with developmental amnesia and selective hippocampal damage

Debate continues regarding the possible role of the hippocampus across short-term and working memory tasks. The current study examined the possibility of a hippocampal contribution to precise, high-resolution cognition and conjunctive memory. We administered visual working memory tasks featuring a continuous response component to a well-established developmental amnesic patient with relatively selective bilateral hippocampal damage (Jon) and healthy controls. The patient was able to produce highly accurate response judgments regarding conjunctions of color and orientation or color and location, using simultaneous or sequential presentation of stimuli, with no evidence of any impairment in working memory binding, categorical accuracy, or continuous precision. These findings indicate that hippocampal damage does not necessarily lead to deficits in high-resolution cognitive performance, even when the damage is severe and bilateral.

Analyses of n-back high-load working memory in early Parkinson's disease: An fMRI study

BACKGROUND

Cognitive impairment is common in Parkinson's disease (PD). Working memory is a group of basic processes that is part of many cognitive operations (from complex decision-making to selective attention). To date, there is not sufficient research on working memory impairment in early PD patients, especially patients under high load.

AIM

The aim of this study was to observe the activation of brain regions in early PD patients during verbal high-load cognitive tasks and to explore the correlation between cognitive behavior indexes and gray matter areas of brain structure.

METHODS

A total of 45 participants were included: 15 healthy controls and 30 PD patients without dementia. They underwent BOLD imaging during a digital N-back working memory task in a 3 T scanner, and their high-resolution T1-weighted three-dimensional brain anatomical images were collected. The SPM12 standard method was used to generate a comparison map with 2 plus 3-back greater than 0-back, and with age, sex, levodopa equivalent daily dose (LEDD) and education as covariables, and multiple regression analysis was conducted to analyze the correlation between MOCA, MES and brain gray matter volume.

RESULTS

Compared with the control group, the frontoparietal region was significantly activated in patients with PD under the background of high working memory burden. In addition, there was a widespread area correlation between the brain structure and basic cognitive function.

CONCLUSIONS

The frontoparietal cortex is the core hub of working memory, and there may be a compensation mechanism in PD patients without dementia with a high-load of working memory.

Working Memory Swap Errors Have Identifiable Neural Representations

Working memory is an essential component of cognition that facilitates goal-directed behavior. Famously, it is severely limited and performance suffers when memory load exceeds an individual's capacity. Modeling of visual working memory responses has identified two likely types of errors: guesses and swaps. Swap errors may arise from a misbinding between the features of different items. Alternatively, these errors could arise from memory noise in the feature dimension used for cueing a to-be-tested memory item, resulting in the wrong item being selected. Finally, it is possible that so-called swap errors actually reflect informed guessing, which could occur at the time of a cue, or alternatively, at the time of the response. Here, we combined behavioral response modeling and fMRI pattern analysis to test the hypothesis that swap errors involve the active maintenance of an incorrect memory item. After the encoding of six spatial locations, a retro-cue indicated which location would be tested after memory retention. On accurate trials, we could reconstruct a memory representation of the cued location in both early visual cortex and intraparietal sulcus. On swap error trials identified with mixture modeling, we were able to reconstruct a representation of the swapped location, but not of the cued location, suggesting the maintenance of the incorrect memory item before response. Moreover, participants subjectively responded with some level of confidence, rather than complete guessing, on a majority of swap error trials. Together, these results suggest that swap errors are not mere response-phase guesses, but instead result from failures of selection in working memory, contextual binding errors, or informed guesses, which produce active maintenance of incorrect memory representations.

Cognition in prodromal Parkinson's disease

One characteristic of Parkinson's disease (PD) is a prodromal phase, lasting many years during which both pre-clinical motor and non-motor symptoms occur. Around one-fifth of patients with PD manifest mild cognitive impairment at time of clinical diagnosis. Thus, important challenges are to define the time of onset of cognitive dysfunction in the prodromal phase of PD, and to define its co-occurrence with other specific characteristics. Evidence for cognitive change in prodromal PD comes from various study designs, including both longitudinal and cross-sectional approaches with different target groups. These studies support the concept that changes in global cognitive function and alterations in executive functions occur, and that these changes may be present up to 6 years before clinical PD diagnosis. Notably, this evidence led to including global cognitive impairment as an independent prodromal marker in the recently updated research criteria of the Movement Disorder Society for prodromal PD. Knowledge in this field, however, is still at its beginning, and evidence is sparse about many aspects of this topic. Further longitudinal studies including standardized assessments of global and domain-specific cognitive functions are needed to gain further knowledge about the first appearance, the course, and the interaction of cognitive deficits with other non-motor symptoms in prodromal stage PD. Treatment approaches, including non-pharmacological interventions, in individuals with prodromal PD might help to prevent or delay cognitive dysfunction in early PD.

Parkinsonism and dementia

The aim of the present review is to summarize literature data on dementia in parkinsonian disorders. Cognitive decline and the gradual development of dementia are considered to be key features in the majority of parkinsonian conditions. The burden of dementia in everyday life of parkinsonian patients and their caregivers is vast and can be even more challenging to handle than the motor component of the disease. Common pathogenetic mechanisms involve the aggregation and spreading of abnormal proteins like alpha-synuclein, tau or amyloid in cortical and subcortical regions with subsequent dysregulation of multiple neurotransmitter systems. The degree of cognitive deterioration in these disorders is variable and ranges from mild cognitive impairment to severe cognitive dysfunction. There is also variation in the number and type of affected cognitive domains which can involve either a single domain like executive or visuospatial function or multiple ones. Novel genetic, biological fluid or imaging biomarkers appear promising in facilitating the diagnosis and staging of dementia in parkinsonian conditions. A significant part of current research in Parkinson's disease and other parkinsonian syndromes is targeted towards the cognitive aspects of these disorders. Stabilization or amelioration of cognitive outcomes represents a primary endpoint in many ongoing clinical trials for novel disease modifying treatments in this field. This article is part of the Special Issue "Parkinsonism across the spectrum of movement disorders and beyond" edited by Joseph Jankovic, Daniel D. Truong and Matteo Bologna.

Exploring the Genotype-Phenotype Correlation in GBA-Parkinson Disease: Clinical Aspects, Biomarkers, and Potential Modifiers

Variants in the glucocerebrosidase (GBA) gene are the most common genetic risk factor for Parkinson disease (PD). These include pathogenic variants causing Gaucher disease (GD) (divided into “severe,” “mild,” or “complex”—resulting from recombinant alleles—based on the phenotypic effects in GD) and “risk” variants, which are not associated with GD but nevertheless confer increased risk of PD. As a group, GBA-PD patients have more severe motor and nonmotor symptoms, faster disease progression, and reduced survival compared with noncarriers. However, different GBA variants impact variably on clinical phenotype. In the heterozygous state, “complex” and “severe” variants are associated with a more aggressive and rapidly progressive disease. Conversely, “mild” and “risk” variants portend a more benign course. Homozygous or compound heterozygous carriers usually display severe phenotypes, akin to heterozygous “complex” or “severe” variants carriers. This article reviews genotype–phenotype correlations in GBA-PD, focusing on clinical and nonclinical aspects (neuroimaging and biochemical markers), and explores other disease modifiers that deserve consideration in the characterization of these patients.

A new toolbox to distinguish the sources of spatial memory error

Studying the sources of errors in memory recall has proven invaluable for understanding the mechanisms of working memory (WM). While one-dimensional memory features (e.g., color, orientation) can be analyzed using existing mixture modeling toolboxes to separate the influence of imprecision, guessing, and misbinding (the tendency to confuse features that belong to different memoranda), such toolboxes are not currently available for two-dimensional spatial WM tasks. Here we present a method to isolate sources of spatial error in tasks where participants have to report the spatial location of an item in memory, using two-dimensional mixture models. The method recovers simulated parameters well and is robust to the influence of response distributions and biases, as well as number of nontargets and trials. To demonstrate the model, we fit data from a complex spatial WM task and show the recovered parameters correspond well with previous spatial WM findings and with recovered parameters on a one-dimensional analogue of this task, suggesting convergent validity for this two-dimensional modeling approach. Because the extra dimension allows greater separation of memoranda and responses, spatial tasks turn out to be much better for separating misbinding from imprecision and guessing than one-dimensional tasks. Code for these models is freely available in the MemToolbox2D package and is integrated to work with the commonly used MATLAB package MemToolbox.

Different patterns of short-term memory deficit in Alzheimer's disease, Parkinson's disease and subjective cognitive impairment

It has recently been proposed that short-term memory (STM) binding deficits might be an important feature of Alzheimer's disease (AD), providing a potential avenue for earlier detection of this disorder. By contrast, work in Parkinson's disease (PD), using different tasks, has suggested that the STM impairment in this condition is characterised by increased random guessing, possibly due to fluctuating attention. In the present study, to establish whether a misbinding impairment is present in sporadic late-onset AD (LOAD) and increased guessing is a feature of PD, we compared the performance of these patient groups to two control populations: healthy age-matched controls and individuals with subjective cognitive impairment (SCI) with comparable recruitment history as patients. All participants performed a sensitive task of STM that required high resolution retention of object-location bindings. This paradigm also enabled us to explore the underlying sources of error contributing to impaired STM in patients with LOAD and PD using computational modelling of response error. Patients with LOAD performed significantly worse than other groups on this task. Importantly their impaired memory was associated with increased misbinding errors. This was in contrast to patients with PD who made significantly more guessing responses. These findings therefore provide additional support for the presence of two doubly dissociable signatures of STM deficit in AD and PD, with binding impairment in AD and increased random guessing characterising the STM deficit in PD. The task used to measure memory precision here provides an easy-to-administer assessment of STM that is sensitive to the different types of deficit in AD and PD and hence has the potential to inform clinical practice.

Glucocerebrosidase Defects as a Major Risk Factor for Parkinson's Disease

Heterozygous mutations of the GBA1 gene, encoding for lysosomal enzyme glucocerebrosidase (GCase), occur in a considerable percentage of all patients with sporadic Parkinson's disease (PD), varying between 8% and 12% across the world. Genome wide association studies have confirmed the strong correlation between PD and GBA1 mutations, pointing to this element as a major risk factor for PD, possibly the most important one after age. The pathobiological mechanisms underlying the link between a defective function of GCase and the development of PD are still unknown and are currently the focus of intense investigation in the community of pre-clinical and clinical researchers in the PD field. A major controversy regards the fact that, despite the unequivocal correlation between the presence of GBA1 mutations and the risk of developing PD, only a minority of asymptomatic carriers with GBA1 mutations convert to PD in their lifetime. GBA1 mutations reduce the enzymatic function of GCase, impairing lysosomal efficiency and the cellular ability to dispose of pathological alpha-synuclein. Changes in the cellular lipidic content resulting from the accumulation of glycosphingolipids, triggered by lysosomal dysfunction, may contribute to the pathological modification of alpha-synuclein, due to its ability to interact with cell membrane lipids. Mutant GCase can impair mitochondrial function and cause endoplasmic reticulum stress, thereby impacting on cellular energy production and proteostasis. Importantly, reduced GCase activity is associated with clear activation of microglia, a major mediator of neuroinflammatory response within the brain parenchyma, which points to neuroinflammation as a major consequence of GCase dysfunction. In this present review article, we summarize the current knowledge on the role of GBA1 mutations in PD development and their phenotypic correlations. We also discuss the potential role of the GCase pathway in the search for PD biomarkers that may enable the development of disease modifying therapies. Answering these questions will aid clinicians in offering more appropriate counseling to the patients and their caregivers and provide future directions for PD preclinical research.

Visual self-motion cues are impaired yet overweighted during visual-vestibular integration in Parkinson's disease

Parkinson's disease is prototypically a movement disorder. Although perceptual and motor functions are highly interdependent, much less is known about perceptual deficits in Parkinson's disease, which are less observable by nature, and might go unnoticed if not tested directly. It is therefore imperative to seek and identify these, to fully understand the challenges facing patients with Parkinson's disease. Also, perceptual deficits may be related to motor symptoms. Posture, gait and balance, affected in Parkinson's disease, rely on veridical perception of one's own motion (self-motion) in space. Yet it is not known whether self-motion perception is impaired in Parkinson's disease. Using a well-established multisensory paradigm of heading discrimination (that has not been previously applied to Parkinson's disease), we tested unisensory visual and vestibular self-motion perception, as well as multisensory integration of visual and vestibular cues, in 19 Parkinson's disease, 23 healthy age-matched and 20 healthy young-adult participants. After experiencing vestibular (on a motion platform), visual (optic flow) or multisensory (combined visual-vestibular) self-motion stimuli at various headings, participants reported whether their perceived heading was to the right or left of straight ahead. Parkinson's disease participants and age-matched controls were tested twice (Parkinson's disease participants on and off medication). Parkinson's disease participants demonstrated significantly impaired visual self-motion perception compared with age-matched controls on both visits, irrespective of medication status. Young controls performed slightly (but not significantly) better than age-matched controls and significantly better than the Parkinson's disease group. The visual self-motion perception impairment in Parkinson's disease correlated significantly with clinical disease severity. By contrast, vestibular performance was unimpaired in Parkinson's disease. Remarkably, despite impaired visual self-motion perception, Parkinson's disease participants significantly overweighted the visual cues during multisensory (visual-vestibular ) integration (compared with Bayesian predictions of optimal integration) and significantly more than controls. These findings indicate that self-motion perception in Parkinson's disease is affected by impaired visual cues and by suboptimal visual-vestibular integration (overweighting of visual cues). Notably, vestibular self-motion perception was unimpaired. Thus, visual self-motion perception is specifically impaired in early-stage Parkinson's disease. This can impact Parkinson's disease diagnosis and subtyping. Overweighting of visual cues could reflect a general multisensory integration deficit in Parkinson's disease, or specific overestimation of visual cue reliability. Finally, impaired self-motion perception in Parkinson's disease may contribute to impaired balance and gait control. Future investigation into this connection might open up new avenues of alternative therapies to better treat these difficult symptoms.

Systematic review of genetic variants associated with cognitive impairment and depressive symptoms in Parkinson's disease

OBJECTIVE

Cognitive impairment and depression are among the most prevalent and most disabling non-motor symptoms in Parkinson's disease (PD). The genetic factors that are associated with these symptoms remain uncertain. This systematic review aims to summarise the prevailing evidence from all genetic association studies investigating the genetic variants associated with cognitive impairment and depressive symptoms in people with PD.

METHOD

A systematic review using five online databases: PubMed, PsycINFO, CINAHL, EMBASE and OpenGrey (PROSPERO protocol: CRD42017067431). We completed the quality assessment using the Q-Genie tool.

RESULTS

2353 articles were screened, and 43 articles were found to be eligible to be included. A meta-analysis of studies investigating LRRK2 rs34637584 confirmed that the minor allele carriers had significantly less cognitive impairment (p = 0.015). Further meta-analyses showed that GBA variants rs76763715 (p < 0.001) and rs421016 (p = 0.001) were significantly associated with more cognitive impairment in people with PD. Minor alleles of GBA variants rs76763715, rs421016, rs387906315 and rs80356773 were associated with more depressive symptoms in PD. Moreover, APOE ε4 allele has been associated with more cognitive impairment in PD. BDNF (rs6265) and CRY1 (rs2287161) variants have been associated with more depressive symptoms in people with PD.

CONCLUSIONS

PD carriers of GBA variants are at high risk for cognitive decline and depression. Screening for these variants may facilitate early identification and effective management of these non-motor symptoms. The molecular mechanisms underlying favourable cognitive functioning in LRRK2 rs34637584 variant carriers warrant further investigation.

Effects of Home-Based Working Memory Training on Visuo-Spatial Working Memory in Parkinson's Disease: A Randomized Controlled Trial

BACKGROUND

Cognitive impairment is a very frequent and severe nonmotor symptom of Parkinson's disease (PD). Early intervention in this at-risk group for cognitive decline may be crucial for long-term preservation of cognitive functions. Computerized working memory training (WMT) has been proven beneficial in non-PD patient populations, but such evidence is still needed for patients with PD.

OBJECTIVE

This study aimed to evaluate the effect of WMT on visuo-spatial working memory (WM) in cognitively unimpaired patients with PD.

METHODS

A single-blind randomized controlled trial encompassing 76 patients with PD but no cognitive impairment according to level II diagnostic criteria was conducted. Thirty-seven patients engaged in home-based adaptive WMT 5 times per week for a period of 5 weeks, whereas the remaining patients were in the waiting list arm of the study (control group [CG]). Working memory performance was evaluated using a computerized task before and after intervention and at 14-week follow-up, allowing to quantify the precision of WM on a continuous scale, ie, to test not only if an item was remembered but also how well the location of this item was retained.

RESULTS

Coincidently, the WMT group showed slightly worse WM performance compared with the CG at baseline, which was ameliorated after WMT. This training-induced effect remained stable until follow-up.

CONCLUSION

Patients showing relatively low WM performance, despite not formally diagnosable as Parkinson's disease with mild cognitive impairment (PD-MCI), seem to benefit from home-based WMT. Thus, WMT could potentially be implemented in future trials as a time- and cost-efficient route to counteract subtle cognitive changes in early disease stages.

TRIAL REGISTRATION

German Clinical Trial Register (drks.de, DRKS00009379).

Dopamine affects short-term memory corruption over time in Parkinson's disease

Cognitive deficits are a recognised component of Parkinson's disease (PD). However, particularly within the domain of short-term memory, it is unclear whether these impairments are masked, or caused, by patients' dopaminergic medication. The effect of medication on pure maintenance in PD patients has rarely been explored, with most assessments examining maintenance intercalated between other executive tasks. Moreover, few studies have utilised methods that can measure the quality of mental representations, which can enable the decomposition of recall errors into their underlying neurocognitive components. Here, we fill this gap by examining pure maintenance in PD patients in high and low dopaminergic states. Participants had to encode the orientation of two stimuli and reproduce these orientations after a short (2 s) or long (8 s) delay. In addition, we also examined the performance of healthy, age-matched older adults to contextualise these effects and determine whether PD represents an exacerbation of the normal ageing process. Patients showed improved recall OFF compared to ON their dopaminergic medication, but only for long-duration trials. Moreover, PD patients OFF their medication actually performed at a level superior to age-matched controls, indicative of a paradoxical enhancement of memory in the low dopaminergic state. The application of a probabilistic model of response selection suggested that PD patients made fewer misbinding errors in the low, compared with high, dopaminergic state for longer-delay trials. Thus, unexpectedly, the mechanisms that prevent memoranda from being corrupted by misbinding over time appear to be enhanced in PD patients OFF dopaminergic medication. Possible explanations for this paradoxical effect are discussed.

Visual short-term memory for coherent motion in video game players: evidence from a memory-masking paradigm

In this study, we investigated visual short-term memory for coherent motion in action video game players (AVGPs), non-action video game players (NAVGPs), and non-gamers (control group: CONs). Participants performed a visual memory-masking paradigm previously used with macaque monkeys and humans. In particular, we tested whether video game players form a more robust visual short-term memory trace for coherent moving stimuli during the encoding phase, and whether such memory traces are less affected by an intervening masking stimulus presented 0.2 s after the offset of the to-be-remembered sample. The results showed that task performance of all groups was affected by the masking stimulus, but video game players were affected to a lesser extent than controls. Modelling of performance values and reaction times revealed that video game players have a lower guessing rate than CONs, and higher drift rates than CONs, indicative of more efficient perceptual decisions. These results suggest that video game players exhibit a more robust VSTM trace for moving objects and this trace is less prone to external interference.

[The genetic predictors of cognitive impairment in Parkinson's disease]

Parkinson's disease (PD) is a common neurodegenerative disorder that can be both sporadic and familial. A number of studies are devoted to the study of non-motor symptoms in PD today. Cognitive deficits, and especially dementia, are one of the most severe and disabling non-motor symptoms of PD. More than a quarter of patients in the early stages of PD have a moderate cognitive impairment, more than half of patients with PD develop dementia within 10 years from the date of diagnosis. Using genome-wide association studies (GWAS), a number of genes associated with cognitive impairment have been identified based on a comparison of genetic and clinical phenotypes. These genes can be divided into three groups: genes that lead to the development of PD and are inherited according to the laws of Mendel (SNCA), genes that are risk factors for PD development (GBA, MAPT) and genes associated with the development of cognitive impairment, but not with PD (COMT, APOE, BDNF). This review examines the effect of genetic variants in the above-mentioned genes on cognitive functions in patients with PD. The elucidation of the genetic basis of cognitive deficits in PD could help in choice of treatment tactics and in development of new therapeutic strategies.

Binding deficits in visual short-term memory in patients with temporal lobe lobectomy

Classical views of the medial temporal lobe (MTL) have established that it plays a crucial role in long-term memory (LTM). Here we demonstrate, in a sample of patients who have undergone anterior temporal lobectomy for the treatment of pharmacoresistant epilepsy, that the MTL additionally plays a specific, causal role in short-term memory (STM). Patients (n=22) and age-matched healthy control participants (n=26) performed a STM task with a sensitive continuous report measure. This paradigm allowed us to examine recall memory for object identity, location and object-location binding, independently on a trial-by-trial basis. Our findings point to a specific involvement of MTL in object-location binding, but, crucially, not retention of either object identity or location. Therefore the MTL appears to perform a specific computation: binding disparate features that belong to a memory. These results echo findings from previous studies, which have identified a role for the MTL in relational binding for LTM, and support the proposal that MTL regions perform such a function for both STM and LTM, independent of the retention duration. Furthermore, these findings and the methodology employed here may provide a simple, sensitive and clinically valuable means to test memory dysfunuction in MTL disorders.

Working Memory in Alzheimer's Disease and Parkinson's Disease

Working memory impairments are frequently observed in patients with Alzheimer's disease (AD) and Parkinson's disease (PD). Recent research suggests that the mechanisms underlying these deficits might be dissociable using sensitive tasks, specifically those that rely on the reproduction of the exact quality of features held in memory.In patients with AD, working memory impairments are mainly due to an increase in misbinding errors. They arise when patients misremember which features (e.g., color, orientation, shape, and location) belong to different objects held in memory. Hence, they erroneously report features that belong to items in memory other than the one they are probed on. This misbinding of features that belong to different objects in memory can be considered a form of interference between stored items. Such binding errors are evident even in presymptomatic individuals with familial AD (due to gene mutations) who do not have AD yet. Overall, these findings are in line with the role of the medial temporal lobes, and specifically the hippocampus, in retention of feature bindings, regardless of retention duration, i.e., in both short- or long-term memory.Patients with PD, on the other hand, do not show increased misbinding. Their working memory deficits are associated with making more random errors or guesses. These random responses are not modulated by manipulations of their dopaminergic medication and hence may reflect involvement of non-dopaminergic neurotransmitters in this deficit. In addition, patients with PD demonstrate impairments in gating of information into relevant vs. irrelevant items in memory, a cognitive operation that is modulated by dopaminergic manipulation in line with a frontal executive effect of this neurotransmitter. Thus, although AD and PD are both associated with working memory impairments, these surface manifestations appear to be underpinned by very different mechanisms.

Glucocerebrosidase mutations and synucleinopathies: Toward a model of precision medicine

Glucocerebrosidase is a lysosomal enzyme. The characterization of a direct link between mutations in the gene coding for glucocerebrosidase (GBA1) with the development of Parkinson's disease and dementia with Lewy bodies has heightened interest in this enzyme. Although the mechanisms through which glucocerebrosidase regulates the homeostasis of α-synuclein remains poorly understood, the identification of reduced glucocerebrosidase activity in the brains of patients with PD and dementia with Lewy bodies has paved the way for the development of novel therapeutic strategies directed at enhancing glucocerebrosidase activity and reducing α-synuclein burden, thereby slowing down or even preventing neuronal death. Here we reviewed the current literature relating to the mechanisms underlying the cross talk between glucocerebrosidase and α-synuclein, the GBA1 mutation-associated clinical phenotypes, and ongoing therapeutic approaches targeting glucocerebrosidase. © 2018 International Parkinson and Movement Disorder Society.

Dopamine guides competition for cognitive control: Common effects of haloperidol on working memory and response conflict

Several lines of evidence suggest that dopamine modulates working memory (the ability to faithfully maintain and efficiently manipulate information over time) but its specific role has not been fully defined. Nor is it clear whether any effects of dopamine are specific to memory processes or whether they reflect more general cognitive mechanisms that extend beyond the working memory domain. Here, we examine the effect of haloperidol, principally a dopamine D₂ receptor antagonist, on the ability of humans to ignore distracting information or update working memory contents. We compare these effects to performance on an independent measure of cognitive control (response conflict) which has minimal memory requirements. Haloperidol did not selectively affect the ability to ignore or update, but instead reduced the overall quality of recall. In addition, it impaired the ability to overcome response conflict. The deleterious effect of haloperidol on response conflict was selectively associated with the negative effect of the drug on ignoring - but not updating - suggesting that dopamine affects protection of working memory contents and inhibition in response conflict through a common mechanism. These findings provide new insights into the role of dopamine D₂ receptors on human cognition. They suggest that D₂ receptor effects on protecting the memory contents from distraction might be related to a more general process that supports inhibitory control in contexts that do not require working memory.

Ignoring versus updating in working memory reveal differential roles of attention and feature binding

Ignoring distracting information and updating current contents are essential components of working memory (WM). Yet, although both require controlling irrelevant information, it is unclear whether they have the same effects on recall and produce the same level of misbinding errors (incorrectly joining the features of different memoranda). Moreover, the likelihood of misbinding may be affected by the feature similarity between the items already encoded into memory and the information that has to be filtered out (ignored) or updated into memory. Here, we investigate these questions. Participants were sequentially presented with two pairs of arrows. The first pair of arrows always had to be encoded into memory, but the second pair either had to be ignored (ignore condition) or allowed to displace the previously encoded items (update condition). To investigate the effect of similarity on recall, we also varied, in a factorial manner, whether the items that had to be ignored or updated were presented in the same or different colours and/or same or different spatial locations to the original memoranda. By applying a computational model, we were able to quantify the levels of misbinding. Ignoring, but not updating, increased overall recall error as well as misbinding rates, even when accounting for the retention period. This indicates that not all manipulations of attention in WM are equal in terms of their effects on recall and misbinding. Misbinding rates in the ignore condition were affected by the colour and spatial congruence of relevant and irrelevant information to a greater extent than in the update condition. This finding suggests that attentional templates are used to evaluate relevant and irrelevant information in different ways during ignoring and updating. Together, the results suggest that differences between the two functions might occur due to higher levels of attentional compartmentalisation - or protection - during updating compared to ignoring.

Fractionating the Neurocognitive Mechanisms Underlying Working Memory: Independent Effects of Dopamine and Parkinson's Disease

Deficits in working memory (WM) in Parkinson’s disease (PD) are often considered to be secondary to dopaminergic depletion. However, the neurocognitive mechanisms by which dopamine causes these deficits remain highly contested, and PD is now also known to be associated with nondopaminergic pathology. Here, we examined how PD and dopaminergic medication modulate three components of WM: maintenance over time, updating contents with new information and making memories distracter-resistant. Compared with controls, patients were disproportionately impaired when retaining information for longer durations. By applying a probabilistic model, we were able to reveal that the source of this error was selectively due to precision of memory representations degrading over time. By contrast, replenishing dopamine levels in PD improved executive control over both the ability to ignore and update, but did not affect maintenance of information across time. This was due to a decrease in guess responses, consistent with the view that dopamine serves to prevent WM representations being corrupted by irrelevant information, but has no impact on information decay. Cumulatively, these results reveal a dissociation in the neural mechanisms underlying poor WM: whereas dopamine reduces interference, nondopaminergic systems in PD appear to modulate processes that prevent information decaying more quickly over time.

Glucocerebrosidase Mutations in Parkinson Disease

Following the discovery of a higher than expected incidence of Parkinson Disease (PD) in Gaucher disease, a lysosomal storage disorder, mutations in the glucocerebrocidase (GBA) gene, which encodes a lysosomal enzyme involved in sphingolipid degradation were explored in the context of idiopathic PD. GBA mutations are now known to be the single largest risk factor for development of idiopathic PD. Clinically, on imaging and pharmacologically, GBA PD is almost identical to idiopathic PD, other than certain features that can be identified in the specialist research setting but not in routine clinical practice. In patients with a known GBA mutation, it is possible to monitor for prodromal signs of PD. The clinical similarity with idiopathic PD and the chance to identify PD at a pre-clinical stage provides a unique opportunity to research therapeutic options for early PD, before major irreversible neurodegeneration occurs. However, to date, the molecular mechanisms which lead to this increased PD risk in GBA mutation carriers are not fully elucidated. Experimental models to define the molecular mechanisms and test therapeutic options include cell culture, transgenic mice and other in vivo models amenable to genetic manipulation, such as drosophilia. Some key pathological pathways of interest in the context of GBA mutations include alpha synuclein aggregation, lysosomal-autophagy axis changes and endoplasmic reticulum stress. Therapeutic agents that exploit these pathways are being developed and include the small molecule chaperone Ambroxol. This review aims to summarise the main features of GBA-PD and provide insights into the pathological relevance of GBA mutations on molecular pathways and the therapeutic implications for PD resulting from investigation of the role of GBA in PD.

Glucocerebrosidase mutations and neuropsychiatric phenotypes in Parkinson's disease and Lewy body dementias: Review and meta-analyses

Heterozygous mutations in glucocerebrosidase gene (GBA) are a major genetic risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Recently, there has been a considerable focus on the relationship between GBA mutations and emergence of cognitive impairment and neuropsychiatric symptoms in these diseases. Here, we review the literature in this area, with a particular focus, including meta-analysis, on the key neuropsychiatric symptoms of cognitive impairment, psychosis, and depression in Parkinson's disease. Our meta-analysis demonstrated that GBA mutations are associated with a 2.4-fold increased risk of cognitive impairment. In addition, our novel meta-analyses of psychosis and depression showed a 1.8- and 2.2-fold increased risk respectively associated with GBA mutations, although due to possible bias and heterogeneity the depression findings should be interpreted with caution. While the precise mechanisms which increase susceptibility to neurodegeneration in GBA carriers are not known, evidence of greater cortical Lewy body pathology, reduced patterns of cortical activation, and hippocampal pathology in animal models are all consistent with a direct effect of GBA mutations on these symptoms. Extension of this work in DLB and individuals without neurodegeneration will be important in further characterizing how GBA mutations increase risk for PD and DLB and influence disease course.

Glucocerebrosidase and Parkinson Disease: Molecular, Clinical, and Therapeutic Implications

Parkinson disease (PD) is a complex neurodegenerative disease characterised by multiple motor and non-motor symptoms. In the last 20 years, more than 20 genes have been identified as causes of parkinsonism. Following the observation of higher risk of PD in patients affected by Gaucher disease, a lysosomal disorder caused by mutations in the glucocerebrosidase (GBA) gene, it was discovered that mutations in this gene constitute the single largest risk factor for development of idiopathic PD. Patients with PD and GBA mutations are clinically indistinguishable from patients with idiopathic PD, although some characteristics emerge depending on the specific mutation, such as slightly earlier onset. The molecular mechanisms which lead to this increased PD risk in GBA mutation carriers are multiple and not yet fully elucidated, they include alpha-synuclein aggregation, lysosomal-autophagy dysfunction and endoplasmic reticulum stress. Moreover, dysfunction of glucocerebrosidase has also been demonstrated in non-GBA PD, suggesting its interaction with other pathogenic mechanisms. Therefore, GBA enzyme function represents an interesting pharmacological target for PD. Cell and animal models suggest that increasing GBA enzyme activity can reduce alpha-synuclein levels. Clinical trials of ambroxol, a glucocerebrosidase chaperone, are currently ongoing in PD and PD dementia, as is a trial of substrate reduction therapy. The aim of this review is to summarise the main features of GBA-PD and discuss the implications of glucocerebrosidase modulation on PD pathogenesis.

Effects of Parkinson's disease and dopamine on digit span measures of working memory

RATIONALE

Parkinson's disease (PD) impairs working memory (WM)-the ability to maintain items in memory for short periods of time and manipulate them. There is conflicting evidence on the nature of the deficits caused by the disease, and the potential beneficial and detrimental effects of dopaminergic medication on different WM processes.

OBJECTIVES

We hypothesised that PD impairs both maintenance and manipulation of items in WM and dopaminergic medications improve this in PD patients but impair it in healthy older adults.

METHODS

We tested 68 PD patients ON and OFF their dopaminergic medication, 83 healthy age-matched controls, and 30 healthy older adults after placebo and levodopa administration. We used the digit span, a WM test with three components (forwards, backwards, and sequence recall) that differ in the amount of manipulation required. We analysed the maximum spans and the percentage of lists correctly recalled, which probe capacity of WM and the accuracy of the memory processes within this capacity, respectively.

RESULTS

PD patients had lower WM capacity across all three digit span components, but only showed reduced percentage accuracy on the components requiring manipulation (backwards and sequence spans). Dopaminergic medication did not affect performance in PD patients. In healthy older adults, levodopa did not affect capacity, but did impair accuracy on one of the manipulation components (sequence), without affecting the other (backwards).

CONCLUSIONS

This suggests that the deficit of maintenance capacity and manipulation accuracy in PD patients is not primarily a dopaminergic one and supports a potential "overdosing" of intact manipulation mechanisms in healthy older adults by levodopa.

Cognitive changes in prodromal Parkinson's disease: A review

Although other nonmotor phenomena representing possible prodromal symptoms of Parkinson's disease have been described in some detail, the occurrence and characteristics of cognitive decline in this early phase of the disease are less well understood. The aim of this review is to summarize the current state of research on cognitive changes in prodromal PD. Only a small number of longitudinal studies have been conducted that examined cognitive function in individuals with a subsequent PD diagnosis. However, when we consider data from at-risk groups, the evidence suggests that cognitive decline may occur in a substantial number of individuals who have the potential for developing PD. In terms of specific cognitive domains, executive function in particular and, less frequently, memory scores are reduced. Prospective longitudinal studies are thus needed to clarify whether cognitive, and specifically executive, decline might be added to the prodromal nonmotor symptom complex that may precede motor manifestations of PD by years and may help to update the risk scores used for early identification of PD. © 2017 International Parkinson and Movement Disorder Society.

Cognitive and motor functioning in elderly glucocerebrosidase mutation carriers

Mutations in the glucocerebrosidase (GBA) gene are a strong genetic risk factor for the development of Parkinson's disease and dementia with Lewy Bodies. However the penetrance of GBA mutations is low for these diseases in heterozygous carriers. The aim of this study was to examine the relationship between mutation status and cognitive and motor functioning in a sample of community-dwelling older adults. Using linear mixed effects models, we examined the effect of heterozygous mutation status on 736 community-dwelling older adults (≥70 years) without dementia or Parkinson's disease assessed over an average of 6 years, 28 of whom had a single GBA mutation (primarily N370S). Verbal memory was measured using the picture version of the Free and Cued Selective Reminding Test, and carriers showed significantly (p < 0.05) greater decline in verbal memory over time. There was no difference in motor function or any other cognitive domain. Taken together, these results suggest an effect, but an overall limited burden, of harboring a single GBA mutation in aging mutation carriers.

Large-scale exploratory genetic analysis of cognitive impairment in Parkinson's disease

Cognitive impairment is a common and disabling problem in Parkinson's disease (PD). Identification of genetic variants that influence the presence or severity of cognitive deficits in PD might provide a clearer understanding of the pathophysiology underlying this important nonmotor feature. We genotyped 1105 PD patients from the PD Cognitive Genetics Consortium for 249,336 variants using the NeuroX array. Participants underwent assessments of learning and memory (Hopkins Verbal Learning Test-Revised [HVLT-R]), working memory/executive function (Letter-Number Sequencing and Trail Making Test [TMT] A and B), language processing (semantic and phonemic verbal fluency), visuospatial abilities (Benton Judgment of Line Orientation [JoLO]), and global cognitive function (Montreal Cognitive Assessment). For common variants, we used linear regression to test for association between genotype and cognitive performance with adjustment for important covariates. Rare variants were analyzed using the optimal unified sequence kernel association test. The significance threshold was defined as a false discovery rate-corrected p-value (P_FDR) of 0.05. Eighteen common variants in 13 genomic regions exceeded the significance threshold for one of the cognitive tests. These included GBA rs2230288 (E326K; P_FDR = 2.7 × 10^-4) for JoLO, PARP4 rs9318600 (P_FDR = 0.006), and rs9581094 (P_FDR = 0.006) for HVLT-R total recall, and MTCL1 rs34877994 (P_FDR = 0.01) for TMT B-A. Analysis of rare variants did not yield any significant gene regions. We have conducted the first large-scale PD cognitive genetics analysis and nominated several new putative susceptibility genes for cognitive impairment in PD. These results will require replication in independent PD cohorts.

Genetics of Parkinson's Disease: Genotype-Phenotype Correlations

Since the first discovery of a specific genetic defect in the SNCA gene, encoding for α-synuclein, as a causative factor for Parkinson's disease 20 years ago, a multitude of other genes have been linked to this disease in rare cases with Mendelian inheritance. Furthermore, the genetic contribution to the much more common sporadic disease has been demonstrated through case control association studies and, more recently, genome-wide association studies. Interestingly, some of the genes with Mendelian inheritance, such as SNCA, are also relevant to the sporadic disease, suggesting common pathogenetic mechanisms. In this review, we place an emphasis on Mendelian forms, and in particular genetic defects which present predominantly with Parkinsonism. We provide details into the particular phenotypes associated with each genetic defect, with a particular emphasis on nonmotor symptoms. For genetic defects for whom a sufficient number of patients has been assessed, there are evident genotype-phenotype correlations. However, it should be noted that patients with the same causative mutation may present with distinctly divergent phenotypes. This phenotypic variability may be due to genetic, epigenetic or environmental factors. From a clinical and genetic point of view, it will be especially interesting in the future to identify genetic factors that modify disease penetrance, the age of onset or other specific phenotypic features.

Dementia in Parkinson's disease

Dementia can occur in a substantial number of patients with Parkinson's disease with a point prevalence close to 30%. The cognitive profile is characterized by predominant deficits in executive, visuospatial functions, attention and memory. Behavioral symptoms are frequent such as apathy, visual hallucinations and delusions. The most prominent associated pathology is Lewy body-type and biochemical deficit is cholinergic. Placebo-controlled randomized trials with cholinesterase inhibitors demonstrated modest but significant benefits in cognition, behavioral symptoms and global functions.

Dopamine Alters the Fidelity of Working Memory Representations according to Attentional Demands

Capacity limitations in working memory (WM) necessitate the need to effectively control its contents. Here, we examined the effect of cabergoline, a dopamine D₂ receptor agonist, on WM using a continuous report paradigm that allowed us to assess the fidelity with which items are stored. We assessed recall performance under three different gating conditions: remembering only one item, being cued to remember one target among distractors, and having to remember all items. Cabergoline had differential effects on recall performance according to whether distractors had to be ignored and whether mnemonic resources could be deployed exclusively to the target. Compared with placebo, cabergoline improved mnemonic performance when there were no distractors but significantly reduced performance when distractors were presented in a precue condition. No significant difference in performance was observed under cabergoline when all items had to be remembered. By applying a stochastic model of response selection, we established that the causes of drug-induced changes in performance were due to changes in the precision with which items were stored in WM. However, there was no change in the extent to which distractors were mistaken for targets. Thus, D₂ agonism causes changes in the fidelity of mnemonic representations without altering interference between memoranda.

Cognitive Performance Patterns in Healthy Individuals with Substantia Nigra Hyperechogenicity and Early Parkinson's Disease

Introduction: Hyperechogenicity of the substantia nigra (SN+) is a risk marker for Parkinson’s disease (PD) which can be detected before the diagnosis. In healthy individuals, SN+ has been associated with slight deficits in specific cognitive functions, suggesting cognitive impairment as a possible pre-diagnostic marker for PD. However, the pattern of cognitive deficits associated with SN+ has not yet been compared with those present in PD.

Methods: Data of 262 healthy individuals with normal echogenicity (SN-) and 48 healthy individuals with SN+ were compared with 82 early stage PD patients using the “Consortium to Establish a Registry for Alzheimer’s disease” test battery. First, the test clusters (factors) were identified using a principal component analysis (PCA). Mean group performance of cognitive tests belonging to distinct factors, according to the PCA, and single subtest performances were compared using analyses of variance. Second, the number of individuals with abnormal cognitive performances (z-score < -1.0) were compared between groups.

Results: Verbal memory, semantic and executive function, and praxis were identified as components of cognitive performances. The SN+ group performed significantly worse than the SN- group in tests assessing semantic and executive function, with a non-significant decrease in verbal memory. On the subtest level, individuals of the SN+ group scored significantly lower than the SN- group on the Boston Naming Test (BNT; p = 0.008). In all subtests, the percentages of PD patients with values below the cut-off for abnormal performance were higher than in the SN- group. Moreover, more individuals from the SN+ group scored below the cut-off in the BNT (SN- = 8.4%, SN+ = 20.8%, p = 0.01) and TMT-B (SN- = 6.9%, SN+ = 16.7%, p = 0.02), compared to the SN- group.

Conclusion: This study confirms poorer performance of healthy individuals with SN+ compared to SN- in specific cognitive domains. However, against the SN- group, the cognitive profile of the SN+ group was not fully consistent with the profile of early PD patients. Our data argues that cognitive impairment associated with SN+ might differ slightly from that seen in early PD. Compensational mechanisms in the early phases of neurodegeneration, and the fact that only a subgroup of SN+ will develop PD, may partly explain these differences.

Visual dysfunction in Parkinson's disease

Patients with Parkinson's disease have a number of specific visual disturbances. These include changes in colour vision and contrast sensitivity and difficulties with complex visual tasks such as mental rotation and emotion recognition. We review changes in visual function at each stage of visual processing from retinal deficits, including contrast sensitivity and colour vision deficits to higher cortical processing impairments such as object and motion processing and neglect. We consider changes in visual function in patients with common Parkinson's disease-associated genetic mutations including GBA and LRRK2 . We discuss the association between visual deficits and clinical features of Parkinson's disease such as rapid eye movement sleep behavioural disorder and the postural instability and gait disorder phenotype. We review the link between abnormal visual function and visual hallucinations, considering current models for mechanisms of visual hallucinations. Finally, we discuss the role of visuo-perceptual testing as a biomarker of disease and predictor of dementia in Parkinson's disease.

A cognitive fMRI study in non-manifesting LRRK2 and GBA carriers

Mutations in the GBA and LRRK2 genes account for one-third of the prevalence of Parkinson's disease (PD) in Ashkenazi Jews. Non-manifesting carriers (NMC) of these mutations represent a population at risk for future development of PD. PD patient who carry mutations in the GBA gene demonstrates more significant cognitive decline compared to idiopathic PD patients. We assessed cognitive domains using fMRI among NMC of both LRRK2 and GBA mutations to better understand pre-motor cognitive functions in these populations. Twenty-one LRRK2-NMC, 10 GBA-NMC, and 22 non-manifesting non-carriers (NMNC) who participated in this study were evaluated using the standard questionnaires and scanned while performing two separate cognitive tasks; a Stroop interference task and an N-Back working memory task. Cerebral activation patterns were assessed using both whole brain and predefined region of interest (ROI) analysis. Subjects were well matched in all demographic and clinical characteristics. On the Stroop task, in spite of similar behavior, GBA-NMC demonstrated increased task-related activity in the right medial frontal gyrus and reduced task-related activity in the left lingual gyrus compared to both LRRK2-NMC and NMNC. In addition, GBA-NMC had higher activation patterns in the incongruent task compared to NMNC in the left medial frontal gyrus and bilateral precentral gyrus. No whole-brain differences were noted between groups on the N-Back task. Paired cognitive and task-related performance between GBA-NMC, LRRK2-NMC, and NMNC could indicate that the higher activation patterns in the incongruent Stroop condition among GBA-NMC compared to LRRK2-NMC and NMNC may represent a compensatory mechanism that enables adequate cognitive performance.