Brain networks in posterior cortical atrophy: a single case tractography study and literature review

Allochiria for spatial landmarks as the presenting feature of posterior cortical atrophy

Allochiria refers to the mislocation of stimuli to the corresponding position on the opposite side of the body or hemispace. It is most often, although not exclusively, reported in the tactile modality and typically in association with unilateral neglect. We describe a patient presenting with a 2-year history of topographical disorientation without other cognitive complaints. We conducted a systematic exploration of his topographical problems to identify their cognitive substrate. Standard neuropsychological examination revealed no abnormalities. Notably, he performed well on perceptual, spatial, and constructional tasks. No signs of neglect were elicited. A tailored battery of tests was administered, involving road maps and landmarks, and designed to replicate the situations in which he experienced symptoms. The experimental tests showed no evidence of topographical agnosia or amnesia for landmarks and their spatial relationships and no hemispatial neglect. Nevertheless, the patient exhibited a systematic tendency to translocate topographical landmarks sited on the left to the right side. The phenomenon, consistent with representational allochiria, occurred exclusively for topographical landmarks, and was present along both personally familiar and new learned routes. Over the next two years more widespread visuoperceptual and spatial deficits emerged, with Balint and Gerstmann syndromes. Functional imaging revealed hypoperfusion of the occipito-parietal regions and amyloid PET the presence of amyloid plaques. A diagnosis was made of posterior cortical atrophy, the visual variant of Alzheimer's Disease. To our knowledge this is the first case of topographical disorientation presenting with selective representational allochiria and the first report of allochiria as an early sign of posterior cortical atrophy. The case sheds light on the cognitive basis of allochiria and on a puzzling clinical presentation of neurodegenerative brain disease.

Right-side spatial neglect and white matter disconnection after left-hemisphere strokes

Spatial neglect usually concerns left-sided events after right-hemisphere damage. Its anatomical correlates are debated, with evidence suggesting an important role for fronto-parietal white matter disconnections in the right hemisphere. Here, we describe the less frequent occurrence of neglect for right-sided events, observed in three right-handed patients after a focal stroke in the left hemisphere. Patients were tested 1 month and 3 months after stroke. They performed a standardized paper-and-pencil neglect battery and underwent brain MRI with both structural and diffusion tensor (DT) sequences, in order to assess both grey matter and white matter tracts metrics. Lesions were manually reconstructed for each patient. Patients presented signs of mild right-sided neglect during visual search and line bisection. One patient also showed pathological performance in everyday life. Structural MRI demonstrated left parietal strokes in two patients, in the region extending from the postcentral gyrus to the temporo-parietal junction. One of these two patients also had had a previous occipital stroke. The remaining patient had a left frontal stroke, affecting the precentral, the postcentral gyri and the basal ganglia. DT MRI tractography showed disconnections in the fronto-parietal regions, concerning principally the superior longitudinal fasciculus (SLF). These results suggest an important role for left SLF disconnection in right-side neglect, which complements analogous evidence for right SLF disconnection in left-side neglect.

Comparative Quantification of Diffusion Tensor Tractography Using Automated Whole Brain MRI Tractography for Intracranial Tumor Surgery: Technical Note

With the improvement of diffusion tensor imaging (DTI) and algorithms, diffusion tensor tractography (DTT) may provide quantitative information on white matter tracts (WMT) that may help quantitatively assess WMT integrity and distortion, which may help with correlations of neurologic function or prognosis. This manuscript is the first to describe a technical method for quantitative analysis of clinically relevant white matter tracts during intracranial tumor surgery. The authors quantitatively analyzed relevant proximal WMT, pre and postoperatively, in a patient undergoing cranial surgery using DTT software to evaluate fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD), geodesic anisotropy (GA), tract count, and tract volume. A method was then established to formulate quantitative comparisons between pre and postoperative WMT. Quantitative assessment of the corticospinal and optic radiation tracts revealed significant increases in the FA, GA, and tract count in the corticospinal and optic radiations postoperatively (p<.0001). MD, RD, and AD were found to be significantly diminished postoperatively (p<.0001). The postoperative optic radiations showed diminished volume as a result of damage to the tract pathway. To conclude, the utilization of white matter tractography provides a technical advancement that allows for quantitative comparative assessments of white matter tracts, which could assess the degree of brain changes following tumor surgery.

An action-concept processing advantage in a patient with a double motor cortex

Patients with atrophy in motor brain regions exhibit selective deficits in processing action-related meanings, suggesting a link between movement conceptualization and the amount of regional tissue. Here we examine such a relation in a unique opposite model: a rare patient with a double cortex (due to subcortical band heterotopia) in primary/supplementary motor regions, and no double cortex in multimodal semantic regions. We measured behavioral performance in action- and object-concept processing as well and resting-state functional connectivity. Both dimensions involved comparisons with healthy controls. Results revealed preserved accuracy in action and object categories for the patient. However, unlike controls, the patient exhibited faster performance for action than object concepts, a difference that was uninfluenced by general cognitive abilities. Moreover, this pattern was accompanied by heightened functional connectivity between the bilateral primary motor cortices. This suggests that a functionally active double motor cortex may entail action-processing advantages. Our findings offer new constraints for models of action semantics and motor-region function at large.

The temporal lobe in typical and atypical Alzheimer disease

Alzheimer disease (AD) is defined neuropathologically by abnormal extra-cellular β-amyloid plaques combined with intraneuronal tau aggregation. Patients sharing the same neuropathological features but presenting different clinical manifestations and evolutions have led to the notion of AD spectrum. This spectrum encompasses typical and atypical forms of AD. For all of them, specific parts of the temporal lobes, as well as their structural and functional connections with other brain regions, are affected. In typical amnestic late-onset Alzheimer's disease (>65 years old; LOAD), tau pathology gradually spreads to the brain from the medial temporal lobe (MTL). MTL is an inhomogeneous structure consisting of several subregions densely connected to each other and to other cortical and subcortical brain regions. These regions play a crucial role in the storage of information in episodic memory. In less common early-onset AD (<65 years old; EOAD), a large proportion of patients presents atypical clinical manifestations, in which memory impairment is not inaugural and predominant. Instead, these patients have predominant and/or isolated deficits in language, visuospatial, motor, or executive/behavioral functions. In atypical variants, brain damage is mainly centered on the posterior regions, with relative sparing of the MTL. However, the temporal lobe also appears to be variably and specifically damaged in some subtypes of EOAD. For example, the left superior temporal gyrus is the core of brain damage in the language variant, as well as the ventral regions of the temporal lobe play an important role in the clinic of the visual variant.

From competition to cooperation: Visual neglect across the hemispheres

Visuospatial neglect is a frequent and disabling consequence of injuries to the right hemisphere. Patients with neglect show signs of impaired attention for left-sided events, which depends on dysfunction of fronto-parietal networks. After unilateral injury, such as stroke, these networks and their contralateral homologs can reorganize following multiple potential trajectories, which can be either adaptive or maladaptive. This article presents possible factors influencing the profile of evolution of neglect towards recovery or chronicity, and highlights potential mechanisms that may constrain these processes in time and space. The integrity of white matter pathways within and between the hemisphere appears to pose crucial connectivity constraints for compensatory brain plasticity from remote brain regions. Specifically, the availability of a sufficient degree of inter-hemispheric connectivity might be critical to shift the role of the undamaged left hemisphere in spatial neglect, from exerting maladaptive effects, to promoting compensatory activity.

Splenial white matter integrity is associated with memory impairments in posterior cortical atrophy

Posterior cortical atrophy is an atypical form of Alzheimer’s disease characterized by visuospatial impairments and predominant tissue loss in the posterior parieto-occipital and temporo-occipital cortex. Whilst episodic memory is traditionally thought to be relatively preserved in posterior cortical atrophy, recent work indicates that memory impairments form a common clinical symptom in the early stages of the disease. Neuroimaging studies suggest that memory dysfunction in posterior cortical atrophy may originate from atrophy and functional hypoconnectivity of parietal cortex. The structural connectivity patterns underpinning these memory impairments, however, have not been investigated. This line of inquiry is of particular interest, as changes in white matter tracts of posterior cortical atrophy patients have been shown to be more extensive than expected based on posterior atrophy of grey matter. In this cross-sectional diffusion tensor imaging MRI study, we examine the relationship between white matter microstructure and verbal episodic memory in posterior cortical atrophy. We assessed episodic memory performance in a group of posterior cortical atrophy patients (n = 14) and a group of matched healthy control participants (n = 19) using the Free and Cued Selective Reminding Test with Immediate Recall. Diffusion tensor imaging measures were obtained for 13 of the posterior cortical atrophy patients and a second control group of 18 healthy adults. Patients and healthy controls demonstrated similar memory encoding performance, indicating that learning of verbal information was preserved in posterior cortical atrophy. However, retrieval of verbal items was significantly impaired in the patient group compared with control participants. As expected, tract-based spatial statistics analyses showed widespread reductions of white matter integrity in posterior cortical regions of patients compared with healthy adults. Correlation analyses indicated that poor verbal retrieval in the patient group was specifically associated with microstructural damage of the splenium of the corpus callosum. Post-hoc tractography analyses in healthy controls demonstrated that this splenial region was connected to thalamic radiations and the retrolenticular part of the internal capsule. These results provide insight into the brain circuits that underlie memory impairments in posterior cortical atrophy. From a cognitive perspective, we propose that the association between splenial integrity and memory dysfunction could arise indirectly via disruption of attentional processes. We discuss implications for the clinical phenotype and development of therapeutic aids for cognitive impairment in posterior cortical atrophy.

Profound regional spectral, connectivity, and network changes reflect visual deficits in posterior cortical atrophy: an EEG study

Patients with posterior cortical atrophy (PCA-AD) show more severe visuospatial and perceptual deficits than those with typical AD (tAD). The aim of this study was to investigate whether functional alterations measured by electroencephalography can help understand the mechanisms that explain this clinical heterogeneity. 21-channel electroencephalography recordings of 29 patients with PCA-AD were compared with 29 patients with tAD and 29 controls matched for age, gender, and disease severity. Patients with PCA-AD and tAD both showed a global decrease in fast and increase in slow oscillatory activity compared with controls. This pattern was, however, more profound in patients with PCA-AD which was driven by more extensive slowing of the posterior regions. Alpha band functional connectivity showed a similar decrease in PCA-AD and tAD. Compared with controls, a less integrated network topology was observed in PCA-AD, with a decrease of posterior and an increase of frontal hubness. In PCA-AD, decreased right parietal peak frequency correlated with worse performance on visual tasks. Regional vulnerability of the posterior network might explain the atypical pattern of neurodegeneration in PCA-AD.

Functional brain connectome in posterior cortical atrophy

This study investigated the functional brain connectome architecture in patients with Posterior Cortical Atrophy (PCA). Eighteen PCA patients and 29 age- and sex- matched healthy controls were consecutively recruited in a specialized referral center. Participants underwent neurologic examination, cerebrospinal fluid (CSF) examination for Alzheimer's disease (AD) biomarkers, cognitive assessment, and brain MRI. For a smaller subset of participants, FDG-PET examination was available. We assessed topological brain network properties and regional functional connectivity as well as intra- and inter-hemispheric connectivity, using graph analysis and connectomics. Supplementary analyses were performed to explore the association between the CSF AD profile and the connectome status, and taking into account hypometabolic, atrophic, and spared regions (nodes). PCA patients showed diffuse functional connectome alterations at both global and regional level, as well as a connectivity breakdown between the posterior brain nodes. They had a widespread loss of both intra- and inter-hemispheric connections, exceeding the structural damage, and including the frontal connections. In PCA, connectome alterations were identified in all the brain nodes irrespectively of their structural and metabolic classification and were associated with a connectivity breakdown between damaged and spared areas. Taken together, these findings suggest the potentially high sensitivity of graph-analysis and connectomic in capturing the progression and maybe early signs of neurodegeneration in PCA patients.

Parietal Involvement in the Semantic Variant of Primary Progressive Aphasia with Alzheimer's Disease Cerebrospinal Fluid Profile

Semantic variant of primary progressive aphasia (svPPA) is typically associated with non-Alzheimer's disease (AD) pathology. However, some anatomopathological studies have found AD lesions in those patients. We compared brain perfusion SPECT of 18 svPPA patients with cerebrospinal fluid (CSF) biomarkers indicative of non-AD pathology (svPPA-nonAD) and three svPPA patients with CSF biomarkers indicative of underlying AD (svPPA-AD). All svPPA patients had severe left temporopolar hypoperfusion. SvPPA-nonAD had additional anterior cingulate and mediofrontal hypoperfusion, whereas svPPA-AD had greater left parietal and posterior cingulate involvement. Parietal damage in svPPA constitutes a biomarker for underlying Alzheimer pathology thus refining the classification of this PPA variant.

Longitudinal neuroanatomical and cognitive progression of posterior cortical atrophy

Posterior cortical atrophy is a clinico-radiological syndrome characterized by progressive decline in visual processing and atrophy of posterior brain regions. With the majority of cases attributable to Alzheimer's disease and recent evidence for genetic risk factors specifically related to posterior cortical atrophy, the syndrome can provide important insights into selective vulnerability and phenotypic diversity. The present study describes the first major longitudinal investigation of posterior cortical atrophy disease progression. Three hundred and sixty-one individuals (117 posterior cortical atrophy, 106 typical Alzheimer's disease, 138 controls) fulfilling consensus criteria for posterior cortical atrophy-pure and typical Alzheimer's disease were recruited from three centres in the UK, Spain and USA. Participants underwent up to six annual assessments involving MRI scans and neuropsychological testing. We constructed longitudinal trajectories of regional brain volumes within posterior cortical atrophy and typical Alzheimer's disease using differential equation models. We compared and contrasted the order in which regional brain volumes become abnormal within posterior cortical atrophy and typical Alzheimer's disease using event-based models. We also examined trajectories of cognitive decline and the order in which different cognitive tests show abnormality using the same models. Temporally aligned trajectories for eight regions of interest revealed distinct (P < 0.002) patterns of progression in posterior cortical atrophy and typical Alzheimer's disease. Patients with posterior cortical atrophy showed early occipital and parietal atrophy, with subsequent higher rates of temporal atrophy and ventricular expansion leading to tissue loss of comparable extent later. Hippocampal, entorhinal and frontal regions underwent a lower rate of change and never approached the extent of posterior cortical involvement. Patients with typical Alzheimer's disease showed early hippocampal atrophy, with subsequent higher rates of temporal atrophy and ventricular expansion. Cognitive models showed tests sensitive to visuospatial dysfunction declined earlier in posterior cortical atrophy than typical Alzheimer's disease whilst tests sensitive to working memory impairment declined earlier in typical Alzheimer's disease than posterior cortical atrophy. These findings indicate that posterior cortical atrophy and typical Alzheimer's disease have distinct sites of onset and different profiles of spatial and temporal progression. The ordering of disease events both motivates investigation of biological factors underpinning phenotypic heterogeneity, and informs the selection of measures for clinical trials in posterior cortical atrophy.

Neural Mechanisms of Visual Dysfunction in Posterior Cortical Atrophy

Posterior cortical atrophy (PCA) is characterized predominantly by visual dysfunction that arises from bilateral impairments in occipital, parietal, and temporal regions of the brain. PCA is clinically identified based primarily on visual symptoms and neuroimaging findings. Region-specific gray and white matter deficits have been discussed in detail, and are associated with clinical manifestations that present with similar patterns of perfusion and metabolic findings. Here, we discuss both structural and functional changes in the ventral and dorsal visual streams along with their underlying relationships. We also discuss the most recent developments in neuroimaging characteristics and summarize correlations between distinct neuroimaging presentations.

The Progressive Acalculia Presentation of Parietal Variant Alzheimer's Disease

BACKGROUND

Many patients with early-onset Alzheimer's disease (EOAD; age of onset <65 years) have non-amnestic presentations involving language (logopenic primary progressive aphasia, lvPPA), visuospatial abilities (posterior cortical atrophy, PCA), and even asymmetric symptoms consistent with corticobasal syndrome (CBS). An inferior parietal lobule variant of EOAD commonly presents with progressive difficulty with calculations.

METHODS

We reviewed 276 EOAD patients for presentations with predominant acalculia. These patients were diagnosed with clinically probable Alzheimer's disease (AD) verified by positron emission tomography (PET) or cerebrospinal fluid amyloid-β or tau biomarkers.

RESULTS

We identified 18 (9M/9F) (6.5%) EOAD patients with progressive acalculia that did not meet most criteria for lvPPA, visual PCA, or CBS. Their ages of onset and presentation were 56.6 (5.0) and 59.4 (6.5), respectively. Their acalculia was consistent with a primary acalculia ("anarithmetia") not explained by language or visuospatial impairments. Many also had anomia (14/18), ideomotor apraxia (13/18), and the complete Gerstmann's syndrome (7/18). Visual analysis of their diverse magnetic resonance imaging disclosed biparietal atrophy, disproportionately worse on the left.

CONCLUSIONS

Primary acalculia may be the most common manifestation of an inferior parietal presentation of EOAD affecting the left intraparietal sulcus. This parietal variant also commonly involves progressive anomia, ideomotor apraxia, and other elements of Gerstmann's syndrome. The early recognition of patients with this variant, which is distinguishable from lvPPA, visual PCA, or CBS, would be facilitated by its recognition as a unique subtype of EOAD.

Functional brain network centrality is related to APOE genotype in cognitively normal elderly

INTRODUCTION

Amyloid plaque deposition in the brain is an early pathological change in Alzheimer's disease (AD), causing disrupted synaptic connections. Brain network disruptions in AD have been demonstrated with eigenvector centrality (EC), a measure that identifies central regions within networks. Carrying an apolipoprotein (APOE)-ε4 allele is a genetic risk for AD, associated with increased amyloid deposition. We studied whether APOE-ε4 carriership is associated with EC disruptions in cognitively normal individuals.

METHODS

A total of 261 healthy middle-aged to older adults (mean age 56.6 years) were divided into high-risk (APOE-ε4 carriers) and low-risk (noncarriers) groups. EC was computed from resting-state functional MRI data. Clusters of between-group differences were assessed with a permutation-based method. Correlations between cluster mean EC with brain volume, CSF biomarkers, and psychological test scores were assessed.

RESULTS

Decreased EC in the visual cortex was associated with APOE-ε4 carriership, a genetic risk factor for AD. EC differences were correlated with age, CSF amyloid levels, and scores on the trail-making and 15-object recognition tests.

CONCLUSION

Our findings suggest that the APOE-ε4 genotype affects brain connectivity in regions previously found to be abnormal in AD as a sign of very early disease-related pathology. These differences were too subtle in healthy elderly to use EC for single-subject prediction of APOE genotype.

Executive dysfunction contributes to verbal encoding and retrieval deficits in posterior cortical atrophy

Posterior Cortical Atrophy (PCA) is a neurodegenerative syndrome that typically presents with predominant visual and spatial impairments. The early diagnostic criteria specify a relative sparing of functioning in other cognitive domains, including executive functions, language, and episodic memory, yet little is known of the cognitive profile of PCA as the disease progresses. Studies of healthy adults and other posterior cortical lesion patients implicate posterior parietal and temporal regions in executive functions of working memory and verbal fluency, both of which may impact episodic memory. Relatively little has been reported about these cognitive functions in PCA, and to our knowledge there has not yet been a study of the impact of such deficits on memory function in PCA. We sought to examine PCA patients' performance on tests of executive function and the associations to verbal episodic memory encoding, storage, and delayed recall. Nineteen individuals with PCA underwent neuropsychological and neuroimaging evaluations as part of a comprehensive clinical assessment. We developed a novel consensus rating method-the Neuropsychological Assessment Rating (NAR) scale-to grade the severity of test performance impairments in selected cognitive domains and subdomains. Hypothesis-driven analyses demonstrated relative deficits in working memory and lexical-semantic retrieval. Preliminary analyses suggested associations between both deficits and atrophy in the left-hemisphere inferior parietal lobule. These executive deficits were also associated with impairments in verbal encoding and delayed recall, but not with recognition discriminability. We conclude that deficits in verbal executive functions impact verbal episodic memory in PCA. Our findings also support theories emphasizing the role of the posterior parietal cortex in supporting executive and lexical-semantic contributions to verbal episodic memory.

Progressive Compromise of Nouns and Action Verbs in Posterior Cortical Atrophy

Processing of nouns and action verbs can be differentially compromised following lesions to posterior and anterior/motor brain regions, respectively. However, little is known about how these deficits progress in the course of neurodegeneration. To address this issue, we assessed productive lexical skills in a patient with posterior cortical atrophy (PCA) at two different stages of his pathology. On both occasions, he underwent a structural brain imaging protocol and completed semantic fluency tasks requiring retrieval of animals (nouns) and actions (verbs). Imaging results were compared with those of controls via voxel-based morphometry (VBM), whereas fluency performance was compared to age-matched norms through Crawford's t-tests. In the first assessment, the patient exhibited atrophy of more posterior regions supporting multimodal semantics (medial temporal and lingual gyri), together with a selective deficit in noun fluency. Then, by the second assessment, the patient's atrophy had progressed mainly toward fronto-motor regions (rolandic operculum, inferior and superior frontal gyri) and subcortical motor hubs (cerebellum, thalamus), and his fluency impairments had extended to action verbs. These results offer unprecedented evidence of the specificity of the pathways related to noun and action-verb impairments in the course of neurodegeneration, highlighting the latter's critical dependence on damage to regions supporting motor functions, as opposed to multimodal semantic processes.

Functional and structural brain networks in posterior cortical atrophy: A two-centre multiparametric MRI study

This study identified structural and functional brain connectivity alterations in two independent samples of patients along the posterior cortical atrophy (PCA) disease course. Twenty-one PCA patients and 44 controls were recruited from two expert centres. Microstructural damage of white matter (WM) tracts was assessed using probabilistic tractography; resting state (RS) functional connectivity of brain networks was explored using a model free approach; grey matter (GM) atrophy was investigated using voxel-based morphometry. Compared with controls, common patterns of damage across PCA patients included: GM atrophy in the occipital-temporal-parietal regions; diffusion tensor (DT) MRI alterations of the corpus callosum and superior (SLF) and inferior longitudinal fasciculi (ILF) bilaterally; and decreased functional connectivity of the occipital gyri within the visual network and the precuneus and posterior cingulum within the default mode network (DMN). In PCA patients with longer disease duration and greater disease severity, WM damage extended to the cingulum and RS functional connectivity alterations spread within the frontal, dorsal attentive and salience networks. In PCA, reduced DMN functional connectivity was associated with SLF and ILF structural alterations. PCA patients showed distributed WM damage. Altered RS functional connectivity extends with disease worsening from occipital to temporo-parietal and frontostriatal regions, and this is likely to occur through WM connections. Future longitudinal studies are needed to establish trajectories of damage spreading in PCA and whether a combined DT MRI/RS functional MRI approach is promising in monitoring the disease progression.

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PCA patients showed distributed WM damage.

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In PCA, WM damage is associated with longer disease duration ad greater severity.

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In PCA, altered RS functional connectivity extends with disease worsening.

Cortico-thalamic disconnection in a patient with supernumerary phantom limb

Supernumerary phantom limb (SPL) designates the experience of an illusory additional limb occurring after brain damage. Functional neuroimaging during SPL movements documented increased response in the ipsilesional supplementary motor area (SMA), premotor cortex (PMC), thalamus and caudate. This suggested that motor circuits are important for bodily related cognition, but anatomical evidence is sparse. Here, we tested this hypothesis by studying an extremely rare patient with chronic SPL, still present 3 years after a vascular stroke affecting cortical and subcortical right-hemisphere structures. Anatomical analysis included an advanced in vivo reconstruction of white matter tracts using diffusion-based spherical deconvolution. This reconstruction demonstrated a massive and relatively selective disconnection between anatomically preserved SMA/PMC and the thalamus. Our results provide strong anatomical support for the hypothesis that cortico-thalamic loops involving motor-related circuits are crucial to integrate sensorimotor processing with bodily self-awareness.

Early-Onset Alzheimer Disease

Early-onset Alzheimer disease (EOAD), with onset in individuals younger than 65 years, although overshadowed by the more common late-onset AD (LOAD), differs significantly from LOAD. EOAD comprises approximately 5% of AD and is associated with delays in diagnosis, aggressive course, and age-related psychosocial needs. One source of confusion is that a substantial percentage of EOAD are phenotypic variants that differ from the usual memory-disordered presentation of typical AD. The management of EOAD is similar to that for LOAD, but special emphasis should be placed on targeting the specific cognitive areas involved and more age-appropriate psychosocial support and education.

Drawing Disorders in Alzheimer's Disease and Other Forms of Dementia

Drawing is a multicomponential process that can be impaired by many kinds of brain lesions. Drawing disorders are very common in Alzheimer's disease and other forms of dementia, and can provide clinical information for the distinction of the different dementing diseases. In our review we started from an overview of the neural and cognitive bases of drawing, and from a recollection of the drawing tasks more frequently used for assessing individuals with dementia. Then, we analyzed drawing disorders in dementia, paying special attention to those observed in Alzheimer's disease, from the prodromal stages of the amnesic mild cognitive impairment to the stages of full-blown dementia, both in the sporadic forms with late onset in the entorhino-hippocampal structures and in those with early onset in the posterior neocortical structures. We reviewed the drawing features that could differentiate Alzheimer's disease from vascular dementia and from the most frequent forms of degenerative dementia, namely frontotemporal dementia and Lewy body disease. Finally, we examined some peculiar aspects of drawing disorders in dementia, such as perseverations, rotations, and closing-in. We argue that a careful analysis of drawing errors helps to differentiate the different forms of dementia more than overall accuracy in drawing.

Lateralized occipital degeneration in posterior cortical atrophy predicts visual field deficits

Background

Posterior cortical atrophy (PCA), the visual variant of Alzheimer's disease, leads to high-level visual deficits such as alexia or agnosia. Visual field deficits have also been identified, but often inconsistently reported. Little is known about the pattern of visual field deficits or the underlying cortical changes leading to this visual loss.

Methods

Multi-modal magnetic resonance imaging was used to investigate differences in gray matter volume, cortical thickness, white matter microstructure and functional activity in patients with PCA compared to age-matched controls. Additional analyses investigated hemispheric asymmetries in these metrics according to the visual field most affected by the disease.

Results

Analysis of structural data indicated considerable loss of gray matter in the occipital and parietal cortices, lateralized to the hemisphere contralateral to the visual loss. This lateralized pattern of gray matter loss was also evident in the hippocampus and parahippocampal gyrus. Diffusion-weighted imaging showed considerable effects of PCA on white matter microstructure in the occipital cortex, and in the corpus callosum. The change in white matter was only lateralized in the occipital lobe, however, with greatest change in the optic radiation contralateral to the visual field deficit. Indeed, there was a significant correlation between the laterality of the optic radiation microstructure and visual field loss.

Conclusions

Detailed brain imaging shows that the asymmetric visual field deficits in patients with PCA reflect the pattern of degeneration of both white and gray matter in the occipital lobe. Understanding the nature of both visual field deficits and the neurodegenerative brain changes in PCA may improve diagnosis and understanding of this disease.

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Patients with posterior cortical atrophy show asymmetric visual field deficits manifesting as hemianopia.

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Both gray and white matter show lateralized degeneration corresponding to the most affected visual field.

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Laterality of microstructure in the optic radiation correlates with visual field loss.

Botallo's error, or the quandaries of the universality assumption

One of the founding principles of human cognitive neuroscience is the so-called universality assumption, the postulate that neurocognitive mechanisms do not show major differences among individuals. Without negating the importance of the universality assumption for the development of cognitive neuroscience, or the importance of single-case studies, here we aim at stressing the potential dangers of interpreting the pattern of performance of single patients as conclusive evidence concerning the architecture of the intact neurocognitive system. We take example from the case of Leonardo Botallo, an Italian surgeon of the Renaissance period, who claimed to have discovered a new anatomical structure of the adult human heart. Unfortunately, Botallo's discovery was erroneous, because what he saw in the few samples he examined was in fact the anomalous persistence of a fetal structure. Botallo's error is a reminder of the necessity to always strive for replication, despite the major hindrance of a publication system heavily biased towards novelty. In the present paper, we briefly discuss variations and anomalies in human brain anatomy and introduce the issue of variability in cognitive neuroscience. We then review some examples of the impact on cognition of individual variations in (1) brain structure, (2) brain functional organization and (3) brain damage. We finally discuss the importance and limits of single case studies in the neuroimaging era, outline potential ways to deal with individual variability, and draw some general conclusions.

Non-pharmacological intervention for posterior cortical atrophy

Posterior cortical atrophy (PCA) is a rare neurodegenerative condition characterized by progressive visual-perceptual deficits. Although the neurocognitive profile of PCA is a growing and relatively well-established field, non-pharmacological care remains understudied and to be widely established in clinical practice. In the present work we review the available literature on non-pharmacological approaches for PCA, such as cognitive rehabilitation including individual cognitive exercises and compensatory techniques to improve autonomy in daily life, and psycho-education aiming to inform people with PCA about the nature of their visual deficits and limits of cognitive rehabilitation. The reviewed studies represented a total of 7 patients. There is a scarcity of the number of studies, and mostly consisting of case studies. Results suggest non-pharmacological intervention to be a potentially beneficial approach for the partial compensation of deficits, improvement of daily functionality and improvement of quality of life. Clinical implications and future directions are also highlighted for the advancement of the field, in order to clarify the possible role of non-pharmacological interventions, and its extent, in PCA.

Loss of functional connectivity is greater outside the default mode network in nonfamilial early-onset Alzheimer's disease variants

The common and specific involvement of brain networks in clinical variants of Alzheimer's disease (AD) is not well understood. We performed task-free ("resting-state") functional imaging in 60 nonfamilial AD patients, including 20 early-onset AD (age at onset <65 years, amnestic/dysexecutive deficits), 24 logopenic aphasia (language deficits), and 16 posterior cortical atrophy patients (visual deficits), as well as 60 healthy controls. Seed-based connectivity analyses were conducted to assess differences between groups in 3 default mode network (DMN) components (anterior, posterior, and ventral) and 4 additional non-DMN networks: left and right executive-control, language, and higher visual networks. Significant decreases in connectivity were found across AD variants compared with controls in the non-DMN networks. Within the DMN components, patients showed higher connectivity in the anterior DMN, in particular in logopenic aphasia. No significant differences were found for the posterior and ventral DMN. Our findings suggest that loss of functional connectivity is greatest in networks outside the DMN in early-onset and nonamnestic AD variants and may thus be a better biomarker in these patients.

Allocentric spatial neglect with posterior cortical atrophy

Patients with posterior cortical atrophy (PCA) have been reported to have neglect in the egocentric/ body-centered reference frame. This report describes a woman with PCA who had a right-sided stimulus-based form of allocentric visual neglect on cancellation, reading, and drawing tests. Her brain imaging revealed left parietal atrophy. The pathophysiology of this disorder may be related to an impairment of the ventral "what" stream's ability to interact with the dorsal "where" stream that mediates the allocation of spatial attention, or a deficit in the systems that allocate contralateral focal attention. Further research is needed to better understand the mechanisms of this disorder and to optimally treat it.

Posterior cortical atrophy and Alzheimer's disease: a meta-analytic review of neuropsychological and brain morphometry studies

This paper presents the first systematic review and meta-analysis of neuropsychological and brain morphometry studies comparing posterior cortical atrophy (PCA) to typical Alzheimer's disease (tAD). Literature searches were conducted for brain morphometry and neuropsychological studies including a PCA and a tAD group. Compared to healthy controls (HC), PCA patients exhibited significant decreases in temporal, occipital and parietal gray matter (GM) volumes, whereas tAD patients showed extensive left temporal atrophy. Compared to tAD patients, participants with PCA showed greater GM volume reduction in the right occipital gyrus extending to the posterior lobule. In addition, PCA patients showed less GM volume loss in the left parahippocampal gyrus and left hippocampus than tAD patients. PCA patients exhibit significantly greater impairment in Immediate Visuospatial Memory as well as Visuoperceptual and Visuospatial Abilities than patients with tAD. However, tAD patients showed greater impairment in Delayed Auditory/Verbal Memory than patients with PCA. PCA is characterized by significant atrophy of the occipital and parietal regions and severe impairments in visuospatial functioning.

Alteration of functional connectivity within visuospatial working memory-related brain network in patients with right temporal lobe epilepsy: a resting-state fMRI study

PURPOSE

This study aimed to investigate the resting-state brain network related to visuospatial working memory (VSWM) in patients with right temporal lobe epilepsy (rTLE). The functional mechanism underlying the cognitive impairment in VSWM was also determined.

METHOD

Fifteen patients with rTLE and 16 healthy controls matched for age, gender, and handedness underwent a 6-min resting-state functional MRI session and a neuropsychological test using VSWM_Nback. The VSWM-related brain network at rest was extracted using multiple independent component analysis; the spatial distribution and the functional connectivity (FC) parameters of the cerebral network were compared between groups. Behavioral data were subsequently correlated with the mean Z-value in voxels showing significant FC difference during intergroup comparison.

RESULTS

The distribution of the VSWM-related resting-state network (RSN) in the group with rTLE was virtually consistent with that in the healthy controls. The distribution involved the dorsolateral prefrontal lobe and parietal lobe in the right hemisphere and the partial inferior parietal lobe and posterior lobe of the cerebellum in the left hemisphere (p<0.05, AlphaSim corrected). Between-group differences suggest that the group with rTLE had a decreased FC within the right superior frontal lobe (BA8), right middle frontal lobe, and right ventromedial prefrontal lobe compared with the controls (p<0.05, AlphaSim corrected). The regions of increased FC in rTLE were localized within the right superior frontal lobe (BA11), right superior parietal lobe, and left posterior lobe of the cerebellum (p<0.05, AlphaSim corrected). Moreover, patients with rTLE performed worse than controls in the VSWM_Nback test, and there were negative correlations between ACCmeanRT (2-back) and the mean Z-value in the voxels showing decreased or increased FC in rTLE (p<0.05).

CONCLUSION

The results suggest that the alteration of the VSWM-related RSN might underpin the VSWM impairment in patients with rTLE and possibly implies a functional compensation by enlarging the FC within the ipsilateral cerebral network.

Individualized Gaussian process-based prediction and detection of local and global gray matter abnormalities in elderly subjects

Structural imaging based on MRI is an integral component of the clinical assessment of patients with potential dementia. We here propose an individualized Gaussian process-based inference scheme for clinical decision support in healthy and pathological aging elderly subjects using MRI. The approach aims at quantitative and transparent support for clinicians who aim to detect structural abnormalities in patients at risk of Alzheimer's disease or other types of dementia. Firstly, we introduce a generative model incorporating our knowledge about normative decline of local and global gray matter volume across the brain in elderly. By supposing smooth structural trajectories the models account for the general course of age-related structural decline as well as late-life accelerated loss. Considering healthy subjects' demography and global brain parameters as informative about normal brain aging variability affords individualized predictions in single cases. Using Gaussian process models as a normative reference, we predict new subjects' brain scans and quantify the local gray matter abnormalities in terms of Normative Probability Maps (NPM) and global z-scores. By integrating the observed expectation error and the predictive uncertainty, the local maps and global scores exploit the advantages of Bayesian inference for clinical decisions and provide a valuable extension of diagnostic information about pathological aging. We validate the approach in simulated data and real MRI data. We train the GP framework using 1238 healthy subjects with ages 18–94 years, and predict in 415 independent test subjects diagnosed as healthy controls, Mild Cognitive Impairment and Alzheimer's disease.

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We propose an approach to support individualized clinical decisions in elderlies.

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Gaussian process models are used to build a normative generative model of aging.

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It affords probabilistic predictions of local gray matter volume in subjects.

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We validate the model using simulated and large real MRI data samples.

Novel brain imaging approaches to understand acquired and congenital neuro-ophthalmological conditions

PURPOSE OF REVIEW

The arrival of large datasets and the on-going refinement of neuroimaging technology have led to a number of recent advances in our understanding of visual pathway disorders. This work can broadly be classified into two areas, both of which are important when considering the optimal management strategies. The first looks at the delineation of damage, teasing out subtle changes to (specific components of) the visual pathway, which may help evaluate the severity and extent of disease. The second uses neuroimaging to investigate neuroplasticity, via changes in connectivity, cortical thickness, and retinotopic maps within the visual cortex.

RECENT FINDINGS

Here, we give consideration to both acquired and congenital patients with damage to the visual pathway, and how they differ. Congenital disorders of the peripheral visual system can provide insight into the large-scale reorganization of the visual cortex: these are investigated with reference to disorders of the optic chiasm and anophthalmia (absence of the eyes). In acquired conditions, we consider the recent work describing patterns of degeneration, both following single insult and in neurodegenerative conditions. We also discuss the developments in functional neuroimaging, with particular reference to work on hemianopia and the controversial suggestion of cortical reorganization following acquired retinal injury.

SUMMARY

Techniques for comparing neuro-ophthalmological conditions with healthy visual systems provide sensitive metrics to uncover subtle differences in grey and white matter structure of the brain. It is now possible to compare the massive reorganization present in congenital conditions with the apparent lack of plasticity following acquired damage.

Deprivation-related and use-dependent plasticity go hand in hand

Arm-amputation involves two powerful drivers for brain plasticity-sensory deprivation and altered use. However, research has largely focused on sensory deprivation and maladaptive change. Here we show that adaptive patterns of limb usage after amputation drive cortical plasticity. We report that individuals with congenital or acquired limb-absence vary in whether they preferentially use their intact hand or residual arm in daily activities. Using fMRI, we show that the deprived sensorimotor cortex is employed by whichever limb individuals are over-using. Individuals from either group that rely more on their intact hands (and report less frequent residual arm usage) showed increased intact hand representation in the deprived cortex, and increased white matter fractional anisotropy underlying the deprived cortex, irrespective of the age at which deprivation occurred. Our results demonstrate how experience-driven plasticity in the human brain can transcend boundaries that have been thought to limit reorganisation after sensory deprivation in adults. DOI: http://dx.doi.org/10.7554/eLife.01273.001.

Distributed circuits, not circumscribed centers, mediate visual recognition

Increasingly, the neural mechanisms that support visual cognition are being conceptualized as a distributed but integrated system, as opposed to a set of individual, specialized regions that each subserve a particular visual behavior. Consequently, there is an emerging emphasis on characterizing the functional, structural,and computational properties of these broad networks [corrected]. We present a novel theoretical perspective, which elucidates the developmental emergence, computational properties, and vulnerabilities of integrated circuits using face and word recognition as model domains. Additionally, we suggest that, rather than being disparate and independent, these neural circuits are overlapping and subject to the same computational constraints. Specifically, we argue that both word and face recognition rely on fine-grained visual representations but, by virtue of pressure to couple visual and language areas and to keep connection length short, the left hemisphere becomes more finely tuned for word recognition and, consequently, the right hemisphere becomes more finely tuned for face recognition. Thus, both hemispheres ultimately participate in both forms of visual recognition, but their respective contributions are asymmetrically weighted.

White matter atrophy in Alzheimer's disease variants

BACKGROUND

In comparison with late-onset Alzheimer's disease (LOAD, onset, >65 years), early-age-of-onset Alzheimer's disease (EOAD, onset, <65 years) more often presents with language, visuospatial, and/or executive impairment, often occurring earlier than a progressive memory deficit. The logopenic variant of primary progressive aphasia (lv-PPA) and posterior cortical atrophy (PCA) have recently been described as possible atypical variants of EOAD. Lv-PPA is characterized by isolated language deficit, whereas PCA is characterized by predominant visuospatial deficits. Severe hemispheric gray matter (GM) atrophy associated with EOAD, lv-PPA, and PCA has been described, but regional patterns of white matter (WM) damage are still poorly understood.

METHODS

Using structural magnetic resonance imaging and voxel-based morphometry, we investigated WM damage in patients with EOAD (n = 16), PCA (n = 13), lv-PPA (n = 10), and LOAD (n = 14) at presentation and 72 age-matched control subjects.

RESULTS

In patients with EOAD, PCA, and lv-PPA, WM atrophy was centered on the lateral temporal and parietal regions, including the cingulum and posterior corpus callosum. Compared with control subjects, patients with lv-PPA showed more severe left parietal damage, and patients with PCA showed more severe occipital atrophy. Moreover, patients with EOAD had greater cingulum atrophy compared with those with LOAD. LOAD showed WM damage in the medial temporal regions and less extensive hemispheric involvement.

CONCLUSION

Patterns of WM damage in EOAD, lv-PPA, and PCA are consistent with the clinical syndromes and GM atrophy patterns. WM injury in AD atypical variants may contribute to symptoms and disease pathogenesis.

When the single matters more than the group: very high false positive rates in single case Voxel Based Morphometry

Voxel Based Morphometry (VBM) studies typically involve a comparison between groups of individuals; this approach however does not allow inferences to be made at the level of the individual. In recent years, an increasing number of research groups have attempted to overcome this issue by performing single case studies, which involve the comparison between a single subject and a control group. However, the interpretation of the results is problematic; for instance, any significant difference might be driven by individual variability in neuroanatomy rather than the neuropathology of the disease under investigation, or might represent a false positive due to the data being sampled from non-normally distributed populations. The aim of the present investigation was to empirically estimate the likelihood of detecting significant differences in gray matter volume in individuals free from neurological or psychiatric diagnosis. We compared a total of 200 single subjects against a group of 16 controls matched for age and gender, using two independent datasets from the Neuroimaging Informatics Tools and Resources Clearinghouse. We report that the chance of detecting a significant difference in a disease-free individual is much higher than previously expected; for instance, using a standard voxel-wise threshold of p<0.05 (corrected) and an extent threshold of 10 voxels, the likelihood of a single subject showing at least one significant difference is as high as 93.5% for increases and 71% for decreases. We also report that the chance of detecting significant differences was greatest in frontal and temporal cortices and lowest in subcortical regions. The chance of detecting significant differences was inversely related to the degree of smoothing applied to the data, and was higher for unmodulated than modulated data. These results were replicated in the two independent datasets. By providing an empirical estimation of the number of significant increases and decreases to be expected in each cortical and subcortical region in disease-free individuals, the present investigation could inform the interpretation of future single case VBM studies.

Early-onset Alzheimer's disease: nonamnestic subtypes and type 2 AD

Patients with Alzheimer's disease (AD), the most prevalent neurodegenerative dementia, are usually elderly; however, ∼4-5% develop early-onset AD (EOAD) with onset before age 65. Most EOAD is sporadic, but about 5% of patients with EOAD have an autosomal dominant mutation such as Presenilin 1, Presenilin 2, or alterations in the Amyloid Precursor Protein gene. Although most Alzheimer's research has concentrated on older, late-onset AD (LOAD), there is much recent interest and research in EOAD. These recent studies indicate that EOAD is a heterogeneous disorder with significant differences from LOAD. From 22-64% of EOAD patients have a predominant nonamnestic syndrome presenting with deficits in language, visuospatial abilities, praxis, or other non-memory cognition. These nonamnestic patients may differ in several ways from the usual memory or amnestic patients. Patients with nonamnestic EOAD compared to typical amnestic AD have a more aggressive course, lack the apolipoprotein Eɛ4 (APOE ɛ4) susceptibility gene for AD, and have a focus and early involvement of non-hippocampal areas of brain, particularly parietal neocortex. These differences in the EOAD subtypes indicate differences in the underlying amyloid cascade, the prevailing pathophysiological theory for the development of AD. Together the results of recent studies suggest that nonamnestic subtypes of EOAD constitute a Type 2 AD distinct from the usual, typical disorder. In sum, the study of EOAD can reveal much about the clinical heterogeneity, predisposing factors, and neurobiology of this disease.

Ventral and dorsal visual streams in posterior cortical atrophy: a DT MRI study

Using diffusion tensor magnetic resonance imaging tractography, ventral (inferior longitudinal fasciculus) and fronto-occipital (inferior fronto-occipital fasciculus) and dorsal (fronto-parietal superior longitudinal fasciculus) visual pathways were assessed in 7 patients with posterior cortical atrophy (PCA), showing either predominantly ventral or additional dorsal cognitive deficits. Corpus callosum and corticospinal tracts were also studied. Gray and white matter atrophy was assessed using voxel-based morphometry. In all PCA patients, abnormal diffusivity indexes were found in bilateral inferior longitudinal fasciculus and inferior fronto-occipital fasciculus, with a left-side predominance. Patients also had mild microstructural damage to the corpus callosum. The 2 patients with more dorsal symptoms also showed right fronto-parietal superior longitudinal fasciculus abnormalities. Corticospinal tracts were normal, bilaterally. When studied separately, patients with ventral clinical impairment showed a pattern of atrophy mainly located in the ventral occipitotemporal regions, bilaterally; patients with both ventral and dorsal clinical deficits showed additional atrophy of the bilateral inferior parietal lobe. Magnetic resonance imaging patterns of abnormalities mirror closely the clinical phenotypes and could provide reliable ante mortem markers of tissue damage in PCA.

Structural connectivity in a single case of progressive prosopagnosia: the role of the right inferior longitudinal fasciculus

Progressive prosopagnosia (PP) is a clinical syndrome characterized by a progressive and selective inability to recognize and identify faces of familiar people. Here we report a patient (G.S.) with PP, mainly related to a prominent deficit in recognition of familiar faces, without a semantic (cross-modal) impairment. An in-depth evaluation showed that his deficit extended to other classes of objects, both living and non-living. A follow-up neuropsychological assessment did not reveal substantial changes after about 1 year. Structural MRI showed predominant right temporal lobe atrophy. Diffusion tensor imaging was performed to elucidate structural connectivity of the inferior longitudinal fasciculus (ILF) and the inferior fronto-occipital fasciculus (IFOF), the two major tracts that project through the core fusiform region to the anterior temporal and frontal cortices, respectively. Right ILF was markedly reduced in G.S., while left ILF and IFOFs were apparently preserved. These data are in favour of a crucial role of the neural circuit subserved by right ILF in the pathogenesis of PP.

The paradox of syndromic diversity in Alzheimer disease

Variant syndromes of Alzheimer disease (AD), led by deficits that extend beyond memory dysfunction, are of considerable clinical and neurobiological importance. Such syndromes present major challenges for both diagnosis and monitoring of disease, and serve to illustrate the apparent paradox of a clinically diverse group of disorders underpinned by a common histopathological substrate. This Review focuses on the most common variant AD phenotypes: posterior cortical atrophy, logopenic variant primary progressive aphasia and frontal variant AD. The neuroanatomical, molecular and pathological correlates of these phenotypes are highlighted, and the heterogeneous clinical presentations of the syndromes are discussed in the context of the emerging network paradigm of neurodegenerative disease. We argue that these apparently diverse clinical phenotypes reflect the differential involvement of a common core temporoparietofrontal network that is vulnerable to AD. According to this interpretation, the network signatures corresponding to AD variant syndromes are produced by genetic and other modulating factors that have yet to be fully characterized. The clinical and neurobiological implications of this network paradigm in the quest for disease-modifying treatments are also explored.

Brain networks of visuospatial attention and their disruption in visual neglect

Visual neglect is a multi-component syndrome including prominent attentional disorders. Research on the functional mechanisms of neglect is now moving from the description of dissociations in patients' performance to the identification of the possible component deficits and of their interaction with compensatory strategies. In recent years, the dissection of attentional deficits in neglect has progressed in parallel with increasing comprehension of the anatomy and function of large-scale brain networks implicated in attentional processes. This review focuses on the anatomy and putative functions of attentional circuits in the brain, mainly subserved by fronto-parietal networks, with a peculiar although not yet completely elucidated role for the right hemisphere. Recent results are discussed concerning the influence of a non-spatial attentional function, phasic alertness, on conscious perception in normal participants and on conflict resolution in neglect patients. The rapid rate of expansion of our knowledge of these systems raises hopes for the development of effective strategies to improve the functioning of the attentional networks in brain-damaged patients.

Posterior cortical atrophy

Posterior cortical atrophy (PCA) is a neurodegenerative syndrome that is characterised by progressive decline in visuospatial, visuoperceptual, literacy, and praxic skills. The progressive neurodegeneration affecting parietal, occipital, and occipitotemporal cortices that underlies PCA is attributable to Alzheimer's disease in most patients. However, alternative underlying causes, including dementia with Lewy bodies, corticobasal degeneration, and prion disease, have also been identified, and not all patients with PCA have atrophy on clinical imaging. This heterogeneity has led to inconsistencies in diagnosis and terminology and difficulties in comparing studies from different centres, and has restricted the generalisability of findings from clinical trials and investigations of factors that drive phenotypic variability. Important challenges remain, including the identification of factors associated not only with the selective vulnerability of posterior cortical regions but also with the young age of onset of PCA. Greater awareness of the syndrome and agreement over the correspondence between syndrome-level and disease-level classifications are needed to improve diagnostic accuracy, clinical management, and the design of research studies.