The relationship of topographical memory performance to regional neurodegeneration in Alzheimer's disease

The network activated during normal route learning shares considerable homology with the network of degeneration in the earliest symptomatic stages of Alzheimer's disease (AD). This inspired the virtual route learning test (VRLT) in which patients learn routes in a virtual reality environment. This study investigated the neural basis of VRLT performance in AD to test whether impairment was underpinned by a network or by the widely held explanation of hippocampal degeneration. VRLT score in a mild AD cohort was regressed against gray matter (GM) density and diffusion tensor metrics of white matter (WM) (n = 30), and, cerebral glucose metabolism (n = 26), using a mass univariate approach. GM density and cerebral metabolism were then submitted to a multivariate analysis [support vector regression (SVR)] to examine whether there was a network associated with task performance. Univariate analyses of GM density, metabolism and WM axial diffusion converged on the vicinity of the retrosplenial/posterior cingulate cortex, isthmus and, possibly, hippocampal tail. The multivariate analysis revealed a significant, right hemisphere-predominant, network level correlation with cerebral metabolism; this comprised areas common to both activation in normal route learning and early degeneration in AD (retrosplenial and lateral parietal cortices). It also identified right medio-dorsal thalamus (part of the limbic-diencephalic hypometabolic network of early AD) and right caudate nucleus (activated during normal route learning). These results offer strong evidence that topographical memory impairment in AD relates to damage across a network, in turn offering complimentary lesion evidence to previous studies in healthy volunteers for the neural basis of topographical memory. The results also emphasize that structures beyond the mesial temporal lobe (MTL) contribute to memory impairment in AD—it is too simplistic to view memory impairment in AD as a synonym for hippocampal degeneration.

Social cognitive deficits and their neural correlates in progressive supranuclear palsy

Although progressive supranuclear palsy is defined by its akinetic rigidity, vertical supranuclear gaze palsy and falls, cognitive impairments are an important determinant of patients’ and carers’ quality of life. Here, we investigate whether there is a broad deficit of modality-independent social cognition in progressive supranuclear palsy and explore the neural correlates for these. We recruited 23 patients with progressive supranuclear palsy (using clinical diagnostic criteria, nine with subsequent pathological confirmation) and 22 age- and education-matched controls. Participants performed an auditory (voice) emotion recognition test, and a visual and auditory theory of mind test. Twenty-two patients and 20 controls underwent structural magnetic resonance imaging to analyse neural correlates of social cognition deficits using voxel-based morphometry. Patients were impaired on the voice emotion recognition and theory of mind tests but not auditory and visual control conditions. Grey matter atrophy in patients correlated with both voice emotion recognition and theory of mind deficits in the right inferior frontal gyrus, a region associated with prosodic auditory emotion recognition. Theory of mind deficits also correlated with atrophy of the anterior rostral medial frontal cortex, a region associated with theory of mind in health. We conclude that patients with progressive supranuclear palsy have a multimodal deficit in social cognition. This deficit is due, in part, to progressive atrophy in a network of frontal cortical regions linked to the integration of socially relevant stimuli and interpretation of their social meaning. This impairment of social cognition is important to consider for those managing and caring for patients with progressive supranuclear palsy.

Multicenter stability of diffusion tensor imaging measures: a European clinical and physical phantom study

Diffusion tensor imaging (DTI) detects white matter damage in neuro-psychiatric disorders, but data on reliability of DTI measures across more than two scanners are still missing. In this study we assessed multicenter reproducibility of DTI acquisitions based on a physical phantom as well as brain scans across 16 scanners. In addition, we performed DTI scans in a group of 26 patients with clinically probable Alzheimer's disease (AD) and 12 healthy elderly controls at one single center. We determined the variability of fractional anisotropy (FA) measures using manually placed regions of interest as well as automated tract based spatial statistics and deformation based analysis. The coefficient of variation (CV) of FA was 6.9% for the physical phantom data. The mean CV across the multicenter brain scans was 14% for tract based statistics, and 29% for deformation based analysis. The degree of variation was higher in less organized fiber tracts. Our findings suggest that a clinical and physical phantom study involving more than two scanners is indispensable to detect potential sources of bias and to reliably estimate effect size in multicenter diagnostic trials using DTI.

Quantification of receptor-ligand binding potential in sub-striatal domains using probabilistic and template regions of interest

Sub-striatal regions of interest (ROIs) are widely used in PET studies to investigate the role of dopamine in the modulation of neural networks implicated in emotion, cognition and motor function. One common approach is that of Mawlawi et al. (2001) and Martinez et al. (2003), where each striatum is divided into five sub-regions. This study focuses on the use of two spatial normalization-based alternatives to manual sub-striatal ROI delineation per subject: manual ROI delineation on a template brain and the production of probabilistic ROIs from a set of subject-specific manually delineated ROIs. Two spatial normalization algorithms were compared: SPM5 unified segmentation and ART. The ability of these methods to quantify sub-striatal regional non-displaceable binding potential (BP(ND)) and BP(ND) % change (following methylphenidate) was tested on 32 subjects (16 controls and 16 ADHD patients) scanned with the dopamine D(2)/D(3) ligand [(18)F]fallypride. Probabilistic ROIs produced by ART provided the best results, with similarity index values against subject-specific manual ROIs of 0.75-0.89 (mean 0.84) compared to 0.70-0.85 (mean 0.79) for template ROIs. Correlations (r) for BP(ND) and BP(ND) % change between subject-specific manual ROIs and these probabilistic ROIs of 0.90-0.98 (mean 0.95) and 0.98-1.00 (mean 0.99) respectively were superior overall to those obtained with template ROIs, although only marginally so for BP(ND) % change. The significance of relationships between BP(ND) measures and both behavioural tasks and methylphenidate plasma levels was preserved with ART combined with both probabilistic and template ROIs. SPM5 virtually matched the performance of ART for BP(ND) % change estimation but was inferior for BP(ND) estimation in caudate sub-regions. ART spatial normalization combined with probabilistic ROIs and to a lesser extent template ROIs provides an efficient and accurate alternative to time-consuming manual sub-striatal ROI delineation per subject, especially when the parameter of interest is BP(ND) % change.

What the left and right anterior fusiform gyri tell us about semantic memory

The study of patients with semantic dementia, a variant of frontotemporal lobar degeneration, has emerged over the last two decades as an important lesion model for studying human semantic memory. Although it is well-known that semantic dementia is associated with temporal lobe degeneration, controversy remains over whether the semantic deficit is due to diffuse temporal lobe damage, damage to only a sub-region of the temporal lobe or even less severe damage elsewhere in the brain. The manner in which the right and left temporal lobes contribute to semantic knowledge is also not fully elucidated. In this study we used unbiased imaging analyses to correlate resting cerebral glucose metabolism and behavioural scores in tests of verbal and non-verbal semantic memory. In addition, a region of interest analysis was performed to evaluate the role of severely hypometabolic areas. The best, indeed the only, strong predictor of semantic scores across a set of 21 patients with frontotemporal lobar degeneration with semantic impairment was degree of hypometabolism in the anterior fusiform region subjacent to the head and body of the hippocampus. As hypometabolism in the patients' rostral fusiform was even more extreme than the abnormality in other regions with putative semantic relevance, such as the temporal poles, the significant fusiform correlations cannot be attributed to floor-level function in these other regions. More detailed analysis demonstrated more selective correlations: left anterior fusiform function predicted performance on two expressive verbal tasks, whereas right anterior fusiform metabolism predicted performance on a non-verbal test of associative semantic knowledge. This pattern was further supported by an additional behavioural study performed on a wider cohort of patients with semantic dementia, in which the patients with more extensive right-temporal atrophy (when matched on degree of naming deficit to a set of cases with more extensive left temporal atrophy) were significantly more impaired on the test of non-verbal semantics. Our preferred interpretation of this laterality effect involves differential strength of connectivity between different regions of a widespread semantic network in the human brain.

Absolute diffusivities define the landscape of white matter degeneration in Alzheimer's disease

Recent imaging evidence in Alzheimer's disease suggests that neural involvement in early-stage disease is more complex than is encapsulated in the commonly held position of predominant mesial temporal lobe degeneration-there is also early posterior cingulate cortex and diencephalic damage. These findings suggest that early clinical Alzheimer's disease is underpinned by damage to an inter-connected network. If correct, this hypothesis would predict degeneration of the white matter pathways that connect this network. This prediction can be tested in vivo by diffusion magnetic resonance imaging. Most diffusion tensor imaging studies of white matter in neurodegenerative disorders such as Alzheimer's disease have concentrated on fractional anisotropy reductions and increased 'apparent' diffusivity; however, there is a lack of empirical biological evidence to assume that fractional anisotropy changes will necessarily capture the full extent of white matter changes in Alzheimer's disease. In this study, therefore, we undertook a comprehensive investigation of diffusion behaviour in Alzheimer's disease by analysing each of the component eigenvalues of the diffusion tensor in isolation to test the hypothesis that early Alzheimer's disease is associated with degeneration of a specific neural network. Using tract-based spatial statistics, we performed voxel-wise analyses of fractional anisotropy, axial, radial and mean diffusivities in 25 Alzheimer's disease patients compared with 13 elderly controls. We found that increased absolute (axial, radial and mean) diffusivities in Alzheimer's disease were concordant in a distribution consistent with the network hypothesis, highly statistically significant and far more sensitive than fractional anisotropy reductions. The former three measures identified confluent white matter abnormalities in parahippocampal gyrus and posterior cingulum, extending laterally into adjacent temporo-parietal regions as well as splenium and fornix. The caudal occipital lobe, temporal pole, genu and prefrontal white matter were relatively preserved. This distribution is highly consistent with expected predictions of tract degeneration from grey matter lesions identified by fluorodeoxyglucose positron emission tomography and structural magnetic resonance imaging. Concordant with results from these other imaging modalities, this pattern predominantly involves degeneration of the tracts connecting the circuit of Papez. These findings also highlight that early neuropathological processes are associated with changes of the diffusion ellipsoid that are predominantly proportional along all semi-principal axes.

Registration accuracy for VBM studies varies according to region and degenerative disease grouping

Voxel-based morphometry studies are frequently cited as having the advantage of being objective compared to region-of-interest methods. This statement assumes, however, that all regions are treated equally both in controls and diseased cohorts. This study aimed to test whether this statement is correct by analyzing fiducial landmarks in controls, Alzheimer's disease (as a model of mild generalized atrophy model); Frontotemporal Dementia (focal atrophy model) and Semantic Dementia (extreme focal atrophy model). Standard SPM5 and DARTEL were evaluated using either raw or skull-stripped/bias corrected scans. The results indicated that with all methods there was variability in the degree of misregistration across regions and that there was a disease grouping interaction-most severely in the extreme focal atrophy model (Semantic Dementia). Preprocessing improved VBM outputs both with standard SPM and DARTEL. In the latter case, this occurred to an extreme degree-DARTEL using raw data was grossly insensitive to a ground truth (manually verified hippocampal atrophy in AD) whereas DARTEL after preprocessing yielded excellent results with respect to this yardstick.

Atrophy patterns in histologic vs clinical groupings of frontotemporal lobar degeneration

OBJECTIVE

Predictable patterns of atrophy are associated with the clinical subtypes of frontotemporal dementia (FTD): behavioral variant (bvFTD), semantic dementia (SEMD), and progressive nonfluent aphasia (PNFA). Some studies of pathologic subtypes have also suggested specific atrophy patterns; however, results are inconsistent. Our aim was to test the hypothesis that clinical, but not pathologic, classification (FTD with ubiquitin inclusions [FTD-U] and FTD with tau inclusions [FTD-T]) is associated with predictable patterns of regional atrophy.

METHODS

Magnetic resonance scans of nine FTD-U and six FTD-T patients (histologically confirmed) were compared with 25 controls using voxel-based morphometry (VBM). Analyses were conducted with the patient group classified according to histologic or clinical variant. Additionally, three Alzheimer pathology patients who had the syndrome of SEMD in life (FTD-A) were analyzed.

RESULTS

The VBM studies in clinical variants confirmed established patterns of atrophy (SEMD, rostral temporal; bvFTD, mesial frontal; PNFA, left insula). FTD-U and FTD-T VBM results were very similar, showing severe atrophy in the temporal poles, mesial frontal lobe, and insulae. A conjunction analysis confirmed this similarity. Subgroup analysis found that SEMD associated with either FTD-T or FTD-U was associated with similar rostral temporal atrophy; however, FTD-A had a qualitatively different pattern of left hippocampal atrophy.

CONCLUSIONS

While there is predictable atrophy for clinical variants of frontotemporal dementia (FTD), histologic FTD variants show no noticeable differences. Reports of specific atrophy profiles are likely the result of idiosyncrasies in small groups. Semantic dementia associated with Alzheimer pathology, however, presented a distinct atrophy pattern.

Understanding social dysfunction in the behavioural variant of frontotemporal dementia: the role of emotion and sarcasm processing

Social interaction is profoundly affected in the behavioural form of frontotemporal dementia (bvFTD) yet there are few means of objectively assessing this. Diagnosis of bvFTD is based on informant report, however a number of individuals with a clinical profile consistent with the disease have no imaging abnormality and seem to remain stable, with doubt about the presence of underlying neurodegenerative pathology. We aimed to quantify aspects of the behavioural disorder and link it to the underlying level of atrophy in socially relevant brain regions. We tested individuals with either bvFTD (N = 26) or Alzheimer's disease (N = 9) and 16 controls using The Awareness of Social Inference Test (TASIT) to assess their ability to identify emotion and sarcasm in video vignettes. A subset of bvFTD patients (N = 21) and controls (N = 12) were scanned using MRI within 6 months of assessment. There was marked impairment in the ability of bvFTD patients whose scans showed abnormalities to recognize sarcastic, but not sincere statements. Their capacity to interpret negative emotion was also impaired, and this appeared to be a major factor underlying the deficit in sarcasm recognition. Clinically diagnosed bvFTD patients whose scans were normal, Alzheimer's disease patients and controls had no difficulty in appreciating both types of statement. In a multivariate imaging analysis it was shown that the sarcasm (and emotion recognition) deficit was dependent on a circuit involving the lateral orbitofrontal cortex, insula, amygdala and temporal pole, particularly on the right. Performance on a more global test of cognitive function, the Addenbrooke's Cognitive Examination did not have a unique association with these regions. The TASIT is an objective test of social dysfunction in bvFTD which indexes the frontotemporal volume loss in bvFTD patients and provides an objective measure for separating behavioural patients who are likely to decline from those who may remain stable. These results provide additional evidence for the role of the orbitofrontal cortex and related structures in the processing of socially relevant signals, particularly those where negative emotion recognition is important.

Transient epileptic amnesia: regional brain atrophy and its relationship to memory deficits

Transient epileptic amnesia (TEA) is a recently recognised form of epilepsy of which the principle manifestation is recurrent, transient episodes of isolated memory loss. In addition to the amnesic episodes, many patients describe significant interictal memory difficulties. Performance on standard neuropsychological tests is often normal. However, two unusual forms of memory deficit have recently been demonstrated in TEA: (i) accelerated long-term forgetting (ALF): the excessively rapid loss of newly acquired memories over a period of days or weeks and (ii) remote autobiographical memory loss: a loss of memories for salient, personally experienced events of the past few decades. The neuroanatomical bases of TEA and its associated memory deficits are unknown. In this study, we first assessed the relationship between subjective and objective memory performance in 41 patients with TEA. We then analysed MRI data from these patients and 20 matched healthy controls, using manual volumetry and voxel-based morphometry (VBM) to correlate regional brain volumes with clinical and neuropsychological data. Subjective memory estimates were unrelated to performance on standard neuropsychological tests but were partially predicted by mood, ALF and remote autobiographical memory. Manual volumetry identified subtle hippocampal volume loss in the patient group. Both manual volumetry and VBM revealed correlations between medial temporal lobe atrophy and standard anterograde memory scores, but no relation between atrophy and ALF or remote autobiographical memory. These results add weight to the hypothesis that TEA is a syndrome of mesial temporal lobe epilepsy. Furthermore, they suggest that although standard anterograde memory test performance is related to the degree of mesial temporal lobe damage, this is not true for ALF and autobiographical amnesia. It is possible that these unusual memory deficits have a more diffuse physiological basis rather than being a consequence of discrete structural damage.

Semantic dementia and fluent primary progressive aphasia: two sides of the same coin?

Considerable controversy exists regarding the relationship between semantic dementia (SD) and progressive aphasia. SD patients present with anomia and impaired word comprehension. The widely used consensus criteria also include the need for patients to exhibit associative agnosia and/or prosopagnosia: many authors have used the label SD for patients with non-verbal, as well as verbal, semantic deficits on formal testing even if they recognize the objects and people encountered in everyday life; others interpret the criterion of agnosia to require pervasive recognition impairments affecting daily life. According to this latter view, SD patients have pathology that disrupts both a bilateral ventrotemporal-fusiform network (resulting in agnosia) and the left hemisphere language network (resulting in profound aphasia). These authors suggest that this profile is different to that seen in the fluent form of primary progressive aphasia (fPPA), a neurodegenerative disease primarily affecting language function. We present data on seven patients who met the diagnostic criteria for fPPA. All seven showed deficits relative to matched controls on both verbal and non-verbal measures of semantic memory, and these deficits were modulated by degree of anomia, concept familiarity and item typicality. Voxel-based morphometry revealed reduced grey matter density in the temporal lobes bilaterally (more widespread on the left), with the severity of atrophy in the left inferior temporal lobe being significantly related to performance on both the verbal and non-verbal measures. Together these findings suggest that patients who meet the diagnostic criteria for fPPA, can also meet the diagnostic criteria for early-stage SD provided that the impact of concept familiarity and typicality is taken into account. In addition, these findings support a claim that the patients' deficits on both verbal and non-verbal tasks reflect progressive deterioration of an amodal integrative semantic memory system critically involving the rostral temporal lobes, rather than a combination of atrophy in the left language network and a separate bilateral ventrotemporal-fusiform network.