Evan Fletcher

White matter lesions impair frontal lobe function regardless of their location

Objective: To analyze the effect of white matter lesions in different brain regions on regional cortical glucose metabolism, regional cortical atrophy, and cognitive function in a sample with a broad range of cerebrovascular disease and cognitive function. Methods: Subjects (n = 78) were recruited for a study of subcortical ischemic vascular disease (SIVD) and Alzheimer disease (AD) contributions to dementia. A new method was developed to define volumes of interest from high-resolution three-dimensional T1-weighted MR images. Volumetric measures of MRI segmented white matter signal hyperintensities (WMH) in five different brain regions were related to regional PET glucose metabolism (rCMRglc) in cerebral cortex, MRI measures of regional cortical atrophy, and neuropsychological assessment of executive and memory function. Results: WMH was significantly higher in the prefrontal region compared to the other brain regions. In all subjects, higher frontal and parietal WMH were associated with reduced frontal rCMRglc, whereas occipitotemporal WMH was only marginally associated with frontal rCMRglc. These associations were stronger and more widely distributed in nondemented subjects where reduced frontal rCMRglc was correlated with WMH for all regions measured. In contrast, there was no relationship between WMH in any brain region and rCMRglc in either parietal or occipitotemporal regions. WMHs in all brain regions were associated with low executive scores in nondemented subjects. Conclusions: The frontal lobes are most severely affected by SIVD. WMHs are more abundant in the frontal region. Regardless of where in the brain these WMHs are located, they are associated with frontal hypometabolism and executive dysfunction.

Anatomical Mapping of White Matter Hyperintensities (WMH): Exploring the Relationships Between Periventricular WMH, Deep WMH, and Total WMH Burden

Background and Purpose— MRI segmentation and mapping techniques were used to assess evidence in support of categorical distinctions between periventricular white matter hyperintensities (PVWMH) and deep WMH (DWMH). Qualitative MRI studies generally identify 2 categories of WMH on the basis of anatomical localization. Separate pathophysiologies and behavioral consequences are often attributed to these 2 classes of WMH. However, evidence to support these empirical distinctions has not been rigorously sought. Methods— MRI analysis of 55 subjects included quantification of WMH volume, mapping onto a common anatomical image, and spatial localization of each WMH voxel. WMH locations were then divided into PVWMH and DWMH on the basis of distance from the lateral ventricles and correlations, with total WMH volume determined. Periventricular distance histograms of WMH voxels were also calculated. Results— PVWMH and DWMH were highly correlated with total WMH (R2>0.95) and with each other (R2>0.87). Mapping of all WMH revealed smooth expansion from around central cerebrospinal fluid spaces into more distal cerebral white matter with increasing WMH volume. Conclusion— PVWMH, DWMH, and total WMH are highly correlated with each other. Moreover, spatial analysis failed to identify distinct subpopulations for PVWMH and DWMH. These results suggest that categorical distinctions between PVWMH and DWMH may be arbitrary, and conclusions regarding individual relationships between causal factors or behavior for PVWMH and DWMH may more accurately reflect total WMH volume relationships.

Extent and distribution of white matter hyperintensities in normal aging, MCI, and AD

Objective: To analyze the extent and spatial distribution of white matter hyperintensities (WMH) in brain regions from cognitively normal older individuals (CN) and patients with mild cognitive impairment (MCI) and Alzheimer disease (AD). Methods: We studied 26 mild AD, 28 MCI, and 33 CN. MRI analysis included quantification of WMH volume, nonlinear mapping onto a common anatomic image, and spatial localization of each WMH voxel to create an anatomically precise frequency distribution map. Areas of greatest frequency of WMH from the WMH composite map were used to identify 10 anatomic regions involving periventricular areas and the corpus callosum (CC) for group comparisons. Results: Total WMH volumes were associated with age, extent of concurrent vascular risk factors, and diagnosis. After correcting for age, total WMH volumes remained significantly associated with diagnosis and extent of vascular risk. Regional WMH analyses revealed significant differences in WMH across regions that also differed significantly according to diagnosis. In post-hoc analyses, significant differences were seen between CN and AD in posterior periventricular regions and the splenium of the CC. MCI subjects had intermediate values at all regions. Repeated measures analysis including vascular risk factors in the model found a significant relationship between periventricular WMH and vascular risk that differed by region, but regional differences according to diagnosis remained significant and there was no interaction between diagnosis and vascular risk. Conclusions: Differences in white matter hyperintensities (WMH) associated with increasing cognitive impairment appear related to both extent and spatial location. Multiple regression analysis of regional WMH, vascular risk factors, and diagnosis suggest that these spatial differences may result from the additive effects of vascular and degenerative injury. Posterior periventricular and corpus callosum extension of WMH associated with mild cognitive impairment and Alzheimer disease indicate involvement of strategic white matter bundles that may contribute to the cognitive deficits seen with these syndromes.

Covariations in ERP and PET Measures of Spatial Selective Attention in Human Extrastriate Visual Cortex

In a previous study using positron emission tomography (PET), we demonstrated that focused attention to a location in the visual field produced increased regional cerebral blood flow in the fusiform gyrus contralateral to the attended hemifield (Heinze et al. [1994]: Nature 372:543). We related these effects to modulations in the amplitude of the P1 component (80–130 msec latency) of the visual event‐related brain potentials (ERPs) recorded from the same subjects, under the identical stimulus and task conditions. Here, we replicate and extend these findings by showing that attention effects in the fusiform gyrus and the P1 component were similarly modulated by the perceptual load of the task. When subjects performed a perceptually demanding symbol‐matching task within the focus of spatial attention, the fusiform activity and P1 component of the ERP were of greater magnitude than when the subjects performed a less perceptually demanding task that required only luminance detection at the attended location. In the latter condition, both the PET and ERP attention effects were reduced. In addition, in the present data significant activations were also obtained in the middle occipital gyrus contralateral to the attended hemifield, thereby demonstrating that multiple regions of extrastriate visual cortex are modulated by spatial attention. The findings of covariations between the P1 attention effect and activity in the posterior fusiform gyrus reinforce our hypothesis that common neural sources exist for these complementary, but very different measures of human brain activity. Hum. Brain Mapping 5:273–279, 1997.

White Matter Changes Compromise Prefrontal Cortex Function in Healthy Elderly Individuals

Changes in memory function in elderly individuals are often attributed to dysfunction of the prefrontal cortex (PFC). One mechanism for this dysfunction may be disruption of white matter tracts that connect the PFC with its anatomical targets. Here, we tested the hypothesis that white matter degeneration is associated with reduced prefrontal activation. We used white matter hyperintensities (WMH), a magnetic resonance imaging (MRI) finding associated with cerebrovascular disease in elderly individuals, as a marker for white matter degeneration. Specifically, we used structural MRI to quantify the extent of WMH in a group of cognitively normal elderly individuals and tested whether these measures were predictive of the magnitude of prefrontal activity (fMRI) observed during performance of an episodic retrieval task and a verbal working memory task. We also examined the effects of WMH located in the dorsolateral frontal regions with the hypothesis that dorsal PFC WMH would be strongly associated with not only PFC function, but also with areas that are anatomically and functionally linked to the PFC in a task-dependent manner. Results showed that increases in both global and regional dorsal PFC WMH volume were associated with decreases in PFC activity. In addition, dorsal PFC WMH volume was associated with decreased activity in medial temporal and anterior cingulate regions during episodic retrieval and decreased activity in the posterior parietal and anterior cingulate cortex during working memory performance. These results suggest that disruption of white matter tracts, especially within the PFC, may be a mechanism for age-related changes in memory functioning.

Longitudinal Changes in White Matter Disease and Cognition in the First Year of the Alzheimer Disease Neuroimaging Initiative

OBJECTIVE To evaluate relationships between magnetic resonance imaging (MRI)-based measures of white matter hyperintensities (WMHs), measured at baseline and longitudinally, and 1-year cognitive decline using a large convenience sample in a clinical trial design with a relatively mild profile of cardiovascular risk factors. DESIGN Convenience sample in a clinical trial design. SUBJECTS A total of 804 participants in the Alzheimer Disease Neuroimaging Initiative who received MRI scans, cognitive testing, and clinical evaluations at baseline, 6-month follow-up, and 12-month follow-up visits. For each scan, WMHs were detected automatically on coregistered sets of T1, proton density, and T2 MRI images using a validated method. Mixed-effects regression models evaluated relationships between risk factors for WMHs, WMH volume, and change in outcome measures including Mini-Mental State Examination (MMSE), Alzheimer Disease Assessment Scale-Cognitive Subscale (ADAS-Cog), and Clinical Dementia Rating Scale sum of boxes scores. Covariates in these models included race, sex, years of education, age, apolipoprotein E genotype, baseline clinical diagnosis (cognitively normal, mild cognitive impairment, or Alzheimer disease), cardiovascular risk score, and MRI-based hippocampal and brain volumes. RESULTS Higher baseline WMH volume was associated with greater subsequent 1-year increase in ADAS-Cog and decrease in MMSE scores. Greater WMH volume at follow-up was associated with greater ADAS-Cog and lower MMSE scores at follow-up. Higher baseline age and cardiovascular risk score and more impaired baseline clinical diagnosis were associated with higher baseline WMH volume. CONCLUSIONS White matter hyperintensity volume predicts 1-year cognitive decline in a relatively healthy convenience sample that was similar to clinical trial samples, and therefore should be considered as a covariate of interest at baseline and longitudinally in future AD treatment trials.

Effects of systolic blood pressure on white-matter integrity in young adults in the Framingham Heart Study: a cross-sectional study

BACKGROUND Previous studies have identified effects of age and vascular risk factors on brain injury in elderly individuals. We aimed to establish whether the effects of high blood pressure in the brain are evident as early as the fifth decade of life. METHODS In an investigation of the third generation of the Framingham Heart Study, we approached all participants in 2009 to ask whether they would be willing to undergo MRI. Consenting patients underwent clinical assessment and cerebral MRI that included T1-weighted and diffusion tensor imaging to obtain estimates of fractional anisotropy, mean diffusivity, and grey-matter volumes. All images were coregistered to a common minimum deformation template for voxel-based linear regressions relating fractional anisotropy, mean diffusivity, and grey-matter volumes to age and systolic blood pressure, with adjustment for potential confounders. FINDINGS 579 (14·1%) of 4095 participants in the third-generation cohort (mean age 39·2 years, SD 8·4) underwent brain MRI between June, 2009 and June, 2010. Age was associated with decreased fractional anisotropy and increased mean diffusivity in almost all cerebral white-matter voxels. Age was also independently associated with reduced grey-matter volumes. Increased systolic blood pressure was linearly associated with decreased regional fractional anisotropy and increased mean diffusivity, especially in the anterior corpus callosum, the inferior fronto-occipital fasciculi, and the fibres that project from the thalamus to the superior frontal gyrus. It was also strongly associated with reduced grey-matter volumes, particularly in Brodmanns area 48 on the medial surface of the temporal lobe and Brodmanns area 21 of the middle temporal gyrus. INTERPRETATION Our results suggest that subtle vascular brain injury develops insidiously during life, with discernible effects even in young adults. These findings emphasise the need for early and optimum control of blood pressure. FUNDING National Institutes of Health and National Heart, Lung, and Blood Institute; National Institute on Aging; and National Institute of Neurological Disorders and Stroke.

Dissociating top-down attentional control from selective perception and action

Research into the neural mechanisms of attention has revealed a complex network of brain regions that are involved in the execution of attention-demanding tasks. Recent advances in human neuroimaging now permit investigation of the elementary processes of attention being subserved by specific components of the brains attention system. Here we describe recent studies of spatial selective attention that made use of positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and event-related brain potentials (ERPs) to investigate the spatio-temporal dynamics of the attention-related neural activity. We first review the results from an event-related fMRI study that examined the neural mechanisms underlying top-down attentional control versus selective sensory perception. These results defined a fronto-temporal-parietal network involved in the control of spatial attention. Activity in these areas biased the neural activity in sensory brain structures coding the spatial locations of upcoming target stimuli, preceding a modulation of subsequent target processing in visual cortex. We then present preliminary evidence from a fast-rate event-related fMRI study of spatial attention that demonstrates how to disentangle the potentially overlapping hemodynamic responses elicited by temporally adjacent stimuli in studies of attentional control. Finally, we present new analyses from combined neuroimaging (PET) and event-related brain potential (ERP) studies that together reveal the timecourse of activation of brain regions implicated in attentional control and selective perception.

Sub-Regional Hippocampal Injury is Associated with Fornix Degeneration in Alzheimer's Disease.

We examined in vivo evidence of axonal degeneration in association with neuronal pathology in Alzheimer’s disease (AD) through analysis of fornix microstructural integrity and measures of hippocampal subfield atrophy. Based on known anatomical topography, we hypothesized that the local thickness of subiculum and CA1 hippocampus fields would be associated with fornix integrity, reflecting an association between AD-related injury to hippocampal neurons and degeneration of associated axon fibers. To test this hypothesis, multi-modal imaging, combining measures of local hippocampal radii with diffusion tensor imaging (DTI), was applied to 44 individuals clinically diagnosed with AD, 44 individuals clinically diagnosed with mild cognitive impairment (MCI), and 96 cognitively normal individuals. Fornix microstructural degradation, as measured by reduced DTI-based fractional anisotropy (FA), was prominent in both MCI and AD, and was associated with reduced hippocampal volumes. Further, reduced fornix FA was associated with reduced anterior CA1 and antero-medial subiculum thickness. Finally, while both lesser fornix FA and lesser hippocampal volume were associated with lesser episodic memory, only the hippocampal measures were significant predictors of episodic memory in models including both hippocampal and fornix predictors. The region-specific association between fornix integrity and hippocampal neuronal death may provide in vivo evidence for degenerative white matter injury in AD: axonal pathology that is closely linked to neuronal injury.

Regional pattern of white matter microstructural changes in normal aging, MCI, and AD

Objective: To cross-sectionally compare the regional white matter fractional anisotropy (FA) of cognitively normal (CN) older individuals and patients with mild cognitive impairment (MCI) and Alzheimer disease (AD), separately focusing on the normal-appearing white matter (NAWM) and white matter hyperintensities (WMH), and to test the independent effects of presumed degenerative and vascular process on FA differences. Methods: Forty-seven patients with AD, 73 patients with MCI, and 95 CN subjects received diffusion tensor imaging and vascular risk evaluation. To properly control normal regional variability of FA, we divided cerebral white matter into 4 strata as measured from a series of young healthy individuals (H1 = highest; H2 = intermediate high; H3 = intermediate low; H4 = lowest anisotropy stratum). Results: For overall cerebral white matter, patients with AD had significantly lower FA than CN individuals or patients with MCI in the regions with higher baseline anisotropy (H1, H2, and H3), corresponding to long corticocortical association fibers, but not in H4, which mostly includes heterogeneously oriented fibers. Vascular risk showed significant independent effects on FA in all strata except H1, which corresponds to the genu and splenium of the corpus callosum. Similar results were found within NAWM. FA in WMH was significantly lower than NAWM across all strata but was not associated with diagnosis or vascular risk. Conclusions: Both vascular and Alzheimer disease degenerative process contribute to microstructural injury of cerebral white matter across the spectrum of cognitive ability and have different region-specific injury patterns.