29 research outputs found
Treatment-induced neuroplasticity after anomia therapy in post-stroke aphasia : a systematic review of neuroimaging studies
We systematically reviewed the literature on neural changes following anomia treatment post-stroke. We conducted electronic searches of CINAHL, Cochrane Trials, Embase, Ovid MEDLINE, MEDLINE-in-Process and PsycINFO databases; two independent raters assessed all abstracts and full texts. Accepted studies reported original data on adults with post-stroke aphasia, who received behavioural treatment for anomia, and magnetic resonance brain imaging (MRI) pre- and post-treatment. Search results yielded 2481 citations; 33 studies were accepted. Most studies employed functional MRI and the quality of reporting neuroimaging methodology was variable, particularly for pre-processing steps and statistical analyses. The most methodologically robust data were synthesized, focusing on pre- versus post-treatment contrasts. Studies more commonly reported increases (versus decreases) in activation following naming therapy, primarily in the left supramarginal gyrus, and left/bilateral precunei. Our findings highlight the methodological heterogeneity across MRI studies, and the paucity of robust evidence demonstrating direct links between brain and behaviour in anomia rehabilitation
Visual and auditory perceptual strength norms for 3,596 French nouns and their relationship with other psycholinguistic variables
Perceptual experience plays a critical role in the conceptual representation of words. Higher levels of semantic variables such as imageability, concreteness, and sensory experience are generally associated with faster and more accurate word processing. Nevertheless, these variables tend to be assessed mostly on the basis of visual experience. This underestimates the potential contributions of other perceptual modalities. Accordingly, recent evidence has stressed the importance of providing modality-specific perceptual strength norms. In the present study, we developed French Canadian norms of visual and auditory perceptual strength (i.e., the modalities that have major impact on word processing) for 3,596 nouns. We then explored the relationship between these newly developed variables and other lexical, orthographic, and semantic variables. Finally, we demonstrated the contributions of visual and auditory perceptual strength ratings to visual word processing beyond those of other semantic variables related to perceptual experience (e.g., concreteness, imageability, and sensory experience ratings). The ratings developed in this study are a meaningful contribution toward the implementation of new studies that will shed further light on the interaction between linguistic, semantic, and perceptual systems
Test-retest reliability of diffusion measures extracted along white matter language fiber bundles using HARDI-based tractography
High angular resolution diffusion imaging (HARDI)-based tractography has been
increasingly used in longitudinal studies on white matter macro- and micro-structural
changes in the language network during language acquisition and in language
impairments. However, test-retest reliability measurements are essential to ascertain that
the longitudinal variations observed are not related to data processing. The aims of this
study were to determine the reproducibility of the reconstruction of major white matter
fiber bundles of the language network using anatomically constrained probabilistic
tractography with constrained spherical deconvolution based on HARDI data, as well as
to assess the test-retest reliability of diffusion measures extracted along them. Eighteen
right-handed participants were scanned twice, one week apart. The arcuate, inferior
longitudinal, inferior fronto-occipital, and uncinate fasciculi were reconstructed in the left
and right hemispheres and the following diffusion measures were extracted along each
tract: fractional anisotropy, mean, axial, and radial diffusivity, number of fiber orientations,
mean length of streamlines, and volume. All fiber bundles showed good morphological
overlap between the two scanning timepoints and the test-retest reliability of all diffusion
measures in most fiber bundles was good to excellent. We thus propose a fairly simple,
but robust, HARDI-based tractography pipeline reliable for the longitudinal study of white
matter language fiber bundles, which increases its potential applicability to research on
the neurobiological mechanisms supporting language
The challenge of mapping the human connectome based on diffusion tractography
Tractography based on non-invasive diffusion imaging is central to the study of human brain connectivity. To date, the approach has not been systematically validated in ground truth studies. Based on a simulated human brain data set with ground truth tracts, we organized an open international tractography challenge, which resulted in 96 distinct submissions from 20 research groups. Here, we report the encouraging finding that most state-of-the-art algorithms produce tractograms containing 90% of the ground truth bundles (to at least some extent). However, the same tractograms contain many more invalid than valid bundles, and half of these invalid bundles occur systematically across research groups. Taken together, our results demonstrate and confirm fundamental ambiguities inherent in tract reconstruction based on orientation information alone, which need to be considered when interpreting tractography and connectivity results. Our approach provides a novel framework for estimating reliability of tractography and encourages innovation to address its current limitations
Predicting early post-stroke aphasia outcome from initial aphasia severity
Background: The greatest degree of language recovery in post-stroke aphasia takes
place within the first weeks. Aphasia severity and lesion measures have been shown to be
good predictors of long-term outcomes. However, little is known about their implications
in early spontaneous recovery. The present study sought to determine which factors
better predict early language outcomes in individuals with post-stroke aphasia.
Methods: Twenty individuals with post-stroke aphasia were assessed <72 h (acute)
and 10â14 days (subacute) after stroke onset. We developed a composite score (CS)
consisting of several linguistic sub-tests: repetition, oral comprehension and naming.
Lesion volume, lesion load and diffusion measures [fractional anisotropy (FA) and axial
diffusivity (AD)] from both arcuate fasciculi (AF) were also extracted using MRI scans
performed at the same time points. A series of regression analyses were performed to
predict the CS at the second assessment.
Results: Among the diffusion measures, only FA from right AF was found to be a
significant predictor of early subacute aphasia outcome. However, when combined in two
hierarchical models with FA, age and either lesion load or lesion size, the initial aphasia
severity was found to account for most of the variance (R
2 = 0.678), similarly to the
complete models (R
2 = 0.703 and R
2 = 0.73, respectively).
Conclusions: Initial aphasia severity was the best predictor of early post-stroke aphasia
outcome, whereas lesion measures, though highly correlated, show less influence on the
prediction model. We suggest that factors predicting early recovery may differ from those
involved in long-term recovery
Brain white matter damage and its association with neuronal synchrony during sleep
The restorative function of sleep partly relies on its ability to deeply synchronize cerebral networks to create large slow oscillations observable with EEG. However, whether a brain can properly synchronize and produce a restorative sleep when it undergoes massive and widespread white matter damage is unknown. Here, we answer this question by testing 23 patients with various levels of white matter damage secondary to moderate to severe traumatic brain injuries (ages 18â56; 17 males, six females, 11â39 months post-injury) and compared them to 27 healthy subjects of similar age and sex. We used MRI and diffusion tensor imaging metrics (e.g. fractional anisotropy as well as mean, axial and radial diffusivities) to characterize voxel-wise white matter damage. We measured the following slow wave characteristics for all slow waves detected in N2 and N3 sleep stages: peak-to-peak amplitude, negative-to-positive slope, negative and positive phase durations, oscillation frequency, and slow wave density. Correlation analyses were performed in traumatic brain injury and control participants separately, with age as a covariate. Contrary to our hypotheses, we found that greater white matter damage mainly over the frontal and temporal brain regions was strongly correlated with a pattern of higher neuronal synchrony characterized by slow waves of larger amplitudes and steeper negative-to-positive slopes during non-rapid eye movement sleep. The same pattern of associations with white matter damage was also observed with markers of high homeostatic sleep pressure. More specifically, higher white matter damage was associated with higher slow-wave activity power, as well as with more severe complaints of cognitive fatigue. These associations between white matter damage and sleep were found only in our traumatic brain injured participants, with no such correlation in controls. Our results suggest that, contrary to previous observations in healthy controls, white matter damage does not prevent the expected high cerebral synchrony during sleep. Moreover, our observations challenge the current line of hypotheses that white matter microstructure deterioration reduces cerebral synchrony during sleep. Our results showed that the relationship between white matter and the brainâs ability to synchronize during sleep is neither linear nor simple
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Deformation-based shape analysis of the hippocampus in the semantic variant of primary progressive aphasia and Alzheimerâs disease
Background
Increasing evidence shows that the semantic variant of primary progressive aphasia (svPPA) is characterized by hippocampal atrophy. However, less is known about disease-related morphological hippocampal changes. The goal of the present study is to conduct a detailed characterization of the impact of svPPA on global hippocampus volume and morphology compared with control subjects and patients with Alzheimerâs disease (AD).
Methods
We measured hippocampal volume and deformation-based shape differences in 22 patients with svPPA compared with 99 patients with AD and 92 controls. Multiple Automatically Generated Templates Brain Segmentation Algorithm (MAGeT-Brain) was used on MRI images obtained at the diagnostic visit.
Results
Comparable left and right hippocampal atrophy were observed in svPPA and AD. Deformation-based shape analysis showed a common pattern of morphological deformation in svPPA and AD compared with controls. More specifically, both svPPA and AD showed inward deformations in the dorsal surface of the hippocampus, from head to tail on the left side, and more limited to the anterior portion of the body in the right hemisphere. These results also pointed out that both diseases are characterized by a lateral displacement of the central part (body) of the hippocampus.
Discussion
Our study provides critical new evidence of hippocampal morphological changes in svPPA, similar to those found in AD. These findings highlight the importance of considering morphological hippocampal changes as part of the anatomical profile of patients with svPPA
Sleep spindles are resilient to extensive white matter deterioration
Sleep spindles are an essential part of non-rapid eye movement sleep, notably involved in sleep consolidation, cognition, learning
and memory. These oscillatory waves depend on an interaction loop between the thalamus and the cortex, which relies on a structural backbone of thalamo-cortical white matter tracts. It is still largely unknown if the brain can properly produce sleep spindles
when it underwent extensive white matter deterioration in these tracts, and we hypothesized that it would affect sleep spindle generation and morphology. We tested this hypothesis with chronic moderate to severe traumatic brain injury (n Œ 23;
30.5 6 11.1 years old; 17 m/6f), a unique human model of extensive white matter deterioration, and a healthy control group
(n Œ 27; 30.3 6 13.4 years old; 21m/6f). Sleep spindles were analysed on a full night of polysomnography over the frontal, central
and parietal brain regions, and we measured their density, morphology and sigma-band power. White matter deterioration was
quantified using diffusion-weighted MRI, with which we performed both whole-brain voxel-wise analysis (Tract-Based Spatial
Statistics) and probabilistic tractography (with High Angular Resolution Diffusion Imaging) to target the thalamo-cortical tracts.
Group differences were assessed for all variables and correlations were performed separately in each group, corrected for age and
multiple comparisons. Surprisingly, although extensive white matter damage across the brain including all thalamo-cortical tracts
was evident in the brain-injured group, sleep spindles remained completely undisrupted when compared to a healthy control group.
In addition, almost all sleep spindle characteristics were not associated with the degree of white matter deterioration in the braininjured group, except that more white matter deterioration correlated with lower spindle frequency over the frontal regions. This
study highlights the resilience of sleep spindles to the deterioration of all white matter tracts critical to their existence, as they conserve normal density during non-rapid eye movement sleep with mostly unaltered morphology. We show that even with such a severe traumatic event, the brain has the ability to adapt or to withstand alterations in order to conserve normal sleep spindles
Cerebral white matter diffusion properties and freeâwater with obstructive sleep apnea severity in older adults
Characterizing the effects of obstructive sleep apnea (OSA) on the aging brain could be key in our understanding of neurodegeneration in this population. Our objective was to assess white matter properties in newly diagnosed and untreated adults with mild to severe OSA. Sixtyâfive adults aged 55 to 85 were recruited and divided into three groups: control (apneaâhypopnea index â€5/hr; n = 18; 65.2â±â7.2âyears old), mild (>5 to â€15 hr; n = 27; 64.2â±â5.3âyears old) and moderate to severe OSA (>15/hr; n = 20; 65.2â±â5.5âyears old). Diffusion tensor imaging metrics (fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity, and mean diffusivity) were compared between groups with TractâBased Spatial Statistics within the white matter skeleton created by the technique. Groups were also compared for white matter hyperintensities volume and the freeâwater (FW) fraction. Compared with controls, mild OSA participants showed widespread areas of lower diffusivity (pâ<â.05 corrected) and lower FW fraction (pâ<â.05). Participants with moderate to severe OSA showed lower AD in the corpus callosum compared with controls (pâ<â.05 corrected). No betweenâgroup differences were observed for FA or white matter hyperintensities. Lower white matter diffusivity metrics is especially marked in mild OSA, suggesting that even the milder form may lead to detrimental outcomes. In moderate to severe OSA, competing pathological responses might have led to partial normalization of diffusion metrics
Test-Retest Reliability of Diffusion Measures Extracted Along White Matter Language Fiber Bundles Using HARDI-Based Tractography
High angular resolution diffusion imaging (HARDI)-based tractography has been increasingly used in longitudinal studies on white matter macro- and micro-structural changes in the language network during language acquisition and in language impairments. However, test-retest reliability measurements are essential to ascertain that the longitudinal variations observed are not related to data processing. The aims of this study were to determine the reproducibility of the reconstruction of major white matter fiber bundles of the language network using anatomically constrained probabilistic tractography with constrained spherical deconvolution based on HARDI data, as well as to assess the test-retest reliability of diffusion measures extracted along them. Eighteen right-handed participants were scanned twice, one week apart. The arcuate, inferior longitudinal, inferior fronto-occipital, and uncinate fasciculi were reconstructed in the left and right hemispheres and the following diffusion measures were extracted along each tract: fractional anisotropy, mean, axial, and radial diffusivity, number of fiber orientations, mean length of streamlines, and volume. All fiber bundles showed good morphological overlap between the two scanning timepoints and the test-retest reliability of all diffusion measures in most fiber bundles was good to excellent. We thus propose a fairly simple, but robust, HARDI-based tractography pipeline reliable for the longitudinal study of white matter language fiber bundles, which increases its potential applicability to research on the neurobiological mechanisms supporting language