Publications

Abstract

A large amount of variability exists across human brains; revealed initially on a small scale by postmortem studies and,
more recently, on a larger scale with the advent of neuroimaging. Here we compared structural variability between human
and macaque monkey brains using grey and white matter magnetic resonance imaging measures. The monkey brain was
overall structurally as variable as the human brain, but variability had a distinct distribution pattern, with some key areas
showing high variability. We also report the first evidence of a relationship between anatomical variability and evolutionary
expansion in the primate brain. This suggests a relationship between variability and stability, where areas of low variability
may have evolved less recently and have more stability, while areas of high variability may have evolved more recently and
be less similar across individuals. We showed specific differences between the species in key areas, including the amount of
hemispheric asymmetry in variability, which was left-lateralized in the human brain across several phylogenetically recent
regions. This suggests that cerebral variability may be another useful measure for comparison between species and may add
another dimension to our understanding of evolutionary mechanisms.

Abstract

Patients with stroke offer a unique window into understanding human brain function. Mapping stroke lesions poses several challenges due to the complexity of the lesion anatomy and the mechanisms causing local and remote disruption on brain networks. In this prospective longitudinal study, we compare standard and advanced approaches to white matter lesion mapping applied to acute stroke patients with aphasia. Eighteen patients with acute left hemisphere stroke were recruited and scanned within two weeks from symptom onset. Aphasia assessment was performed at baseline and six-month follow-up. Structural and diffusion MRI contrasts indicated an area of maximum overlap in the anterior external/extreme capsule with diffusion images showing a larger overlap extending into posterior perisylvian regions. Anatomical predictors of recovery included damage to ipsilesional tracts (as shown by both structural and diffusion images) and contralesional tracts (as shown by diffusion images only). These findings indicate converging results from structural and diffusion lesion mapping methods but also clear differences between the two approaches in their ability to identify predictors of recovery outside the lesioned regions.

Abstract

Structural imaging based on computerized tomography (CT) and magnetic resonance imaging (MRI) has progressively replaced traditional post‐mortem studies in the process of identifying the neuroanatomical basis of language. In the clinical setting, the information provided by structural imaging has been used to confirm the exact diagnosis and formulate an individualized treatment plan. In the research arena, neuroimaging has permitted to understand neuroanatomy at the individual and group level. The possibility to obtain quantitative measures of lesions has improved correlation analyses between severity of symptoms, lesion load, and lesion location. More recently, the development of structural imaging based on diffusion MRI has provided valid solutions to two major limitations of more conventional imaging. In stroke patients, diffusion can visualize early changes due to a stroke that are otherwise not detectable with more conventional structural imaging, with important implications for the clinical management of acute stroke patients. Beyond the sensitivity to early changes, diffusion imaging tractography presents the possibility of visualizing the trajectories of individual white matter pathways connecting distant regions. A pathway analysis based on tractography is offering a new perspective in neurolinguistics. First, it permits to formulate new anatomical models of language function in the healthy brain and allows to directly test these models in the human population without any reliance on animal models. Second, by defining the exact location of the damage to specific white matter connections we can understand the contribution of different mechanisms to the emergence of language deficits (e.g., cortical versus disconnection mechanisms). Finally, a better understanding of the anatomical variability of different language networks is helping to identify new anatomical predictors of language recovery. In this chapter we will focus on the principles of structural MRI and, in particular, diffusion imaging and tractography and present examples of how these methods have informed our understanding of variance in language performances in the healthy brain and language deficits in patient populations.

Abstract

Insight in the developmental trajectory of the neuroanatomical reading correlates is important to understand related cognitive processes and disorders. In adults, a dual pathway model has been suggested encompassing a dorsal phonological and a ventral orthographic white matter system. This dichotomy seems not present in pre-readers, and the specific role of ventral white matter in reading remains unclear. Therefore, the present longitudinal study investigated the relation between ventral white matter and cognitive processes underlying reading in children with a broad range of reading skills (n = 61). Ventral pathways of the reading network were manually traced using diffusion tractography: the inferior fronto-occipital fasciculus (IFOF), inferior longitudinal fasciculus (ILF) and uncinate fasciculus (UF). Pathways were examined pre-reading (5–6 years) and after two years of reading acquisition (7–8 years). Dimension reduction for the cognitive measures resulted in one component for pre-reading cognitive measures and a separate phonological and orthographic component for the early reading measures. Regression analyses revealed a relation between the pre-reading cognitive component and bilateral IFOF and left ILF. Interestingly, exclusively the left IFOF was related to the orthographic component, whereas none of the pathways was related to the phonological component. Hence, the left IFOF seems to serve as the lexical reading route, already in the earliest reading stages.

Abstract

The parietal lobe has a unique place in the human brain. Anatomically, it is at the crossroad between the frontal, occipital, and temporal lobes, thus providing a middle ground for multimodal sensory integration. Functionally, it supports higher cognitive functions that are characteristic of the human species, such as mathematical cognition, semantic and pragmatic aspects of language, and abstract thinking. Despite its importance, a comprehensive comparison of human and simian intraparietal networks is missing.

In this study, we used diffusion imaging tractography to reconstruct the major intralobar parietal tracts in twenty-one datasets acquired in vivo from healthy human subjects and eleven ex vivo datasets from five vervet and six macaque monkeys. Three regions of interest (postcentral gyrus, superior parietal lobule and inferior parietal lobule) were used to identify the tracts. Surface projections were reconstructed for both species and results compared to identify similarities or differences in tract anatomy (i.e., trajectories and cortical projections). In addition, post-mortem dissections were performed in a human brain.

The largest tract identified in both human and monkey brains is a vertical pathway between the superior and inferior parietal lobules. This tract can be divided into an anterior (supramarginal gyrus) and a posterior (angular gyrus) component in both humans and monkey brains. The second prominent intraparietal tract connects the postcentral gyrus to both supramarginal and angular gyri of the inferior parietal lobule in humans but only to the supramarginal gyrus in the monkey brain. The third tract connects the postcentral gyrus to the anterior region of the superior parietal lobule and is more prominent in monkeys compared to humans. Finally, short U-shaped fibres in the medial and lateral aspects of the parietal lobe were identified in both species. A tract connecting the medial parietal cortex to the lateral inferior parietal cortex was observed in the monkey brain only.

Our findings suggest a consistent pattern of intralobar parietal connections between humans and monkeys with some differences for those areas that have cytoarchitectonically distinct features in humans. The overall pattern of intraparietal connectivity supports the special role of the inferior parietal lobule in cognitive functions characteristic of humans.

Abstract

Background: Functional neuroimaging supports the hypothesis that auditory verbal hallucinations (AVH) in schizophrenia result from altered functional connectivity between perisylvian language regions, although the extent to which AVH are also associated with an altered tract anatomy is less clear.

Methods: Twenty-eight patients with schizophrenia subdivided into 17 subjects with a history of AVH and 11 without a history of hallucinations and 59 age- and IQ-matched healthy controls were recruited. The number of streamlines, fractional anisotropy (FA), and mean diffusivity were measured along the length of the arcuate fasciculus and its medial and lateral components.

Results: Patients with schizophrenia had bilateral reduction of FA relative to controls in the arcuate fasciculi (p < .001). Virtual dissection of the subcomponents of the arcuate fasciculi revealed that these reductions were specific to connections between posterior temporal and anterior regions in the inferior frontal and parietal lobe. Also, compared with controls, the reduction in FA of these tracts was highest, and bilateral, in patients with AVH, but in patients without AVH, this reduction was reported only on the left.

Conclusions: These findings point toward a supraregional network model of AVH in schizophrenia. They support the hypothesis that there may be selective vulnerability of specific anatomical connections to posterior temporal regions in schizophrenia and that extensive bilateral damage is associated with a greater vulnerability to AVH. If confirmed by further studies, these findings may advance our understanding of the anatomical factors that are protective against AVH and predictive of a treatment response.

Abstract

Right hemisphere dominance for visuospatial attention is characteristic of most humans, but its anatomical basis remains unknown. We report the first evidence in humans for a larger parieto-frontal network in the right than left hemisphere, and a significant correlation between the degree of anatomical lateralization and asymmetry of performance on visuospatial tasks. Our results suggest that hemispheric specialization is associated with an unbalanced speed of visuospatial processing.