Human language requires constant learning of new words, leading to the acquisition of an average vocabulary of more than 30,000 words in adult life. The ability to learn new words is highly variable ...and may rely on the integration between auditory and motor information. Here, we combined diffusion imaging tractography and functional MRI to study whether the strength of anatomical and functional connectivity between auditory and motor language networks is associated with word learning ability. Our results showed that performance in word learning correlates with microstructural properties and strength of functional connectivity of the direct connections between Broca's and Wernicke's territories in the left hemisphere. This study suggests that our ability to learn new words relies on an efficient and fast communication between temporal and frontal areas. The absence of these connections in other animals may explain the unique ability of learning words in humans.
The frontal aslant tract is a direct pathway connecting Broca's region with the anterior cingulate and pre-supplementary motor area. This tract is left lateralized in right-handed subjects, ...suggesting a possible role in language. However, there are no previous studies that have reported an involvement of this tract in language disorders. In this study we used diffusion tractography to define the anatomy of the frontal aslant tract in relation to verbal fluency and grammar impairment in primary progressive aphasia. Thirty-five patients with primary progressive aphasia and 29 control subjects were recruited. Tractography was used to obtain indirect indices of microstructural organization of the frontal aslant tract. In addition, tractography analysis of the uncinate fasciculus, a tract associated with semantic processing deficits, was performed. Damage to the frontal aslant tract correlated with performance in verbal fluency as assessed by the Cinderella story test. Conversely, damage to the uncinate fasciculus correlated with deficits in semantic processing as assessed by the Peabody Picture Vocabulary Test. Neither tract correlated with grammatical or repetition deficits. Significant group differences were found in the frontal aslant tract of patients with the non-fluent/agrammatic variant and in the uncinate fasciculus of patients with the semantic variant. These findings indicate that degeneration of the frontal aslant tract underlies verbal fluency deficits in primary progressive aphasia and further confirm the role of the uncinate fasciculus in semantic processing. The lack of correlation between damage to the frontal aslant tract and grammar deficits suggests that verbal fluency and grammar processing rely on distinct anatomical networks.
Experimental data on monkeys and functional studies in humans support the existence of a complex fronto-parietal system activating for cognitive and motor tasks, which may be anatomically supported ...by the superior longitudinal fasciculus (SLF). Advanced tractography methods have recently allowed the separation of the three branches of the SLF but are not suitable for their functional investigation. In order to gather comprehensive information about the functional organisation of these fronto-parietal connections, we used an innovative method, which combined tractography of the SLF in the largest dataset so far (129 participants) with 14 meta-analyses of functional magnetic resonance imaging (fMRI) studies. We found that frontal and parietal functions can be clustered into a dorsal spatial/motor network associated with the SLF I, and a ventral non-spatial/motor network associated with the SLF III. Further, all the investigated functions activated a middle network mostly associated with the SLF II. Our findings suggest that dorsal and ventral fronto-parietal networks are segregated but also share regions of activation, which may support flexible response properties or conscious processing. In sum, our novel combined approach provided novel findings on the functional organisation of fronto-parietal networks, and may be successfully applied to other brain connections.
•We used a novel approach to investigate fronto-parietal functions.•These are segregated into a dorsal spatial and a ventral non-spatial network.•These networks rely on the superior longitudinal fasciculus (1st and 3rd branch).•They overlap on areas with flexible response properties that rely on the 2nd branch.
Stroke-induced aphasia is associated with adverse effects on quality of life and the ability to return to work. For patients and clinicians the possibility of relying on valid predictors of recovery ...is an important asset in the clinical management of stroke-related impairment. Age, level of education, type and severity of initial symptoms are established predictors of recovery. However, anatomical predictors are still poorly understood. In this prospective longitudinal study, we intended to assess anatomical predictors of recovery derived from diffusion tractography of the perisylvian language networks. Our study focused on the arcuate fasciculus, a language pathway composed of three segments connecting Wernicke's to Broca's region (i.e. long segment), Wernicke's to Geschwind's region (i.e. posterior segment) and Broca's to Geschwind's region (i.e. anterior segment). In our study we were particularly interested in understanding how lateralization of the arcuate fasciculus impacts on severity of symptoms and their recovery. Sixteen patients (10 males; mean age 60 ± 17 years, range 28-87 years) underwent post stroke language assessment with the Revised Western Aphasia Battery and neuroimaging scanning within a fortnight from symptoms onset. Language assessment was repeated at 6 months. Backward elimination analysis identified a subset of predictor variables (age, sex, lesion size) to be introduced to further regression analyses. A hierarchical regression was conducted with the longitudinal aphasia severity as the dependent variable. The first model included the subset of variables as previously defined. The second model additionally introduced the left and right arcuate fasciculus (separate analysis for each segment). Lesion size was identified as the only independent predictor of longitudinal aphasia severity in the left hemisphere beta = -0.630, t(-3.129), P = 0.011. For the right hemisphere, age beta = -0.678, t(-3.087), P = 0.010 and volume of the long segment of the arcuate fasciculus beta = 0.730, t(2.732), P = 0.020 were predictors of longitudinal aphasia severity. Adding the volume of the right long segment to the first-level model increased the overall predictive power of the model from 28% to 57% F(1,11) = 7.46, P = 0.02. These findings suggest that different predictors of recovery are at play in the left and right hemisphere. The right hemisphere language network seems to be important in aphasia recovery after left hemispheric stroke.
Early anatomically based models of language consisted of an arcuate tract connecting Broca's speech and Wernicke's comprehension centers; a lesion of the tract resulted in conduction aphasia. ...However, the heterogeneous clinical presentations of conduction aphasia suggest a greater complexity of perisylvian anatomical connections than allowed for in the classical anatomical model. This article re‐explores perisylvian language connectivity using in vivo diffusion tensor magnetic resonance imaging tractography. Diffusion tensor magnetic resonance imaging data from 11 right‐handed healthy male subjects were averaged, and the arcuate fasciculus of the left hemisphere reconstructed from this data using an interactive dissection technique. Beyond the classical arcuate pathway connecting Broca's and Wernicke's areas directly, we show a previously undescribed, indirect pathway passing through inferior parietal cortex. The indirect pathway runs parallel and lateral to the classical arcuate fasciculus and is composed of an anterior segment connecting Broca's territory with the inferior parietal lobe and a posterior segment connecting the inferior parietal lobe to Wernicke's territory. This model of two parallel pathways helps explain the diverse clinical presentations of conduction aphasia. The anatomical findings are also relevant to the evolution of language, provide a framework for Lichtheim's symptom‐based neurological model of aphasia, and constrain, anatomically, contemporary connectionist accounts of language. Ann Neurol 2005
OBJECTIVETo combine MRI-based cortical morphometry and diffusion white matter tractography to describe the anatomical correlates of repetition deficits in patients with primary progressive aphasia ...(PPA).
METHODSThe traditional anatomical model of language identifies a network for word repetition that includes Wernicke and Broca regions directly connected via the arcuate fasciculus. Recent tractography findings of an indirect pathway between Wernicke and Broca regions suggest a critical role of the inferior parietal lobe for repetition. To test whether repetition deficits are associated with damage to the direct or indirect pathway between both regions, tractography analysis was performed in 30 patients with PPA (64.27 ± 8.51 years) and 22 healthy controls. Cortical volume measurements were also extracted from 8 perisylvian language areas connected by the direct and indirect pathways.
RESULTSCompared to healthy controls, patients with PPA presented with reduced performance in repetition tasks and increased damage to most of the perisylvian cortical regions and their connections through the indirect pathway. Repetition deficits were prominent in patients with cortical atrophy of the temporo-parietal region with volumetric reductions of the indirect pathway.
CONCLUSIONSThe results suggest that in PPA, deficits in repetition are due to damage to the temporo-parietal cortex and its connections to Wernicke and Broca regions. We therefore propose a revised language model that also includes an indirect pathway for repetition, which has important clinical implications for the functional mapping and treatment of neurologic patients.
Spherical deconvolution methods have been applied to diffusion MRI to improve diffusion tensor tractography results in brain regions with multiple fibre crossing. Recent developments, such as the ...introduction of non-negative constraints on the solution, allow a more accurate estimation of fibre orientations by reducing instability effects due to noise robustness. Standard convolution methods do not, however, adequately model the effects of partial volume from isotropic tissue, such as gray matter, or cerebrospinal fluid, which may degrade spherical deconvolution results. Here we use a newly developed spherical deconvolution algorithm based on an adaptive regularization (damped version of the Richardson–Lucy algorithm) to reduce isotropic partial volume effects. Results from both simulated and in vivo datasets show that, compared to a standard non-negative constrained algorithm, the damped Richardson–Lucy algorithm reduces spurious fibre orientations and preserves angular resolution of the main fibre orientations. These findings suggest that, in some brain regions, non-negative constraints alone may not be sufficient to reduce spurious fibre orientations. Considering both the speed of processing and the scan time required, this new method has the potential for better characterizing white matter anatomy and the integrity of pathological tissue.
Abstract
Humans show a preference for using the right hand over the left for tasks and activities of everyday life. While experimental work in non-human primates has identified the neural systems ...responsible for reaching and grasping, the neural basis of lateralized motor behavior in humans remains elusive. The advent of diffusion imaging tractography for studying connectional anatomy in the living human brain provides the possibility of understanding the relationship between hemispheric asymmetry, hand preference, and manual specialization. In this study, diffusion tractography was used to demonstrate an interaction between hand preference and the asymmetry of frontoparietal tracts, specifically the dorsal branch of the superior longitudinal fasciculus, responsible for visuospatial integration and motor planning. This is in contrast to the corticospinal tract and the superior cerebellar peduncle, for which asymmetry was not related to hand preference. Asymmetry of the dorsal frontoparietal tract was also highly correlated with the degree of lateralization in tasks requiring visuospatial integration and fine motor control. These results suggest a common anatomical substrate for hand preference and lateralized manual specialization in frontoparietal tracts important for visuomotor processing.
The occipital and frontal lobes are anatomically distant yet functionally highly integrated to generate some of the most complex behaviour. A series of long associative fibres, such as the ...fronto-occipital networks, mediate this integration via rapid feed-forward propagation of visual input to anterior frontal regions and direct top–down modulation of early visual processing.
Despite the vast number of anatomical investigations a general consensus on the anatomy of fronto-occipital connections is not forthcoming. For example, in the monkey the existence of a human equivalent of the ‘inferior fronto-occipital fasciculus’ (iFOF) has not been demonstrated. Conversely, a ‘superior fronto-occipital fasciculus’ (sFOF), also referred to as ‘subcallosal bundle’ by some authors, is reported in monkey axonal tracing studies but not in human dissections.
In this study our aim is twofold. First, we use diffusion tractography to delineate the in vivo anatomy of the sFOF and the iFOF in 30 healthy subjects and three acallosal brains. Second, we provide a comprehensive review of the post-mortem and neuroimaging studies of the fronto-occipital connections published over the last two centuries, together with the first integral translation of Onufrowicz's original description of a human fronto-occipital fasciculus (1887) and Muratoff's report of the ‘subcallosal bundle’ in animals (1893).
Our tractography dissections suggest that in the human brain (i) the iFOF is a bilateral association pathway connecting ventro-medial occipital cortex to orbital and polar frontal cortex, (ii) the sFOF overlaps with branches of the superior longitudinal fasciculus (SLF) and probably represents an ‘occipital extension’ of the SLF, (iii) the subcallosal bundle of Muratoff is probably a complex tract encompassing ascending thalamo-frontal and descending fronto-caudate connections and is therefore a projection rather than an associative tract.
In conclusion, our experimental findings and review of the literature suggest that a ventral pathway in humans, namely the iFOF, mediates a direct communication between occipital and frontal lobes. Whether the iFOF represents a unique human pathway awaits further ad hoc investigations in animals.
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