Built on an analogy between the visual and auditory systems, the following dual stream model for language processing was suggested recently: a dorsal stream is involved in mapping sound to articulation, and a ventral stream in mapping sound to meaning. The goal of the study presented here was to test the neuroanatomical basis of this model. Combining functional magnetic resonance imaging (fMRI) with a novel diffusion tensor imaging (DTI)-based tractography method we were able to identify the most probable anatomical pathways connecting brain regions activated during two prototypical language tasks. Sublexical repetition of speech is subserved by a dorsal pathway, connecting the superior temporal lobe and premotor cortices in the frontal lobe via the arcuate and superior longitudinal fascicle. In contrast, higher-level language comprehension is mediated by a ventral pathway connecting the middle temporal lobe and the ventrolateral prefrontal cortex via the extreme capsule. Thus, according to our findings, the function of the dorsal route, traditionally considered to be the major language pathway, is mainly restricted to sensory-motor mapping of sound to articulation, whereas linguistic processing of sound to meaning requires temporofrontal interaction transmitted via the ventral route.DTI ͉ extreme capsule ͉ fMRI ͉ language networks ͉ arcuate fascicle ͉ extreme capsule

In the present study, we identified the most probable trajectories of point-to-point segregated connections between functional attentional centers using a combination of functional magnetic resonance imaging and a novel diffusion tensor imaging-based algorithm for pathway extraction. Cortical regions activated by a visuospatial attention task were subsequently used as seeds for probabilistic fiber tracking in 26 healthy subjects. Combining probability maps of frontal and temporoparietal regions yielded a network that consisted of dorsal and ventral connections. The dorsal connections linked temporoparietal cortex with the frontal eye field and area 44 of the inferior frontal gyrus (IFG). Traveling along superior longitudinal and arcuate fascicles, these fibers are well described in relation to spatial attention. However, the ventral connections, which traveled in the white matter between insula (INS) cortex and putamen parallel to the sylvian fissure, were not previously described for visuospatial attention. Linking temporoparietal cortex with anterior INS and area 45 of IFG, these connections may provide an anatomical substrate for crossmodal cortical integration needed for stimulus perception and response in relation to current intention. The newly anatomically described integral network for visuospatial attention might improve the understanding of spatial attention deficits after white matter lesions.

The neural mechanisms underlying spatial neglect are still disputed. Abnormal left parietal hyperactivation is proposed to lead to the rightward attentional bias, a clinical hallmark of neglect. Extinction, another deficit of visuospatial attention, is regarded as either a 'mild' form of neglect or a distinct syndrome. Although both neglect and extinction are typical syndromes of acute right hemispheric stroke, all imaging studies investigating these syndromes were conducted at least several weeks after stroke onset, in a phase when brain reorganization has already progressed. The present study aimed at comparing the activation patterns in acute stroke patients with neglect and extinction during visuospatial processing. Using functional magnetic resonance imaging, we examined the functional state of the attention system in 33 patients with a first ever stroke (53 ± 5 h after stroke onset) and age-matched healthy subjects (n = 15). All patients had embolic infarcts within the territory of the right middle cerebral artery. Patients were divided into three groups: (i) normal visuospatial processing (control patients, n = 11); (ii) patients with visual extinction but with no signs of neglect (n = 9); and (iii) patients with visual neglect (n = 13). While undergoing functional magnetic resonance imaging, patients performed a Posner-like task for visuospatial attention with detection of the targets in the left and right visual hemifields. Patients with neglect showed the expected imbalance in the left versus right parietal activation, which however, was present also in control and extinction patients, thus representing an epiphenomenon of the acute structural lesion in the right hemisphere. Compared with control patients, neglect was characterized by reduced activation in the right parietal and lateral occipital cortex, as well as in the left frontal eye field. In contrast, the activation pattern in patients with extinction differed from all other groups by an increased activation of the left prefrontal cortex. In both patients with neglect and extinction, detection of targets in the left hemifield correlated with an activation in the left prefrontal and parietal cortex. Thus at least in acute stroke, a relative hyperactivation of the left parietal cortex is not a particular characteristic of neglect. The specific signature of neglect is represented by the dysfunction of the right parietal and lateral occipital cortex. The function of the left attentional centres might provide a compensatory role after critical right hemisphere lesions and be relevant for the contralesional spatial processing.

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