Disconnection syndromes are often conceptualized exclusively within cognitive box-and-arrow diagrams unrelated to brain anatomy. In a patient with alexia in his left visual field resulting from a posterior callosal lesion, we illustrate how diffusion tensor imaging can reveal the anatomical bases of a disconnection syndrome by tracking the degeneration of neural pathways and relating it to impaired fMRI activations and behavior. Compared to controls, an abnormal pattern of brain activity was observed in the patient during word reading, with a lack of activation of the left visual word form area (VWFA) by left-hemifield words. Statistical analyses of diffusion images revealed a damaged fiber tract linking the left ventral occipito-temporal region to its right homolog across the lesioned area of corpus callosum and stopping close to the areas found active in fMRI. The behavioral disconnection syndrome could, thus, be related functionally to abnormal fMRI activations and anatomically to the absence of a connection between those activations. The present approach, based on the “negative tracking” of degenerated bundles, provides new perspectives on the understanding of human brain connections and disconnections.

We measured cerebral activation with functional magnetic resonance imaging at 3 Tesla while eight healthy volunteers performed various number processing tasks known to be dissociable in brain-lesioned patients: naming, comparing, multiplying, or subtracting single digits. The results revealed the activation of a circuit comprising bilateral intraparietal, prefrontal, and anterior cingulate components. The extension and lateralization of this circuit was modulated by task demands. The intraparietal and prefrontal activation was more important in the right hemisphere during the comparison task and in the left hemisphere during the multiplication task and was intensely bilateral during the subtraction task. Thus, partially distinct cerebral circuits with the dorsal parietal pathway underlie distinct arithmetic operations.