Able to name, unable to compare: the visual abilities of a posterior split-brain patient

[Hemispheric specialisation versus inter-hemispheric communication]

The first part of this article covers the main discoveries that led to the concept of hemispheric specialisation, from Egyptian antiquity to present times, through the pivotal XIXth century period that saw the attribution of specific cognitive functions to the left and right hemispheres. Next, this dichotomous conception of cerebral function, attributing a given process to a hemisphere and hypothesising callosal transmission, is discussed in the light of recent studies on language comprehension. Present day knowledge suggesting an alternative to the structuralist view of hemispheric specialisation in the form of dynamic, complementary sharing of labour, and of cooperation through transcortical neural networks, is then considered. Finally, the role of the corpus callosum in interhemispheric communication is briefly covered. An emphasis is placed on the diversity of this structure that is at the origin of highly different functions (fibre size, homotopic vs heterotopic connections). Ultimately, we contrast the view of a corpus callosum serving as an information transmitting channel with that of a fibre tract co-activating the non-engaged hemisphere and preparing it for potential stimulation. In this manner, the corpus callosum minimises disparities in the distribution of attention between the two hemispheres.

Functional connectivity: integrating behavioral, diffusion tensor imaging, and functional magnetic resonance imaging data sets

In the present study, we combined 2 types of magnetic resonance technology to explore individual differences on a task that required the recognition of objects presented from unusual viewpoints. This task was chosen based on previous work that has established the necessity of information transfer from the right parietal cortex to the left inferior cortex for its successful completion. We used reaction times (RTs) to localize regions of cortical activity in the superior parietal and inferior frontal regions (blood oxygen level-dependent [BOLD] response) that were more active with longer response times. These regions were then sampled, and their signal change used to predict individual differences in structural integrity of white matter in the corpus callosum (using diffusion tensor imaging). Results show that shorter RTs (and associated increases in BOLD response) are associated with increased organization in the splenium of the corpus callosum, whereas longer RTs are associated with increased organization in the genu.

Visualizing the neural bases of a disconnection syndrome with diffusion tensor imaging

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.