The Right Hand Draws the Trees, But the Left Draws the Forest?

Spatial processing is lateralized: the right hemisphere is optimized for perceiving global aspects of space (“seeing the forest”), while the left hemisphere specializes in perceiving local aspects of space (“seeing the trees”). However, less is known about how the information is shared across the hemispheres and which areas within the corpus callosum are required for transferring and integrating visuospatial information. Here, we report a 60 year old woman with a mass lesion in the splenium of the corpus callosum who demonstrated visuospatial processing deficits that were out-of-proportion to the rest of her neurological examination. Remarkably, in the Rey-Osterrieth Complex figure task, she copied with her left hand the outlines of the figure (global aspects), whereas with her right hand she drew the details of that figure (local aspects). While hemispheric lesions have demonstrated single dissociations of spatial processing, these results indicate that a lesion in the corpus callosum can produce a double dissociation for high-level spatial tasks, as local and global spatial perception are further dissociated with handedness. Interestingly, as little as the posterior third of the corpus callosum is required for proper visuospatial information transfer and integration, which provides important insight into the interhemispheric functional anatomy that underlies visuospatial perception.