The neuropsychological model for the NLD syndrome was developed by Rourke (1989) based on the hemispheric differences proposed by Goldberg and Costa (1981). The model focuses on dysfunction in cerebral white matter, the long myelinated fibers thought to be responsible for association and integration of cognition (Fisher, DeLouca, & Rourke, 1997).

This figure shows an example where the brain white matter structure is reconstructed using 3D streamtube rendering. (Color denotes location along the superior-inferior axis. For reference, the inset shows the inferior hemisphere of the same brain). from Hsu & Provenzale, 2004

This image of white matter structure was created using diffusion tensor imaging (DTI).  It clearly shows the intricate network created by the white matter fibers in the brain and helps to understand their function as integrators across brain regions.

Goldberg and Costa (1981) found that the proportion of white matter to grey matter is greater in the right hemisphere of the brain than the left hemisphere. They suggested that this proportional difference impacts the processing capabilities of the two hemispheres by making the right hemisphere more capable of intermodal integration and processing of novel stimuli. The left hemisphere, with its lesser amount of white matter, they suggested, is responsible for unimodal and motor processing and storage of processes that have already been encoded by the right hemisphere (Goldberg & Costa, 1981).

Rourke (1989; Rourke & Tsatsanis, 1996) focuses on this hemispheric difference in his delineation of the subtypes of LD, proposing that the deficits exhibited by individuals with NLD are related to the creation of new processes to handle novel tasks, a capability handled by the right hemisphere. He and colleagues suggest that the more cerebral white matter that is dysfunctional, and thus the greater disorder of the right hemisphere, the more likely the NLD syndrome will be manifest (Rourke & Tsatsanis, 1996). This model has led to NLD also being referred to as developmental right-hemisphere syndrome (DRHS) (Gross-Tsur, Shalev, Manor, & Amir, 1995).

Rourke does not appear to have integrated more recent research about hemispheric specialization and white matter into his neuropsychological model, but the last 23 years have not produced findings that directly contradict Goldberg and Costa (1981). While currently popular theories of hemispheric specialization focus on verbal-spatial and analytic-holistic dichotomies rather than on the encoding of processes to handle novel tasks (Gazzaniga, Ivry, & Mangun, 2002), the right hemisphere is still believed to have a higher concentration of white matter, and white matter is even more clearly associated with integrative functions (Filley, 2001).

from Barrick, Mackay, Crow & Roberts (2001)

More recent research shown in this figure supports Goldberg & Costa's (1981) hypothesis that the right hemisphere has more concentrated white matter than the left.