Left superior parietal cortex involvement in writing: integrating fMRI with lesion evidence.

Writing is a uniquely human skill that we utilize nearly everyday. Lesion studies in patients with Gerstmann's syndrome have pointed to the parietal cortex as being critical for writing. Very little information is, however, available about the precise anatomical location of brain regions subserving writing in normal healthy individuals. In this study, we used functional magnetic resonance imaging (fMRI) to investigate parietal lobe function during writing to dictation. Significant clusters of activation were observed in left superior parietal lobe (SPL) and the dorsal aspects of the inferior parietal cortex (IPC) bordering the SPL. Localized clusters of activation were also observed in the left premotor cortex, sensorimotor cortex and supplementary motor area. No activation cluster was observed in the right hemisphere. These results clearly indicate that writing appears to be primarily organized in the language-dominant hemisphere. Further analysis revealed that within the parietal cortex, activation was significantly greater in the left SPL, compared to left IPC. Together with lesion studies, findings from the present study provide further evidence for the essential role of the left SPL in writing. Deficits to the precise left hemisphere parietal cortex regions identified in the present study may specifically underlie disorders of writing observed in Gerstmann's syndrome and apractic agraphia.