Superior parietal lobule
Area of the cortex involved in peripheral processes specific to handwriting.
Damage to the left superior parietal lobule, should generate distorted graphemes but not misspelled words, while damage to other areas of the cortex like the frontal lobe should produce alterations in written and oral spelling without distorted graphemes.
Magrassi et al. describe the clinical and neuropsychological features of a patient with combined agraphia for handwriting and typewriting bearing a small glioblastoma in the left parietal lobe. His agraphia resolved after antiedema therapy and they tested by bipolar cortical stimulation his handwriting abilities during an awake neurosurgical procedure.
They could reversibly re-induce the same defects of writing by stimulating during surgery a limited area of the superior parietal gyrus in the same patient and in an independent patient that was never agraphic before the operation. In those patients stimulation caused spelling errors, poorly formed letters and in some cases a complete cessation of writing with minimal or no effects on oral spelling.
The results suggest that stimulating a specific area in the superior parietal gyrus can generate different patterns of agraphia. Moreover, the findings also suggest that some of the central processes specific for typing and handwriting converge with motor processes at least in the limited portion of the superior parietal gyrus were thw patient was mapped 1).
The superior parietal lobule was inconstantly involved in calculation processing (40% of cases in the left and 75% in the right side) 2).