Inaccurate reaching associated with a superior parietal lobe tumor.

A man with a tumor in the right superior parietal lobule had difficulty reaching for visualized objects. There were no significant deficits in visual sensation, visual attention, somatosensory function, elementary motility, praxis, or visuospatial performance. If allowed to visually fixate the target before reaching, he misreached only with his left arm and only when he was not allowed to observe the reaching limb. If he was required to maintain central visual fixation while reaching into his peripheral visual fields, his left arm misreached into both visual hemifields but his right arm misreached only into the left visual hemifield. These results demonstrate abnormalities, referable to both the contralateral arm and the contralateral visual field, that can neither be reduced to elementary disturbances of visual or somatosensory function nor to an elementary disturbance of motility. This pattern of misreaching has not been previously reported in human subjects or in experimental animals, but this may be attributable to differences of methodology. The misreaching observed in this patient may correspond to loss of posterior parietal neurons serving a supramodal integrative function.