Astereognosis. Tactile discrimination after localized hemispheric lesions in man.

The specific object of this study was to determine which cortical areas have to be damaged or disconnected to cause an impairment of somatosensory discrimination (astereognosis). Ninety-three patients with verified unilateral and circumscribed lesions took part in a somatosensory two-alternative forced-choice discrimination of size and shape. The results failed to support earlier views of astereognosis. Only direct damage to or an undercutting of the anterior part of the middle third of the postcentral gyrus caused impairment of size and shape discrimination (astereognosis) contralateral to the lesion. When the hand is used for stereognostic discrimination, the integration of somatosensory impulse patterns into spatial information about the objects is believed to take place in the contralateral somatosensory hand area.