Role of the Parietal Cortex for Sensorimotor Transformation

Somatosensory and motor functions of both hands were examined in 10 patients with unilateral parietal cortex lesions as revealed by computerized tomography. Tests of elementary somatosensory functions comprised surface sensibility, vibration, and position sense. For evaluation of complex somatosensory functions, the recognition of surface textures and object forms was tested. Motor performance was examined by measuring force and position control as well as manipulative and explorative motor behaviour. All patients showed varying degrees of sensory disturbance. Although the patients had no or minor paresis, they all had motor deficits concerning force control, fine movements, and manipulation with the hand contralateral to the lesion. Patients with more posteriorly located parietal lesions revealed predominantly severe disturbances of complex sensibility, precision grip, manipulation, and explorative finger movements. The characteristic and consistent feature of the disturbed motor behaviour was the loss of the purposive nature of the motor acts which were no longer adequate for the tasks (tactile apraxia). Paresis or ataxia could not account for these deficits.