Somatosensory and motor disturbances in patients with parietal lobe lesions.

Lesion studies show that a wide range of integrative sensorimotor functions can be selectively disturbed in patients with parietal lobe damage. Lesions restricted to the somatosensory representations on the anterior parietal lobe produce somatosensory deficits that resemble deafferentated states, including the secondary effects on motor control. Slightly more posterior lesions often are associated with impairment of more complex synthetic somatosensory functions similar to those observed after dorsal column lesions. Damage of the posterior parietal lobe can selectively interfere with virtually every aspect of somatosensory function. These perceptive and cognitive disturbances may or may not be associated with complex motor disturbances of the apractic type. The frequent association of astereognosia and tactile apraxia illustrate the mutual interdependence of the sensorimotor processes involved in active touch and the proximity of the respective processing modules. Parietal lobe function is critical for the control of force and posture, and for the formation of the body image and its relation to external space (the guidance of movements, including the eyes, to external objects). Imaging studies underscore the prominent role of the parietal cortex as a sensorimotor interface and provide complementary information about the interrelationship between perception and action. Action observation activates premotor cortex, but parietal cortex is also recruited whenever an action involves objects, thus emphasizing the significance of parietal cortex for object-directed motor behavior. In contrast to the ventral-dorsal route processing dichotomy in the visual system, both the perceptual-cognitive and motor aspects of somatosensory processing are compromised after parietal damage, demonstrating a different functional architecture of the two sensory systems. The preservation of the comprehension of the meaning of gestures or of object use in patients with lesions restricted to the parietal lobe reveals that the semantic aspects of motor behavior are mediated in the temporal lobe.