Two hundred and thirty-four patients with unilateral cerebral pathology and 175 control subjects were examined with a sensitive test for tactile extinction. Damage to the right hemisphere was associated with extinction slightly (but not significantly) more often than damage to the left hemisphere; the asymmetry may be due to selective exclusion of aphasics with damage to the left hemisphere. Extinction of the left side of the body, however, was significantly more common than of the right side; this asymmetry could not be accounted for by exclusion of untestable aphasics, but was a consequence of frequent ipsilateral (left side) extinction among the group with damage to the left hemisphere while the group with damage to the right hemisphere extinguished the contralateral (left) side almost exclusively. Although the hemispheres as a whole did not differ in their association with extinction, lesions in the right parietal lobe were significantly more effective in producing extinction than lesions in the left; in both cases the contralateral side of the body was affected. By contrast, lesions in the left frontal lobe were moderately but not significantly more effective in producing extinction than right frontal damage; in almost all these cases the left side of the body was affected, regardless of which frontal lobe was damaged. A relationship between extinction and pathology in the vicinity of the anterior callosum, as determined from CT scan and angiography, was found among the frontal cases. We propose an anatomical model to explain tactile extinction and its asymetric characteristics in the human. During the extinction tests a response mechanism in the left (speech) hemisphere bases its perceptual output on the relative strengths of two simultaneous sensory inputs. Damage at any point in the channel from the periphery to the response mechanism weakens one signal in comparison to the other, resulting in a response bias favouring the stronger stimulus. Tactile information from the left hand, after reaching the somatosensory zone in the right hemisphere, is transmitted to the left hemisphere by a diffuse, widespread network including the frontal lobes and the anterior callosum. This anatomical arrangement renders left-hand information more vulnerable to chance lesions than right-hand information, which has direct access to the response mechanism via a more compact projection system.
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