Lateralization of brain mechanisms underlying automatic and controlled forms of spatial orienting of attention

Unilateral Spatial Neglect (USN) can be considered as a defect in spatial orienting of attention toward the half space contralateral to the damaged hemisphere. Since the classical work of Brain, W.R. Brain 64:224-272 (1941), several authors have emphasized that this defect is definitely more frequent and severe in patients with right brain damage, but the reason for this hemispheric asymmetry has remained controversial. Several investigations conducted with different experimental paradigms in different laboratories have shown that only automatic orienting of attention toward stimuli arising in the half space contralateral to the damaged hemisphere is disrupted in USN, whereas controlled, volitional orienting can be more or less completely spared. The hypothesis that at the level of the right hemisphere, spatial orienting of attention may be mainly prompted by automatic mechanisms may, therefore, be advanced. By contrast, the left hemisphere might play a leading role in mechanisms underlying volitional orienting of attention. According to this model, recovery from USN could be due to substitution of the lost automatic orienting mechanisms with the spread volitional orienting mechanisms subserved by the intact left hemisphere.