Neural consequences of acting in near versus far space: a physiological basis for clinical dissociations

We used PET to determine which brain regions are implicated when normal vol unteers bisect horizontal lines and point to dots in near (peripersonal) or far (extrapersonal) space. Studies of line bisection in patients with righ t hemisphere lesions have shown that bisection performance can be severely impaired in either near or far space while remaining within normal limits i n the other spatial domain. Likewise, clinical dissociations between pointi ng to objects in near and far space have been reported. The normal function al anatomy of these dissociations has not been demonstrated convincingly. R egional cerebral blood flow measurements using PET were carried out in 12 h ealthy right-handed male volunteers who bisected lines or pointed to dots i n near or far space, using a laser pen. Subjects performing either task in near space showed neural activity in the left dorsal occipital cortex, left intraparietal cortex, left ventral premotor cortex and left thalamus, In f ar space, subjects performing either task showed activation of the ventral occipital cortex bilaterally and the right medial temporal cortex. These da ta provide physiological support for the clinically observed dissociations demonstrating that attending to and acting in near space differentially emp loys dorsal visuomotor processing areas, whereas attending to and acting in far space differentially draws on ventral visuoperceptual processing areas , even when the motor components of the tasks are identical when performed in the two spaces.