Pseudoneglect in a patient with partial callosal disconnection.

Each hemisphere may not only be responsible for processing stimuli from the contralateral sensory systems and for programming movements of the contralateral hand, but may also be important for mediating attention-intention within the contralateral hemispace. If each hemisphere mediates attention-intention within contralateral hemispace, both hemispheres must work in concert especially when an arm is crossed and used in opposite hemispace. If both hemispheres did not work together, the directional bias of each hemisphere would become manifest and induce the contralateral extremity to err toward its own hemispace, inducing pseudoneglect . To test this hypothesis, we examined a patient who had a partial callosal disconnection. In trying visually and somaesthetically to bisect lines with her left hand in right hemispace, the patient made systematic errors to the left; in bisecting lines in left hemispace with her right hand, she made systematic errors to the right. The patient also made less dramatic systematic errors even when a hand was used in its own hemispace, which suggests that each hemisphere mediates attention-intention within and towards the contralateral hemispace.