Neglect in a patient with partial callosal disconnection.

A right-handed woman suffered destruction of the anterior two thirds of the corpus callosum and the adjacent medial frontal lobes as a sequel of bleeding from an anterior cerebral artery aneurysm. When drawing and writing with her right hand, the patient displayed neglect of the left hemispace. In line bisection each hand deviated towards the side ipsilateral to the lesion, but the size of error decreased when the lines were placed to the ipsilateral side of the body. When tested for her verbal report of tachistoscopically presented, horizontal pairs of simple geometrical figures, she showed a systematic rightward shift of attention when stimuli were presented to the left hemispace or to the centre, but not with presentation to the right hemispace. These results cannot be attributed to a mere deficit in motor control of the hand, nor can they be entirely referred to perceptual rivalry between simultaneous bilateral stimuli. It seems that a directional bias in favour of the right side is introduced when the left hemisphere processes sensory information to prepare a motor or verbal response.