Left hemisphere involvement in left spatial neglect from right-sided lesions.

Current views of the unilateral neglect syndrome caused by right-sided focal lesions generally imply that the left hemisphere is lacking in awareness for the left half of the body and surrounding space. Questioning this assumption on the basis of previous split-brain and hemispherectomy observations, we applied lateralized tests for left hemineglect in three subjects with complete forebrain commissurotomy. Results for the left disconnected hemisphere revealed substantial awareness for the left side of the body and also for extrapersonal space, far greater than suggested by the unilateral lesion data. It is inferred that the left hemisphere, functioning independently, possesses the requisite cognitive mechanisms needed to prevent the appearance of the typical neglect syndrome as observed following right-sided lesions. It is proposed that the eventual explanation of this disorder must therefore account for the lack of left hemisphere compensation.