Preserved auditory spatial localization following cerebral hemispherectomy.

Auditory spatial function was assessed in six patients who had undergone unilateral cerebral hemispherectomy for the relief of intractable epilepsy. Separate localization and discrimination tests were carried out. In the first test, subjects localized the azimuthal position of a brief click presented at one of 13 positions in a free field, by pointing to its perceived position. All six patients demonstrated some preservation of localization ability, with some individual subjects performing at normal or near-normal levels. However, as a group, the patient sample localized less accurately than normal controls at extreme azimuthal positions, especially contralateral to the removal. In the second test, two clicks were presented either from the same location or from two locations separated by 30 degrees, and a same-different judgment was required. In this test the patient group performed worse than control subjects overall, but also demonstrated preserved discrimination ability in both hemifields. These findings demonstrate the expected existence of contralateral localization deficits, but the relative sparing of function contrasts with reports from animal behavioural studies, in which severe and persistent localization deficits contralateral to auditory cortex excision are described. Our findings suggest that extensive, early damage to one hemisphere may permit reorganization of function to occur, with auditory spatial ability being mediated by cortical systems in the remaining hemisphere and/or by subcortical structures.