Spatial Localization after Excision of Human Auditory Cortex

Neurophysiological and animal ablation studies concur that primary auditory cortex is necessary for computation of the spatial coordinates of a sound source. Human studies have reported conflicting findings but have often suffered from inadequate psychophysical measures and/or poor lesion localization. We tested patients with unilateral temporal lobe excisions either encroaching on or sparing Heschl's gyrus (HG), quantifying lesion extent using anatomical magnetic resonance imaging measures. Subjects performed two tasks. In the localization task, they heard single clicks in a free-field spatial array subtending 180 degrees of azimuth and indicated the perceived location with a laser pointer. In the discrimination task, two clicks were presented, and subjects indicated if they were in the same or different position. As a group, patients with right temporal excision, either encroaching onto HG or not, were significantly impaired in both hemifields in both tasks, although this was not true for all individuals. Patients with left temporal resections generally performed normally, although some of the patients with left HG excision showed impaired performance bilaterally, especially in the discrimination task. This pattern stands in marked contrast to previous studies showing significant preservation of localization in hemispherectomized patients. We conclude that (1) contrary to hypotheses derived from animal studies, human auditory spatial processes are dependent primarily on cortical areas within right superior temporal cortex, which encompass both spatial hemifields; (2) functional reorganization may not take place after restricted focal damage but only after more extensive early damage; and (3) the existence of individual differences likely illustrates differential patterns of functional lateralization and/or recovery.